Mechanism and growth rate of underfill delaminations~1)

摘要

Any well-informed prognosis of the expected service life of flip-chips must be preceded by and based on a deep understanding of the effects causing a delamination of the under-fills. In this article, we have set out to present a new approach for the analysis of such delaminations. For this purpose, a flip-chip is equipped with a Pyrex plate instead of the conventionally used Si-chip. This makes it possible to create a pictorial record of the interactions in the underfill-passivation-interface with the assistance of a digital camera and a microscope. This method permits a precise, cost-efficient and rapid analysis of underfill delaminations. Comparative studies may help to find reliable material combinations and geometrical guidelines for low-delamination flip-chips.On the basis of this method, we carried out a number of experiments presented in the second half of this article. Delaminations caused by thermal cycling tests in the course of our studies always started at the feet of the contacts and were distributed evenly across the chip surface. No cases for cracks spreading inward from the chip edge were recorded. In test environments of high humidity, a well-formed meniscus at the chip edge served to delay the onset of the delamination process.The study of geometrical parameters showed that circuit-boards of different thickness tended to favour different crack growth patterns and mechanisms. The shape of the crack growth curve was either linear or similar to that of a square root. The analysis of different chip sizes showed that large chips (100 mm~2) did not suffer from proportionally more delaminations than small chips (25 mm~2). If the manufacturing environment allows the production of larger chips, these can under certain circumstances be more reliable than small ones. There were no indications for low contact heights (50 #mu#m) being potentially more hazardous for the underfill-passivation interface than higher ones (100 #mu#m).In the conduct of thermal cycling tests, the transition time between the temperatures needs to be taken into account since it affects the delamination of the underfills from the passivation.The technique presented in this article makes it possible to examine there reliability of flip-chip-assemblies with regard to delaminations of the underfill from the passivation. In the context of comparative studies, suitable material combinations and geometrical guidelines may be established.