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上周在圣地亚哥(San Diego), 业界一年一度的盛会APEX落下了帷幕。Indium公司一如既往地参加和支持APEX，并在会上发表4篇技术文章。

在展会上，我们介绍的重点仍然是Indium8.9系列的焊接产品。这些产品分有卤素和无卤素的(halogen contained or halogen free)，每种产品都有自己突出的特点，但是整个系列的产品都是针对免洗无铅(no-clean Pb-free)而设计的，可以与fine powder 兼容，能够很好的帮助客户解决枕头效应(head-in-pillow), graping 等问题。

Indium发表的技术文章可以在我们公司的网站上免费下载：http://www.indium.com/technical-documents/whitepaper/

• QFN Voding Control Via Solder Mask Patterning on Thermal Pad
• Material and Process optimization for HIP Defect Elimination
• Voiding Mechanism and Control in Mixed Solder Alloy System
• The Effects of Human-Induced Contamination on PCB Assembly Electrical Reliability

我很高兴能参加这次盛会，并见到了许多新老客户。期待明年4月份Las Vegas 的APEX.

Cheers!

PS: 隆重祝贺Indium公司的好朋友和我的好友，Intel公司的Raiyo Aspandiar 荣获Distinguished Committee Service Award (from IPC at IPC APEX EXPO in February in recognition of Raiyo’s outstanding contributions to the development of IPC-7095C, Design and Assembly Process Implementation for BGAs.) 实至名归！！…… 有的书把Steve Jobs 05年Stanford 演讲的名言”Stay hungry, stay foolish” 翻译成 “求知如渴，虚怀若谷”。 我觉得这是绝妙的翻译，也是Raiyo 的真实写照！！ 恭喜你Raiyo!!

Pic: Indium Corp

NanoFoil^® is a great localized heat source, but it can cause some ugly looking aesthetic defects if the process is not set up correctly. Let’s take a look at how you can use high temperature tape to mask off parts during NanoBonding.

With certain NanoFoil^® thicknesses and assembly pressures, solder may be ejected from the bond area. To protect these parts from “solder spitting”, simply mask them off with high temperature tape. In the picture shown here (right), excess solder and pressure were used with a higher energy version of NanoFoil^®.

Similar parts were again prepared for bonding (left), this time with high temperature tape covering the area around the solder joint.

After a similar reaction, we see the telltale signs of excessive pressure and solder. This time, however, we can simply peel away the excess solder (right).

After bonding, the tape can be peeled off the surfaces, revealing the fresh, clean surface underneath.

This is a very flexible solution - to mask parts during reflow. In high volume, metal shims and shutters can be used to keep solder where it belongs. The best solution is to optimize the solder thickness, assembly pressure, and NanoFoil^® thickness. We can help you do that.

* This post is part of the NanoFoil^® Do-It-Yourself Tips and Tricks series

We’ve heard about the solder paste “graping” defect, but the same oxidation challenge occurs in other solder forms as well, such as solder washer preforms used to attach small connectors. 

Solder graping is a defect that arose as a result of small solder paste deposits and higher (than were used in the past) reflow temperatures.  The peak reflow temperature for SnPb alloys were just over 200°C, but. with SnAgCu alloys, that peak is as high as 260°C.  New solder pastes have been developed that address this defect by utilizing flux chemistries that function as an oxidation barrier.

But what about other solder forms? 

I was recently working with a customer who was evaluating solder preforms for a connector application.  These solder washers (outside diameter of 0.025”), like the small solder paste deposits, had a large surface area with a tendency to oxidize on their heated journey to melting.  These washers were made of SnAgCu, so the peak temperature required of them was no different than it would be for Pb-Free solder paste.

I found that the same 1°C ramp that causes graping of solder paste can do so for preforms as well.  The difference though, is that the non-molten portion of solder takes the shape of its un-reflowed form, that of a small washer in this case, as opposed to the “grapes” of aggregated solder powder.  

