Indium Corporation Blogs

Over the years, solder alloy choices have been pretty stable.  In the last century, SN63 and SN62 could be found at any company making any kind of electronic device, and both alloys were the backbone of every company making solders.

But, when lead was identified as causing health issues, it was legislated out of everything from paint to gasoline to electronics, including solders.  In 2003, RoHS (Restriction of Hazardous Substances) was passed in Europe to restrict the use of lead (as well as mercury, cadmium, hexavalent chromium, and polybrominated diphenyl ethers: PBDE) in electronics and electronic equipment.

The electronics industry is now focused on SAC alloys (so named because they contain Sn, Ag, and Cu).  But, there is also SnAg, which was used in the lead era when a higher melting point was required.  The addition of the copper (in SAC) offers the benefit of improving wetting and potentially reducing the silver content from a non-copper alloy like 96.5Sn 3.5Ag. 

But, there are many applications where SnAg works well. Changing from it would require customer and/or government approval, and that involves a lot of extra money and time. This lead-free alloy works well in the assembly of a variety of medical devices that use non-traditional metallizations and fluxes.  The Cu addition (in a SAC alloy) probably would not improve the results enough to warrant the cost of requalifying an existing medical device through government agencies, so they stay with what works. 

So, if you are using 96.5Sn 3.5Ag (or 96Sn 4Ag), don't be afraid to stick with it.  Indium Corporation offers both of these solder alloys (and over 250 other alloys) in a variety of forms: preforms, wire, paste, and ribbon.   And, if you want to look at the SAC alloy family to see if it works better in your application, we will help you with that, too.  Just contact our Application Engineering Staff for help.

Carol

I’m just back from Malaysia, where I visited one of our larger customers who has been using our high-lead (high-Pb) dispensable NC-SMQ75 solder paste for many years. No surprises there, but what many people don’t realize is that the NC-SMQ75 solder paste can be used as a no-clean material in many power die-attach applications. I [ACM] spoke to my friend SzePei Lim [SPL], our Area Technical Manager, based in Kuala Lumpur, about the revolutionary NC-SMQ75 paste:

[ACM] Please tell me about NC-SMQ75. What makes it a unique material?

[SPL] Indium's NC-SMQ75 no-clean die-attach solder paste is one of an emerging class of materials from Indium Corporation based on “ULR” (ultra-low residue) fluxes: these have residue levels of 4% or less after  reflow. NC-SMQ75 leaves only about 4%, by weight, of flux: therefore around 0.4%, by weight, of a 90%w/w metal solder paste. This is the lowest residue solder paste we know of that is widely used in the power semiconductor assembly industry. NC-SMQ75 no-clean solder paste is our best seller in the die-attach application on leadframes for power devices, such as clip-on-leadframe and leadframe-based clip-bonded stacked die. It can be applied by either dispensing or printing, and can be reflowed under either a forming gas or a nitrogen environment.

[ACM] What does “no-clean” mean in high-temperature power semiconductor die-attach applications? They are very different from standard no-clean solder paste usages.

[SPL] There are some big differences: the current flow in power semiconductors is vertical (from top to bottom or vice versa), rather than between adjacent conductors, like in surface mount technology (SMT), plus the package is overmolded with a solid-filled epoxy-based material.

A high voltage and thin die therefore combine to give a significant field strength across the die. A ULR flux with benign, hard residues and low resistivity (good electrical properties) is, therefore, critical. This type of residue also allows for good bonding to the overmolding compound, to prevent delamination during thermal cycling and MSL testing. Customers using this paste in no-clean applications report that, once the reflow profile has been optimized to minimize both voiding and residue levels, the final overmolded component is suitable for use in many different type of application, including automotive.

[ACM] Is there a tradeoff between a ULR no-clean solder paste and reduced voiding?

[SPL] A customer has to be careful to optimize their reflow profile to minimize voiding. That is true for the ULR pastes as well as other types. However, NC-SMQ75 has repeatedly proven itself to be able to reflow with less than 5% total voids in many smaller die applications, especially those less than 10 x 10mm.

[ACM] Solder pastes typically “spit” badly when reflowed, leaving undesirable flux spatter on wirebonding pads. Is it possible to use this as a no-clean paste even in a wirebonded application?

[SPL] Yes. Perhaps surprisingly, these ultra low residue characteristics enable NC-SMQ75 to be used as a true no-clean solder paste, even in the power die-attach application where subsequent steps include  wire-bonding. We have experience with several customers, where the design and placement of the paste deposit can be optimized to minimize the issue of flux spitting onto wirebond pads. And we can provide guidance where needed. This usually works best in applications where there are fewer than 5 wirebond pads per component. 

[ACM] Are there any special precautions that need to be taken when evaluating the NC-SMQ75 for no-clean power applications?

[SPL] Power semiconductor device types are undergoing rapid evolution, as the electrical demands of the devices drive customers away from thin wirebonds towards more robust copper-clip-based applications. Die are also becoming thinner: down to 50 microns, in some cases. As with all applications where there is no single set of applicable industry standard test methods, large-scale testing of multiple batches of components and paste batches is recommended, to establish sufficient data to allow clear decisions to be made on the usefulness of the solder paste in the final assembly process.

Occasional incompatibility with a specific type of semiconductor die may be seen, but it is something that we know about as a rare issue. Indium Corporation technical personnel can assist during the evaluation process, to guide customers on what to look out for. Additionally, I, and several of my colleagues, have extensive experience using NC-SMQ75 in no-clean die-attach applications. The compatibility of the final reflowed flux residue with different types of overmolding compounds is usually very good, with the Sumitomo G700 series appearing to be one of the best types, although Hitachi, Panasonic, and others may also be suitable.

