Solder Post Attachment of Ceramic Chip Carriers to Ceramic Film Integrated Circuits

Several advantages are obtained when the standoff height of soldered leadless ceramic chip carriers is as large as 0.010-0.020 In, rather than 0.001 or 0.002 in. These advantages include easier cleaning and encapsulation under the carrier, more compliant joint configurations, better accommodation of nonplanarity, and relief from problems associated with differential thermal expansion. The larger solder volume also improves the self-alignment feature and decreases the gold embrittlement problem. This standoff height can be obtained by solder "bumping" the chip carriers prior to their attachment to film integrated circuits. A controlled solder volume is applied to each pad by using spherical preforms, which are solid phase bonded to the chip carrier pads and subsequently reflowed to produce the bumps. For maximum height, a sphere with diameter equal to 3/4 the center-to-center spacing is used, and the final standoff height is about half the sphere diameter. For smaller spheres the final height decreases by about 0.001 in for each 0.001 in of reduction in sphere diameter. This is consistent with an analytical model which calculates the height as a function of solder volume and carrier weight. If the carrier has castellations, then the solder has an easy "escape route," and there is a critical weight per pad which can cause the solder to squirt out and the carrier to collapse onto the substrate. The occurrence of this effect can be minimized by controlling soldering parameters and can be eliminated by using pads that do not extend to the edge of the chip carrier. The integrity of these joints is judged by electrical resistance and the torque required to twist a carrier off. The torque amounts to 20 inch Lb for a 24-pad carrier, corresponding to 4000 Lbf/in
²
of shear stress. Temperature cycling between -40 and + 130°C shows no degradation after 1000 cycles. Long-term aging at 150°C shows a 40 percent loss of strength after 3000 h, but no measurable resistance change. Arrhenius extrapolations indicate adequate strength and negligible failure rates after 50 years at 80°C.