Fortunately, the issue can be addressed in 3 ways, and I encouraged my customer in this case to process their assembly using the combination:

1. Adjust the reflow profile. Ed Briggs' recommendations work well here.  Further, use a fast ramp to peak. I tested the reflow on a hot plate set to 250°C; this improved the wetting.  Preforms are unique from solder paste in that they are solid metal, as opposed to a mixture of metal and flux.  They do not benefit from the same heating controls that solder paste requires. 
2. Reflow in nitrogen.  By purging out the environmental oxygen during the reflow process, the solder preform will not oxidize.
3. Apply a tacky RMA flux, such as Tac007.  RMA fluxes provide stronger oxidation barriers than other non-rosin flux types. 

In the second image here, it is evident the improvement these changes made in terms of the spread and coalescence of the solder preforms. Note that the addition of tacky flux left an amber-colored no-clean residue, however, this can easily be washed away using a mild solvent.

Any questions?  AskUs@indium.com!

It seems like a fairly simple thing to do.  What could be difficult about soldering a wire to a pad? 

Well, I hear three common complaints and expressions of frustration:

POSITIONING: Typically, in this process, a soldering iron is used. The first problem arises from trying to hold onto the soldering iron AND the wire to be joined to the prefluxed pad AND the solid-core solder wire you are using.  An extra hand would be nice! Some people use a system of fixtures or clips to hold the wire and the pad in the appropriate position. (see image and link, below)*.

COLD SOLDER JOINT: Another common complaint is that, after soldering, the wire easily pulls out of the solder joint.  This is due to the poor wetting of the solder to the wire and the pad - it never really "soldered".  A solution that I share is to pretin both the pad and the wire with the solder, using a flux.  To pretin the wire, I suggest melting some of the solder in a crucible or solder pot.  Dip the wire in the flux and then into the molten solder.  A teardrop should form on the end of the wire.  It can also be pretinned using the soldering iron. Next, pretin the pad. Both pretinned surfaces will have a coating of post-reflow flux residue.  If required, this residue can easily be removed using a suitable solvent.  Now that you have pretinned both surfaces, the pad should be heated with the soldering iron and, when the proper temperature is reached, the pretinned wire should be pressed to the pretinned pad.  The solder on both the pad and the wire will melt together and, when the heat is removed, the joint will be formed.  Usually this can be accomplished without adding additional flux.

INCONSISTENT VOLUME: A third issue is that the volume of solder in the joint is not uniform from piece to piece. If this is your concern, consider using a flux-coated solder preform. They can be produced with the exact solder volume, and the precise dimensions to fit onto the wire you are joining to the pad.  Similar to the process described above, when the pad and the wire are heated, the flux will be activated (removing the oxides) and the solder preform will melt, forming a consistent and perfect solder joint.

Please contact our technical support group with any questions you may have.  We are always ready to help you solve your soldering problem, whether it is large or small.

For more background, read these blog posts on hand soldering:

• soldering iron tip temperature
• hand soldering flux selection
• hand soldering tech support
• the importance of a clean soldering iron tip

Paul Socha

*Image: Harbor Freight sells a product called "Helping Hands" for (US) $6.99, as of this writing. Other companies offer similar products. Consider buying more clamps to hold the wire in place, freeing you to hold only the solder wire and the soldering iron.

Printed circuit boards containing both surface mounted and through-hole components are common, and are often referred to as "mixed technology" boards.  In mixed technology assembly, solder paste is used to attach the components to the surfaces and wave soldering attaches the components that are inserted through holes in the board.  For low volume production, hand soldering is often utilized - for the attachment of through-hole components.  Both of these methods require additional steps after the original reflow of the solder paste.