Customers using a standard convection oven modified for high-temp applications need to ensure the N₂  flow rate is stable and that there is a controlled, low-ppm oxygen level throughout the oven.

[ACM] I understand that there are new, lower voiding, ultralow residue, no-clean pastes being developed for power semiconductor devices: is that true?

[SPL] Yes, our US- and China-based research and development teams, led by Dr Ning-Cheng Lee, are developing even more solder pastes for no-clean die-attach in this market. Some of these may also be applicable for our new HTPbF (high-temperature lead-free) drop-in die-attach paste, the BiAgX material, but that is still a few months away from implementation.

SzePei, thank you for teaching us. Many thanks for your gift of mooncake last month, and please enjoy your Zhongqiu celebration! 

Folks,

There is a lot of interest in cleaning PCBs that have been assembled with no-clean solder pastes. 

Recently I discussed the topic with my good friend Mike Bixenman of Kyzen.

Dr. Ron (DR)

Mike, many of the best performing lead-free and lead containing solder pastes today are no-cleans.  They have been designed to solve assembly problems like graping and the head-in-pillow defect.  For the vast majority of applications, the small amount of residue left by a no-clean is not a problem.  However, some assemblers want the performance of no-cleans, but need to clean the no-clean residue as they have extreme reliability or cosmetic requirements.  Are there cleaning solutions for these situations?

Mike Bixenman (MB)

Absolutely!

DR

Can you tell use a little bit about these cleaning solutions?

MB

Several factors come into consideration when engineering electronics assembly cleaning agents. Design factors include the soil make-up, heat exposure, Z-axis clearance under bottom termination components, material compatibility, and cleaning equipment. Typical process goals require that all flux be removed in one cleaning cycle, shiny solder joints (no chemical attack to the alloy), fast production speed, no material effect to labels and other materials of construction, long chemistry bath life, and low operating concentrations.  

Cleaning solutions vary depending on the cleaning equipment. For solvent systems, a solvent cleaning agent is needed - with properties that allow for non-flammability, constant boiling mixture, and being environmentally-friendly to workers and the environment. For solvent cleaning agents that are rinsed with water, the cleaning agent requires a solvent mixture that can be rinsed with water while matching up to the soil and cleaning equipment. For aqueous cleaning agents, the cleaning agent is engineered with properties that provide solvency for the soil, polarity for inducing a dipole and/ or to oxidize and reduce the soil, low surface tension to reduce the wetting angle, buffers to stabilize pH, defoaming to reduce the tendency to foam at high pressures, and inhibitors to widen the passivation range on metallic alloys.

The property most critical is the nature of the soil. As soldering temperatures rise and the time exposed to higher temperatures increase, solder paste material supplies must improve the oxygen barrier and prevent flux burn out. This requires higher molecular weight compositions that may change the nature of the soil and the cleaning solution needed to remove the soil. Other factors such as processing conditions and how these conditions can change the soil’s cleaning properties must be considered. For example, excessive exposure to heat may polymerize the flux residue rending the soil uncleanable. To better understand and plan for these factors, solubility testing and matching the cleaning agent to the soil assist formulators in designing cleaning agents that are effective on a wide range of soldering material residues.

DR

What type of equipment is typically needed?

MB

Two key factors must be matched to clean:

1: Potential energy of the cleaning agent for the soil and

2: Kinetic energy of cleaning machine for delivering the cleaning agent to the soil necessary to create a flow channel needed to rapidly displace the soil.  

The cleaning machine requires energy to deliver the cleaning fluid across a distance and create enough force to deflect fluids under the Z-Axis. The capillary attraction for moving the cleaning fluid into an out of tight gaps is created by fluid flow, spray impingement pressure and surface tension effects. When cleaning under tight standoffs, cleaning agents that wet (form small droplets) improves capillary action, penetration and wetting of the residue. The solubility rate is dependent on the soil, temperature effects and concentration of the cleaning agent needed to dissolve the soil. Hard soils clean at a slower rate and remove the soil in a concentric (tunneling effect) manner. Soft soils clean at a fast rate and remove the soil in a channeling (multiple tunnels) effect.

The Z-Axis gap height has a direct correlation to the energy required to penetrate and remove the soil under components, time required to clean the soil and wash temperature. The irony is that lower Z-axis gaps increase capillary action of the flux for underfilling the bottom side of the component. When this occurs, flux residue dams up and closes any flow channels under the component. Research findings indicate that high pressure coherent spray jets are needed since energy drop is less and defective energy is higher. The wash time needed to clean under a 1-2 mil gap as compared to a 4-6 mil gap can range from 4-8 times longer. Higher wash temperatures increase the softening effect and aid in penetrating and removing the soil. The net effect is that, as components decrease in size, the Z-Axis gap height reduces and the cleaning factors needed to clean the soil increase. These effects favor spray-in-air cleaning equipment over immersion cleaning equipment.

DR

How are the results of cleaning assessed, so that we know that the boards are truly clean?

MB

The first level that we judge cleaning performance by is the visual presence of the residue post cleaning. Most cleaning processes have no problem with removing surface residue from the assembly. The issue is the residue under the bottom side of the component. This complicates the issue since the residue under a specific component is where most failures occur. These site-specific failures may reduce the confidence in existing IPC standards that correlate anion and cation ionic residues over the entire board surface area. So, when designing the cleaning process, we use test cards with bottom termination components and judge cleaning performance by the level of flux residue remaining under those components. To achieve this value, all components are removed and the surface area of the residue under components is graded and statistically analyzed.

