Stress, strain and displacement

For a thin plate to bend, as in the image below, the top of the plate must stretch, while the bottom must be compressed. The stretching and compression of this plate depend on the displacement created by force P.

Strain is measured in millionths of a unit. For example, 500 microstrains represents a strain of:

Thus, 500 microstrains represent a strain of 0.0005" over a distance of one inch. The components installed on a PCB suffer the same strains. Typically, through-hole components are not very sensitive to strain because of their flexible lugs.  However, large surface mount components such as BGAs are very sensitive to strain.

Certain manufacturing processes such as Electroless Nickel/Immersion Gold Plating, widely used for installation of BGAs, render the solder joints very brittle under certain conditions. Since they have a rigid body and are installed directly on the PCB surface, these components withstand stretching to such an extent that they will suffer solder fracture failures. It has also been proved (1) that the increasingly widespread use of lead-free soldering renders the solder joint even more brittle. The Bansal, Yoon & Mahadev study (2) shows that the most exposed solder joints are located at the edges of the components. Large components soldered to a card, such as BGAs, will have the effect of reinforcing the PCB at this location. The stresses exerted on them will then be distributed to the weakest locations, the outer edges.