A cost effective PoP structure with high I/O density
Authors: Wang Hongjie , Huang Weidong , Geng Fei , Lu Yuan , Zhang Bo , Yin Wen , Wang Haidong 
Published: October 2015

Package-on-package (PoP) structure is widely used in smart phones and tablets in which memory package is directly attached to the top of the application processor. As the market demands more speed and bandwidth, memory devices need more than 1000 I/Os to support future requirements. However← since the package size also becomes smaller and smaller, finer I/O pitch is absolutely required. Although using some new technology can achieve finer I/O pitch, it increases the manufacturing cost. Using traditional mature technology can reduce manufacturing cost, but has limitation in finer I/O pitch. So, it demands a reasonable balance between design, process and cost to develop an applicable PoP structure.
In this paper we proposed a novel and cost effective PoP interconnection structure and a multi-layer PoP model. The PoP interconnection was formed by the solder ball on the top package connected to the solder bumps on the bottom package. The solder bump was made of a smaller solder ball attached on a Cu stud bump on the top of bottom substrate. The Cu stud bump was made through wire bonding machines and was coined so that the small solder ball can be attached to it. Using film assist molding technology, a half of the solder ball is exposed outside of molding compound, which can be connected with the solder ball of the top package through reflow process. This PoP interconnection structure was named solder bump through molding (BTM).
A three layer PoP vehicle package was designed in our experiments. The top package was a wire bonding BGA, the middle and bottom packages were both flip chip BGA with BTM interconnection structure. The package size of these three packages was 10×10mm2 and ball pitch was 0.4mm. The assembly process of top package was as normal as other wire bonding BGA. The assembly processes of middle and bottom packages were as follows: The Cu stud bumps were first bonded to the top surface of the substrate using wire bonding machines. Small solder balls were attached to the top of Cu stud bumps using stencil tool and then reflowed. After solder bumps were made, all chips were flip bonded to the substrates. Then, using film assist molding and MUF technology, the chips were encapsulated and Cu stud bumps were half exposed. After all the packages were ready, the package stacking and reflow was performed one by one from top to the bottom and the overall three layer PoP was formed. C-scan test and cross section analysis showed that the encapsulation had no voids in most samples. Electrical test results showed the interconnection was good. Reliability study will be also discussed in this paper, which is still in research now.
In BTM structure, both Cu stud and solder ball attach can be easily realized. The ball pitch can be 0.4mm or smaller and the process is also applicable for more layer PoP. Thus, BTM PoP structure provides a good solution considering the balance among cost, performance and manufacturing for 3D package.
Acknowledgments
The authors acknowledge the support of National Science and Technology Major Project (Project numberï¼š2013ZX02501003).