To increase your profits (saving you time and money while improving your quality and productivity) InTEGRATED PREFORMS^® have found a place in mixed technology assembly.  InTEGRATED PREFORMS^® are interconnected solder washers, designed to fit the pin pattern of a through-hole component.  These arrayed solder washers are sized to deliver the precise solder volume required to fill the holes and to produce excellent solder fillets at each joint. 

In some cases, to add even more solder, solder paste is deposited over the holes and the InTEGRATED PREFORM^® is placed into the paste. The component is then inserted through the solder preform, the solder paste, and the hole. 

In other applications, TacFlux® (that is compatible with the solder paste's flux vehicle) is applied to the preform before it is placed on the component's pins, or is placed directly on the board, and the component is inserted as described above.  Whichever method is used, only one reflow step and only one cleaning step are required.

In traditional wave soldering, components with long pins are a special challenge because they are very difficult to attach without getting alloy on the pins during wave or hand soldering.  InTEGRATED PREFORMS^® can be applied to the top or bottom side of the board and are reflowed along with the components held down with solder paste.

InTEGRATED PREFORMS^®  are designed and built to address the unique characteristics of each specific application.  To build your InTEGRATED PREFORMS® we require the following information, so the solder volume and washer spacing are correct for your specific pin configuration:

• Hole size
• Pin size
• Board thickness
• Center to center spacing of the pins (within the row, and row to row)
• Solder Alloy
• Is the preform going to be used to add to the volume of solder from the paste?

Separate (individual) solder washers can also be used in place of connected InTEGRATED PREFORMS^®.  They can be designed to deliver the same consistent volume of solder required for each joint.   Care must be taken, however, to place only one preform on a pin, and not miss any.  This is what makes InTEGRATED PREFORMS^® desirable.  The solder washer array is designed and manufactured to fit the pin configuration so only one washer goes on a pin.  If extra solder volume is required, InTEGRATED PREFORMS^® can be easily stacked.

With today's drive to optimize profits, InTEGRATED PREFORMS^® present an excellent opportunity.  The biggest advantage of InTEGRATED PREFORMS^® is the fact that quality can be improved while costs are reduced.  If you are looking for any easy way to cut costs, increase production, improve quality, improve customer satisfaction, and, ultimately, increase your profits, talk to me about InTEGRATED PREFORMS^®.

Paul Socha psocha@indium.com

BONUS: Read our white papers regarding InTEGRATED PREFORMS^® .

As mentioned in a separate post, uniform pressure across a NanoBond^® interface is critical for maximum solder bond strength. Uniform pressure is much easier to achieve with flat, coplanar bonding surfaces.

If you are in the design stage of your NanoFoil^® application, now is the time to make sure that the parts you will be bonding are flat – so you can apply even pressure to the entire bondline. This may necessitate machining, or otherwise conditioning, the parts so they are flat, smooth, and clean.

The obvious question I would assume you will ask is, “How flat should these surfaces be?” For target bonding and other large area bonding applications, we use this general rule: ≤1mm/m. For thinner parts, the pressure during assembly may help compensate for surface variations.

As far as the surface roughness, parts definitely don’t need to be polished – I’ve even used coarse-grit emery paper to flatten surfaces before bonding. Remember though, it’s recommended you clean your surfaces with isopropyl alcohol to remove oils and debris after any sanding/grinding operation.

*This post is part of the NanoBond^® Process series

The first time I was taught how to solder (as a child), I was told: “All the surfaces need to be mechanically cleaned and chemically cleaned.” The person who told me this was referring to pipes, I was learning about plumbing. (I would have never thought we'd be using nanotechnology to create solder joints!) Although your application is probably far from a plumbing job, the basics of soldering remain the same. The best solder bonds are formed when oxides and contaminants are not present.

These two points are taken care of in traditional electronics soldering by using a flux. Flux can flush away light contaminants like dust, and reduce oxides on certain metal surfaces. But, in the NanoBond^® process, we aren’t using flux.

