Development of a new Package-on-Package (PoP) structure for next-generation portable electronics

Package-on-Package (PoP) is one of the major 3D packaging approaches. It vertically combines discrete memory and logic Ball Grid Array (BGA) packages, where one package rests on the top of the other. Recently, PoP technologies have attracted more interests, especially for portable electronics related products and applications. ASTRI has developed a new PoP structure, which employs a new bottom package which is over molded Fine-pitch Ball Gird Array (FBGA) format with mechanically balanced package structure. For the top package, it is a commercial FBGA format with two die stacked inside. Since the new bottom package structure is based on an existing FBGA format, the mold chase is independent of the mold cap and size of the bottom PoP package. The final mold layer for bottom package has a thickness of 0.50 mm. The top interface of the bottom PoP package has 136 pads with 0.65 mm pitch and a two-row peripheral format, while the bottom interface has 272 pads with 0.65 mm pitch and a four-row peripheral format. The size of both top and bottom packages is 14 × 14mm
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. The 3D finite element (FE) simulator Ansoft HFSS was used to evaluate the high-frequency performance of the new PoP structure. The low-loss and low cross-coupling results indicate that the above PoP offers a promising solution for communication-related products. Physical configuration of the package was evaluated by cross section & scanning electron microscopy (SEM) examination, which showed a favorable interconnection structure. The warpage of the bottom PoP package after assembly process was characterized by a shadow moiré system. The warpage was well controlled by adopting a fully molded structure. The average package warpage value was around 30 μm that was well below the warpage target of 80 μm widely used in the industry. In this work, the moisture sensitivity level-3 (MSL-3) test was applied to evaluate the reliability of the above package. Board-level reliability including temperature cycling (TC) & drop tests were also evaluated. The results of the reliability tests showed good performance and demonstrated that the above PoP is a viable design. In summary, the above PoP design exhibited promising RF performance, excellent warpage performance, qualified package-level and board-level reliability. It offers an effective solution for high-density packages used in the portable consumer electronics.