This is a response to an amazing number of postings on March 13 and 14. Apparently there has been a pent-up demand for the new tin whiskers forum. I list below some comments from a number of those postings, and my response. I will start with the general comment that it helps to keep in mind that the real goal is not to prevent tin whiskers but to prevent short circuits caused by tin whiskers. I await someone who can do the math on how statistically significantly conformal coatings "mitigate" tin whiskers. (Bob Landman) Bob, you?re going to have to wait longer than you can afford to get a statistically significant answer to your mitigation question. First, you?re going to have to live with the fact that conformal coating will not entirely prevent short circuits caused by tin whiskers. But that really doesn?t matter. The requirement is not that all such short circuits be prevented, but that their frequency of occurrence is acceptably low ? no higher than the frequency of occurrence of other kinds of circuit failure. I believe, for reasons elaborated on below, that this is a reasonable expectation. If a whisker can grow from one pin on an IC package, then certainly, it can also grow from adjacent pins as well and then don't we have the perfect opportunity for shorts? (Bob Landman) Two whiskers can grow through conformal coating and meet. The presence of an electrical potential between the whiskers increases the probability of their meeting, but as Henning has already commented, there?s still a question of whether they will actually meet, and if they do, whether (because of their oxide coatings) they will cause a short circuit. Keeping in mind my comment above, I believe that the probability of two whiskers penetrating a conformal coating and meeting is low enough for most high-rel applications. ??any trace of silicon on the surface?? Raymond Bennett Silicone on a surface will prevent the proper adhesion of a conformal coating. However, the primary concern is how this lack of adhesion prevents the coating from performing its primary function, which is to adhere to the surface so as to prevent the opportunity for a continuous layer of liquid water from forming. I don?t think anyone knows how much a poorly adhering conformal coating affects the probability of penetration of a whisker from below, but in preventing penetration from above (i.e., from the outside) it will probably work as well as a well-adhered coating (see Joe Kane?s point 5). For typical voltages (<50 V) the thickness required to prevent a short circuit is probably on the order of tens of nanometers. ?I seem to remember somewhere that the tips of the whiskers are so thin, that they can bore right through a conformal coating...no matter how well applied.? Phil Shewmaker There?s been no such evidence that I know of. Certainly a thin whisker is sharper than a thick one, but it also buckles more easily. ??there has not been adequate research into the use of ternary or quaternary lead-free solder alloys for whisker mitigation?? John Wayt The research on what elements reduce the tendency of tin to form whiskers was done fifty years ago. The only element that has a well-established ability is lead. Bismuth may have a small reducing effect. That?s the complete list. ?The tin crystallizes and grows?? Steve Smith Tin is crystalline when it is electroplated, and it is crystalline when it freezes. There?s no way to get amorphous tin. ?Remember that we don't have an intact layer of tin oxide on the surface. The thermal cycling continually shatters it, just as colliding sheets of ice on the ocean break and reform creating a jagged ice-sheet.? Steve Smith The time required for an oxide coating to start forming on a freshly exposed layer of metallic tin is measured in microseconds. The layer quickly grows to a few nanometers and then effectively stops growing. ?Moisture has been shown to be a major driver of whiskering, so any idea that any conformal coating can prevent whiskering I would hold up in the air between thumb and forefinger and at arms length, and drop into a high-humidity environment for a few thousand hours, and then come back and gingerly poke it with a long stick.? Steve Smith Moisture does seem to accelerate whisker growth, but it is not necessary. The greater concern for moisture should be what it does to the properties of conformal coatings. They all absorb moisture to some extent and therefore become softer. That still might not make any difference on how easy it is for a whisker to penetrate from above. ?As far as I am aware the promotion of use of conformal coatings to aid RoHS issues were not to help tin whiskers, but stop oxidisation of crystalline tin joints. Using a conformal coating stops the oxidation process and improves short term reliability.? Mark Vaughn Conformal coatings are applied to prevent dendritic growth between adjacent metalizations at different electrical potentials. It does not stop oxidation, nor does it need to. ?Tin loves to form a crystal structure, plumbers tell the difference between tin and solder bar, by bending the bar and listening to it, the crystalline structure of tin will crackle.? Mark Vaughn Pure tin makes a noise called ?tin cry? when it is bent. That has no relevance to anything being discussed in this forum. ?With Pb joints the eutectic point, that is the potentially crystalline phase during cooling lasts for about a 2 degree temperature change, with a Tin based lead free solder that is around 39 to 42 degrees.? Mark Vaughn I have no idea what Mark is intending to say. ?This makes it very difficult to ensure a manually soldered joint is a good one, especially when the joint looks so damn awful in lead free.? Mark Vaughn Manually soldered solder connections have remained satisfactory for many decades. If there?s enough of it and the wetting to the surfaces to be connected is satisfactory, their appearance (i.e., texture and luster) is unimportant. ?So there is a high probability that manual joints with contain crystalline fishures that over time usually a few days to weeks will oxidise and form a bad/dry joint.? Mark Vaughn Manually soldered connections do not contain fissures, and their oxidation is virtually complete by the time the solder has cooled to room temperature. ?I expect it's more related to the electrical current flowing and voltage charges across the tin lattice structure. Some crystalline structures are