This is a reasonable start, certainly autoritive but nothing about
in-service vibration and/or temperature on PbF embrittlement

from 2006
http://ntrs.nasa.gov/archive/nasa/ca...268_2008037503.
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.
21. 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 There are various anecdotal, historical
references to tin pest.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
[12].
Russian winter during his retreat from Moscow
Figure 2. Transformation of Beta-Tin into Alpha-Tin in
Sn-O.5Cu at T = -18°C [13]
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).
4.
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.