Soldering Process

Soldering is a process by which two metals or alloys are joined together with a third metal or alloy. The third metal or alloy has a much lower melting point compared to the first two metals. Soldering is different from adhesive joining. Adhesives bond by mechanical attraction. In soldering, there is also a chemical reaction in addition to physical reaction.


Soldering is primarily used to provide a metallurgical joint with good electrical contact and/or seal against leakage. Solders typically do not provide high mechanical strength, given the soft nature of popular solder materials. Soldering is used extensively in the electronics industry and printed circuit boards. It is also used in joining metals in industries such as cutlery, tools, metal box making etc.


In order to have a good soldering joint, one must form inter-metallic layers between the solder material and the base metal. Solder that simply solidifies over the base metal without forming any bonds are very weak in mechanical strength and also poorly conductive.


Within each inter-metallic layer, there are actually a number of different compounds formed by the solder materials and the base metal. These compounds are typically quite brittle and will adversely affect the integrity of the solder joint. As the joint is subject to stress, thermal cycles, vibration, or shock, the inter-metallic layers are usually where it starts to fail. Since the inter-metallic layers are inevitable, it is best to keep it as thin as possible.


We have to solder fast. This requires knowing the properties of the base metal and the speed it dissolves in the solder material. 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.




Inter-metallic layer is a essential to create a strong 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.