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Reliability Impact of Partial Pad Craters
Reliability Impact of Partial Pad Craters
PCB pad craters are associated with single overstress events that cause immediate failure. This examines the effects of partial pad craters on reliability.
Analysis Lab

Authored By:
Brian Roggeman and David Rae
A.R.E.A. Consortium
Universal Instruments Corporation
Binghamton, NY, USA
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Transcript
PCB pad craters are generally associated with single overstress events that cause immediate failure.

However, significant reliability concerns are raised when partial non-catastrophic cracks are present under the pads of an assembly at the beginning of service life.

This investigation empirically examines the effects of partial pad craters on board reliability. Partial pad craters were created through bending over-stress events which simulate damage modes experienced during assembly, handling, test, and shipment.

Crack frequency and crack area distributions were measured for a variety of board flexure magnitudes. Final long-term reliability was then characterized for specific pre-damage levels in cyclic bending environments.

The results revealed a dramatic degradation in reliability which was then correlated back to the initial crack distribution from the pre-reliability characterization damage event.

Furthermore, the relative effect of pre-damage on reliability was markedly greater as the fatigue stress levels were reduced.
Summary
PCB pad craters are generally associated with single overstress events that cause immediate failure. However, significant reliability concerns are raised when partial non-catastrophic cracks are present under the pads of an assembly at the beginning of service life. This investigation empirically examines the effects of partial pad craters on board reliability. Partial pad craters were created through bending over-stress events which simulate damage modes experienced during assembly, handling, test, and shipment. Crack frequency and crack area distributions were measured for a variety of board flexure magnitudes. Final long-term reliability was then characterized for specific pre-damage levels in cyclic bending environments.

The results revealed a dramatic degradation in reliability which was then correlated back to the initial crack distribution from the pre-reliability characterization damage event. Furthermore, the relative effect of pre-damage on reliability was markedly greater as the fatigue stress levels were reduced.
Conclusions
Bending strain level during a single bend damage event was strongly correlated to observed partial pad craters in assemblies following dye-and-pry analysis. Even with all the pads exhibiting partial pad craters and some with 100% of the crack extending across the pad area, in this investigation, the electrical continuity of the circuit was not compromised. With increased bending strain, an increased proportion of pads exhibited cracks. The crack area distribution also exhibited a transition from no cracks at the lowest strain levels tested up through the majority of pads showing cracks extending over >80% of the pad area.

Single-sided cyclic bending fatigue testing revealed that the number of cycles to first electrical failure was well described by an exponential functional relationship to board strain magnitude for both the undamaged assemblies and damaged assemblies. The undamaged subset of assemblies was tested across a range of board strain levels (1000-3500) and the failure mode was largely attributed to trace cracking. However, at the lowest strain level (1000) a portion of the assemblies failed by solder fatigue as revealed by dye-and-pry destructive failure mode analysis.

Assemblies damaged by a single 3500 bend event revealed a marked reduction in cyclic bending lifetime. The relative impact of the damage event on reliability became greater at reduced cyclic bending strain magnitudes. This finding suggests that many 'accelerated' tests may not sufficiently uncover latent damage risks.
Initially Published in the SMTA Proceedings
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