Decomposition Temperature of OSP Films
Certainly, the ability of the OSP film to minimize oxygen penetration down onto the base copper is critical when maintaining solderability. This becomes increasingly significant when lead-free assembly temperatures are involved. Thermal stability of the film is related to the coating's ability to maintain a solderable surface under lead-free assembly conditions.
The latest generation based on Arylphenylimidazole shows much higher heat stability and can easily withstand the peak temperature of reflow for typical lead-free solders. Thermogravimetry (TG) curve indicates the weight increase and decrease, and differential thermal analysis (DTA) curve indicates the exothermic and endothermic reaction of OSP coating itself (Figure 1). An OSP coating composed of an Alkyl benzimidazole compounds, which are still widely used in the market begins to decompose around 250°C as shown in Figure 1 (top graph) This is the typical peak temperature for lead-free soldering. However, the latest OSP composed of Aryl-phenylimidazole technology shows much higher decomposition temperature. (approximately 350°C) shown in the lower graph in Figure 1. The fabricator should carefully investigate the choice of an OSP process. It is certainly righteous to ask the supplier to disclose these types of decomposition graphs.
Basically, if lead-free assembly is the norm, then choosing the OSP with a higher decomposition temperature makes sense. The question though is does thickness of the OSP film play a role in solderability preservation? As stated earlier, one should not be mesmerized with the idea that thicker is better. Again, the highest weighting this author can place on this requirement is that the quality of the OSP film depends on the uniformity of the film over the copper and the ability of the OSP to minimize oxygen penetration.Figure 1: Thermogravimetry (TG) curve indicates the weight increase and decrease, and differential thermal analysis (DTA) curve indicates the exothermic and endothermic reaction of OSP coating itself.