Folks,
In a recent post, I shared my perspective on the pluses, minuses and neutral aspects of lead-free solder assembly. In the minus category, I listed tin whiskers. A few people commented that tin whiskers were the biggest concern in lead-free assembly. I have trouble understanding this perspective. I’m not saying these folks are wrong, just that I don’t understand their viewpoint.
First, let me say that I appreciate the concern for tin whiskers in mission critical electronics such as military, aerospace and medical. I am also sympathetic to the fact that, even though these types of electronics are exempt from RoHS, they may have to use RoHS compliant products because non-RoHS compliant products may not be available.
When I discuss the topic of tin whiskers, people will point me to NASA’s tin whisker failures website . However, when one goes to the site, there are only about twenty tin whisker fails referenced, many due to bright tin plate. Bright tin plate should never be used in mission critical electronics as it is virtually assured of producing tin whiskers. In addition, many of the articles referenced do not talk about tin whisker fails. Few if any fails are discussed relevant to RoHS (i.e. almost all fails discussed are prior to July 2006.)
I do not want to minimize the significance of tin whisker fails, some of them cost 100s of millions of dollars (e.g., satellite failures). In addition, there have been a few papers that have discussed the formation of tin whiskers even if mitigation techniques are used. Tin whiskers clearly can cause problems, but do not appear to be common, especially if mitigation techniques are used.
So here is my question, who knows of any verified tin whisker fails when tin whisker mitigation techniques were used? Tin whisker mitigation techniques typically use 2% bismuth or antimony in the tin, assure that the tin has a matte finish and use a nickel strike plating between the copper and the tin to minimize copper diffusion into the tin.
Surely if tin whiskers are a major concern, there should be many fails in the over $3 trillion worth of RoHS compliant electronics manufactured since July 2006.
Cheers,
Dr. Ron
I think the paper “Copper Tin Intermetallic Crystals and Their Role in the Formation of Microbridges between the Leads of Hand Reworked Fine Pitch Components” by
Jeff Kukelhan
as published in the 2010 IPC Printed Circuit Expo, APEX & Designer Summit Proceedings may be relevant.
He reports electrical failures during testing due to the formation of microbridges (whiskers). While the exact chemical species may be different between pure “tin whiskers” and “copper tin intermetallic microbridges”; perhaps there is some underlying commonality to the two phenomena.
The most recent report on the subject comes … not from NASA’s prior work.
It comes from NASA’s recent report on Toyota cars.
While searching for the cause of “sudden acceleration” (at Congresses request).. what did the find?
You guess it..
Even though their sample lot size was limited, they found many tin whiskers in the control electronics of the Toyota cars (ECU) they reviewed…
Yea, none of the failures caused by tin whisker growth caused the “sudden acceleration”.
So, Toyota was off the hook for the problem they were looking for.
But our industry isn’t cleared of producing un-reliable products when using ROHS.
And I am reasonably sure Toyota was running their processes to industry standards (cleanliness, quality parts, protection of assemblies, etc…)
“Surely if tin whiskers are a major concern, there should be many fails in the over $3 trillion worth of RoHS compliant electronics manufactured since July 2006.”…..
Apparently there ARE many failure due to ROHS..
and the mass media hasn’t noticed…yet.
The only known mitigation technique I am aware of involves Lead.
Conformal coatings?…. do little.
Other metal additives? .. much more expense or bigger problem with environment.
bottom line … we really don’t fully understand the issue and don’t have a proven/viable solution that doesn’t involve lead.
and yet… ROHS is what we have to work with.
Today, you have no choice but to use RoHS compliant components in your products. (Yes, some specialized markets can re-tin parts with a SnPb, but that is not the norm or cost effective.) Not all components use any mitigation… Ni underplating while a good method, is not widly used, used mostly on connectors. Annealing is used on most IC lead frame type components. These mitigation methods help reduce or at least delay Sn whisker growth. But a product is made up of lots of components… 10 to 1000’s, leaving lots of whisker opportunity.
No company wants to go public with a Sn whisker problem… what’s the corrective action? So, when a company has a whisker event, they quietly deal with it and don’t talk about it. Most consumer products have a toss and replace life cycle. Rarely, would a manufacturer take back a product and do a deep dive on the cause for failure. Only the Hi-Rel folks can afford to do that. So that’s why you haven’t read about any problems.
Many Sn whisker problems will come from plating mistakes that result in a higher propensity for whiskers. This is particularly easy to do on mechanical components like shields and brackets that get plated in smaller run plating companies.
Most conformal coatings will not prevent whiskers from penetrating. But they are an additional layer of dielectric on the adjacent surface, and will help in many situations. Wait for it, sooner of later, a whisker event will make the news…
Doctor Ron.
Please see the NASA report included in the NHTSA report on Toyota sudden acceleration. While tin whiskers were NOT the primary cause of engine acceleration, the presence of tin whiskers in the pedal assembly must be of concern to Toyota. I see a further recall of 2,000,000 plus autos to fix “floor mats”. I wonder if there is acctually an exchange of small pedal electronics modules going on? Any Toyota owners out there?
http://www.nhtsa.gov/staticfiles/nvs/pdf/NASA-UA_report.pdf
But…
If you do a search for the term “whiskers” in the full report , you will see that failure due to whiskers did happen.
Section 6.6.2.3 (page 112) for example
We can’t “Bagdad Bob” this situation… the whiskers are here and more are coming.
Tin whiskers are a real phenomenon, but are difficult to spot. Last year my freezer failed just out of guarantee, I had to scrap and replace it quickly before food spoiled, but now I wish I had the electronic board to investigate properly. How many consumer products are analysed for cause of failure? How many manufacturers would want to publicise the reasons for guarantee claims, even if they did investigate?
My freezer might not be “mission critical” but it is important to me, and a million similar faults out of a billion consumer products will cost more than a satellite failure, but won’t get the same attention.
The number of whisker related failures is probably much higher than the ones made public. Besides being difficult to spot and correctly identify, do you think the manufacturer of your mp3 player would be highly worried if you throw away yours because a whisker made it useless and buy a new one?.
Remember the Swatch case. As of January 2006, 30% of the quartz crystal oscillators of their watches had tin whiskers and 5% have experienced short circuits. I don’t have the figure of the total number of watches affected but I guess it might have been pretty high. Enough at least to deserve Swatch a RoHS exemption.
Just the fraction we know out of what I guess is the tip of the whisker-induced failures iceberg.
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