Interfacial stress is the stress that is generated as a result of soldering two different metals together. Different metals have different CTEs (coefficient of thermal expansion) and thus expand and contract by different amounts as the temperature changes. When two different metals are soldered together, changes in temperature can cause one metal to expand more than the other. Stress caused due to this phenomenon is called Interfacial Stress. An example of this would be joining the leads of a component to a copper pad on a PCB board. The component leads can be a different material from the copper pads. The component leads are soldered onto the copper pads to form an electrical connection. An increase in temperature can cause both metals to expand by different amounts and thus interfacial stress can cause the solder joint to crack and result in a bad connection.

Another area where Interfacial Stress is found in PCBs is within the solder materials itself, due to Intermetallics. Intermetallics are compounds formed when impurities like nickel, gold, palladium and other metals get added into molten solder. These metals are usually present in the leads of surface mount components. In the soldering process, these metals mix with molten solder to form an intermetallic layer (IML). This intermetallic layer is not good because it has a different rate of thermal expansion compared to the layer over and under it. Which can cause interfacial stress failure.

The analysis of stress caused by joining two different metals together is called interfacial stress analysis.