High Quality Solder Pastes:

Abstract

As electronic devices have become more complex and interconnection density has increased, electronics manufacturers are facing new challenges to solder SMD packages with pitches down to 0.3 mm or less. To achieve positive results, all parameters throughout the soldering process have to be optimised. The first step on the SMT line is the application of solder paste. Any faults at this stage (material, equipment or process related) will be carried through the entire production line. Solder paste is one of the most important factors in the whole chain. It is important to understand the influence of the metal powders, activators, solvents and additives on soldering of ultra‐fine pitch SMDs. Special attention must be paid to the powder (fine pitch devices demand a fine grain in the solder paste). The reliability of the soldered joints is mainly dependent (apart from on the solder paste) on the solder quantities applied to the component pads, the tolerance regarding the shape and size of stencils + PCBs + SMDs, the accuracy of mounting and printing, and on the reflow profile. It is important to design the stencil apertures with sufficient surface area to provide enough surface tension (between the paste and the component pad) to pull the solder paste out of the stencil, while keeping the component pad small enough to match the lead of the component. As the wetting of fine pitch components is especially critical, it is necessary to pay more attention to the design of the reflow profile. It is recommended to solder ultra‐fine pitch components under nitrogen, as this enlarges the process window considerably.