SMT PoP Process Explained for Your PCB Assembly

The SMT PoP Process plays a vital role as electronic devices continue to shrink in size and increase in complexity. This method involves stacking multiple integrated circuits (ICs) to optimize space without compromising performance. By understanding PoP assembly, manufacturers can improve the reliability of printed circuit boards, leading to efficient designs that align with the requirements.

PoP (Part-on-Part) Assembly

PoP (Part-on-Part) Assembly allows you to stack components on a PCB. For instance, a processor can have memory positioned directly on top. This approach is beneficial for conserving space in high-density designs. However, it requires additional steps during the assembly process. You might need more placement stages and reflow cycles, which can raise costs and extend lead times.

What is PoP in SMT?

Package on Package (PoP) technology combines logic and memory chips to save space and enhance signal speed. It’s commonly found in devices like smartphones and cameras. PoP utilizes the Surface Mount Technology (SMT) process, which allows chips to be mounted directly onto the circuit board. Additional chips can then be stacked on top, minimizing the size of the PCB while maintaining performance. One of the advantages of PoP is that it enables separate testing and replacement of chips. If one chip fails, you can replace just that one instead of the entire stack, making SMT assembly more adaptable.

Types of PoP Structures

Solder Ball Connected PoP

This type of PoP utilizes small solder balls to connect stacked chips. These balls form strong electrical connections between the layers. It improves signal flow and overall device performance. This also conserves space on the PCB for compact and high-density circuit designs. Advanced electronic devices like smartphones use this PoP.

MLP Connected PoP

MLP (Molding Laser Package) PoP features a copper lead frame, ensuring a robust and reliable electrical connection. The process begins with sealing the bottom chip in plastic, followed by a laser that removes material to reveal the solder. This step simplifies the connection of upper components and enables the creation of thinner, more efficient PoP structures. You’ll see this technology in devices that require smaller, faster chips.

Folded and BVA Connected PoP

This type of PoP utilizes flexible circuits that can fold to fit into compact spaces. It allows for the connection of multiple package layers without requiring additional room. The BVA (Bond Via Array) technology enhances connections and minimizes pin spacing. This design is instrumental in producing thinner, high-performance devices. It is particularly effective in electronics that require numerous I/O connections.

The SMT POP Manufacturing Process

Non-PoP Component Assembly

Before stacking PoP components, the PCB goes through SMT assembly. This ensures a solid base for PoP placement.

Step 1: Stencil Printing

Solder paste is applied to PCB pads using a stencil. This ensures the right amount of paste goes where needed. You need proper application is key for strong connections.

Step 2: Component Placement

Machines place surface-mount components onto the soldered pads. Precision is crucial to ensure proper alignment. Misplaced parts can cause electrical failures. Automated pick-and-place systems help keep everything accurate.

Step 3: Reflow Soldering

The PCB moves through a high-temperature reflow oven. Heat melts the solder paste and bonds the components. This process secures everything firmly to the board. Controlled heating and cooling prevent weak or cracked joints.

Step 4: Inspection

Every board goes through Automated Optical Inspection (AOI). This checks for bad solder joints and misaligned parts. Even tiny defects can affect performance.

PoP Component Assembly

After ensuring that you assembly the non-PoP component correctly, you’ll now proceed to assemble the PoP part of the electronics.

Step 1: Screenprinting

In this step, solder paste is applied to the PCB pads for PoP. This paste helps create strong electrical connections. A stencil ensures the right amount of paste is used. Proper printing prevents weak or missing joints.

Step 2: Pick-up Bottom PoP

A machine picks up the bottom PoP component carefully. It uses vacuum suction for a secure grip. The component is positioned for precise placement.

Step 3: Placement Bottom PoP

The bottom PoP is placed onto the soldered pads. This must be done with high precision. Misalignment can cause electrical failure. A pick-and-place machine ensures accurate positioning.

Step 4: Fluxing Top PoP

The top PoP is dipped in flux before placement. Flux removes oxidation and improves solder flow. It also helps create strong and stable joints.

Step 5: Placement Top PoP

The top PoP is carefully placed on the bottom PoP. Proper alignment is crucial for good solder joints. Any misalignment can cause circuit defects. Automated machines handle this step with high accuracy.

Step 6: Reflow

The entire PCB is heated in a reflow oven. This melts the solder, securing all components. The heat profile must be carefully controlled. Overheating can damage delicate PoP structures.

