Finite element analysis of ultra thin BGA package: first and second level reliability

In the case of UBGA packages (ultra thin ball grid array), the first level interconnection makes use of mechanical contact force between the pads on the chip and the substrate; there is no metallurgical connection between the two. An adhesive (conductive or non conductive) is used to join the chip to the substrate thereby ensuring a good contact. In order to explore the limits of this technology, flip chip attach at wire bond pitches was attempted. For pad pitches as small as 40 μm, this task becomes challenging, especially when using contact force as a means of ensuring first level connectivity. The FEA results point to the critical parameters that affect the connection performance at such low pitches. The effect of properties of the different materials used in construction of the package has been studied.. Several parameter studies have been performed to evaluate the effect on both first and second level interconnections. In general, the second level reliability of the devices is expected to be very high since a very low amount of inelastic strain is accumulated in the solder joints during thermal cycling. There is a good correlation between the observed and predicted locations of the failure region for the solder joints.