[carlsonmark]

Disclaimer: I'm a software guy

Another source of cracking could be related to moisture in the SMD components. We do primarily small volume PCB runs where I work. In house, with parts stored here, pick-and-placed here, etc. We have had components crack as well, and I recall one of the suspicions being the source of the components. They were a supposedly reliable supplier, but components from them failed more often than those from another supplier so we stopped getting parts from them.

I am not sure if resistors are as prone to humidity effects, but if plastic parts are not humidity controlled before going into a reflow oven, they are prone to cracking. Even as a software guy, I have seen several brand new boards with cracked components on them over the last 15 years. Often the cracked components are difficult to find, since they are so small and do not fail completely (they are just off by a wide margin.)

https://www.ti.com/lit/an/snoa300a/snoa300a.pdf