Thermal Fatigue Analysis? 2

[Foreman]

I'm a structures guy, but I've been asked to look at thermal fatigue for a surface mounted transistor. To give you some background, it is company policy not to use surface mounted components on electronics boards. But the electrical guys want to use a specific surface mounted transistor, but cannot perform a thermal cycle test on any board with the transistor until they have some analysis showing the transistor good.

I'm pretty ignorant to electrical components, so any information would be greatly appreciated. I have a book by Steinberg, called Preventing Thermal Cycling and Vibration Failures in Electronic Equipement, but I've heard his method is flawed and is not neccessarily a conservative analysis.

How does one go about doing one of these analyses? Does anybody have fatigue properties for Solder? Does anybody have good advice for me?

Thanks in Advance

 

[Jchristensen]

You have asked a very difficult question that has taken the electronics industry a couple decades to resolve. Since this is new ground for you and your company, my advice is to stick with the Steinberg guidelines. As for your specific issues: Actual thermal cycling tests are generally preferred over analysis. But, in some cases analysis is OK. For example, the simplest case you could ever hope for is to have the maximum principle stress to be 250 psi or less. In this case, the solder joint will never fail, regardless of the number of cycles, or duration of the stress. Good luck meeting that criterion. One way to “cheat” is to make the solder joint thicker, so that you reduce the shear stresses during each cycle. As for more realistic analysis, one must model the strain behavior of solder as a function of time and temperature. When cold, solder is very creep resistant, but when warm, it is like ”silly putty”. Companies who have developed these methods are not generally willing to give out that information. Other things to consider are: reliability and cost requirements of the project. Can you afford to make a circuit card that matches the coefficient of thermal expansion of the transistor? That is the approach generally used for high reliability assemblies.

 

[rorschach]

I've had some experience with high temp boards (135-150C) for use in downhole instrumentation. just how hot will your boards be getting? solder strength falls off very rapidly with increasing temp. you will have the CTE of the board, the lead frame and the solder itself to account for. staying away from surface mount components whenever possible is a very good design philosophy for high temp service. We found that using a ceramic board with controlled CTE (Raychem I beleive made it. it was baby blue but I don't remember the designation number) in conjunction with the use of Indium corp Induim 182 solder (20 karat tin/gold) allowed the use of the few surface mount components we were forced to use. warning, Indium 182 forms brittle intermetallics with lead so you must make sure that there is no lead residue on the board, the component or the soldering equipment. Indium 182 has the highest melting point and the highest strength of all the solders we tested, but it is a stone cold you know what to work with.