Let me finish by adding that highly dense interconnects assembled onto circuit boards is advancing at a rapid pace. Traditional SMT component spacing between conductors was larger. No-clean post soldering residues posed minimal risks to reliability. The information age has spoiled us in expecting higher functionality in smaller spaces. As assembles reduce in size and increase the levels of functionality, cleaning becomes more important.  I hope that the cleaning factors discussed in this interview provide insight into cleaning process design considerations that may be of help.

DR

Mike, thanks.  Who should folks contact if they would like more information on cleaning boards assembled with no-clean solder pastes.

MB

Thanks for letting me share with your readers.   I would be glad to help anyone with the cleaning challenges they face.  Contact me at mikeb@kyzen.com.

Cheers,

Dr. Ron 

Folks,

I’m taking a few moments from Wassail Weekend , held annually in my village, Woodstock VT, “The prettiest small town in America”, to write a post about last week’s workshops at ACI.

Indium colleague Ed Briggs and I gave a 3 hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analysis of “graping” and the “head-in-pillow” defect is the best around. 

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e. very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g. OSP Immersion silver, electroless nickel gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of 5 years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years in harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in the Fred's chart (above). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being a confirmed one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many "gaps" that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100M program in 2013.

For those that missed this free workshop, ACI host Mike Prestoy is planning another one in 6 months.

Cheers,

Dr. Ron

Here is a list of tricks to help you overcome the issues that can arise while hand soldering silicon-based solar cells (and other applications as well). Some of these ideas are obvious for most, but all the suggestions can help you form a better solder joint - and build a better final product:

 

1)    Use the correct soldering tip. I’ve made the mistake of using an inappropriate solder tip before, and so have many of my customers. It’s a frustrating problem you will only let happen to you once: everything is set up perfectly but nothing will melt, until you notice the solder tip is not the correct size or shape. This has happened to many of my customers who were initially using cone point soldering tips when they were working with 2mm wide solder coated tabbing ribbon. Simply changing the tip to a 2mm wide chisel point made all the difference, and promoted soldering readily. Why such a big difference in performance? The chisel tip allows heat to flow across the ribbon, instead of only heating a single point. More heat flow = more heat in your solder joint.

2)    Pre-tin the soldering iron. Just as an appropriately sized soldering tip will distribute heat across the soldering surface, a bit of molten alloy can help create a thermal interface to maximize heat transfer. Remember to melt a small amount of solder onto the tip of your iron before soldering, and be sure it’s the same alloy you are soldering with. (Leave the custom alloying to us ;)

3)    Consider the alloy you are soldering. All the heat your typical soldering iron can produce will not be enough to melt some of the highest temperature alloys. Be sure to have a good understanding of the alloy you have selected. In some cases with low-temperature alloys (like bismuth or indium alloys), excessive soldering temperature can de-wet the alloy and char low temperature fluxes.

4)    Use the correct flux. Fluxes are quite different, I’ve spent my entire soldering career trying to get that point across. There are fluxes for high temperatures or low temperatures, cleaning with water or not cleaning at all. There are specialty fluxes for specialty alloys and there are fluxes for different soldering surfaces. Use the correct flux. If you don’t know what the best flux for the application is - just ask; that’s what I am here for.

5)    Use a bottom side heater. Silicon is known to pull heat away – that c-Si solar cell that needs to be soldered is a heatsink! Some solder equipment vendors also provide underside heating pads to help prevent excessive heat loss.

6)    Keep your soldering iron clean. That black crud that builds up on your soldering iron tip, it’s not helping you form a good solder joint. Those oxides and charred flux residues can easily be removed by wiping the hot iron across the wet sponge (that should be at your soldering station). A clean tip will lead to better heat transfer, and it will make the fluxes you use more effective.

 

This is the soldering station I use, it’s a PS-900 supplied by OK International. Just about any soldering iron will work, but they won’t all work as well – or come with as good support.

 

I’m still learning all the tricks to hand soldering, so feel free to share any you have learned over the years!

 

~Jim

Folks,

The Guadalajara (GDL), MX Chapter of the SMTA held their first meeting on November 9 and 10 at CETI in GDL.
Approximately 70 engineers from local companies attended. It was a great success. 

The agenda was:

November 9^th, 2011

0830-0900am    Registration and Exhibits Open

0900-0915am    Welcoming Remarks and Exhibition starts

0915-1045am    Inventec: “Reliability Assessment regarding Flux residues"

1045-1215pm    Sanmina-SCI: “Capabilities of a Failure Analysis Lab”

1215-1315pm    LUNCH

1315-1445pm    DEK: "Optimizing the Print Process for Mixed Technology"

1445 -1615pm   Vitronics Soltec: “How to Choose a Robust Configuration for Equipment
                          for Defect Free Soldering - Reflow, Wave and Selective”

1615-1745pm    KIC: “Fixing Reflow and Wave Related Defects as Well as How to Avoid 
                          Them in the First Place”

 

 November 10^th, 2011

0900-1030am    Sanmina-SCI: “Process development of 01005 components”

1030-1200pm    Indium: "Lead-Free Assembly for High Yields and Reliability."

1200-1300pm    LUNCH

1300-1430pm    Universal Instruments: "Tutorial on Failure Analysis"

1430-1600pm    Zestron: “PCB cleaning before conformal coating”

1600-1730pm    Kester: “Understanding Soldering Chemistries - Reducing Costly
                          Defects, Increasing Yields and Reliability.

 

I spoke on “Lead-Free Assembly for High Yields and Reliability." We had several raffles and gave away autographed copies of my book “The Adventures of Patty and the Professor,” which has just recently been formally published. 

