Reliability of Tin Lead Solder vs. Lead Free Solder

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Reliability of Tin Lead Solder vs. Lead Free Solder

The issue of tin-lead solder versus lead-free solder reliability and whether lead in solders used for circuit board assembly impacts the reliability.
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers, pick and place. Coming to you from the ITM cave high above Mount Rialto.

Wherever Mount Rialto is. We are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. Jim, today we have a question on lead-free reliability, SAC vs. tin-lead.

The old one short of the meaning of life. This is from L.B. L.B. is saying could you please give me some further explanation of your answers regarding the tin-lead versus lead-free versus unknown leaded alloy reliability issue?

Jim
What answers were they?

Phil
I don't know. We must have said something, which was true. We must have parted some wisdom.

L.B. goes on to say from all of the reliability problems that I have read about with lead-free alloys for circuit board assembly it seems like almost any amount of lead is more reliable than none.

Jim
I know where L.B. is coming from and I can guess what I said. This idea that any amount of lead is better than none addresses one specific reliability issue and that is whiskers, tin whiskers.

The presence of lead, even in a small amount 1 or 2%, is the best way to mitigate tin whiskers. So if you are building space satellites or pacemakers or any ultra-high reliability where you are concerned about shorting due to tin whiskers adding any amount of lead will significantly reduce, if not eliminate, that reliability issue of tin whiskers.

What I am assuming I was talking about was the many more common reliability issues such as drop shock, vibration and thermal cycling where in most cases lead-free has been found to be at least as reliable as tin-lead, except for some high-rel, high-stress environments where the jury is still out and we are still trying to understand the complete issue to be certain that we can build a pacemaker or something like that, assuming we can mitigate the whisker issues, in terms of thermal cycle reliability being the biggest one.

So that was what I was concerned with. When I say unknown lead-free reliability that is for these ultra-high stress, high-reliability, long-life factors.

Again, the more typical failure modes of thermal cycling, drop shock and vibration where we just don't understand the nature of the failure mechanism to have complete confidence in the lead-free alloys in terms of the long-term reliability. But yes, if you are worried about whiskers, lead is the way to go. That is why people building satellites, all of the tin finishes on the leads come off and are replaced with tin-lead.

Phil
This is a question that persists. It started in the dark days prior to 2005 and the implementation of ROHS was still being debated and kicked around.

More and more papers are being written to the contrary of what our participant has asked here. It is all pointing in the direction that Jim mentioned.

This is the stuff we have been reading and more and more long-term data is being harvested as we go on. It is now long-term data. We are now 11 years past.

Jim
That's right. An article published by IBM referring to servicing that nobody is asking for an extension of the server exemption for lead-free because all of the problems have been solved. They are going to make even enterprise-level servers with lead-free because they have developed the knowledge to have confidence in their reliability.

Which has taken a lot of work. They have just achieved this in the last couple of years. They have been working continuously since before ROHS to understand the problems to achieve the confidence in the reliability in these different server products which are of course very critical and long-life, very high reliability.

Phil
That is very significant and telling.

Jim
Nobody is building hard pacemakers for lead-free yet.

Phil
Well you have to ask yourself, would you want one? With what we know so far and everything else? Do you feel lucky?

We hope we steered you in the right direction on that one. You have been listening to Board Talk with Phil Zarrow and Jim Hall. Remember Board Talk melts in your mind not in your ears. And whatever you do...

Jim
Don't solder like my brother.

Phil
And please don't solder like my brother.

Comments

To Back-up what the brothers say, between 2007 and 2021 I was very involved with the building of many thousands of very complex boards being mounted in military vehicles or theater deployed. All of them were Pb free assemblies. Over that time, not one was ever found to have a failure related to solder joint failure of the SAC alloy.

Alan Woodford, Zentech Manufacturing

I can tell you, from "field" evidence, that the early lead-free solders and/or component lead finishes are NOT reliable in high-vibration environments with extremes of temperature. I've had multiple modules fail due to cracked solder joints in my early-2000s (just prior to phase-in of RoHS) vehicle.

Ben Bertz, Red Seal Measurement

The long-term reliability differences might indeed be minor (or even negligible for most use cases) but they are not the only consideration with regard to quality and cost. As mentioned above, it took IBM over a decade to solve "all of the problems" associated with Pb-free solders. And these challenges brought forth the best in human ingenuity, no doubt. However, some of our "solutions" are workarounds. Process challenges endure, which raise costs and lower yields.

For example, the higher melting points of Pb-free solders has resulted in novel and enduring SMT process challenges. The enhanced demands on flux and paste chemistries have been substantial. Board and component deflection tendencies have been aggravated, necessitating special fixturing and more robust designs. In general, process windows are now permanently smaller. All of these (and similar) issues constitute an increased burden on our industry, and arguably divert limited resources from their best and highest use. It's not a stretch to think this might also have an effect on long term reliability.

Alan Couchman, Process Sciences, Inc.

For more information about tin whiskers, take a look at this: http://mcblogs.montgomerycollege.edu/insights/2016/12/06/nasa-we-have-a-problem/

Leo Ranta, Emerson Automation Solutions

I think you have failed to mention some of the fundamanetal properties of typical SAC type Pb free solders that leads one to the conclusion that they are leass relaible. I site the papaer "Evolution of Lead Free solder material behavior under elevated temp aging" by Ma,Suhling,Zhang,Lall & Bozak Figures 17 vs Figure 21 as evidence that sac based Pb free solders continue to loose strength over extended periods of exposure to high temps. After very long exposures to elevated temps you will get failure. Is this a problem in the real world...perhaps not, as there are several other time based failure modes in most electonics packaging, first and foremost is the Au/Al interface on you semiconductior wirebond.

I believe the whisker concern while real can be controlled with surface finish. Ni/Pd finish used by TI among otheres is pretty imune to whiskers as is Ni/Pd/Au and Ni/Au(super thin). ENIG can be used, but that introduces the potential for black pad and any finish with Au brings forth the Au embrittlement concern which is another topic of discussion. I also want to point out that while the pacemaker example is good interms of the catatrophic outcome of failure, the environment is very benign in terms of temp extreme and mechanical shock loads. I would site automotive as probably your toughest environment as there is an expectation of long life in arguably a very tough environement. Please keep up the good work and interesting topics of discussion.

Gifford Plume, Patriot Marine Services