Selective Soldering - Flux Amount Control for Consistent Process Quality

Flux is one of the most critical parameters in the selective soldering process. Flux deposition on the board needs to be carefully controlled. It should have the right balance between solderability and reliability. Flux has a major impact on barrel filling and defect production in challenging thermal applications. Robust flux design for selective soldering is therefore a critical factor. Partnership between flux and machine designers is a key component for success.

It is important that the right amount of flux is applied on the right spot with a defined spread. Flux must not penetrate into SMD areas. Selective solder flux may not be compatible with solder paste residues. Flux that is not activated may also affect reliability in the long term because of potential electro migration.

The application of flux is done by drop jet systems. Many high-volume applications require a high throughput. Short cycle-times can be achieved with high-frequency drop jets mounted on robots that can accelerate and decelerate quickly. Any clogging of the drop jet may result in solder defects like bridging or open joints. Spraying therefore becomes critical, and the flux supply may also be affected by outer noise such as clogging, flux pollution, pressure, temperature and other changes. Controlling the amount of flux may require a closed-loop system to guarantee a consistent process quality. This study evaluates the flux application process and tries to monitor the impact of drifts and changes. A method is shown where the process can be robust without being sensitive to noise. Statistical tools are used to prove repeatability in a selective soldering process and between different machines.