FR4 PCB CTE vs. Steel screw/nut joint CTE

[dmsang]

New to the forum . . .
I am wondering how a .093 thk FR4 board Z direction CTE would compare to a steel screw CTE.

The issue is that there is a heatsink bolted to a .093 FR4 board using a steel shoulder screw and nut.

Any ideas what the CTE mismatch between the bolted joint and FR4 Z direction might be (per C) ?

THanks !

 

[sreid]

I believe the CTE for FR4 is about 18 x 10^-6 and the CTE for mild steel is about 11 x 10^-6.