Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Thermal Fatigue Analysis?

Thermal Fatigue Analysis?

Thermal Fatigue Analysis?

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

RE: Thermal Fatigue Analysis?

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.  

RE: Thermal Fatigue Analysis?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close