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How to deal with tin whiskers

Posted: 29 Nov 2011     Print Version  Bookmark and Share

Keywords:tin whiskers  Restriction of Hazardous Substances  SnPb 

So tin whiskers grow at random and interfere with system/subassembly performance. What can you do? To develop a mitigation strategy, the first thing we need is an understanding of what causes tin whiskers. Unfortunately, there is no accepted explanation of how they form, but a number of theories exist. Some postulate that the whiskers form in response to residual stresses within the tin plating and are caused by the chemistry of the plating. They point to the residual stress that results from the bright (small grain) electroplate process finishes as making those finishes prone to whiskers, yet large-grain finishes (matte) are also known to grow whiskers. Other theories hold that recrystallisation and abnormal grain growth may impact the lattice spacing leading to whiskers.

Stresses can come from many places and are accepted in the lead world, but these same stresses seem to induce whiskers in the pure tin world. Sources of stress include compressive forces from external activity like tightening a fastener; bending or stretching that might occur in the formation of the leads; and even nicks or scratches created in normal handling. Finally, a seemingly mundane difference in the coefficient of thermal expansion between the lead-frame base material and the tin-plating material has been cited as a possible source for stress that causes the whisker problem.1 Annealed matte tin seems to be the most successful finish for reducing stress and thus is often used by component companies as a lead-free finish.3

Where does that leave us? Many experiments have been conducted with inconsistent results. The present consensus is that influences that increase the stress or promote diffusion tend to induce whisker formation. In summary, the industry really does not know what causes tin whiskers to form.

Is lead really the problem?
Changing pace just a bit, consider the lead question from a different perspective. How much lead is really being consumed each year? According to the International Lead and Zinc Study Group, worldwide usage of lead in 2010 was 9.595 million metric tons, up from 8.966 million metric tons in 2009.4 (This increase is understandable, given the slowed economy in 2009.) Of that lead usage, 80% is consumed in lead-acid batteries, an application that remains exempt from RoHS compliance. Note also that before the RoHS directive, only 0.5% of lead was consumed in electronic solder and a mere 0.05% was consumed in electroplate for ICs.

What do all these statistics tell us? The 2010 usage of lead, in all applications, was approximately 21 million pounds. Of that, 16.8 million pounds was consumed in batteries and only about 10,500 pounds would have been consumed in IC lead finish if the RoHS directive were not in force for electronics.

Recall that the expected environmental harm from lead in electronics was the impetus behind the RoHS legislative action. Lead was feared as a contaminant to groundwater. Many well-intentioned people overlook one important fact, however: Elemental lead is not water soluble. Other sources concur: "Lead does not break down in the environment. Once lead falls onto soil, it usually sticks to the soil particles."5 When burned in an open-fire recycling operation, lead was feared to cause a poisonous vapor if inhaled. From NASA6, the facts are:
1. An open-fire temperature is approximately 1000°C, but lead boils at 1740°C.
2. Thus, the vapor pressure of lead would be negligible, presenting little possibility of lead-vapor poisoning.
3. Workers who solder with tin lead (SnPb) solder do not have high lead levels in their blood.

In the end, there is no evidence that lead in electronics presents a health risk or causes environmental harm. Ironically, many of the proposed lead-free solutions do pose environment problems and many are much worse for the environment.

Lead-free electronics
The move to lead-free products meant that the electronics industry has had to develop lead-free solders and terminal finishes compatible with those solders. Manufacturers have tried a number of different lead-free alloys and some very sophisticated binary, ternary, and quaternary alloys and discovered that these alloys are both expensive and hard to use. Additionally, several tin-silver alloys like tin-silver-copper, tin-silver-bismuth, tin-silver-copper-bismuth, and various other combinations have also been investigated. Bismuth-209 is slightly radioactive, so it posed its own set of issues.

In all, there had been many serious problems converting to lead-free electronics, but I will not go into a diatribe on all of them today. There are, however, two solutions worth mentioning.

Pure Tin (Sn) is inexpensive and readily available, is not chemically hazardous, and is easy to use. Most lead-free terminal finishes today are annealed matte tin as compared to bright tin or small-grain tin. The known and anticipated issue with pure tin has been presented above: whiskers. They will form over time, at random, and can eventually cause shorts or worse. Whiskers grow fairly slowly at sea level but more rapidly at higher altitude. There are mitigation techniques which I will discuss momentarily.

Nickel Palladium Gold (NiPdAu) is a popular lead-free finish material being used more and more widely. Maxim Integrated Products offers it on over 5000 different part numbers today. It is more expensive than pure tin and requires high-temperature lead-free solder.

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