Luckily, NanoFoil^® can power through the soldering process as long as the proper surface finishes are used, and they are ready to be soldered to. Different surfaces are prepared in different ways. Here is a list of some common surface finishes and what preparation they require:

• Gold – Wipe with isopropyl alcohol if aged
• Silver – Wipe with isopropyl alcohol if aged
• Tin – Remove oxides with 10% HCl if aged
• Solder coating - Remove oxides with 10% HCl if aged
• Aluminum – Add solder coating
• Molybdenum – Add solder coating
• Titanium – Add solder coating
• Naval Brass – Add solder coating

And for any surface you don’t see, feel free to contact askus@indium.com so we can find a solution for you.

*This post is part of the NanoBond^® Process series

While on a recent trip to Malaysia, I interviewed two colleagues regarding trends in semiconductor assembly. My previously-published interview with Sze-Pei Lim appears here.

This time, while on a visit to a logic device manufacturer in the North West, I [ACM] talked briefly to Sehar Samiappan [SS], Indium Corporation's Area Technical Manager, about our recently-developed water-soluble pin-transfer Ball-Attach Flux, WS446-NRD, which is designed for BGA applications of 0.5mm pitch, and greater than 1500  I/Os.

[ACM] What is the origin of WS446-NRD?

[SS] The development was driven by a customer need for a guaranteed good quality BGA (ball-grid array) solder joint, but with reduced environmental impact. Our very quality-focused customer uses several different suppliers of organic FC-BGA substrates with copper OSP pads. The customer had serious concerns about occasional poor solderability of SAC305 solder spheres onto substrates. The key defect seen was poor wetting onto the OSP-coated copper pad, which would give rise to variability in both joint strength and bump coplanarity,  and even (in worse cases) missing-ball / “big ball” effects. Some of the pad finishes were seen to be highly oxidized, severely restricting solder wetting during reflow. Variability in the surface finish was found to be not just from supplier to supplier, but also showed up as lot-to-lot variability from lower cost suppliers.

Some of the differences seen could be attributed to the method of mask desmear from the C4 “cage” of the flip-chip (top side) area, which was either a plasma-based desmear or an oxidizing inorganic acid dip, that was clearly having effects on solderability of the opposite (bottom) BGA side of the substrate.

[ACM] What steps have customers previously taken to get around this issue?

[SS] This is a serious issue for many ball-attach flux users, and some customers have gone to the lengths of using a special fluxing step to remove contaminants such as oxide and OSP (organic solderability protectant) coatings. These liquid fluxes are very reactive, but require  separate spraying, reflow, and cleaning stages that add cost and time. The halogenated ball-attach fluxes of the WS446 series have an established good chemistry that allows wetting of SAC105, 305, and 405 onto a variety of metallizations. In the semiconductor assembly industry, the WS446 fluxes are well-known in Taiwan, and throughout South East Asia, for their good solderability and long pot-life in a variety of FC-BGA applications.

[ACM] What was different about WS446-NRD, and why was it developed?

[SS] WS446 fluxes are all colored, using a bright red dye, so the flux can be seen by eye and automatically detected by vision systems. Red coloration also allows automated ball-attach flux dipping replenishment systems to detect flux levels. Normally, colored fluxes are not a problem, but the customer had some environmental concerns with the red color contaminating the water-wash equipment, and building up in their water-recycling system. WS446-NRD was developed from the basic WS446 flux series chemistry, but  without the red dye. The solderability performance of WS446-NRD was excellent, eliminating the variations in OSP solderability without requiring any additional processing steps. WS446-NRD also passed internal process and product requirements, such as cleanability, and the customer was very pleased with Indium’s ability to rapidly tailor a chemistry to their specific requirements.

[ACM] Sehar: thank you. I look forward to sharing a durian with you again when they are back in season.

