Improvement of connectivity in Cu/OSP flip chip package using NCP

A recent trend toward thinner, lighter, smaller flip chip packages has been driving the gap and pitch narrower and smaller. For encapsulation of such packages, capillary underfill (CUF) is facing many challenges and no longer the mainstream underfill. Instead, non conductive paste (NCP), one of pre-applied underfill materials (PAM), is being used for it. Another trend toward a lower cost of the materials for flip chip packages is leading to a shift from Cu/Ni/Au electrodes to Cu/OSP (Organic Solderability Preservative). OSP is a treatment agent to inhibit the oxidation of Cu. Coating with OSP acts as an antioxidant. However, OSP on an electrode surface often causes interference with thermal compression bonding between solder and Cu. This paper discusses the solution for that OSP bonding issue by optimizing pre-treatments. OSP surfaces were observed by focused ion beam (FIB). Scanning electron microscope (SEM) was used to evaluate the bonding performance.