Novel method for PTH soldering driven by the thermal restrictions of a PTH LCD device

Abstract

The approaching deadline for the RoHS Server exemption expiration provides incentive to the server manufacturing industry to accelerate the Pb-free transition for complex manufacturing processes. The Pb-free soldering process requires a higher reflow temperature than standard eutectic SnPb solder to achieve proper solder joint formation, but introduces more thermal stress to thermally/temperature sensitive components (TSC), especially for the pin through hole (PTH) components attached by surface mount soldering/intrusive soldering or legacy wave soldering processes. Hand soldering and solder fountain (solder pot or selective wave) are two methods used as replacements for legacy wave-soldering to avoid thermal damage to TSC's. There are some challenges with hand soldering that include achieving the 50% hole fill requirements and consistently performing a manual operation during high volume mass production. Solder fountain has become the most reliable method for thermal sensitive PTH components to ensure temperature limitations. In one application that it was required to use solder fountain was the soldering of a Liquid Crystal Display (LCD) PTH component. This particular LCD component had very severe temperature limitations restricting its exposure to 90∼100°C. The method presented in this publication is to reduce the thermal exposure of this specific TSC PTH component, a process utilizing a solder fountain with some additional tools to reduce the heat transfer. In order to further control heat transfer via conduction, convection, or radiation, the idea of applying a controllable air flow between the component body and heated pins was conceived. The air flow between the heat source and component body formed an air wall thermally isolating the component without causing a disturbance with the soldering process. A design of experiments (DOE) using manufacturing parameters that influence component body temperature was completed and provided optimized settings for the “air wall's” effect. This paper documents the experiment varying contact cycles, contact time and air flow that generated an effective air wall process and discusses how the air wall successfully reduces heat transfer to the component body. It will compare the thermal impact on the LCD component body of hand soldering, solder fountain, and solder fountain with the “air wall” mechanism.