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 

It is not often that we get a request for SIR (Surface Insulation Resistance) and/or ECM (Electro-Chemical Migration) results for a water washable/soluble flux or solder paste, but often enough to "inspire" me to blog about it. Generally, it is a contract manufacturer that is being asked to provide the data by their customer. So, it is difficult to get my following "argument" to flow upstream to the original requester. Often times the CM does not care. The CM has been pushed into a CYA scenario and just wants to please their customer. That is not a criticism....it is just the reality of it....and we've all been there at one time or another.

The IPC J-STD-004, for example, does provide instructions for performing SIR on water washable materials. It is definitely a valid and meaningful test. However, asking for such results from the flux or paste supplier is a little bit like asking an on-line dating participant for a photograph. Why do I say that? The SIR/ECM results of a water washable/soluble material are heavily dependent on how well the test coupons are cleaned after reflow. So, you can bet that the supplier is going to do a complete and thorough job of cleaning the test coupons for the sake of producing the best SIR and/or ECM results possible. It is only to be expected that the supplier is going to want to show the best performance of their product. (BIG) But!!!! It does not represent how well assemblies are going to be cleaned on the factory floor; and, hence, the suppliers' SIR/ECM results probably will differ from the sort of performance that will be had from an assembly cleaned on the factory floor. Depending on how well the factory cleans, it could be better or it could be worse.

So, the message is, YOUR MILEAGE MAY VARY.....if the factory is using water washable/soluble solder pastes or fluxes, and there is true concern about the SIR/ECM performance of their products, it is best to perform the test on coupons that have been processed on the factory floor with standard operating procedures, equipment, and equipment settings.  

Many thanks to Covington for making shirts that have pockets handily sized to accommodate IPC-B-24 SIR test boards ; )

A few months back, while discussing tricks for hand soldering c-Si solar cells, I mentioned flux . One tip I didn’t mention is how to APPLY flux before soldering.

Tabbing fluxes are commonly sprayed and dipped in automated processes, but flux pen application is best for hand soldering. Flux pens are small plastic containers with a spring loaded felt tip at one end. With a slight amount of pressure, the tip allows flux to wick through the felt and onto the cell.

• Since the tip valve closes between uses, evaporation is kept to a minimum.
• The unit is disposable; there are no needles or micro valves to keep clean.
• Application is uniform so there is less chance of flux inconsistencies.

I use flux pens for non-automated testing of fluxes.

If you are interested in trying one of our fluxes in pen form, send us an email at solar@indium.com.

~Jim

Indalloy Flux #2 is not the only option for soldering to Nitinol.  There is also Indalloy Flux #3.

Both fluxes are strong enough to clean the tenacious oxides off Nitinol, as well as aluminum and stainless steel.

So what are the differences?  To start with, it is the consistency of the material.  Flux #2 is a liquid flux; Flux #3 is much more viscous and is usually applied by brushing it onto the surface.  Flux #2 can also be brushed on, but it can also be sprayed or dispensed.

If you are using a higher temperature solder or have particularly tough oxides, Flux #3 is the right choice.

If your solder contains indium, you will want to choose the Flux #2 because the indium is sensitive to chloride-induced corrosion.  However, if you are using a tin-based solder, Flux #3 is an excellent alternative.

Where these two fluxes are the same is that they both require good cleaning and should not be used in electronics applications.  Both fluxes MUST be cleaned as soon as possible after reflow.  This can be done with warm (not greater than 50°C) water and mechanical scrubbing.  If the water is greater than 50°C, you risk additional reactions and possible pitting of the material.

Both of these fluxes are available online at http://buy.solder.com/Medical-Assembly-Materials/C1036_1/ .  If you have more questions, check out our medical products page or contact me and I will be glad to help!

Carol Gowans

Maria Durham, Indium’s new Technical Specialist in Semiconductor and Advanced Assembly Materials, has been doing some research on indium lead (In/Pb) solder alloys. We chatted about her findings this week. 

 [Andy C. Mackie: ACM] Which indium/lead solder alloys are most common, and what are their properties?

