Microstructure and mechanical properties of electroless copper and their influence on the reliability of printed-circuit boards

The microstructure and mechanical properties of copper deposits used in printed-circuit boards were investigated. The addition of surfactant and stabilizers in the electroless copper plating bath, effectively changed the fine-grained structure obtained from an additive-free bath to a columnar-grain structure. The shape, size and concentration of hydrogen voids on the grain boundaries are also affected by the additives in the plating bath. A comparison of the mechanical properties of commercial and in-house electroless copper deposits indicates that the ductility and tensile toughness of these deposits tested at room temperature can discriminate their behaviors in the Military Float-Solder Test. A minitensile tester was modified for the testing at elevated temperatures. Electroplated copper showed a brittle zone at temperatures higher than 150 centigrade. This brittle zone is similar to that of bulk Tough Pitch copper. However, electroless copper deposits showed an improvement in ductility at elevated temperature. Grain-boundary cracking was observed in electroplated copper tested at elevated temperatures compared to ductile fracture of electroless copper tested under the same conditions. Similarly, dynamic recrystallization is inhibited in the electroplated copper compared to grain growth in the electroless copper deposits. The lack of brittle zone in electroless copper deposits is attributed to the reduction of oxide by co-deposited hydrogen. A thermo-elastic-plastic finite element analysis of the thermal stresses at plated-through-holes in printed-circuit boards was conducted using the measured mechanical properties. The simulation showed that high-toughness electroless copper would pass the Float-Solder Test in agreement with the experimental results. Electroplated copper behaved poorly in a simulation of the Military Thermal-Cycling Test due to its brittle zone at elevated temperatures.