Development of Automotive Body Solders

Lead (Pb) based solders had been used successfully as dent and seam fillers for automotive body panels and were commonly referred to as “body solders”. The usual compositions were 70wt% Pb - 30wt% Sn or 80wt% Pb - 20wt% Sn. But due to the hazards associated with Pb dust and particles from the grinding and sanding, new lead-free body solders were developed in the early 80's. An 82wt%Sn - 15wt%Cu - 3wt%Zn composition was developed to mimic the pasty characteristics and processability of the lead-containing solders. This body solder has the advantages of corrosion resistance, compatibility with e-coat, and a lower processing temperature and lower material cost than an alternative candidate approach that uses a silicon bronze material. The ideal temperature range for applying this solder is between 280 to 350°C, which provides a workable, viscous material. This temperature range is much lower than the MIG welding temperature of the silicon bronze material and results in much less heat distortion of the steel panels. The microstructure is a dendritic structure with a Cu₆Sn₅ intermetallic compound in a Sn-rich matrix. The spacing of the dendrites can be reduced with a small addition of Fe. A zinc chloride (ZnCl₂) and ammonium chloride (NH₄Cl) based flux or tinning paste is required to clean the oxide surface. Any residual flux residue was found to be fully compatible with the phosphate process in a paint compatibility study. There appears to be great potential if such an automated process is implemented.