[Maria Durham: MD] Firstly, the use of lead-(Pb-)containing solders in some soldering applications is restricted due to local environmental and RoHS compliance, but there are still many applications where they are  allowed. Many military, aerospace, and industrial equipment uses, as well as many applications related to vehicles, are exempt. The table below shows the most common indium/lead (In/Pb) alloys (pink) and their properties, sorted by liquidus temperature; the higher of the two melting points (solidus and liquidus) seen for non-eutectic alloys. In blue are three comparison materials.

Indalloy 205 is the most commonly used, probably because it has the closest liquidus temperature to the tin/lead eutectic (183°C), 63Sn/37Pb (Indalloy 106). This means it can be reflowed using a standard Sn/Pb eutectic profile. The next most common alloys that are used are Indalloy7, 204, and 206.  Besides the melting range, indium has comparable thermal and electrical conductivity to standard materials.

[ACM] What makes indium-lead (In/Pb) solders so attractive, and why have we seen a recent resurgence in their usage?

 [MD] One main attraction to using indium/lead (In/Pb) solder alloys in soldering to precious metal surfaces is that, unlike tin-containing solders, they do not leach gold. That is, gold does not dissolve in them to any appreciable extent. During discussions at Semicon West in 2011, one of our California customers reported going through 8 simulated reflows with Indalloy 205 in contact with a gold surface with no loss of joint strength and no joint embrittlement. That is pretty impressive. Note that embrittlement is often caused by gold-intermetallic formation. It has been noted that even at 250°C, 50In/50Pb dissolves Au at a rate 13 times slower than it does into 63Sn/37Pb, although this, of course, is a kinetic, not a solubility limit, study.

The higher melting Indalloy 164 (92.5Pb/5In/2.5Ag) has the lowest coefficient of thermal expansion (CTE) of all of the In/Pb solders and is able to withstand the higher temperature excursions that can be seen in step-soldering type applications (where a very high melting solder is used to form the first joint, followed by a next lowest melting alloy, and so on). This is seen in applications such as power electronics assembly, where the first step solder is often used for die-attach either as a solder paste, wire, or preform. The high melting point helps the solder withstand the operational temperatures associated with under-the-hood electronics, in applications such as engine control modules, where Indalloy 151 (92.5Pb/5Sn/2.5Ag) or Indalloy 163 (95.5Pb/2Sn/2.5Ag) are most commonly used. In/Pb solder is excellent on very rigid structures such as ceramic-to-metal or ceramic-to-ceramic. The desired solidus / liquidus temperature range can be adjusted by changing the indium:lead ratio, making it very easy to “dial in” the alloy to a specific reflow process.

Another attraction to using In/Pb solders is that they exhibit good fatigue resistance in thermal cycling from -55°C to 125°C.  In testing, the 50In50Pb solder joint fatigue life is about 100 times greater than that for 63Sn/37Pb.

 [ACM] What fluxes are used in these applications, and how are they formulated differently?

 [MD] The fluxes most compatible with the lower melting point (<200°C) indium-containing solders are NC-SMQ-80 (solder paste) or the lower-tack TacFlux® 012 (suitable for use with wire, preforms, and spheres). These are no-clean fluxes, specifically formulated for lower temperature reflow.  Under appropriate low temperature reflow these fluxes leave behind benign residues that do not need to be cleaned off (“no-clean” flux), although they are often cleaned off in most practical applications, usually to ensure reliable wirebonds absent of flux spatter.

 To learn more, please contact us.

 Cheers!  Andy

Reducing the surface oxides of Nitinol is just the first step in getting a good solder joint with this versatile medical assembly material.

Next you have to choose the right solder alloy.  You will probably want to stay away from anything containing lead, cadmium, or antimony, particularly in medical applications.  And you will want something with a high tensile strength.

