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package-on-package (PoP)

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Package-on-package (PoP) is a term that describes two or more complete IC packages that are mounted on each other and connected via wire bonds or through-silicon vias (TSVs). PoP has been used since early 2000s. It has been used in consumer devices, such as mobile phone, where it is used for stacking ASICs, baseband chips, and application processors.

PoP combines separate logic and memory packages, which are stacked on top of each other. Often the bottom package is logic and the top is memory. The maximum height of the PoP packages in 2012 was typically 1.4mm to 1.6mm, but the eventual goal is to drive the dimensions down to 1mm and below.

PoP uses flip-chip interconnects with ball grid array (BGA) balls on the bottom of the package. PoP is a 2D-like technology that stacks two or more separate packages on top of each other. In many cases, a memory package is on the top, while an application processor or a baseband die is on the bottom.

In PoP, the top and bottom chips could be individually packaged using wire bonders. In another configuration, the top package could be wire-bonded, while the bottom package could implement copper-pillar flip-chip. Using copper pillar for the bottom package, PoP may be more expensive, but it enables higher I/O densities and increased thermal conductivity.

Toshiba and Sony used some of the first commercial devices using PoPs. Starting in the 2010s, next-generation PoP candidates were developed, including bond via array, embedded PoP, fan-in, fan-out, flip-chip PoP, and even multi-chip modules (MCMs). Many flavors of PoP exist. For example, Amkor‘s Package Stackable Very Thin Fine Pitch BGA (PSvfBGA) launched in 2004. The PSvfBGA has a single and stacked die using wire bond or hybrid (flip chip plus wirebond) stacks. Amkor also has Through Mold Via Package-on-Package (TMV PoP) has with interconnect vias through the mold cap and Package Stackable Flip Chip CSP (PSfcCSP), which makes it possible to use an exposed die bottom package. According to Amkor, the PSfcCSP integrates the package stacking design features of PSvfBGA in an fcCSP (flip chip chip scale package) assembly flow. “PSfcCSP has a thin exposed flip chip die enabling fine pitch stacked interfaces at 0.5 mm pitch, which is a challenge in a center molded PSvfBGA structure,” says Amkor.

In 2020, Amkor developed an RDL-based interposer PoP. More info: RDL-First PoP Fan-Out Wafer-Level Package Process with Chip-to-Wafer Bonding Technology.

 

Fig.01:  Cross sections of Amkor's PSvfBGA PoPs.  Source: Amkor

Fig.01:  Cross sections of Amkor's TMV and PSfcCSP PoPs.  Source: Amkor

Fig.01:  Cross sections of Amkor’s PSvfBGA, TMV, and PSfcCSP PoPs.  Source: Amkor

Fig. 02: Illustration of Interposer PoP: a) Laminate-based; b) RDL-based. Source: Amkor

Fig. 02: Illustration of Interposer PoP: a) Laminate-based; b) RDL-based. Source: Amkor

Fig.01: A schematic of package on package ASIC plus memory PoP, circa 2008 — A typical logic plus memory PoP stack, common to mobile phone SoCs or baseband modems from 2005 onward. Source: Wikipedia/Moody751/CC BY 3.0

Fig.03:  A schematic of package on package ASIC plus memory PoP, circa 2008 — A typical logic plus memory PoP stack, common to mobile phone SoCs or baseband modems from 2005 onward. Source: Wikipedia/Moody751/CC BY 3.0


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