Tensile vs. Shear Load on Solder Joints 

Regular printed circuit board (PCB) expansion due to temperature changes creates a shear load on the solder joint. The mechanism is driven by the CTE mismatch between the PCB (13-17 ppm/°C) and the component. If the PCB and component remain relatively flat then the leads and solder joints need to absorb the shear motion. The metal leads are usually elastic and drive a constant shear force into the solder joint. This is a fully reversed load during temperature cycling.

If the board is loaded in bending then the solder joint is also loaded with a tensile force. Bending can occur due to direct loads or indirectly due to temperature cycling of an over-constrained board, potting, large rigidly attached components and several other causes. A tensile load is usually not reversed so it can experience a "strain ratcheting" mechanism. This mechanism can cause failure in as little as 500 cycles. 

Attaching components to locations that experience board bending is a bad idea.

Copyright Gil Sharon July 1, 2015. All rights reserved.