Mechanical Fatigue
Reliability failure modes involve the gradual cracking of solder joints or traces due to repeated mechanical flexing at a microscopic scale. Such micro-deflection stress degradation often stems from thermal cycling or board handling during assembly. This damage is often invisible to the naked eye but can lead to intermittent electrical failures.
Strain Accumulation
Even small amounts of bending cause strain in the copper and solder that makes up the circuit connections. Over hundreds of cycles, these stresses cause tiny cracks to form at the points of highest tension. These cracks expand until they completely sever the connection, causing a permanent open circuit.
Components with large footprints and many pins are the most susceptible because they provide a large area for the strain to accumulate. The stiffening effects of the solder can focus these forces on the thin copper pads of the board. Manufacturers use stiffeners and careful mounting strategies to limit the range of motion allowed for the substrate.
Reliability Testing
Engineers use strain gauges to measure the amount of deflection during assembly and operation. Thermal shock testing accelerates this degradation to ensure the product can survive its intended lifespan. Boards that exhibit excessive flexing are redesigned with thicker substrates or additional support points.
This analysis prevents field failures in products that experience vibration or extreme temperature changes.