Step 7: Underfill PoP

Underfill material is applied around the PoP components. It strengthens the bond between stacked parts. This prevents stress from weakening the connections. Underfill also protects against vibrations and shocks.

Step 8: Underfill Cure PoP

The underfill is cured to harden and stabilize it. This step ensures long-term reliability. It prevents cracks and weak joints from forming. Proper curing enhances the overall durability of the PoP.

Step 9: Optical and X-ray Inspection of PoP

Automated inspection checks for soldering defects. X-ray imaging reveals hidden joint issues. This checks if there is no missing connections or misalignments.

Step 10: Packaging and Shipping

The final PoP assemblies are cleaned and tested. They are then securely packed to prevent damage. Proper handling ensures safe delivery to customers. It guarantees you receive a high-quality product.

Reflow Soldering Procedure for PoP Assembly

There are two techniques in soldering the PoP for SMT assembly. These are the following:

Single-Pass Reflow

Reflow soldering of PoP requires heating the components to melt the solder. In a single-pass process, both the top and bottom PoP layers are soldered simultaneously. This approach is time-efficient but demands careful temperature management. Excessive heat can lead to warping, so it’s crucial to get the settings just right.

Two-Step Reflow

Two-step reflow begins with soldering the bottom PoP first, followed by the top. This approach helps avoid overheating the sensitive components on top. The bottom receives full heat, which guarantees a solid bond. After that, the top is soldered at a lower temperature. This technique minimizes warping and enhances the quality of the solder.

Choose Between Flux and Paste for PoP Assembly

Solder paste is the best option. Type 5 or finer is ideal, although Type 4 can still be acceptable. The paste gives a solid connection between the bump and pad. It minimizes the risk of open circuits. Finer powder pastes makes a uniform paste, though it has a slower melting rate. Even though it is slow, it helps to equalize the temperature across both PoP layers. This prevents solder from wicking away. However, using finer powder may slightly raise the risk of solder balling.

Advantages of PoP SMT Process

You can combine different storage and logic devices, allowing you to test or replace them individually, which is beneficial for quality control. By stacking PoP vertically, you save space on the board and enhance packaging density. This method also boosts data transfer speeds by connecting devices directly, in contrast to traditional 2D packaging.

Common PoP Electronic Applications

PoP technology is prevalent in today’s devices. For instance, in smartphones such as the Meizu 16S, it links processors and memory. It also drives next-generation wearables, enhancing their size and speed. Electronic watches utilize it to improve performance while maintaining affordability. Additionally, earphones benefit from PoP for increased efficiency. Health watches, like the Amazfit, leverage this technology for advanced functionalities such as blood oxygen monitoring.

FAQs About SMT POP Process

What is the Difference of PoP Process and PiP Process?

PoP involves stacking one package directly on top of another, whereas PiP involves placing one package inside another. PoP is particularly effective for conserving space in devices such as smartphones, as it connects stacked chips using solder balls. In contrast, PiP completely encloses one package within another, making it ideal for deeper integration in compact systems. A typical example of PoP is the stacking of DRAM on an application processor, while PiP resembles the embedding of a sensor chip within a larger processing unit.

Interconnect Materials You Can Use for PoP Assembly

Solder balls are the most popular option for PoP connections. But, you can also use conductive adhesives such as anisotropic conductive films (ACF) and anisotropic conductive pastes. ACF is a unique adhesive that attaches components to circuit boards. It has tiny conductive particles in an insulating resin. If you apply heat and pressure, these particles compress to form a conductive path. The uncompressed particles maintain insulation in other directions.

SMT POP Process on PCBMay

At PCBMay, we provide reliable PoP assembly services for electronics companies. Our advanced SMT technology only uses precise stacking of components. This leads to stable and strong performance. If you’re manufacturing smartphones, wearables, or other small devices, we treat each project with professional engineering and strict quality control. Our experience in surface mount technology (SMT) allows us to create secure interconnections.

Conclusion

SMT PoP assembly has played a vital role in modern PCB production, providing advantages such as space-saving and improved performance. Through proper focus on accurate soldering, meticulous material selection, and thorough inspection, producers attained trustworthy PoP assemblies. For professional-level PCB manufacturing, reach out to PCBMay for quality SMT PoP services tailored to your needs.