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Figure 5: Embedded Passives inside PCB3.
Package-on-Package
The Package-on-Package (PoP) assembly method allows stacking of two components, one on top of another. BGAs are stacked together to provide a shorter signal path between two packages of BGA. This is commonly used to put a BGA containing the memory module, on top of another BGA which contains the microprocessor. The space where one BGA package occupies on the PCBA now contains two components, saving the space for one component.
The initial PoP assembly starts with a 0.65mm pitch BGA, placed on top of a 0.5mm pitch BGA. From 0.5/0.65mm, it evolves into 0.4/0.5mm (0.4mm bottom BGA / 0.5mm top BGA), then to 0.4/0.4mm (both top and bottom BGA at 0.4mm pitch). Regular PoP have a solder ball height spacing between the top and bottom BGA, as the top BGA is soldered onto the pads on top of the bottom BGA. Thru-molded via (TMV) PoP allows the top BGA to solder to vias that replace the pad on top of the bottom BGA. The overall thickness of the PoP after assembly is reduced.
PoP assembly can be performed with an in-line process flow, where the bottom BGA is placed onto the solder paste pads on the PCBA during SMT processing, and the top BGA is either solder or flux dipped then placed onto the bottom BGA. The solder joint between the top and bottom BGA on the PoP is formed at the same time while the PCBA is in the reflow oven. This utilizes existing the SMT machine with the addition of a solder or flux dip module. Nitrogen for reflow is generally required for 0.4/0.5mm pitch PoP. The success of the PoP is more subject to coplanarity of the top and bottom BGA, where at least 80um coplanarity is required. In-line process flow have been successfully used in very high volumes in manufacturing.
Figure 6: PoP inline process flow.
The other PoP assembly option is to do pre-stack. As the name implies, pre-stack will assemble the top and bottom BGA as a component off-line first, then the PoP is installed onto the PCBA as a component. This method is less subject to the coplanarity issue between top and bottom BGA. A customized metal tray is needed to hold the bottom BGA through the SMT machine during pre-stack. This requires manual handling for transferring the bottom BGA from its original tray package onto the customized metal tray. Nitrogen requirement for reflow is the same as the in-line process. Testing of the PoP component after pre-stack, before assembly to the PCBA also poses a challenge.
Figure 7: PoP pre-stack general process flow.
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