The best choice is Indalloy #121 (96.5Sn 3.5Ag).  It has a tensile strength of 5,620 PSI and a melting temperature of 221C and is obviously lead-free.  It wets well to the cleaned Nitinol.

If you need a higher melting temperature solder (one that can withstand autoclave temperatures for example) you should consider Indalloy #182 (80Au 20Sn) which melts at 280C, has a tensile strength of 40,000 PSI, and has long been considered a highly reliable solder.  Additionally, this alloy is available in very fine diameter solder wires to minimize waste.

Soldering temperatures should be 25C to 50C above the liquidus temperature of whichever solder you use and proper cleaning should be always be performed afterwards.

Contact us at medical@indium.com for more information about soldering for medical devices or visit our web site at www.indium.com/medical

Carol

Eric Bastow recently wrote about using our Indalloy Flux #2 for soldering to Nitinol.  He did many tests and wrote an Application Note called Soldering to Nitinol.

Fort Wayne Metals, a leading supplier of medical wire (including Nitinol) also did a test on various fluxes as they relate to break load (maximum load before the joint breaks.

The fluxes tested included:

• Indalloy Flux #2 and Flux #3
• Indalloy Flux #5RMA; #5R; #5RA
• Indalloy Flux #4R
• Flux #400 (no longer commercially available)

The #5 series and the #4R were found to not be strong enough to clean off the tenacious oxides formed on Nitinol. Therefore, they didn't enable the solder to wet the surface properly.

Flux #2 and Flux#3 gave the best results (of the fluxes tested for break load) since they removed more of the oxides and allowed for a stronger solder bond.

Want to know more about soldering to this important medical material?  You can contact Eric Bastow directly at ebastow@indium.com or email us at medical@indium.com. 

Carol Gowans

cgowans@indium.com

With regard to soldering or wetting (coating) with indium, we are often asked to comment on the oxide formation of indium and how to remove it. We are also asked how long will it take for the oxide to reform on the surface. The procedure, below, will help you to better understand indium oxide, its removal, and how to handle it once it has been removed.

Indium is self-passivating. At room temperature, the oxide formation on the surface of the indium will be between 80-100 Angstroms thick.   Generally, this amount of oxide is not considered significant to hamper the wetting of the indium to a substrate, especially if a flux is used. Even if a flux is not used, the indium should not have any difficulty forming a joint or coating a surface.

If the application calls for an oxide-free joint and a flux cannot be used, the indium oxide can be easily removed following these steps:

·         Clean the indium in isopropyl alcohol or acetone to remove any surface organics. Allow to dry.

·         Etch the indium in 10% HCl for 1 minute to remove the surface oxides.

·         Rinse the indium in DI water to remove the acid.

·         Rinse the indium in isopropyl alcohol or acetone to remove the water.

·         Blow dry with dry nitrogen or allow to air dry.

While this etching procedure will remove the oxides, it has also opened up a whole new surface on the indium which will be prone to oxidation. Generally, the formation of oxide will begin on the surface of freshly etched indium as soon as it is exposed to air. At this time the thickness of the oxide layer is between 30-40 Angstroms. After 2-3 days of being exposed to air, the oxide has reached its passivating thickness of 80-100 Angstroms.

Note: 

Indium has the unique ability to cold weld to itself when the oxides have been removed. During the etching process, care must be taken to keep units of indium separated so they will not stick together. If they do stick, it is very difficult to separate them without distorting the indium.

If the etched indium is not going to be used immediately, storage in a nitrogen dry box is recommended . Alternatively, the etched indium can be submerged in clean acetone to prevent exposure to air.

FLUX:
To solder directly to stainless steel, Indalloy #2 Flux (activation range 100-371°C) must be used to remove the surface oxides, allowing a clean surface for the solder to wet. This flux is recommended for mechanical assembly joining only. Due to the corrosiveness, it is not recommend for electrical applications because, if the post reflow flux residue is not thoroughly removed using warm water with mechanical scrubbing, the joint will be compromised due to the potential for corrosion during its life. An alternate solution would be to nickel plate the stainless steel, so a weaker flux (RA, ROL1) can be used that is less corrosive and can be easily removed with an appropriate solvent.   