not complete and are said to have acute voids, in which nucleation sites become seeded, brought on by electrical charge to form crystals that due to differences in angles of the crystal and void become ejected, sounds like a whisker to me.? Mark Vaughn The relationship between potential or current and whisker growth is tenuous at best. The tin plating does not ordinarily contain ?acute voids? (whatever that term might be intended to convey), and while nucleation of whiskers is still not well understood, since metals are electrical conductors, electrical charges are unlikely to have anything to do with it. ?Sadly this is not my field, trying to remember some college stuff 20 years old, and I don't remember the differences with between pure and solvent crystalline grow, so I may be talking bunkum, but I don't think I'm far off.? Mark Vaughn Solvent crystalline growth is irrelevant to tin plating. ?I was also wondering if anyone has evaluated the effects of Hydrogen Sulfide on these connections.? Jacob Brodsky I don?t think that hydrogen sulfide has a measured effect on the growth of tin whiskers (tin sulfide is not a very stable compound), but it has a major effect on the growth of silver whiskers, as discussed at length on the NASA whisker web site. ?Are there conformal coatings that we could put in a purchase specification that might help us here? Is it something we could do ourselves after purchase?? Jacob Brodsky As discussed above and below, conformal coating is the only practical mitigation technique available to the OEM other than solder dipping or (in the case of chip components) replating. ?<While the solder alloys are not pure tin, the soldering process does not wet the entire surface of the parts that have become pure tin plated.> Wetting the entire surface is rare.? Henning Leidecker Small terminations are completely coated with a layer of solder and hence if the solder contains lead they have very little risk of whisker formation. Larger terminations are at risk of whisker growth from any tin-plated area not reached by the solder. Even areas under but near the edge of the solder can grow whiskers since the solder froze before it could completely dissolve the tin. ?If the solder has lead in it, and is applied as a hot liquid, then the lead alloys nicely into the tin and no long whiskers grow. But evaporating or electroplating some lead onto the pure tin is not so effective at getting the lead into the pure tin.? Henning Leidecker Neither evaporating nor electroplating lead onto tin is a commercially available process. There is a proprietary process for replacing existing tin plating with electroplated tin-lead, and that is only practical for chip components. That process does not leave any lead-free tin. ?If the tin surface has already grown some whiskers, and then a leaded alloy is flowed on, the whiskers can sometimes remain: they melt at 232C (and the tough oxide on their surface can maintain them as needles encased in a shell, at up to 260C and sometimes a bit more), while eutectic lead-tin solder melts at 183C and might be applied at less than 250C.? Henning Leidecker While it is true that because of its oxide sheath (thin, but very tough!) a tin whisker can survive heating well above the melting temperature of tin, because of its very tiny cross section it will dissolve instantly upon contact with molten solder. ?Whiskers were able to push through THIN areas of our coating.? ?Warning: Conformal coating processes often suffer from incomplete coating. For example, shadowing effects during spray application, thinning of the coating from surface tension and other forces during curing can leave you with ?less coverage? than the process documentation tells you that you have. Good idea to INSPECT and verify YOUR coating processes ability to coat all areas of interest to you.? Jay Brusse Any conformally coated assembly will have thin areas (anywhere there is a small radius of curvature as well as any shadowed area). Therefore it is only practical to assume that some whiskers will grow on a conformally coated assembly. I see no point of inspecting for thickness since there remains no practical way to make the coating thick in the small-radius-of-curvature regions. See my comments above about how much of a concern thin coating is. ??large grained 100% matte tin plating on copper-lead frames resists whiskering even when exposed to compressive forces and held at elevated temperatures in humid environments. Bright tin, however, had whisker growth reaching 700 µm even at ambient temperatures. Extensive cyclic compression tests with select matte tin chemistries, considered a worst-case evaluation, yielded whisker growth of less than 20 µm.? ?A nickel barrier under tin helps prevent the formation of compressive stresses caused by the growth of CuSn intermetallics and hence reduces the one driving force for tin whisker growth. If a nickel barrier layer is not possible, tin plating is annealed after plating for 1 hour at 150°C to promote controlled intermetallic compound formation and reduced tin whisker risks.? Patrick Lavery, quoted by Bob Landman The problem any OEM has with the type of tin plating being received is that it can?t be inspected to find out if it is ?good? (matte, large grain) or not so ?good? (bright, fine grain). Any assembly will contain components from dozens if not hundreds of component manufacturers, and even one manufacturer can have unnoticed changes in the tin plating process from day to day. Similarly, there is no practical way to determine whether a nickel underplate is present or whether the plating had been reflowed within one hour. (What would you do if you knew?) ?Of these finishes, ENIG has been shown to be the most robust for lead free soldering.? Patrick Lavery, quoted by Bob Landman ENIG won?t grow tin whiskers, but it can have failures due to black pad and brittle fracture. Consider OSP or immersion silver. ?A thick conformal coating (silicone-based polymer with 90% to 92% filled ceramic particles) used in power brick products as part of a thermal management strategy?adds resistance to whisker growth.? Patrick Lavery, quoted by Bob Landman The power brick approach seems to me to be a very effective technique to prevent short circuits due to tin whiskers, provided the coating doesn?t have voids that span the space between tin-plated surfaces at different electrical potentials. Gordon Davy