As promised (you were warned), in this column there will be substantial discussion on the OSP chemistry itself, the issue of thin versus thick OSP coatings and overall OSP performance under lead-free assembly conditions.
The Chemistry Behind OSP
Organic coatings have been used to maintain solderability of copper surfaces in electronic assemblies for over 30 years. Initially, materials based on benzotriazoles (BTA) and imidazole chemistries provided a thin (< 200 angstroms (Å)) protective layer requiring very active flux or inert reflow processes. No-clean assembly materials, air reflow, handling issues and shelf life considerations hindered these thin materials from becoming a true replacement for hot air solder leveling (HASL). The introduction of substituted benzimidazoles (SBI) rendered significant improvements in solderability protection.
Typically, the SBI were thicker coatings (2500-6000 Å) and provided added protection against heat excursions required for complex assembly. While these particular formulations performed adequately with lead-based solders, concerns with the higher peak temperatures required for lead-free assembly arose. Adding to these concerns was that some assembly firms instituted pre-assembly baking procedures to drive out moisture. This issue was primarily driven by the moisture absorption tendencies of some of the newer laminate materials in the market place. Thus, increased protection from added heat was necessary. The mode of operation, then, was to employ the SBI and use the bulk thickness of the coating to resist oxygen penetration to the base copper. These formulations typically contained copper ions that served as a catalyst for organic film thickness growth.
However, as the complex circuit designs continued their evolution, a newer and more robust OSP needed to be developed. With the migration to SAC alloys and peak reflow temperatures reaching and often exceeding 260°C, minimization of oxygen penetration to the base copper was paramount. In addition, the SAC based alloys exhibited less solder paste spread than the lead-based materials. Thus, the issue of preserving the base copper during the lead-free assembly, further required the rethinking of the OSP process. This included looking at all aspects of the process including microetching, OSP film formation and thickness and the stability of the OSP molecule itself.
The issue with OSP film thickness was quite simple. The thicker coatings tend to resist paste spreadability. A thinner coating provides a much easier pathway for the flux vehicle to cut through the film, because OSP is a sacrificial coating. With the lower activity fluxes, removal of thicker organic films is more difficult, and the degree of solder spread is reduced. However, others may be concerned that thin coatings would not suffice under multiple reflows. The research and development work has debunked this concern. Even though the shear bulk thickness is not, in itself, sufficient to protect the underlying copper, the newer aryl-phenyl imidazole (API) organic films are significantly more stable at higher temperatures than SBI. This is critical as higher peak reflow assembly temperatures and dwell times are utilized. Consequently, the API molecule is able to achieve the desired solderability requirements with a thinner coating (1500-3000 Å or 0.15-0.3 microns).