Electronics Failure Analysis

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Electronics Failure Analysis usually involves taking a close look at the solder joints. Most (around 70%) failures are due to failure of solder joints, due to either thermal cycling or vibration. Thermal cycling occurs each time power is applied, since electrical resistance causes heating in the circuit. Stresses build up due to thermal expansion mismatch. Two mechanisms are typically at work: Fatigue & Creep. The cycling of stress due to turning power on & off or vibratory stresses can lead to fatigue.

Creep is typically thought of as a phenomonon that occurs at high temperatue. However, high temperature is relative. Creep usually begins to occur when the absolute temperature reaches 50% of the absolute melting temperature. Since the most common solders typically melt in the 360-370F range, they can be in the creep range even at room temperature. As the solder heats up due to the electrical current, its temperature may reach 80% of the absolute melting temperature.
The photo on the left shows a resistor held in place by two solder joints. The arrows indicate the view of the photo on the right, which shows creep crack growth has occurred due to thermal cycling.

Electronics Failure Analysis
Resistor & Solder Joints
Electronics Failure Analysis
Cracking in Solder Joint

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Ed Pope
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