Pad Cratering Evolution With Multiple-Reflows at Resin-Copper Interfaces Under Large Out-of-Plane Deformation

Pad cratering is a mechanically induced fracture in the resin between the copper foil and the outermost layer of fiberglass of a printed circuit board (PCB). It may be within the resin or at the resin-to-copper or fiberglass interfaces. The propensity for cratering depends on reflow conditions and mechanical loading in the PCB after reflow during operation and handling. These fractures, which often develop due to the application of high mechanical stresses, pose a severe reliability risk for the industry. Pad cratering impacts the production yield as well as long-term reliability. The properties of the bulk resin and resin/copper interfaces evolve during reflow, which further influences the propensity towards pad cratering. A study on resin/copper interface strength with the evolution of bulk resin properties is new. In this study, 10 different bulk resin materials are studied under various reflow conditions. The change in bulk resin properties is computed and used in determining the interfacial fracture strength of resin/copper interfaces. Samples of resin/copper interfaces are subjected to various reflow conditions and tested under dynamic four-point bend loading. The change in failure modes and crack propagation are recorded during the experiment. Interfacial fracture toughness and steady-state energy release rate have been recorded and compared for each testing condition. The evolution of bulk resin properties and resin/copper interfaces has been studied to determine its effect on pad cratering. This study would help recommend resin to copper interface combination, which provides better reliability against pad cratering.