Via-in-Pad in PCB: Advantages and Challenges

In printed circuit board (PCB) design, via-in-pad has become a widely adopted technique, particularly in high-density interconnect (HDI) designs. It is particularly designed for BGAs with a package pitch of 0.8 mm or smaller. It involves placing a via directly within the surface mount pad of a component, offering several design and performance benefits. For example – placement of decoupling capacitors closer to IC pins, improving bypassing and noise performance. Proper via-in-pad implementation ensures thermal, RF shielding, and power application compatibility. However, via-in-pad introduces unique challenges in manufacturing, requiring careful consideration during design and production. This article explores the advantages, challenges, and practical considerations of using via-in-pad in PCB design.

Vias are used to transfer electrical signals between different layers of multi-layer PCBs. Instead of routing a signal away from a pin and placing a traditional via, vias can be made directly on pads. Making vias in pads saves space, simplifies routing, improves thermal management, and also reduces parasitic inductance. Vias in pads are typically more expensive than traditional vias. Designers must verify the design capabilities of their PCB manufacturer and ensure all required files are extracted from the PCB design tool to ensure their boards are manufactured without any hassle.

What is a Pad and Why Use Via-in-Pad?

A pad is a conductive area on the PCB where components are soldered. Pads are critical for ensuring reliable electrical and mechanical connections between components and the board. In traditional designs, vias are placed outside pads to connect different PCB layers. However, in HDI designs with limited space, placing vias within pads referred to as via-in-pad optimizes space utilization and enhances electrical performance.

Via-in-pad is particularly advantageous for:

• High-density designs
• BGA packages
• High-speed designs
• Thermal management

There are many other advantages, which are improvement on signal integrity, we will discuss them below in a separate section.

Design Considerations for Via-in-Pad

Implementing via-in-pad requires adhering to specific design guidelines to ensure manufacturability and reliability:

1. Via Plugging and Capping: Open vias can cause solder to wick through the hole, leading to insufficient solder on the pad. To prevent this, vias must be plugged with epoxy and capped with copper.
2. Pad Definition: Metal-defined pads should fully encapsulate the via to avoid solder spreading, while solder mask-defined pads allow limited via placement within the pad.
3. Thermal and RF Considerations: For power applications, thermal vias within pads enhance heat dissipation. For RF designs, via-in-pad minimizes impedance discontinuities.
4. Copper Thickness: Ensure adequate copper thickness for via plating to withstand mechanical and thermal stress.
5. Solder Mask: Proper solder mask coverage prevents solder bridging and ensures robust solder joints.

Via-in-Pad Increases Component Densities:

The main reason to use via-in-pad is to increase component density in a PCB layout. This comes at the expense of higher cost. Appropriate use of via-in-pad requires plugging the vias and plating over to provide a solderable surface for SMD components. As long as this practice is enforced, via-in-pad can be used without assembly problems. But now you can try, JLCPCB's Free Via-in-Pad Process Boosts PCB Design Efficiency by 200%.

Because of the usage of via-in-pad for high-density PCB layouts, it’s possible that via-in-pad can provide much higher component placement densities. If you do plan to use via-in-pad, the vias must be plugged to prevent solder from wicking to the opposite side of the board and creating assembly defects. As long as this primary guideline is followed, via-in-pad can be used successfully to increase component density in a PCB layout.

Advantages of Via-in-Pad:

As discussed in the previous section, one of the primary reasons for using vias in pads is to save space. Letting designers place vias right under the pad not only frees up space but also makes routing simpler (especially the BGA package). To significantly improve heat dissipation for the heat sources mounted on the PCB. Vias in pads also allow designers to place bypass capacitors close to their components which minimizes inductance. For high-speed designs and interfaces, this results in an improvement in signal integrity. In a more exact way here are the key advantages of via in pad technology:

• Space Optimization: Enables compact layouts, particularly for BGAs and small components.
• Improved Signal Integrity: Reduces trace lengths, minimizing signal loss and interference.
• Enhanced Thermal Performance: Vias in pads act as thermal conduits, dissipating heat efficiently.
• Simplified Routing: Facilitates routing in dense designs by reducing the need for additional traces.
• Proximity of Decoupling Capacitors: Allows capacitors to be placed closer to component pins, improving power delivery.

Challenges and Downsides of Via-in-Pad:

There are a few downsides of using vias in pads which need to be known beforehand to make an informed design choice. It’s always a good idea for PCB designers to review these points before opting for via in pad design. Surface bumps arise when the vias are not placed and filled precisely. These irregularities on the surfaces can complicate the process of soldering components (especially the smaller ones with a fine contact). Thus, when creating vias in pads, manufacturers must ensure all surface bumps are eliminated. And without proper fab techniques, vias in pad are difficult to design and implement.

1. Manufacturing Complexity: Requires advanced processes such as via plugging and planarization, increasing fabrication costs.
2. Soldering Issues: Open vias can cause solder wicking, leading to weak joints.
3. Design Constraints: Improper via placement can result in tombstoning or solder paste spreading.
4. Thermal Expansion: Mismatched thermal expansion between materials can lead to mechanical stress and failure.

Practical Applications:

They are often used with components in a BGA package as there isn’t much room for other vias in there. The size of solder balls and the distance between nearby pins in BGAs can be as small as just a fraction of a millimeter which makes surface routing extremely complicated. Tracks with a very small trace width can be drawn out of each pin to a suitable place for making a via.

But this isn’t the best approach. Pins of components in BGA packaging are systematically arranged as a matrix with each pin having small solder balls. Compared to other commonly used packages such as dual-in-line (DIL) packages, BGAs offer higher packing efficiency and ruggedness. As an alternative, instead of trying to route all pins away and making a via far from the component, vias can be made right below BGA pads to take the signal carried by the component’s pins to the required PCB layers. Some devices which implement this type of design include:

• Smartphones and wearables, where compact designs are critical.
• High-frequency RF circuits, requiring minimal signal degradation.
• Power electronics, for enhanced thermal dissipation.
• Medical devices, where reliability and space efficiency are paramount.

Via in PAD v/s Traditional Vias:

For many who use printed circuit boards in the everyday applications of their industries, the decision between traditional vias and a VIP setup still seems to be shrouded in mystery. Some may question whether it is important to make the right choice, whether it is worth the cost to try VIP, and whether VIP is the future standard that all printed circuit boards will be judged by.

If you’re still not sure whether to go with printed circuit boards with traditional vias or vias in-pad, Millennium Circuits Limited can help. We are the worldwide experts when it comes to supplying printed circuit boards and we know more about printed circuit board design than almost anyone. We have access to talented designers, manufacturers and engineers who know just about all there is to know about every facet of PCB design, including via placement.

JLCPCBs Via-in-Pad Services:

JLCPCB, a leading PCB manufacturer, provides advanced via-in-pad services tailored for modern design requirements. Their services include via plugging, capping, and plating, ensuring minimal solder wicking and reliable connections. With state-of-the-art fabrication processes, JLCPCB offers cost-effective solutions for HDI designs, allowing designers to implement via-in-pad with high precision and efficiency. Checkout our full article on Via in pad design for 6-2- layers.

Conclusion:

Via-in-pad is a powerful technique for modern PCB designs, offering space efficiency, improved electrical performance, and enhanced thermal management. However, it also presents challenges in manufacturing and design, requiring careful planning and collaboration with a trusted PCB manufacturer like JLCPCB. By understanding its advantages and limitations, designers can leverage via-in-pad to achieve high-performance and reliable PCB designs. With proper design practices and advanced manufacturing support, via-in-pad continues to play a pivotal role in advancing PCB technology.