Jim,
There is a raft of information on tin whiskers at one of the NASA sites: Some really great (scary) photos
As you can see on the NASA site, there actually were several communications sat failures due to whiskers. The one that most people remember is the Galaxy 4 Panamsat. That is the one that was carrying most of the pager traffic in the US. I worked on equipment that went on the replacement sat. If ever there was a customer that was jittery and over protective about tin, that one was it. He was right!
As to how the tin got onto the sats, you will see several examples of things that were overlooked. "Everybody" knew that the leads on the ICs, transistors and most electronic parts were tin plated, so we had them solder dipped. We went out of our way to make sure the terminal posts we bought were solder plated, or solder dipped. When we inspected some of the terminal posts that looked odd for solder plate, we found that some were tin plated. At that point we stepped up our inspection for the finish, and we found that the suppliers were ignoring the solder requirement, and sending tin finish because they were in stock. Other structural hardware sometimes was tin plated. Several corrective actions were taken to replace them.
Conformal coat does not protect against tin whiskers. The whiskers are really sharp, and push through the coatings. They also take on all kinds of odd shapes, and have been seen to curl over and touch each other. They also can break off, and become lodged in areas that are not coated.
Harvey
