Critical Cleaning Requirements for Flip Chip Packages

In traditional electronic packages the die and the substrate are interconnected with fine wire. Wire bonding technology is limited to bond pads around the peripheral of the die. As the demand for I/O increases, there will be limitations with wire bonding technology. Flip chip technology eliminates the need for wire bonding by redistributing the bond pads over the entire surface of the die. Instead of wires, the die is attached to the substrate utilizing a direct solder connection. Although several steps and processes are eliminated when utilizing flip chip technology, there are several new problems that must be overcome. The main issue is the mismatch in the coefficient of thermal expansion (CTE) of the silicon die and the substrate. This mismatch will cause premature solder Joint failure. This issue can be compensated for by the use of an underfill material between the die and the substrate. Underfill helps to extend the working life of the device by providing environmental protection and structural integrity. Flux residues may interfere with the flow of underfill encapsulants causing gross solder voids and premature failure of the solder connection. Furthermore, flux residues may chemically react with the underfill polymer causing a change in its mechanical and thermal properties. As flip chip packages decrease in size, cleaning becomes more challenging. While package size continues to decrease, the total number of 1/0 continue to increase. As the I/O increases, the array density of the package increases and as the array density increases, the pitch decreases. If the pitch is decreasing, the standoff is also decreasing. This paper will present the keys to successful flip chip cleaning processes. Process parameters such as time, temperature, solvency, and impingement energy required for successful cleaning will be addressed. Flip chip packages will be cleaned and subjected to JEDEC level 3 testing, followed by accelerated stress testing. The devices will then be analyzed using acoustic microscopy and the results and conclusions reported.