NIR-Assisted Flash Soldering of Electrical Components on Printed Circuits

Recent advances in flexible printed circuits focus on achieving high throughput integration of electrical components and their compatibility with flexible substrates. In this work we demonstrated the near infrared (NIR) technology as an effective tool for flash soldering of a lead-free Sn-Bi-Ag solder paste with low melting temperature for connecting conductive traces printed on flexible substrates such as polyimide and polyester with various electrical components. The NIR technology utilizes the highest energy densities to activate materials with specific absorption properties. Since many polymer substrates have low or negligible absorbance in the near-infrared range, the electromagnetic radiation of the NIR technology enables specific components, especially alloy particles in solder, to absorb the radiation and reach their melting temperature, while the temperature of the substrate remains relatively low. The conditions such as NIR power and the time of exposure were optimized to obtain improved spreading and adhesion of solder on conductive traces and the substrate. The formation of intermetallic region, which represents the degree of soldering process, was studied using cross-sectional SEM and EDX analysis. Mechanical characterization of the soldered joint was performed using universal testing system to correlate with the formation of intermetallic region.