Thermally enhanced PBGA: Effect of heat spreader design on plasma cleaning efficiency and wire sweep

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In a thermally enhanced PBGA (TePBGA), heat spreader is commonly used to fulfill the heat dissipation requirement of the package. The presence of heat spreader reduces plasma cleaning efficiency and increases wire sweep due to narrower gap for plasma gas penetration and mold flow. The impact of insufficient plasma cleaning was found to be greater with certain die surface passivation which could even lead to die top delamination after reliability stressing. As for wire sweep, the issue could lead to high yield loss due to wire shorting especially for ultra fine pitch devices with wire diameter below 25um and multi-tier looping. These challenges can be overcome if the heat spreader design is carefully done and well characterized. A study was conducted on TePBGA with 4 new heat spreader designs to compare with a conventional design as control. In this study, the main response was plasma cleaning efficiency which was measured through contact angle. The 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> key response was wire sweep which was measured using x-ray machine. The 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> response was mold flow balance which was studied through molding short shots. The benchmark or target responses were those of a PBGA without heat spreader. Since the key requirement of a heat spreader is the area on top of the die surface which was needed to provide the intended heat dissipation capability during field application, hence it was not advisable to make any design modification on the heat spreader top surface. Therefore in all the 4 new heat spreader designs, the strategy was to reduce heat spreader peripheral metal volume to allow bigger gap for plasma gas penetration and a more balance mold flow. Each design varies in terms of lid foot design, with the purpose to prevent lid foot delamination, to allow sufficient support to the heat spreader to withstand the compression force during molding, and to provide the least restriction to mold flow. The study started with New Design A with 4×8mm lid foot which showed 0.43% reduction in wire sweep and 18 degree lower in contact angle which were great improvements. However, the design resulted in lid foot delamination due to big lid foot area that caused air trapped under the lid foot. This had lead to New Design B with 4×2mm lid foot which showed greater improvement, with 1.3% reduction in wire sweep and 26 degree lower in contact angle. However, the design suffered mold compound bleeding on top of heat spreader surface due to insufficient lid foot support to withstand the compression force during molding. Next, New Design C was created with 6×1mm lid foot with even bigger improvement, which showed 1.9% reduction in wire sweep and similarly 25 degree lower in contact angle. This design eliminated lid foot delamination and provided sufficient support to withstand mold compression. However, discussion with heat spreader fabricator found that such design may pose mass production issue due to heat spreader stacking/sticking issue. Finally, New Design D was created with 4×8mm lid foot with 1 dimple which showed 1.71% reduction in wire sweep and 26 degree lower in contact angle. Such results were almost comparable to the target responses of a PBGA without heat spreader. Also, the final design was found to overcome heat spreader stacking/sticking issue, while passing MSL3/260C without delamination and mold bleeding issue, hence was recommended as a friendly design for mass production. Molding short shots further confirmed the new designs had more balance mold flow than the conventional design. In conclusion, heat spreader design played a vital role in improving plasma cleaning effectiveness and wire sweep, as well as improving mold flow balance. With a well optimized design, all key responses can be made comparable to those of a PBGA without heat spreader.