Mechanical Stress and Deformation of SMT Components During Temperature Cycling and PCB Bending

Abstract A very common method to predict the reliability of components soldered on printed circuit board (PCB) or substrates is by bending tests and temperature cycle tests, for instance between 55 °C and 125 °C (up to 2000 cycles at 1 h cycle period). Sensitive SMD constructions such as chips with ball grid array mounting or multilayer chip capacitors (MLCC) are often a major issue due to their “flex cracking” problems. This paper describes the real behavior of deformation at temperature cycling and PCB bending of chip components (body size 0603). By using the piezoresistive effect in thick film resistors the effects of stress on the alumina body can be determined and described for the whole temperature range of interest. The complete system of component, PCB/substrate and solder joint will be discussed and different influences will be isolated. It will be shown that CTE-matching of the component and substrate does not lead to an optimum situation. The influence of the solder joint plays an important part. Optimization potentials and design rules for the whole system will be given. The basis of this paper is a quite unusual “measurement tool”, the effect of piezoresistivity. The investigation into that phenomenon will be described very thoroughly first. Special thanks to Maren Steffen who carried out most of the measurements, to my colleague Joachim Aurich for fruitful discussions (he was the first in our company who had discovered and observed the piezoresistive effect on discrete resistors), and Reimer Hinrichs who made most of the drawings.