Indium Corporation Blogs

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,

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

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.

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

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

Ignoring the solder selection as part of your design process is risky business. 

As Terry Costlow, the IPC online editor of EMS Now noted in an article ‘Controlling the Explosion of Lead Free Solders’, the choice of the right solder alloy can affect the manufacturing process, the cost, and the field performance of the product.

Initially it was thought that the move to Pb free solders was just a matter of changing reflow profiles but major issues such as tin whiskers, brittle intermetallic layers and other concerns soon pushed solder selection into the forefront.

With over 200 published alloys and over 300 custom alloys shipped each year, we have seen the need for considering the solder design first.  Before you settle on a solder you have to consider:

·         Surface metallizations

·         Operational temperature of your product or device

·         Form of the solder you want to use (solder paste, solder preform, solder wire, etc.)

·         Temperature of subsequent soldering steps

·         Thermal coefficient of expansion

·         Tensile strength

And these are just a few of the considerations.  Let us help you make the right selection.  Contact us at: askus@indium.com.

Feel free to discuss solder selection with our industry professional, Dr. Lasky on November 11^th, IPC is having a materials conference: Engineering for Compliance in Irvine, CA.

Figure 1: 15% Voiding with air reflow

Figure 2: ~0% Voiding after vacuum reflow

Figure 3: Multiple voids

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

Folks

Folks,

Recently I caught up with Ed Briggs, a member of Indium Corporation's technical staff.  I asked Ed if he would share with us his thoughts on "graping" and how to minimize it.  His comments follow:

Graping is a phenomenon which appears as un-reflowed solder particles atop the solder mass (see figure above). Graping occurrence has increased due to the higher lead-free reflow temperatures, the decrease in volume of the printed paste deposit, and finer powder particle sized solder pastes required in miniaturized electronics. The combination of these factors, puts a lot of "pressure" on the solder paste flux to remove the surface oxides. In addition, during the reflow process, the flux can "run-away" from the solder powder particles, spreading and pooling about the deposit. The exposed powder particles become oxidized. With no flux to protect or remove the oxide, these particles do not coalesce into the solder joint.

A set of wires prepared for connection

Properly joined thermocouple wires

Thermocouples do a lot for us.  We use them for profiling reflow ovens, checking material temperatures, and a host of other temperature related measurements.  They see a lot of abuse and they are bound to break with enough rough use.  So how do we fix broken thermocouple wires?  I have a method that works very well.  It may not be the cheapest way to fix the wires but it has the following advantages:

• Pb-free joint
• Highest thermal conductivity of any current method
• Good for use up to 280°C
• Strongest connection (40,000psi tensile strength)
• Requires no specialized equipment

This method can also be used to convert leaded thermocouple wires to pb-free.

Here’s how:

1) Clip the ends of the thermocouple wires so they are even

2) Strip the sheathing back as shown in the picture (1/4inch)

3) Use a razor blade or emery paper to scrape the oxide layer off the wires, then twist the ends together

4) Put a very thin (~.001”) layer of NC 506 flux on the surface of a ceramic coupon and the exposed thermocouple wires

5) Place an 80Au/20Sn preform or a sphere(s) of the correct volume on the flux layer

6) Place the coupon onto a hotplate set to 400°C

7) Bring the wires over to the Au/Sn (which should now be molten)

8) Dip the wires into the solder

9) The solder should wick onto the wires, when it does – remove the wires.

You can leave the no-clean flux residue on the wires, or wipe it off using a solvent and rag.  You now have a high-temp pb-free thermocouple.

If you'd like to discuss this with me, click here or just give me a call @ (315) 853-4900 x-7592. 

There has been a increase in the number of requests for reflow profiles for lead-free solders. This is a change from last year, where a majority of the requests had been for a low voiding reflow profile. Now, customers are just asking for reflow profiles, in general. Also, a majority of the question for these reflow profiles are coming from companies that are considered RoHS exempt. This trend over the past 2 months tells me that these customers, RoHS exempt companies for tele-communications, medical and military contractors, are now starting to look into lead-free solders.

There is one more surprising thing. Not all of the lead-free solders are the usual Tin / Silver / Copper (Sn/Ag/Cu or SAC). There are many requests for a low temperature lead-free solder, as well.

More information may be found at our Online Help: Indium Knowledge Base.

Folks,

Some have said that I am a fan of lead-free assembly. It's not true. In a world drowning in electronic waste, I am a fan of re-cycling. Removing lead and the other 5 RoHS restricted materials makes re-cycling easier and safer. Before WEEE and RoHS were inacted the world only recycled 10% of "e-waste." We need our recycle target to asymptotically approach 100% in time.

On to lead-free assembly. Some products (network and telecommunications infrastructure, etc) are exempted from the lead restriction in RoHS, these products can contain lead (only in the solder) but must be RoHS compliant in the other 5 substances. Many are now referring to these types of electronics as RoHS 5 products.

RoHS 5 products can face a particular challenge however. Currently it can be difficult to find BGA components with tin-lead solder balls. So, people have to assemble RoHS 5 products with BGAs that have lead-free (SAC) solder balls. This type of assembly is commonly called RoHS 5.5 assembly. The SAC solder in the BGA solder balls doesn't melt at temperatures below 217C. So if the assembler uses a typical tin-lead reflow profile with a peak temperature below 215C, in RoHS 5.5 assembly, the solder joints formed with the BGA will look like the one in the figure above (courtesy C. Key Chung etal). These types of solder joints have been shown to have poor mechanical properties.

Therefore, when assembling RoHS 5.5 products, the reflow oven peak temperature should be about 225C to assure complete reflow of the solder balls which will result in a more robust solder joint.

Cheers,

Dr. Ron

Folks,

I was recently asked to give a presentation and audit an assembly line regarding minimizing "tombstoning" of passives at a major electronic assembler.
As my presentation brought out, tomstoning can be caused by many factors: the reflow profile, the solder metal composition (for lead-free SAC 387 tends to tombstone more than SAC 305), off-center placement, nitrogen reflow atmosphere, buried vias, etc. After giving a two hour presentation, I toured the line that "had a problem with tombstoning."

As I started asking, it became clear that no one knew the magnitude of the problem. "How many passives are on each board?" I asked. No one knew. "How many DPMO (defects per million opportunities) for tombstones have you had recently." This metric was also unknown. As people scurried around to get data, it started to become clear that tombstoning might not be as big an issue as everyone thought. It was more of a local legend.

Finally, we got some data. Each board had about 1000 passives, they had produced 100 boards with a total of two tombstones in the last two hours. Tombstones were the only defect. Hmmmmm two bad boards out of 100 = 98% first pass yield, not bad! From a DPMO perspective, they had 2 defects per 100,000 opportunities or 20 DPMO, which is world class. This level of DPMO would be very difficult to improve on without massive engineering investment. It is "in the noise" and it is likely caused by "common cause" variation. I then asked how much money it costs to repair a tombstone, as expected no one knew. My guess is less than $2. This situation is the rare case where yields are so good, it may not pay to make engineering investment to improve them.

This isn't the point of the story however, whatever is done in a case like this one, needs to be "data driven." Only with the proper failure rate data, plotted in a Pareto chart and understanding costs can the appropriate action plan be developed.

Always be data driven!

Cheers.
Dr. Ron