I think your real question is "What does this test have to do with the life expectancy of the circuit?"
Answer: nothing!
It does provide a certain comfort level to the customer (justified or not!).

What you want is the circuit to past the test. The fastest way would be to put a 5 to 8 mil layer of silicone conformal coating over everything.
This would act as a thermal blanket and allow time for the solder joint to relieve stress. It's not a CTE problem as much as the mismatch in the Delta T between the polymers and the metal. Even if the CTEs matched there would still be joint failures because of this difference.

This has very little to do with the expected life cycle (to many other variables!).

BTW does this equipment experience sudden thermal shocks in the field?

I'm sure Werner will have more to say on this!
 

"Howard P. Stevens" wrote:

A customer of Metallic Resources (manufacturers of bar solder) assembles circuit boards. One of the many tests the boards go through is a thermal cycling test to see if thermal shock can cause failure in the circuit board.

The customer's test specification is Spec Number TSC7514g, section 5.4, reference specification TSC 7010g. (We do not know what this means.) The customer cycles the circuit board from -30 degrees celcius to +80 degrees celcius. The board temperature is dropped to -30 and held for thirty minutes. The board temperature is then raised to +80 and held for thirty minutes, until the temperature is lowered again and so forth for a total of 3,500 hours. The customer is now noticing solder joint failures on this double sided board on the top, which is both through-hole and SMT (failures of solder paste joints on the top), and on the bottom, which is just through-hole (failures of bar solder joints). Solder joints, both top and bottom, show much cracking and some flux residue.

The first question is: is this customer using their own test or is there an IPC test for thermal cycling of a completely assembled circuit board?

The second question is: is the customer's temperature range too extreme, which is causing severe thermal shock, which is then causing joint failures due to different co-efficients of expansion and contraction  (board, solder, lead)?

The third question is: is the use of 3,500 hours of thermal cycling realistic and/or too long?

Any comments would be appreciated.

Sincerely,

METALLIC RESOURCES

Howard P. Stevens
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