In the true sense of the term, tombstoning can only occur during reflow.
Now, if your solder joints are cracking during thermal cycling and opening on one end you still have a problem - it just isn't "tombstoning".
What laminate are you using on your PCBs? Lower grade laminates are less dimensionally stable. I've run 0201s on boards that are 150 degC Tg without any problems during thermal cycle (worst test condition was 150 degC air to 0 degC water for 20 cycles, as well as 50 cycles -40 to 125 degC air to air). Those tests aren't really that stressful - the joys of building Class 2 products.
My point being that if the CTE of the board, solder, and ceramic are different enough, they may be working against each other as they expand and contract.
However, I've run into a problem when we tried to go to off-shore PCB house that used a 135 degC Tg laminate. The PCB itself is so prone to warping during reflow that I was unable to reliably place 0201s on the second pass. I can only imagine what it would have done if we had subjected them to thermal cycling.
Also, what solder paste and alloy are you using? Have you done any x-rays to see if you have voids in your joints before they are thermal cycled? Have you cross-sectioned any parts to see if you are getting good intermetallics?
This message was posted the
Electronics Forum @
reply »