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Understanding Mold Compound Behavior on Flip Chip QFN Packages
Keywords: Flip Chip, Mold voids, Mold
Overmolded Flip Chip QFN is called so because it is primarily being protected by the mold compound. It is a low cost solution to flip chip process, where there is no need for underfill, May it be Pb free or high Pb, a robust leadframe design, quality solder joint formation and an excellent molding process are the 3 factors needed to assemble a high performance flip chip QFN, which has the best of both wirebonded QFN and wafer chip scale devices. Wafer Chip Scale having low resistance but with bad thermals while Wirebonded QFN having good thermals but with high resistance, flip chip QFN both have the positive: low resistance and good thermals. Delamination and voids are the kryptonite of the high performance devices. Delamination can lead to solder cracking which will cause catastrophic failures on the package; on the other hand voids in between bump can lead to solder extrusion which will cause shorting or RDson shift. Voids on a flip chip QFN is hard to detect, it is not easily seen through x-ray or scanning acoustic microscope. It is not a time zero defect that can easily be screened through electrical test. Defining a void free molding process is a tedious activity; a lot of factors need to be considered. Checking for mold voids require destructive failure analysis technique. A proper understanding of a mold compound behavior is needed to fully characterize mold voids and find a solution to eliminate it. This paper will discuss in detail the mold compound behavior on Flip Chip QFN packages having different designs and attributes. Taking for example the turbulent mold flow observed on non-premold LF causing mold voids while smooth and planar flow is being observed on pre-molded LF giving more margins in terms of mold voids. The Cu post height which has direct impact on voids as well as the bump pitch. Unit orientation on leadframe is also considered in the design, whether the buses or bumps should be in parallel with the mold flow to ensure voids will not be encountered. Short shot comparison of wide leadframe having 1 map block versus the standard leadframe size with 4 map blocks to assess the mold flow, will it result to back flow causing voids, or will it push continuously to the vent side causing bleed out? Mold compound material itself has a great contribution to mold voids, having the correct attributes of the mold compound is critical. The mold filler size, which actually decreases the spiral flow value of the mold compound as it decreases its size. The balance and chemistry of having a smaller filler size that easily goes into finer bump pitch, as well as the spiral flow of the compound that has to reach the end of the leadframe before the gelation time have to be properly computed through a series of design of experiments design in full factorial matrix. Mold machine parameters were also tweaked to check if it can resolve mold voids. In theory, lower temperature molding should increase gelation time, which should impact mold voids. Higher transfer pressure should push the mold compound faster to fill in the leadframe strip area fast. Mold vacuum is also needed to help pull in mold fillers and resins in tighter bump spacing, though longer vacuum on time and higher transfer pressure can lead to mold bleed out, which can then be compensated by adjusting the clamping pressure. All these evaluations plus the novel approach of mold voids check thru laser decapsulation and leadframe peel off process to ensure Quality and Robust Process of Flip Chip QFN will be presented.
Ruby Ann M. Camenforte, Packaging Enginner
Texas Instruments
Angeles City , Pampanga
Philippines


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