Strain measurements in thermally loaded circuit boards containing embedded resistors

Four specimens of embedded resistors were tested from room temperature to the processing temperature (177°C) to compare deformations in the printed wiring board (PWB) by means of electron-beam moire. The specimens had different termination sizes and geometries; two with straight approaches to the termination came from one board, and three with indirect approaches came from a second board. In none of the specimens did deformation occur in the embedded resistor or at the interface between the resistor and the termination. However, significant deformation occurred within the PWB and the laser barrier of the specimens with the indirect approach to the termination. Strains as high as 6% were calculated in areas of that PWB. And in two specimen, the mismatch in thermal expansion between the resistor and the laser barrier was so great that a crack developed at that interface.