Residual stress development during relamination of woven composite circuitboards

The manufacture of multilayer circuit boards involves relamination of alter nating B-stage woven glass/epoxy prepregs, which act as bonding sheets, C-s tage cores and copper foils. The high relamination temperature and mismatch of thermo-mechanical properties between the copper and woven glass/epoxy c omposite substrate lead to development of residual stresses in the boards. In the current work, numerical and experimental studies are performed to st udy residual deformation and warpage in a model multilayer circuit board co nstruction of a common composite substrate (7628 fabric style). A numerical procedure based on classical lamination theory with non-isothermal viscoel astic constitutive relations is developed to predict the deformation and re sidual stress state due to relamination. Experimental values of the substra te stress relaxation modulus and coefficients of thermal expansion (CTE) ar e used as inputs in the numerical procedure to predict warpage of model cir cuit 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 d ependent warpage measured using an ultrasonic contour scan technique. Compa rison of the experimental warpage data with numerical predictions provides insight into the effects of processing cycle and substrate properties on re sidual stress development.