Numerical investigation of printed circuit board response during solder float test: Influence of thermal boundary conditions

During qualification, a Printed Circuit Board (PCB) must pass several electrical and thermomechanical tests. Among the tests required by the European Cooperation for Space Standardization (ECSS), the solder float test consists in resting a sample on a 288 °C solder bath. In order to simulate and better understand the latter, the heat transfer coefficient (HTC) of the PCB/solder bath interface has to be characterized.

In this work, an experimental setup has been developed to measure the HTC of the interface between SnPb and a horizontal surface. In addition, a finite element model of the solder float test has been developed in order to study the temperature and stress fields inside the PCB. The temperature field is highly heterogeneous at the beginning of the heating. A direct consequence of this early temperature heterogeneity is the development of stress fields that can be correlated to the observed failure modes. A parametric study revealed the sensitivity of the stress and strain development to changes in the HTC value. The difference observed in finite element simulations between isothermal assumptions and transient regime holds true for any maximum temperature in the range of 100 °C to 288 °C. The present work highlights the importance of considering exact thermal boundary conditions when studying the reliability of PCBs under thermal loading (especially with fast changes).