Flip chips solder joining

In soldering and when an ICA is used, the electrical connection is first established by the solder or the conductive adhesive before the bond is mechanically stabilized by application of an underfill. Using ACA or NCA makes it possible to combine electrical connection and mechanical stabilization in one process step. Electrically conductive connections made with nonconductive adhesives are based on mutual contact between the joining partners. A constant contacting force therefore has to be applied until the adhesive has cured. Contacting flip chips with adhesive... 

Nysaether, J.B. Lai, Z. Liu, J. Isotropically conductive adhesives and solder bumps for flip chip on board circuits—a comparison of lifetime under thermal cychng. Proceedings of the 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Binghamton, NY, September 1998 125-131. 

One of the simplest and most useful tests performed on reflow-attached flip chips to determine the adequacy of the solder joints (i.e., C4) is a tensile pull test (Fig. 31). This is done by adhesively attaching a metal stud to the back of a joined chip and pulling the joints in tension at a slow strain rate (approximately 1.0x10 sec ). The pull force is measured during the test using an appropriate load cell. The pull strength is a useful parameter, but the failure mode is a very important indicator of joint quahty. Planar failure at the solder joint interfaces is indicative of a weak and unacceptable interface condition. 

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