The evolution of organic solderability preservative (OSP) process in PCB application

The implementation of Restriction of Hazardous Substances Directive (RoHS) in 2006 has led to a requirement that soldering materials used in the fabrication of printed circuit boards (PCBs) should be lead-free. This change has raised the peak reflow temperature by 30 C as compared with a tin-lead process. A protective surface finishing should be designed to maintain good solderability, by preventing the copper surface from being oxidized, both during storage after PCB fabrication and during exposures to soldering temperatures. Organic solderability preservative (OSP) is considered as a preferred low cost surface mount technology (SMT)-compatible non-metallic surface finishing method, due to the excellent surface co-planarity of the coated pads. However, the protectiveness of some commercial available OSPs is still poor and their solderability performances are always deteriorated after the multiple lead-free reflow cycles. This article presents experimental data to illustrate the newly developed OSP coating in terms of thermal resistance, copper diffusion suppression and solderability.