Determination of Thermal Stress Resistance of Copper Electroplate by Hot Rupture Tests

Multilayer printed wiring boards make use of electrodeposited copper from two sources. Copper for conductor traces comes from foil manufacturers through thin laminate suppliers. Copper plating for layer interconnection is performed in an in‐house PTH process. Each source makes use of plating chemicals that are obtained from industry suppliers, but production requirements inherent in foil manufacture or in PTH processing can cause variations that occasionally result in copper deposits that exhibit poor hot strength. In a PWMLB, this can result in corner cracks, barrel cracks and inner layer cracks because some of the copper deposits will be susceptible to fissuring under thermal stress conditions. The phenomenon of hot fissuring is caused by the presence of co‐deposited impurities that degrade the hot strength of the deposit due to easy grain boundary separation at elevated temperatures. Stresses imposed by a solder float test, by soldering or by thermal cycling are then sufficient to cause microcracking in a copper plate that is in this condition. In this paper, the author describes how changes in the quality of copper plate can be monitored with a hot rupture mechanical test method. By testing plated copper samples before board fabrication, it is possible to detect and evaluate harmful effects such as fissuring before the electroplated copper is used in a PWMLB assembly.