Solder Joint Behavior in HCC/PWB Interconnections

The majority of effort to improve the reliability of surface mounted hermetic chip carrier to printed wiring board (HCC/PWB) interconneetions has been directed toward developing a PWB with a coefficient of thermal expansion (CTE) matched to the ceramic chip carrier. This approach has been demonstrated as an effective means of reducing stress in the solder joints that occurs during thermal cycling and, therefore, has significantly improved the reliability of these systems. Honeywell's program in this area compared the reliability of chip carriers surface mounted to a standard FR-4 epoxy/glass PWB to those mounted on a FR-4 epoxy/glass PWB with a copper-clad lnvar core. Our results concur with those of others; that is, the use of a CTE matched core significantly improves the resistance of the solder joints to thermal fatigue. Close metallurgical examination, however, reveals that although the solder joints associated with the Cu-clad Invar core PWB do not fail by the mechanism typical for a thermal expansion mismatch (i.e., cracks originating in the high stress region of the solder), they do exhibit intergranular cracking and void formation within the solder structure. These defects appear to be caused at least in part by the thermal degradation of the solder. This indicates that solder joint composition and structure are also important to the reliability of HCC/PWB interconnects. The differences in the solder joint behavior in the thermally matched and mismatched systems are discussed.