Metal core epoxy glass circuit board evaluation program

Leadless hermetic chip carriers are becoming the standard IC package. Their use in combination with epoxy glass PWB's results in unreliable solder joints because of the thermal expansion mismatch between the HCC and epoxy glass PWB. The problem is manifested when assemblies are subjected to repeated thermal excursions. This report describes an approach to reducing the CTE of the epoxy glass PWB (15 to 20 ppm/deg C) to a value close to the CTE of the ceramic HCC (6 to 7 ppm/deg C). This is accomplished by laminating a low expansion metal layer internal to the multilayer board. This stabilized MLB reduces the stresses incurred in the solder joints, and subsequently reduces the number of fractures due to solder fatigue when thermally cycled. Test methods, test results and failure analysis are also included in this report.