Experimental and analytical investigation of thermally induced warpage for printed wiring boards

An analytical approach using finite-element techniques has been developed to simulate PWB (printed wiring board) thermo-mechanical behavior. To validate the analytical approach, a separate experimental approach is being conducted. Four different experimental techniques were investigated for their suitability for PWB warpage measurements: ultrasonics, laser, projection speckle (using vision system), and shadow moire. The shadow moire method has been chosen for measuring warpage of PWBs because it offers good resolution and full-field measurement. Seven test samples with different configurations were carefully designed and fabricated to ensure that the thermally induced warpage would be sufficient for measurement. These test samples consist of copper layers and various FR-4 dielectric layers. Two specimen configurations were tested for a temperature range from 25°C to 100°C. Attempts have also been made to correlate the experimental results with the analytical results, although accurate material properties are not known. A good match was obtained for a set of material properties extracted from a number of publications