As usual, I had dinner at Santo Coyote, one of my favorite restaurants, however my Mexican friends also took me to Sacromonte, claiming it had better food. They were correct. I was convinced to try chicken mole which I liked. It is tough to beat Santo Coyote’s ambiance, however.

I can’t cite data for this, but I am quite sure that GDL has the largest number of workers in electronics assembly outside of Asia. It is great news that they now have an SMTA chapter to help the local engineers network and continue to grow in their skills.  It was great to play a small part in this success, but most of the credit must go to Indium Corporation’s Ivan Castellanos who is chapter president and Kester’s Miguel Vazquez, chapter vice president.

Cheers,

Dr. Ron

Solder wire is generally used for manual soldering operations, including rework.  But, it can also be used in automated applications such as die-attach soldering.  Solder wire can be flux-cored, or solid with a separate flux used.

Each application can have different requirements for the wire.  For example, wire used in die-attach applications needs tight dimensional tolerances to insure an exact, repeatable amount of solder is deposited each time.  Reduced oxides are also critical to eliminate any "splattering" of the molten solder during the deposition process.

Wire can also be used for non-soldering applications. For example, indium (and indium alloys) wire are often used as a sealing material (particularly in cryogenic sealing applications) - more here) and as a thermal interface / management material.

Decades ago, 0.030" (0.76mm) diameter was the standard size, but today we are able to produce diameters as small as 0.001" (0.025mm) in tin silver (Sn Ag), tin silver copper (SAC) and gold tin (Au Sn) alloys.  Considering that a human hair is about 4X that size, that is a very small diameter!  Pure indium wire is limited to 0.010" (0.254mm), but alloys containing indium can be produced smaller than that.

The wide variety of diameters available in Au Sn make this alloy ideal for the complex applications in medical, aerospace, and other high reliability applications.  However, the Sn Ag and the Sn Ag Cu are used across a variety of standard applications that require lead-free materials.  Sn Ag is particularly good in soldering to Nitinol.

At first look, wire seems like a pretty simple product.  But specifying the right alloy, diameter, tolerances, and packaging can make all the difference.  It can help you achieve a repeatable process that gives you high yields, strong solder joints, and enhanced profitability.  For further information - contact me.

Carol Gowans

Many, many thanks to the hundreds of you who came by the Indium Corporation booth at Semicon West this year. Some of you came to hear about our recent global Semiconductor Assembly Materials Roadmap presentations, and all of you wanted to talk about your specific materials needs. Special thanks to those of you who shared the many successes you are having with our growing portfolio of applications-specific materials.


Based on these discussions, just a few of the topics that you will be hearing about in this blog in the coming months are:

- Lead/indium paste for multiple reflow applications onto gold pads
- Tin antimony solder paste
- Fluxes for 2.5D and 3D flip-chip applications
- Waferbumping fluxes for microbumps
- Jetting epoxy fluxes
- Tombstoning in semiconductor applications

Also: a final big THANK YOU to our friends at Nordson/Asymtek for showcasing the Indium halogen-free PoP paste Indium9.88-HF which was still dispensing after over 3 days of continuous usage at room temperature: proving its hard-earned reputation as the Energizer bunny of Pb-free (lead-free) dispense pastes. Here is a picture from the final day.

We look forward to seeing you all in 2012 (Exhibits: July 10-12th, 2012).


Cheers!  Andy

Included in a solder paste's Product Data Sheet, among other things, are general guidelines which aid the customer in designing an SMT reflow profile. The data sheet gives general recommendations, for time above liquidus, peak temperature, and ramp rate.

Example:

 

Figure 1: Example shown Indium8.9 flux with SAC lead-free alloy

The reason for approaching this subject is that often there has been some confusion in regards to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.
























Figure 2: Max Slope

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

Figure 3: Ramp or Average Rate


The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp to spike type profile

From the view point of the solder paste, the lower the ramp rate the better, usually 1-2°C/s. This is to drive off volatiles and help minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size, as we move to 0201’s and smaller discrete components and from 0.5mm pitch area array packages to 0.4mm and smaller. Due to this miniaturization, the observance of graping and head-in-pillow have become more common. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

I'd love to discuss this with you, if this topic is affecting your SMT process. If you'd like, feel free to contact me.

 

 

The week of March 14th Indium Corporation will be exhibiting at the MD&M (Medical Design & Manufacturing) show in Orlando, Florida.  Actually it is one of many MD&M shows held throughout the country.

We have attended previous shows as visitors but this will be our first as an exhibitor.  We will be showcasing our Flux #2 and lead-free solders for soldering to Nitinol.  Eric Bastow recently wrote a blog post on using Flux #2 and either 96.5Sn 3.5Ag or 80Au 20Sn for this application.

Flux #2 cleans off the very tenacious oxides that form on the Nitinol, giving it a clean surface to solder to.  We will be providing further details and samples of the 96.5Sn 3.5Ag in wire form at the show.  You can also request a sample of the Flux #2 by giving us your contact details.  Stop by and see us at Booth 248-250.

Or you can contact Eric Bastow by email at ebastow@indium.com or me by email at cgowans@indium.com.

Nitinol (a nickel titanium alloy) has become a very important material, especially in the medical world. It is often necessary to "attach" Nitinol to another piece of Nitinol or some other material such as platinum or stainless steel. Common high temperature bonding methods, like welding, are not suitable for bonding to Nitinol because high temperatures can ruin Nitinol's shape memory characteristic. However, the temperatures associated with soldering, considerably lower than welding, do not threaten the properties of Nitinol.

