The sheets were soldered using silver solder or some other brazing alloy. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Layers of intermetallics can form between the solder and the soldered material. Another coating consideration is rapid diffusion of coating metal into tin solder. Brazing filler metal is stronger than silver solder, which is stronger than lead-based soft solder. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). A clean unoxidised tip is tinned by applying a little solder and flux. In a traditional die casting process these alloys would solder to the die. Concern arose that the milk contained unsafe levels of lead leached from this solder plug. This causes the solder between this chip and the motherboard to break. The mineral was also employed as a flux to solder precious metals together. Early methods of manufacture employed poisonous lead solder for sealing the cans. This causes the aperture to fill with solder paste. Removal of the oxides from the solder preform is also troublesome. Preheat is also an opportunity for volatile solvents in the solder paste to outgas.