Residual stress development during relamination of woven composite circuit boards

The manufacture of multilayer circuit boards involves relamination of alternating B-stage woven glass/epoxy prepregs, which act as bonding sheets, C-stage cores and copper foils. The high relamination temperature and mismatch of thermo-mechanical properties between the copper and woven glass/epoxy composite substrate lead to development of residual stresses in the boards. In the current work, numerical and experimental studies are performed to study residual deformation and warpage in a model multilayer circuit board construction of a common composite substrate (7628 fabric style). A numerical procedure based on classical lamination theory with non-isothermal viscoelastic constitutive relations is developed to predict the deformation and residual stress state due to relamination. Experimental values of the substrate stress relaxation modulus and coefficients of thermal expansion (CTE) are used as inputs in the numerical procedure to predict warpage of model circuit boards with a non-symmetric lay-up of 7628 style composite substrate. Boards with exactly the same construction as used in the numerical analysis were fabricated according to the prescribed pressing cycle, and the time dependent warpage measured using an ultrasonic contour scan technique. Comparison of the experimental warpage data with numerical predictions provides insight into the effects of processing cycle and substrate properties on residual stress development.