While soldering may be the desired means of attaching to Nitinol, it does not come without its challenges. Nevertheless, with the right material set and equipment, soldering to Nitinol can a robust process.

One of the obstacles to soldering to Nitinol is the inherent titanium-oxide-rich top layer. In order for soldering to take place, the molten solder must have access to clean oxide-free metal. That means that the titanium-oxide-rich layer has to be removed. There are a couple of ways to remove the oxide layer; they can be used in concert with each other and can be repeated as necessary. If the size of the part allows, the oxide layer can be mechanically abraded off. It is also possible to "chemically" remove the oxide layer. That is typically accomplished with a flux. Traditional soldering fluxes are typically designed for relatively pristine surfaces such as cleaned copper. Such a flux would not be effective for soldering to Nitinol. But highly active fluxes, capable of removing the titanium-oxide rich layer, are available.

Furthermore, an appropriate solder alloy must be used. Given that many Nitinol devices are medical in nature, it is intuitive that solders containing Pb (lead) and other toxic metals would not be appropriate. Two solder alloys have emerged as "standard" for soldering to Nitinol:

• 96.5%Sn 3.5%Ag (221C)
• 80%Au 20%Sn (280C).

Which alloy is used is often determined by the expected life of the device and whether or not it will see high temperature autoclaving. Many single-use (disposable) devices use SnAg; whereas long -term or multiple use devices (autoclaved) use AuSn.

Jon major recently joined the Indium Corporation as a Product Manager for Solar back-end assembly products. I greeted him with this impromptu interview.

Jim: First of all Jon, welcome. It’s great to have you as a new addition to the team!

 

Jon: Thank you Jim – it’s an exciting time to be at Indium Corporation and a fantastic time to be a part of the growing solar industry. I am extremely enthusiastic about my new position and am looking forward to making a positive contribution to the solar industry.

 

Jim: I noticed it didn’t take you long to get up to speed. Your time in Silicon Valley must have helped.

 

Jon: Coming from the electronics industry with a focus on product development, new product introduction, manufacturing, and external partner management, I am excited that my past experiences can contribute both to the industry and to Indium Corporation. After joining Indium only a few weeks ago, not only am I getting used to Upstate NY weather, but I have been immersing myself in solar with the goal of gaining a comprehensive understanding of:

 

•       Both rigid and thin-film technologies

•       Technology trends

•       Global and regional markets (EU, China, US, North America)

•       Solar supply chain (Silicon, wafers, cells, module, equipment, inverters, integrators)

•       Equipment manufacturers, contract manufacturers, and how we can collaborate with them to move the industry forward

•       Our products and pricing

•       Our current and future customers

•       Our short and long term opportunities

•       Our competition

•       Our roadmap

•       Our strengths, weaknesses, and threats

•       Our manufacturing capabilities and our QA process

•       Our sales channels, value proposition, key differentiators

•       All Indium processes

 

Jim: I know you've got solar products on your mind. Let our readers know a little bit more about your role here at Indium?

 

Jon: As a Solar Backend Product Manager I will focus (officially) on the business development and growth of Indium’s Solar Back End product offerings.  Now that sounds great but what does it actually mean? I could cut and paste my official job description but I prefer to explain it in my own words. As I think about the first part of that statement, “business development and growth…”, I see my role as:

 

–      Know the market, the customers, the product, and the competition

–      Develop relationships with the Indium team, reps, partners, equipment manufacturers, and, of course, customers

–      Write valuable data sheets, publications, and sales literature

–      Listen to our customers' needs and provide solutions

–      Manage schedules and orders with minimal surprises

–      Build cross-functional collaboration (sales, distribution, marketing, engineering, R&D, QA, production, management)

–      Never let down partners or customers

–      Support all functions of the organization, both internal and external

–      Deliver above & beyond commitments

–      Make great bets – on technology, customers, and opportunities

–      Understand the product life-cycle

–      Ship high quality, consistent product

 

The second part of that statement “..of Indium’s Solar Back End product offerings” is fairly straightforward. Of course this means I will focus on Indium’s current back end products (tabbing ribbon, bus ribbon, metallization paste (or as I prefer to call it – “grid ink”), flux and flux cored wire). With a product development background, this also means I have an opportunity to work with customers, partners, and R&D to develop and bring new products to market that will advance the module assembly industry – very exciting for me personally.

 

Ultimately, I think of my role as both building awareness of Indium’s products and superior technical support available to our customers as well as helping to shape our growing industry.

 

Jim: Okay Jon, you’ve had a while to settle in and get familiar with our Solar Team’s past and present – what are you planning for the future of module assembly?

 

Jon: Regarding the future of module assembly it’s a bit early to know for sure but I am excited about our low-temperature bismuth-containing alloys. These low temperature, lead-free, bismuth-containing alloys reduce the soldering process temperatures, thus reducing thermal stresses. I’m also working with the Indium production team to further reduce our tabbing and bus ribbon yield strength. A lower yield strength will reduce mechanical stress on cells during the assembly process. This is crucial to minimizing the possibility of microcracks and cell breakage during the solar module assembly process.

 

In closing, having lived in California for the last 10 years, I am not 100% familiar with our Upstate New York climate, and especially not all the snow shoveling. I see in my future a solar powered driveway heater!

 

Jon can be reached at jmajor@indium.com

I just visited a customer that was converting from water soluble solder paste to no-clean. Not exactly a slam dunk transition as this customer found out.

During my visit, solder balls and solder beads were observed in the no-clean flux residue adjacent to discrete components (capacitor/resistors). These could potentially be a reliability concern…electrical shorts.

