Lf. Miller, Controlled collapse reflow chip joining (Reprinted from IBM Journal of Research and Development, vol 13, 1969), IBM J RES, 44(1-2), 2000, pp. 93-104
Solder reflow connection of semiconductor devices to substrates has been sh
own to be a reliable, effective, and readily automated technique. Rigid cop
per spheres, which remain rigid during solder reflow, have been used succes
sfully for some time as a major element of the contact joint. However, to e
xpand the capability of such joints to larger devices such as multiple-tran
sistor chips in hybrid components, ductile metallic joining pads can be use
d on the devices instead of the copper spheres to reduce mechanical strains
and permit multi-pad devices to make proper contact to module lands during
reflow. This paper describes a technique that prevents these solder pads f
rom collapsing and permits large scale production. Termed "controlled colla
pse," the method is based on limiting the solderable area of the substrate
lands and chip contact terminals so that surface tension in the molten pad
and land solder supports the device until the joint solidifies. The result
is a sturdy, testable connection of high reliability (bond strength 30-50 g
m, pilot-production yields exceeding 97%, predicted failure rate-based on l
aboratory tests-considerably lower than that of copper ball contacts). The
process is economically adaptable to automation and offers considerable lat
itude in fabrication and control tolerances.