What is Package-on-Package (POP) and Its Benefits

Package-on-Package (PoP) technology, also referred to as stacked packaging, represents a cutting-edge semiconductor packaging approach. It revolves around the vertical integration of discrete logic and memory BGA packages. This entails the stacking of two or more packaging components on one another, interconnected by standardized interfaces for seamless signal transmission. As a result, this technological innovation significantly enhances component density on PCBA while imparting enhanced flexibility to circuit design.

Configuration Modes of PoP Packaging

Currently, two primary PoP packaging configurations are in use:

Pure Memory Stacking: This method entails the stacking of two or more memory packages, typically memory chips, in a vertical arrangement.

Mixed Logic-Memory Stacking: In this particular configuration, logic components, such as the central processing unit (CPU), are housed in the lower package, while memory components, usually memory chips, are integrated into the upper package. To illustrate, the lower package might represent the System-on-Chip (SoC) within a mobile device, encompassing the core logic components. The decision to place the logic package at the bottom is based on its need for a more extensive number of BGA to establish connections with the motherboard.

Configuration Modes of PoP

During the PCB assembly process, the lower package within the PoP stack is directly positioned onto the PCB, with the remaining packages in the stack layered above. The connection and integration of these PoP stack packages occur during the reflow soldering, thus enabling the realization of functional stacking.

PoP Assembly on PCB

In some advanced Package-on-Package applications, memory modules are directly mounted on top of the main processor package. This technology offers significant size advantages for many HDI designs. However, special considerations are essential during the PCB assembly process, which may involve multiple placements and/or reflow soldering cycles. These additional considerations can potentially increase the cost and lead time of PCB assembly services.

Typically, there are two common techniques for PoP assembly: single pass and dual pass. However, FS Technology recommends the use of single-pass Package-on-Package due to its cost-efficiency and similar assembly yield. For most PCB assemblers, PoP devices with fine-pitch BGA packaging, typically 0.5 millimeters or smaller, still pose a relatively new challenge.

Package on Package Assembly

Single-Pass Assembly: In single-pass assembly, the application processor is first mounted on the main PCB, followed by the installation of memory onto the application processor. Finally, the finished PCB undergoes a single reflow soldering process, consolidating all these steps in one pass.

Dual-Pass Assembly: Dual-pass assembly involves an intermediate step where the relevant memory is first mounted onto the application processor. Subsequently, these two components are placed in a carrier, and the first reflow soldering is performed. Afterward, these connected devices are mounted on the main PCB, followed by a second reflow soldering process on the finished PCB.

Advantages of Package-on-Package

Traditionally, components are flatly arranged on a PCB, but Package-on-Package’s vertical stacking structure transforms this conventional format. It efficiently utilizes the PCB’s space, enabling designers to integrate more functions into a smaller PCB footprint without increasing its size, achieving higher-performance component combinations.

The connections between components rely on copper traces, whose length impacts the circuit’s resistance, leading to heat generation and signal delay. Short vertical interconnections between top and bottom packages optimize the distance between components, thereby enhancing the electrical performance of the circuit.

Since memory components reside in separate packages, they can be upgraded or replaced more easily without altering the entire microcontroller or application processor. This approach also permits individual testing of various sections during the production process.

Package-on-Package allows the use of different memory packages under specific requirements, as long as they are compatible with the CPU package. Designers can mix and match microcontrollers and memory packages, thereby increasing design flexibility.

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