Formulation of a New Liquid Flux for High Temperature Soldering

Tony Lentz
FCT Assembly
Greeley, CO, USA

Through-hole soldering is alive and well in the electronics industry, despite its predicted demise at the hands of surface mount technology. Through-hole soldering is largely used for connectors, switches, and other components that require a high solder joint strength. High reliability circuit assemblies will likely use through-hole technology for many years to come.

Thermally demanding circuit boards can be quite difficult to solder due to the high temperatures and long contact times required. Many liquid fluxes available on the market cannot withstand these type of soldering conditions resulting in poor hole fill, bridging and other defects. The goal of this project was to formulate a new liquid flux designed to give optimal soldering performance with thermally demanding circuit boards.

The development of this new liquid flux was guided by these key characteristics.

1. Withstand soldering temperatures up to 290 degrees C with long contact times.
2. Produce optimal hole fill and minimize bridging or other defects.
3. Residue must be easy to wash using de-ionized water, and produce very little foam.
4. Flux must be completely halide and halogen free.
5. Able to be used in leaded and lead-free wave and selective solder systems.

This paper presents the process of developing this new liquid flux. Laboratory and beta-site test results for existing fluxes are compared to this new flux. The result is a unique product that can help solve the challenges of through-hole soldering thermally demanding assemblies.

This process of formulation created a new water soluble liquid flux that meets the desired criteria. This flux works well with wave soldering temperatures of up to 300 degrees C and selective soldering temperatures of over 315 degrees C. Hole fill results are good even with high aspect ratio holes. The residues are easy to wash off with DI water and create minimal foam.

The new flux is completely halide and halogen free, which is relatively new technology for water soluble liquid fluxes. Beta site testing has shown that this flux is usable in leaded and lead-free applications, and in wave and selective solder systems.