Tin Antimony Solder

Folks,

Many people responded to my recent post, In Search of Tin Whisker Fails in Lead-Free Soldering.  A few pointed out that the recent NASA report on the Toyota Unintended Acceleration Issue  discussed numerous tin whiskers that were found, one implicated in a failure.  The tin whiskers were emanating from tin plating.

We don't know, however, if tin whisker mitigation techniques were used. In a mission critical application, such as this, it would appear unwise to use RoHS-compliant electronics, especially since they are not required for automobiles.  In other words, autos are exempt from RoHS.  Let me be very clear, from a tin whisker perspective, I am uncomfortable with RoHS-compliant tin plating in mission critical applications.  Much more work needs to be done before such tin plating should be used in mission critical applications.

In addition, in response to my post, a number of people pointed out the difficulty of proving a tin whisker fail and the reluctance of any manufacturer to admit that their products had them.

But my quest remains unfulfilled; the question remains:

"... who knows of any verified tin whisker fails when tin whisker mitigation techniques where 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."

Restated, here is my point.  Since RoHS, quite a few people take a position something like this:

With RoHS-compliant assembly, even the world of non-mission critical electronics is at considerable risk of numerous catastrophic failures, due to tin whiskers, that will cost $100s billions.

I still maintain, that with mitigation techniques, such as recommended by iNEMI, tin whisker control, for non-critical electronics, can be manageable.

As I pack up to leave my office today at Thayer Engineering School at Dartmouth, I am across the aisle from the chaps that provide our computers and IT support.  They buy millions of dollars of electronics a year.  In chatting with them they state two things:

1. They have noted no difference in electronics reliability since RoHS implementation

2. On the very rare occasion that they get an electronics failure, it is almost always a hard drive.

Bottom line: Except for hard drives, modern electronics are very reliable for their use life.

I expect my quest will uncover some tin whisker fails, even with mitigation, but the fails will most likely be isolated and not a significant threat to the industry at large.

Cheers,

Dr. Ron

The image is from Dr. Henning Leidecker of NASA, one of the world's leading tin whisker experts.

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 where 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.

Annika writes:

Dear Dr. Ron,

I'm a chemist and a PhD student working in the field of atmospheric corrosion of lead and lead-tin alloys in historical organ pipes in Europe.
I read your article on the web about tin pest and Napoleon's Buttons and Lead-free Soldering. In the article you write:

As a general rule of thumb, metals readily soluble in tin suppress or eliminate tin pest formation. Examples of these alloying metals are bismuth, antimony, and lead. As little as 0.5% bismuth or antimony by weight essentially eliminates tin pest, while about 5% lead by weight is needed. Since tin-lead eutectic solder is 37% lead, tin pest has not been an issue in soldering with traditional lead-based solder paste.

Do you think you can give some more information about this "rule of thumb" and that 5% lead will stop the pest?

Annika:

Tin pest in church organ pipes was one of the first known examples of the phenomena. Sadly, little work has been performed on this important topic in recent years. It is an issue of considerable concern for the current move to lead-free solders. I wrote a posting on it some time ago. For those interested, I have a paper with quite an extensive tin pest bibliography, that has the papers with the tables from which I got the "5% lead" information. Send me a note if you would like a copy, rlasky@indium.com.

Cheers,

Dr. Ron