Mechanical Constraint
The maximum allowable deformation of a circuit board under mechanical loading establishes the safety margin for fragile components. Establishing microstrain limits protects ceramic capacitors and solder joints from fracturing during board depaneling, handling, and assembly. These values are measured using strain gauges attached to the board surface near critical areas.
Defection Testing
During in-circuit testing or mechanical routing, the board is subjected to bending forces that stretch the outer fibers of the laminate. Strain gauges record the dynamic deformation in microstrain units, which represent parts-per-million changes in length. IPC standards suggest that maximum strain should not exceed five hundred to one thousand microstrain depending on the thickness of the board and the size of the components.
Exceeding these levels risks fracturing the solder ball connections beneath ball grid array components. This damage is often latent, resulting in intermittent connections that escape initial electrical test but fail in the field. Consequently, strain testing is mandatory when setting up routing and assembly fixtures.
Failure Prevention
By maintaining the deflection within the approved limits, the physical integrity of the circuit is preserved. Fixture pins can be repositioned or squeegee pressure adjusted to reduce board flexure. This mechanical monitoring provides an effective defense against solder joint cracking.
