No abstract

To meet the increasing lifetime requirements of solder joints in thermal cycling, the coefficient of thermal expansion (CTE) of the printed circuit board (PCB) has to be conformed to the CTE of the surface mounted devices (SMD). Novel reinforcing fabrics were successfully used to reduce the CTE of the PCB. Thereby, the mismatch in the CTE of PCB and SMD was reduced. In order to check the expected effect of the improvements, chip resistors were mounted to the PCBs. The samples were aged in a temperature shock test between -40 °C and 125 °C. The characterization of the ageing behavior was done by measuring the required force to shear SMDs off the PCB as well as analysis of crack lengths. The results with the improved CTE showed significant improvement in lifetime of the solder joints. IntroductionThe lifetime requirements of solder joints in the automotive sector are continuously increasing [1]. In nowadays electronic devices, SMD solder joints represent the largest part of all solder joints. These solder joints suffer from thermo-mechanical stresses caused by changing ambient temperature and the mismatch of the CTE of the SMD and the PCB. Electronic devices in automotive applications experience such thermal cycles during operation mainly due to the heating of the combustion engine. In addition to the thermo-mechanical stresses, the heating results in a high homologous temperature T h of the solder material in operation. The homologous temperature is the ratio of operation temperature T to melting temperature T m [2].