Creating a perfect thermal profile for a wearable/IoT PCB undergoing the reflow process is a challenge. That challenge is based on the fact wearable flex PCBs are smaller, and they don’t have sufficient thermal mass compared to conventional rigid PCBs. Also, they have different thicknesses and when it comes to flex, even different thicknesses, where stiffeners are used. Conventional PCBs have certain amounts of copper and pre-pregs in the middle of the board. Those pre-pregs are of certain thicknesses, meaning they absorb a certain amount of heat, thus the thermal profile will greatly differ for a wearable PCB.
The important steps to follow to assure a perfect thermal profile for wearable PCBs are the following. The temperature settings in a conduction oven must be lowered to avoid damaging not only the small boards, but also the components on them. Special profiling care must be given to lead-free temperature settings for wearable PCBs. The best way is to obtain data from the profiler and adjust accordingly. In isolated instances, leaded wearable PCBs need special attention for developing the right thermal profile.
The figure compares a thermal profile for a conventional rigid and wearable flex PCB. In the chart below, the wearable flex circuit shows the lower range of 220 to 225º. The thermal profile for a conventional rigid PCB shows a higher temperature range of 250 to 260º. The thermal profile temperature difference is 20 to 25º. Therefore, process engineering experience comes into play here as a critical factor for creating the perfect thermal profile for a wearable PCB.
Otherwise, assembly of a wearable PCB project may incur any one of several consequences. For example, the process window for thermal profiles for wearable PCBs is normally very tight and needs to be dialed in properly or else risk de-lamination of boards, burned components or shorts under BGA balls.