This is a reasonable start, certainly autoritive but nothing about in-service vibration and/or temperature on PbF embrittlement
pdf DEVELOPING A NASA LEAD-FREE POLICY FOR ELECTRONICS LEARNED LESSONS Michael J. Sampson MD, 20771, USA National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt,
Section 3 ... Tin has two stable allotropes: beta (or "white") tin and alpha (or "grey" tin). At temperatures above 13°C the familiar beta phase is the stable allotrope for tin. Beta tin is a body-centered tetragonal crystal that is ductile and a good electrical conductor. At temperatures below
13°C, the alpha phase is the stable form. Alpha tin is a diamond cubic crystal that is brittle and has semiconductor properties. The transformation from beta to alpha tin is also accompanied by an expansion in volume of -26%. This expansion and the brittle nature of alpha tin produces a wart-like, powdery conversion of the tin commonly referred to as tin pest [Fig.
- The maximum conversion rate reportedly occurs around
- 40°C [ll]. There are various anecdotal, historical references to tin pest. For example the tin buttons of the uniforms of Napoleon's army are supposed to have disintegrated because of tin pest formation in the cold . Russian winter during his retreat from Moscow Figure 2. Transformation of Beta-Tin into Alpha-Tin in Sn-O.5Cu at T = -18°C  Recent experiments have found that Pb-free tin-based alloys can exhibit pest [11,13]. The formation of tin pest seems to be affected by the presence of alloying constituents even at very low concentrations. Like tin whiskers, the most common way to reduce the risk of pest is to alloy the tin with Pb, ideally about 40% Pb. The risk of pest can probably be eliminated or reduced Sn63Pb37 or Sn60Pb40 to acceptable levels by using solder to attach the components making sure as much of the pure tin finish as possible is wetted with the solder. tin- But one must remember that there are many types of coated components that will not be assembled via soldering. For example, mechanical components such as screws, nuts, washers, brackets, shields, connector shells, braids, etc. may be tin-coated and will not be assembled using solders. In fact, sometimes applying solder to components where solder was never intended (e.g., crimping a solder can have deleterious effects coated conductor can result in intermittent electrical contact due to plastic deformation of the solder).
- A SIMPLE PB-FREE POLICY FOR NASA From the preceding discussion it can be seen that NASA does not have to adopt Pb-free systems and would be wise to avoid them as long as possible. It is more straightforward to avoid the use of Pb-free solder than it is to avoid pure tin finishes. At this time there is no need for NASA to accept the use of Pb-free solders except in special circumstances where the use of specialty solders has always been permitted. Such special circumstances tin- include high or low temperature applications where silver, tin-antimony, tin-bismuth or tin-indium solders may be appropriate. The situation is much more complex regarding pure tin finishes. Pure tin is now the solderable termination finish of choice for most commercial electronic parts worldwide. As NASA often needs to utilize commercial parts in order to achieve necessary functionality, mass or size for leading-edge applications, exposure to pure tin terminations is inevitable and in fact is already quite common. The simple NASA policy can therefore be summarized: Tin-Pb solders are required unless Pb-free solder alloys are necessary to meet technical needs such as high (or low) melting points, material compatibility etc. Pure tin termination finishes shall be avoided whenever possible and shall be carefully mitigated against the risk of whisker growth if their use is unavoidable.