Using Thermochemical Analysis in the Quality Control of Prepregs and Laminates for Multilayer PCB Manufacture

Differential Scanning Calorimetry (DSC) is an ideal technique for characterising polymeric materials such as the epoxy resin in epoxy glass prepregs and laminates, the technique having evolved from the older qualitative method of Differential Thermal Analysis (DTA). In DSC the heat flow to or from the sample is measured as a function of temperature or time. Epoxy glass prepregs are used as the bonding/insulating layer in multilayer printed circuit boards. When a sample of the epoxy resin in prepreg is heated, the heat flow that results from the exothermic curing reaction is measured and used to calculate the enthalpy ΔH of the reaction. ΔH is related to the degree of B stage curing and the flow achievable in the laminating press. Methods are given for calculating ΔH and the differences found in two manufacturers' prepreg material are discussed. The effect of ageing on ΔH, which has been found to decrease with time and with the extent of cure, is examined at room temperature and under refrigeration. The difference in the ageing properties of the two prepregs is explained by reference to a plot of the degree of conversion with time. This plot also enables the curing time required in the lamination press and the shelf life of the prepreg at room temperature to be calculated. Monitoring the degree of cure of laminates and cured prepreg using the glass transition temperature of the resin is examined. Insufficient cure may lead to problems of resin smear in multilayer printed circuit boards. Thermochemical analysis is used as a routine quality control method in the Microcircuit Assembly Techniques Facility at Marconi Research Centre for testing incoming prepreg and laminates used in multilayer printed circuit board manufacture.