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Steps to consider when designing a trouble-free solder joint:

Operational temperature

Flux usage
Reflow method
Temperature limitations
Special conditions

Whether the market is communications, automotive, energy, military, medical or aerospace, choosing the correct solder is essential to making a quality product. A solder joint plays a very important role in an electrical or mechanical assembly. How this solder is selected for this joint is extremely important to the application, so following a guideline will insure that this is accomplished.

Before choosing a solder for an application, the operational or service temperature must be known; i.e. the minimum and maximum temperatures the assembly will see, during testing and in service. Continuous operational temperatures above 125°C will require the use of precious metal solders or hard solders such as Au/Sn, Au/Ge, or Au/Si.

Generally, it is best with soft solders to choose one that will melt at least 50°C higher than the expected service temperature of the assembly. (This includes temperature cycling during test and burn-in). If the difference in temperature (Delta T) between operational and the solidus temperature of the solder is too small, this could result in excessive scavenging, leaching and intermetallic growth. These unwanted conditions could lead to problems and eventual joint failure. 

After the correct temperature range for the solder is determined, consideration needs to be given to what metallizations in the joint will interact with the solder.

Metallizations will be addressed next in our series of steps to consider when designing a trouble-free solder joint.