Impact of THT-hole dimensioning on manufacturability in selective wave soldering

Abstract

Higher functional integration and increased electrical and reliability requirements are leading to increasingly complex printed circuit board (PCB) designs. Highly integrated PCBs with mixed SMT/THT assembly cause difficulties in soldering processes due to the thermal mismatch between heat demand and heat supply during soldering and the limited accessibility of THT-solder joints. This directly affects the manufacturability and robustness of the THT-soldering process with respect to sufficient hole fill according to typical quality standards like IPC-A-610. Partial hole fill also affects the mechanical reliability and fatigue resistance of a solder joint as well as its current carrying capacity and high frequency behavior. In the literature, several publications show the positive correlation between a larger hole diameter and vertical hole filling. In this paper, the fundamental relationships between hole diameter, hole filling and process robustness are investigated using design of experiments (DoE). It is shown that hole filling increases with increasing hole diameter, while keeping all other parameters constant. It was found that the process also becomes more robust at shorter solder contact times and lowers preheat temperatures. From these results, a general recommendation for larger THT holes in selective wave soldering can be derived. This is consistent with several other literature reports. In contrast to previous publications, it is not only shown how increasing the gap favors hole filling, but also what this means for the robustness of the soldering process to different PCB copper layer designs and soldering parameters. In addition, the impact of hole diameter is evaluated with respect to robotic component placement and back-end processes such as testing. A Gap-Ratio of 30–40 % is recommended for selective wave soldering to ensure robust soldering processes.