In water soluble processes, solder defects such as solder balling and beading can be washed away in the cleaning process…no worries. However, introducing a no-clean solder paste often requires that the process be “cleaned” up a bit. Here are some ways to do it:

STENCIL DESIGN:
My first step was to investigate the stencil design for these discrete components. Why? Because, since water soluble post-reflow residues (including solder balls & beads) are washed away, many customers will opt to place as much solder (1:1 ratio) as possible on the pads - to achieve a good solder joint. This is especially true for military or medical applications where a robust solder joint fillet is vital. However, because no-clean residues are typically not cleaned, the solder balls and solder beads remain in the flux residue and may produce electrical shorts.

When printing in a 1:1 ratio, especially if the stencil is thicker than average, solder paste is often pushed under the component and onto the solder mask during component placement. Upon reflow, the sub-component solder paste may not pull back into the solder joint. This is one way that solder balls/solder beads are produced.

No one wants to hear that they need to buy new stencils with reduced apertures, but I did recommend, in this case, that some aperture reduction be considered (generally down to 0402 components). Usually a 10-15% reduction, with home-plate or similar design, is common. Many stencil manufacturers are fully aware of the issue and can make suggestions on aperture designs.

REFLOW PROFILE:
Simultaneously, the reflow profile often needs to be adjusted. In the preheat portion of the typical reflow profile, the first few oven zones are used to drive off flux volatiles, making the paste less "mobile". A balance in the ramp rate is vital; too fast - and small “explosions” may cause paste to spatter into other areas; too slow - and two bad things happen: the flux will spread excessively and the flux activity can be exhausted.

Good Starting Points:

• Ramp rate (IMPORTANT: not max slope, see "Best Practices Reflow Profiling for Lead-Free SMT Assembly" for reference): 1°C/s
• Initial first zone setting: 100-110°C

COMMENT to share your solder paste transitioning story or question. Thanks!

I mentioned in a previous a blog posting that the primary driver for halogen-free electronics is ostensibly environmental, but that the confusion about “which halogens and which molecules and what level?” has seemingly decoupled the laudable desire for an improved environment from the reality and made it more of a marketing tool. All this notwithstanding, there remain some instances where the performance of the final product itself can be directly impacted by the presence of halogens, usually as ionic halides. This is the reason why Indium Corporation recently developed what appears, at first glance, to be an odd combination: a high-Pb (high-lead) alloy halogen-free die-attach solder paste, Indium9.72-HF.

 

The halogen-related failure mode for die-attach solder pastes is the corrosion of wirebond pads on the topside of Power Semiconductor die which are soldered to the leadframe with halogen-containing solder paste. Many manufacturers producing high volumes of identical power devices may also use die-attach (sometimes called “soft solder die attach”, SSDA) wire to attach the die to the leadframes in a fluxless process, but many manufacturers prefer the inherent flexibility of a solder paste-based process for medium mix / medium volume applications.

 

Long term blog readers will recall that I did a posting on solderspatter (a.k.a. soldersplatter or soldersplash), and that it can be caused by bubbles of solvent vapor or moisture outgassing from solder paste deposits during reflow. In bursting, the tiny flux droplets or solder particles from the surface of the bubble can be propelled quite a distance (several feet). While solder on wirebond pads is clearly a failure from a reliability viewpoint, certain wirebond pad metallizations may also be subject to corrosion from flux. A poorly maintained reflow oven may also drip flux condensate (usually in the exit – cooling – zone), and this too can be a cause of organic materials on wirebond pads.

 

As long as the bondwire is gold, and wirebond pads are covered in a uniform layer of gold, there is no problem (as long as the flux residue is washed off) since gold is unreactive, even in corrosive environments. Aluminum (Al) or aluminum/silicon (Al/Si) bondpads, however, are potentially reactive. Halogenated materials, such as fluxes and overmolding compounds may react with them to either reduce the wirebond pull strength and/or increase the wirebond junction resistance, leading to localized heating and subsequent thermal-related joint failure. Even covalently-bonded (C-X, where X is a halogen) materials may dissociate at high temperatures: which is how the banned brominated flame retardants work, of course.

 

The biggest danger of halogenated flux corroding wirebond pads is when:

 

1/ Completed assemblies (between the reflow process and the cleaning process) are left for a long time before cleaning; particularly if they are exposed to high humidity (high %RH) before cleaning.

 

2/ The cleaning process is inadequate: either due to poor selection of the cleaning solution, or poor bath maintenance, or inadequate “scrubbing” energy being imparted to the surface to be cleaned, or simply if inadequate time is allowed for cleaning.

 

Note that even optimizing 1/ and 2/ may still lead to bondpad corrosion.

 

The Indium9.72-HF paste is available in both type 3 and 4 powder, in the standard high-Pb alloys, Indalloy 151 (92.5Pb/5Sn/2.5Ag) and Indalloy 163 (95.5/2Sn/2.5Ag), and for larger die that need a higher reliability joint, we also offer the Indalloy 164 (92.5Pb/5In/2.5Ag). A Product Datasheet is available for download, of course.

Cheers! Andy

上週在美國的拉斯韋加斯(Las Vegas), IPC舉辦了美國地區行業的盛會APEX.   Indium公司一如既往的在展會中心安排展位，和業界各位舊友新友交流，與大家分享最新的產品和技術，傾聽大家的反饋和聲音。

 

除此，在人山人海的技術會議交流中心(paper presentation, educational workshop)，Indium公司的五位大將還為大家做了精彩的演講：

• Ning-Cheng Lee, Ph.D, Vice President of Technology 李寧成博士：

²       Lead-Free Flux Technology and Influence on Cleaning.

