Welding of plated, dissimilar metals for RF/EMI shielding

Plated beryllium-copper shield clips are used in the Motorola MicroTAC cellular portable telephone to provide interconnections between the RF ground and the mating shielded and compartmentalized housing cover. Outer shield clip attachment to the plated steel core of the circuit board is accomplished using a resistance weld in a narrow, space-limited area closely adjacent to conductor traces. The production welding process was characterized by widely varying weld strengths, the need for multiple welds to achieve adequate weld strength, ejection of metal during welding, frequent electrode dressing, and frequent electrode replacement. An investigation was performed with the purpose of developing a stronger weld and a more consistent welding process without the limitations mentioned above. It was determined that the process could be improved by (1) the use of projections, (2) larger, harder electrodes, (3) slightly softened shield clips, and (4) an angled top electrode to aid the operator. The improved weld was analyzed using cross sections, optical and scanning electron microscopy, and energy dispersion spectroscopy.<>