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- Intermetallics are frequently brittle, and differences in CTE (coefficient of thermal expansion) between intermetallic and solder can contribute to internal stress
- Depletion of one element of the surface may impair solderability
Figure 2: Cross-section showing the two distinct intermetallics between the copper and tin.
Nevertheless, without an intermetallic layer, there is no valid soldering joint, and once created, the layer grows at any temperature and accelerates exponentially as temperatures increase. This growth continues until the intermetallic compounds (the base metal) occupy the entire joint, and/or the solder is exhausted.
The rules for good solder joints are:
- Solder as quickly as possible
- Use the lowest possible soldering temperature that yields acceptable joints
- Avoid repeated soldering to improve the appearance of the joint since added exposure to high temperatures only increase the intermetallic layer. The joint may look pretty but is weaker or stressed
- Remember that the intermetallic layer grows at any temperature but accelerates exponentially at elevated temperature levels. Rates of dissolution of various metals also will rise with increases in temperature
Reliable Solder Technique
Having established what considerations are necessary to form a reliable solder joint, the next step is to understand how to achieve this and what processes should be followed. While much of the focus in soldering is placed on the tip idle temperature, solder joint temperature is more relevant than of the “no-load” tip idle temperature.
To read the full article, which appeared in the July 2019 issue of SMT007 Magazine, click here.
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