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General Soldering Tips
Inter-metallic layer is a necessary evil. Without it, we don't even have a valid soldering joint. Once created, it grows at any temperature and accelerates exponentially as the temperature goes higher. This growth continues until the entire joint is occupied by the inter-metallic compounds, or either the base metal or the solder is exhausted. Thus the rules of soldering are:

  1. Solder as quickly as possible.
2. Use the lowest possible soldering temperature that yields acceptable joints.
3. Avoid repeated soldering just to make the joint look better. Added exposure to high temperatures only increases the inter-metallic layer. The joint may look pretty but is indeed weaker.
4. Avoid elevated operating temperatures for the end product. Keep it well below the soldering temperature as much as possible. Remember, the inter-metallic layer grows at any temperature, the higher the faster.
Rates of Dissolution
We know that we have to solder fast, but how fast? This question can not be answered without knowing the base metal and the speed it dissolves in the solder material. Of all solder materials, tin based solders, in particular, are especially aggressive dissolving metals. The figure below illustrates the approximate dissolution rates of a few typical base metals in tin.

From the above graph it is clear that Ni has one of the lowest dissolution rates. So it is not surprising that nickel is frequently used as a barrier for soldering.