²       Selection of Dip Transfer Fluxes and Solder Pastes for PoP Assembly.

²       Achieving High Reliability Low-Cost Lead-Free SAC Solder Joints Via Mn or Ce Doping.
 

• Ronald C. Lasky, Ph.D. PE, Senior Technologist

²       Achieving High Reliability for Lead-Free Solder Joints – Materials Consideration

• Mario Scalzo, Senior Technical Support Engineer

²       Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly.

²       Challenges for Implementing a Halogen-Free Process

• Eric Bastow, Senior Technical Support Engineer

²       Understanding SIR

• Chris Anglin, Applications Development Engineer

²       Stencil Printing Transfer Efficiency of Circular vs. Square Apertures with the Same Solder Paste

 這些文章在Indium的技術網站上面，都可以免費下載。

 

Cheers!

 

This blog has been in existence for a little over two years now, and we would like to thank our readers for the feedback and inquiries you have provided. I welcome your comments on what you would like from us. Leave a comment below, or email me at jhisert@indium.com.

 

 

 

And now a look back on past topics of interest:
 

Grid Ink, Silver Ink, Conductive Ink

Bismuth/Tin Tabbing Ribbon, A Low Temperature Pb-Free Alternative

Plated Metallization for C-Si Solar Cells

Increase Packing Density for Evaporation Crucibles

Photon’s 5th PV Tech Show 2010 USA

IPC Solar Standards Update

Solder Shelf Life as Explained by Eric Bastow

Tips to Speed Your Solder and Flux Selection

What's Happening in the Technical Service Department 

A Day in the Life of a Tech Guy

A Clean Laboratory

CIGS for Beginners

3rd Renewable Energy Expo 2009 in New Delhi, India

Solar Products and Representatives

Kleenex®, Google™, FedX®, CIGs?

Indium Solar Products Reunited

Trade Show Visitors Love the Ground Floor

Solar Product Data Sheets

Intersolar 2009 – What Barrier to CIGS Technology?

Concentrator Photovoltaic Systems - Will they reach 50% Efficiency?

Standards for Solar Panel Manufacturing

Solar Panel Certification: “Barrier and Benefit” Reviewed by Eric Bastow

Low Temperature Metallization Paste

What Will Your Interest Be At InterSolar? Meet the Bloggers And Let Us Know.

Share Your Solar Images

SAC vs. Sn/Ag for Solar Soldering

Solder Thickness for PV Interconnect

What is Bus Ribbon?

Standard PV Interconnect Ribbon Sizes

No-Clean Flux

Photovoltaics in EMS Sector

PV Interconnect Products

Eric Bastow - East Coast Technical Support

Mario Scalzo - West Coast Technical Support

Au/Sn Sputtering Targets

SMT Goes Solar

A Trip Down Memory Lane 

More Information About Metallization Paste

A year in the Life of a Solar Blog

CIG Target

23rd European Photovoltaic Solar Energy Conference and Exhibition

TCO choices for CIGS manufacturing 

CIGS Absorber Layer Electroplating

No Slump Metallization Paste

Meet the Bloggers

CIGS - Can sputtering make a breakthrough?

Fluxes for Soldering Tabbing Ribbon

Computer Brain vs. Solar Photovoltaic

Beam it down from space

Selection of the Optimum Lead-Free Solder for Solar Tabbing Ribbon

Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency

Why Thin-Film Solar Cells are Here to Stay

Hot Rooftops to Flashy Digital Cameras

Synchronize Your Solar Cell

Solar Conversion Efficiencies  

Government Support is the Key

It's Just a Beginning ...

Graping is a phenomenon which appears as un-reflowed solder particles, typically seen on the surface of the solder joint.  



             Cross-section of “graped” solder joint





The graping phenomena has become more common due to some of the following issues:

 

1. Reduction of the stencil aperture to accommodate smaller and smaller discrete and passive components (i.e. move from 0603”s to 0402”s to 0201’s)
2. The use of finer particle size solder pastes to accommodate fine feature printing (move from Type 3 to Type 4 to now to Type 5)
3. Higher reflow characteristics for Pb-free soldering
4. The use of water-soluble vs. no-clean solder pastes. No-clean chemistries generally protect the solder powder particles and the metallized surfaces from oxidation during the heating process (after the activator package removes existing oxides). (so how does water-soluble fit into this?)

 

A combination of any of these factors may exhaust the capability of the solder paste flux to remove surface oxides. This depletes the flux and exposes solder paste particles to oxidation, which means the solder particles do not coalesce into the solder joint.

 

To avoid the graping phenomenon, use the following tips in setting up your reflow profile. The intent here is to decrease the amount of heat the solder paste experiences during the reflow process.

1. A ramp to peak profile is better than a soak profile   
2. Decrease total time in oven by adjusting the belt speed. A ramp rate of 1°C/ second from ambient to peak is recommended
3. Use a lower peak temperature - 235°-240°C
4. Shorten the TAL to 40-60seconds

For more information please refer to "Best Practices Reflow Profiling for Lead-free SMT Assembly"

 

Conductive epoxy is a common material choice for bonding components, especially if the assembly process is temperature-sensitive. Tin-based solder paste or preforms with flux are preferred Pb-free bonding materials; however, conductive epoxies arguably provide advantages over these traditional solder assembly materials. 

 

It has been my experience that these advantages are perceived in the absence of an awareness of the full solder assembly materials product offering. Specialty solders can provide the same advantages as conductive epoxies and then some.   

