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Fick's law of diffusion
In soldering with S-Pb solder on a copper plate, the intermetallic compound δ (Cu4 Sn) is formed between copper and tin through diffusion. Fick's law of diffusion provides means for quantitative analysis of this diffusion. The following applies to the J quantity of the dissolved substance (Cu), which diffuses in the melt (solder) through a surface section situated perpendicular to the direction of diffusion: J =-D gradient C, w here gradient indicates the decrease in concentration (C) at that point and D is a constant of proportionality (diffusion coefficient).