PCB Side Plating

Table of Contents

Introduction to PCB Side Plating

Printed circuit boards (PCBs) form the backbone of most electronic devices and products we use today. A PCB provides the mechanical structure to mount and interconnect electronic components using copper traces etched from copper sheets laminated onto a non-conductive substrate.

While traces on the top and bottom of a PCB facilitate component connections, the sides of a PCB were historically left unplated. However, side plating has become an increasingly common finishing technique to extend the conductive copper from the PCB layers to the edges and sides.

Side plating provides several benefits:

Improved electromagnetic interference (EMI) shielding
Lower impedance reference planes
Easier routing of high-speed signals between layers
Heat dissipation through side walls
Improved aesthetic appearance

This article will provide a comprehensive overview of PCB side plating, including:

Types of Side Plating

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There are two main types of side plating used in PCB manufacturing:

Partial Side Plating

Partial side plating involves plating only a portion of the PCB side walls. This focused plating can be used for:

Shielding of specific components or circuits
Heat dissipation in certain areas
Improving routing between layers in a particular location

Partial plating reduces cost compared to full side plating when only selective areas require side metalization. However, it can introduce challenges for assembly if components span plated and non-plated areas.

Full Side Plating

Full side plating, as the name suggests, involves plating the entire perimeter of the PCB. The conductive plating wraps completely around the sides from top to bottom layers.

Full side plating provides maximum EMI protection and lower impedance across the whole board. It also allows components to be mounted across side walls while maintaining grounding.

The tradeoff is increased cost compared to partial plating. Full side plating may also not be necessary in simple, low-speed PCB designs.

Benefits of Side Plating

Side plating a PCB provides several advantages:

EMI Shielding

EMI shielding is one of the primary reasons to use side plating. Input/output signals and high-speed traces can generate significant electromagnetic noise. This can couple to other circuits and traces causing interference.

Side plating creates a continuous conductive shield around circuits encapsulated within a PCB. Plating the side walls provides a direct conduction path to ground for EMI noise.

Full side plating is most effective at blocking EMI. However, strategic partial plating can also be used to isolate critical components.

Lower Impedance

Side plating improves impedance control compared to relying solely on ground planes within a PCB stackup.

Adding a continuous ground reference plane along the side walls reduces loop inductance. This allows tighter control of characteristic impedance for traces routed on inner layers.

Stable, low impedance is critical for high-speed digital and RF circuits to maintain signal integrity. Side plating provides a robust ground return path.

Layer Transitions

Routing traces between layers can be challenging without drilling plated through holes (PTHs). This takes up space and interrupts ground planes.

Side plating enables easier vertical routing between layers. Traces can transition layers by routing out the side wall rather than needing to change layers with a PTH.

This facilitates complex multilayer designs with traces spreading across many layers. Side plating eliminates drill congestion that otherwise results.

Heat Dissipation

Many high power circuits require heatsinking to dissipate heat. Side plating improves thermal performance by turning the entire perimeter into a heatsink.

Plated side walls conduct heat more efficiently out to the board edges. This allows passive cooling using convection and airflow rather than just relying on cooling from the top and bottom surfaces.

Strategic partial plating can also be used to direct heat away from sensitive components. The plated areas will act as thermal conduits.

Aesthetics

A full side plated PCB has a bold, high-tech aesthetic appearance. The shiny metal plating looks impressive and hints at complex internal technology.

While not always functionally necessary, full side plating can provide visual appeal for products where the PCB is visible. Partial plating can also produce unique stylistic results.

Challenges with Side Plating

While offering significant benefits, side plating also introduces some manufacturing challenges:

Increased Cost

The side plating process adds additional steps and material costs during PCB fabrication. This results in notably higher prices compared to conventional unplated boards.

Full side plating in particular can double or triple fabrication costs. Partial plating provides a cost-effective alternative when only select shielded areas are needed.

As side plating becomes more common, economies of scale may help reduce costs in the future. But for now it remains a premium process.

Plating Adhesion

It can be difficult getting plating to adhere evenly along the sides of some PCB materials. This is especially true for flexible circuits and boards with high aspect ratios.

Poor adhesion can lead to flaking, cracking, or peeling during assembly or testing. It requires careful surface preparation and plating chemistry.

Process Complexity

Side plating involves extra steps and handling during fabrication. Masking and etching the side walls for selective plating increases complexity further.

It is essential that side plating integrates cleanly within the overall manufacturing process flow. Test and inspection processes must also confirm plating quality.

Via Crackling

The additional plating process can induce stresses internally within the PCB laminate. This can lead to fracturing around drilled vias, especially those near the board edges.

