A PCB is a multi-material assembly, whose constituents present very different coefficients of thermal expansion. Because of this CTE mismatch, stresses appear inside the PCB when the latter undergoes thermal loading and failure can occur.
During qualification, PCB must pass successfully several tests. Two environmental tests have been considered in this work: thermal cycling and solder float test. During both, the PCB is heated by means of convection: with air in the first case, and with a tin-lead solder bath in the second case. Although the thermal cycling has been modelled and studied in the past, a finite element model of the solder float test still has to be developed.
The solder float test consists in heating the specimen by contact with a solder bath at 288°C. In order to develop a FEM of this qualification test and study the resulting stresses, it is therefore necessary to characterize the boundary conditions, i.e. the heat transfer coefficient value at the PCB/SnPb interface. For this an experimental campaign has been conducted, and allowed for the HTC determination and the development of a solder float test FEM. It appears that the temperature field is highly heterogeneous in the PCB during this test, especially at the beginning of the contact.
From this work, it is confirmed as commonly acknowledged that the temperature remains uniform inside a PCB during thermal cycling tests. It is not the case for the solder float test, leading to consequences from a mechanical point of view. When the heat transfer is considered, the mismatch in thermal conductivity of the different materials can indeed induce stresses and plastic deformation of the copper in the PCB. As a result it can be emphasized that, in contact with the solder bath, the CTE mismatch between the different materials is not the only cause of failure of PCB. For this reason, when studying their reliability under thermal loading, it is important to consider exact environmental conditions.