 

Some claimed advantages to conductive epoxies include:

·         RoHS-compliance

·         Ease of assembly

·         No-clean

·         Low cure temperatures

 

Low-temperature solders such as 58Bi42Sn and 52In48Sn are specialty low-temperature solders which have these same properties including processing temperatures below 150ºC. Both of the referenced alloys are Pb-free, can-be used with no-clean fluxes and are assembled using the traditional solder assembly techniques.

 

It would seem a toss-up between whether to use a conductive epoxy or specialty solder to assemble temperature-sensitive components except that there are additional advantages to a soldered assembly as compared with an epoxy-assembly. These include:

 

·         Thermal cycling reliability

·         Solder material consistency

·         Reworkability

·         Thermal Conductivity

The use of gold layers deposited onto nickel is standard in many industries, from DRAM memory module edge connectors, to electrical test probe contacts, to power semiconductor die metallizations and wirebonding pads. While the role of gold in the final solder joint is well-understood, I wanted to learn more about the gold deposition process from an industry expert, so was given the chance to discuss this with Lenora Toscano, MS, Final Finish Product Manager with MacDermid.

 

Andy Mackie: What role does gold play in protecting surfaces in SMT and semiconductor assembly processes?

Lenora Toscano: Gold does not form an oxide; it protects the nickel from oxidation or passivation. A clean nickel surface has very high solderability for most solder types, but its oxide is very difficult to remove with standard flux types. Also, gold dissolves almost instantaneously into most solders during assembly, thus promoting superior wettability.

 

Andy Mackie: What standards exist on the thickness of gold for different electronics and semiconductor assembly applications?

Lenora Toscano: The main application of ENIG (electroless nickel/immersion gold) coating is in chip-on-board (COB) technology, the typical thickness of the immersion gold layer on the HDI substrate being 3-5 micro-inches.

 

Edge connectors typically require the use of hard gold. Acid gold deposits are used for compliance with MIL-STD-275, which states that gold shall be in accordance with MIL-G-45204, Type II, Class 1. The thickness shall be 50-100 micro-inches, typical thickness is 30-50 micro-inches on 150micro-inches nickel.

 

On the other hand, for solderable surfaces, typical thickness is 5-15 micro-inches on 150micro-inches nickel.

 

For wire bonding, in general, gold plating of a minimum of 30 micro-inches on 200 micro-inches nickel works well. Soft gold is generally preferred. Soft gold processes are also used for boards designed for semiconductor chip (die) attachment. These qualities comply with Type I and III of MIL-G-45204.

  

Andy Mackie:  What are the differences between gold layers deposited by immersion gold and electroplated gold processes?

Lenora Toscano: There are five main differences:

1. The coating thickness is different. Immersion gold is a displacement reaction, gold displaces the nickel on the surface, and is self-limiting as the nickel surface is coated with the immersion gold. Common baths cannot produce thicknesses of much more than 10 micro-inches, while with electroplated gold the thickness depends on current and time. The higher current or longer the plating time the thicker the gold coating.
2. The structure of the gold deposit layers is different. Electroplated gold is denser that the naturally porous immersion deposit.
3. The hardness is usually different. Electroplated gold often has other metals introduced into the plating that make the deposit harder.
4. Porosity is different. Immersion deposits have more porosity that electroplated deposits; it is the nature of the plating system.
5. Deposition composition (purity) varies with additives in the bath. Immersion gold baths contain gold as the only plated metal, while electroplating systems may introduce small amounts of other metals.

Andy Mackie: How thick does gold have to be to fully protect the underlying surface, and what are the trade-offs as customers attempt to reduce their gold costs?

Lenora Toscano: Per IPC-4552 ENIG specification, 1.97 micro-inches is the recommended minimum at +/-4 sigma from the mean, with 3 – 5 micro-inches being typical.

 

The immersion gold deposit is porous by definition. It does offer very good protection to the underlying nickel, but over time the porosity of the deposit results in the passivation of the nickel surface and the wetting forces will be reduced. Of course, this process should take years to occur, but if the gold coating is too thin (below the minimum requirement), it will occur sooner and affect the solderability. 

 

Andy Mackie: What advantage does gold have over silver or other metals?

 

Lenora Toscano: Again, gold has good tarnish resistance and solderability after storage because it does not form an oxide or hydroxides, so it is unaffected by temperature and storage conditions that might reduce the shelf-life of the other finishes. It meets requirements for lead-free (Pb-free) assembly while offering a coplanar surface that is both solderable and aluminum-wire and gold-wire bondable.

 

Gold has good electrical conductivity, and produces a contact surface with low electrical resistance. Electroplated gold is also an excellent etch resist.

 

Electroplated silver is not widely used in the printed circuit industry. Under certain conditions or electrical potential and humidity, silver will migrate along the surface of the deposit and through the body of insulation to produce low-resistance leakage paths. Alkaline cyanide baths for silver electroplating are highly toxic.

 

Immersion silver is susceptible to problems if not correctly stored and even packaged. Packaging materials that contain sulfur or allow exposure to air will result in tarnishing of the surface (sulfide, sulfate, and chloride formation). High levels of surface contamination can detrimentally affect solderability.

---------

Lenora - many thanks for your time, and  for sharing your expertise with us.

Cheers! Andy

While at the Semicon West 2009 show in July, I had a chance to sit down with Herr Klaus Roemer of Pink GmbH. PINK is most famous in the die-attach and power module manufacturing world for their reflow ovens with vacuum, but are also known in the medical and aerospace industries for manufacturing extremely high precision, one-off, vacuum equipment for applications as diverse as particle-accelerators for ion bombardment, and large-volume chambers for helium leak-detection. I asked him some questions about Pink vacuum soldering technology.

Cheers! Andy