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature can be above 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

SOLDER:
The solders usually recommended for stainless steel joining applications contain a considerable amount of tin; however, the actual solder choice has to fit the temperature range of the application. Generally, a low-temperature application may require Indalloy #1E (52In,48Sn) - 118°C (eutectic), while Indalloy #182 (80Au,20Sn)- 280°C (also eutectic) is a great solder choice for high temperature. If you are looking for a solder in the moderate range of temperatures, Indalloy #121 (96.5Sn, 3.5Ag); 221°C (eutectic) is an excellent choice as well as any of the SAC alloys in the same temperature range. There are also many other solders to choose from that will work equally as well. Please see our solder alloy physical properties chart or consult our Applications Engineering staff at Indium Corporation.

We are frequently asked if it is possible to solder to aluminum. The answer is yes, if the following guidelines are followed: 

FLUXES:
Because it is difficult to solder to aluminum, Indium Corporation developed Indalloy Flux #3 (activation temperature is 96-343°C) to remove the tenacious oxides that prevent the solder from wetting to the surface. This flux is very corrosive and is not recommended for electronic applications because, if any of the post-reflow flux residue remains after a warm water rinse with mechanical scrubbing, the joint may be compromised. This flux is recommended for mechanical assembly joining applications only. 

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature is greater than 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

METALLIZATIONS:
An alternate to corrosive fluxes is to nickel plate the aluminum so a weaker flux (RA, ROL1) can be used. These fluxes are less corrosive and can be easily removed with an appropriate solvent.   There are many solder alloys that will wet to nickel. Check out our solder alloy physical properties table.

SOLDER ALLOYS:
The solders that are normally recommended for joining aluminum are:

• Indalloy #201 (91Sn, 9Zn); 199°C E
• Indalloy #176 (95Zn, 5Al); 382°C E. 

Folks,

A reader writes:

My company makes an electronic product that requires a 40 year shelf life. We assemble with tin-lead solder on FR-4 PWBs. The product is to replace older technology (i.e. 1960-70s), but has some newer components such as BGAs, SOICs, and PQFPs. The product will be stored in dry nitrogen at 70F.  We take great care in manufacturing, by cleaning, inspecting, and testing the end product.

My question is, do you know of any studies that would discuss the reliability of products stored or in use for 40 years?

My sense is that our reader will be successful, but his question is profound and hard to answer with confidence. The military would like their electronics to perform for that long, but realistically much of it is replaced every ten years or so. If you look at something like the B-52 bomber, which debuted in 1952, the electronics have been upgraded regularly. So there isn’t as much 40 year electronics experience as one might think. An exception being the IBM AP-101 computer. This computer was kept in service for over 30 years, because it served its function and had survived the rigorous and expensive military qualification testing.

However, anecdotal data might support optimism for 40 year shelf life. In a class I teach at Dartmouth, The Technology of Everyday Things, I have sought out some old transistor radios from the late 1960s and early 70s to show the class how this old technology works. Anytime I have every found an old device like this, they always work, unless the batteries have leaked inside the radio.

This question raises an interesting thought. Although those of us in electronic assembly are concerned with tin-lead and lead-free solder joint life, what about the modern devices inside the components? Forty years is a long time. How will the 3D-22 nanometer copper circuit lines in a modern microprocessor hold up over this amount of time? These circuit lines lines are so fine that the 22 nanometer width is only about 70 atoms.  In addition, copper integrated circuits are still a relatively new technology. I’m sure much accelerated life testing has been done on such circuits, but would such testing confirm 40 years of shelf or service life?

I would appreciate any thoughts that readers have on these questions.

Cheers,

Dr. Ron