Question: We are having a problem with ceramic SMT components (CAPS) showing stress patterns and cracking after thermal testing. Have you ever seen this and if so, what is allowed or acceptable according to IPC standards, even with the lack of verified electrical failure of these components?
Answer: This is an interesting question. Seems like many people are starting to see cracks in their chip capacitors. There are lots of things to look at to make sure the cause is correctly identified.
The list as I would look at it would be:
Z placement force by the pick and place equipment. If the board is thicker than another board and the z axis placement does not catch it, it may apply more force when placing the components and cracking them. Although it may not be seen at this time, the ceramic will fracture when exposed to heat cycles.
Solder paste, Tin/Lead or Lead-Free. Lead-Free is more brittle and the stress relief will be absorbed by the component causing it to crack.
As for the thermal cycles of removing the parts don’t worry too much about the pads, but do concern yourself with the application of heat. There is lots of talk these days about using hot air to reflow the caps as the thermal ramp up with the solder iron is too quick and will potentially crack the end termination from the body of the capacitor.
When you do thermal testing, place some loose caps in the oven and see if they also crack. If not, then a good direction to look would be the installation and soldering process. If the loose components do crack, then I would start with the manufacturer and supplier. They are starting to see some chip caps that are counterfeit components, so this idea cannot be dismissed and needs to be considered.