Cracked vias can create reliability risks and electrical shorts. The risk must be minimized through design strategies and stress relief during fabrication.

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Side Plating Process

Adding side plating to a PCB requires careful integration within the fabrication process. Here is an overview of a typical side plating production flow:

1. Drilling

The process begins like any standard PCB with drilling tooling holes and through holes as needed in the internal layers.

Drilling normally occurs before outer layer imaging so side wall vias can be cleanly formed. Burrs are also removed from via barrels.

2. Layer Imaging

The internal layers are sequentially imaged and etched to form traces similar to a conventional multilayer board fabrication process.

3. Layer Lamination

The cores are then laminated together under heat and pressure. This bonds the layers into a solid board with copper traces embedded internally.

Pre-preg or other bonding materials fill gaps between cores. The lamination must evenly distribute pressure to avoid any internal stresses.

4. Outer Imaging

The top and bottom outer copper layers are coated with photoresist and then imaged with the desired conductor pattern.

Any traces that need to wrap from the top to bottom are imaged onto the side walls. This allows a continuous conductive path.

5. Developing

The board is developed chemically which removes the unexposed photoresist. This leaves behind the desired copper trace pattern on the outer layers and side walls.

6. Panel Plating

The entire panel is submerged into an electroless plating bath. This deposits a thin layer of copper over all exposed surfaces.

For partial side plating, the plating resist masks areas that should remain non-conductive. The boards may be selectively plated in multiple baths.

7. Stripping Resist

Next, the remaining photoresist is stripped away which exposes the underlying copper. This leaves the final plated conductor pattern.

8. Etching

Any unwanted copper without plating is etched away using chemicals. This electrically isolates traces and forms pads for component connections.

9. Test and Inspection

The boards are thoroughly inspected for any plating defects or other anomalies before moving to final finishing. This can involve visual, electrical, and x-ray inspection.

10. Solder Mask and Silkscreen

Solder masking and legend printing steps complete the PCB as required. Special handling may be required for applying solder mask along side walls.

11. Routing

Finally, the individual boards are routed from the larger fabrication panel. Special care must be taken not to damage any side wall plating during routing.

Testing and Inspecting Side Plated PCBs

Verifying side plating quality requires careful testing and inspection:

Visual inspection looks for plating coverage, adhesion issues, cracks, or damage. A microscope checks plating fused into side wall vias.
Cross-sectioning selects sample boards to physically cut and inspect plating thickness and internal quality.
X-Ray imaging reveals hidden plating defects and cracks that may not be visible externally.
Continuity testing uses a multimeter or ohmmeter to electrically verify isolated circuits and that plated areas show good conductivity.
Impedance testing measures impedance between plated side walls and internal ground planes. Impedance should match design targets.
EMI testing verifies side plating effectiveness for shielding electromagnetic interference. Aantom the entire perimeter for full shielding.

Thorough inspection and testing ensures the side plating meets all mechanical, electrical, and shielding requirements before boards ship to customers.

Design Considerations for Side Plating

Maximizing benefits from side plating while avoiding issues requires attention during design:

Use a board cross-section with thick, continuous reference planes. This avoids breaks in the side plated shield.
Space components slightly inward from board edges. This prevents stress cracking during fabrication.
Minimize routing vias near side edges. Use staggered vias if necessary.
Include plated slots, pads, or thermal reliefs along walls for stress relief.
Carefully model side platings effects on impedance for high-speed traces.
Use thicker 2oz+ outer copper if side plating is primarily for shielding versus layer transitions.
Specify solder mask pull back from side walls for easier soldering if components bridge sides.
Utilize side walls to route critical signals on inner layers for short and direct paths.
Design partial plating masks to prevent shadowing and get uniform side coverage.

Proper design is key to avoiding pitfalls and gaining maximum benefit from side plating technology.

Conclusion

PCB side plating provides an array of benefits from EMI shielding to heat dissipation to easier routing between layers. Partial plating delivers these advantages at targeted areas while full side plating maximizes performance.

However, side plating increases fabrication complexity and cost. It also introduces potential risks such as plating adhesion issues or via cracking that must be controlled through design and process optimization.

When utilized appropriately, side plated PCBs enable higher density layouts, accommodate high-speed signaling, and support advanced configurations like rigid-flex boards.

Continuing development of side plating processes and cost reduction will likely make adoption more widespread as PCBs evolve to meet demands for miniaturization, higher frequencies, and improved reliability in the newest electronics.

Frequently Asked Questions

What are the key benefits of side plated PCBs?

The main benefits of side plating are improved EMI shielding, lower impedance reference planes, easier routing between layers, heat dissipation through board edges, and an enhanced aesthetic appearance.

When should partial vs full side plating be used?

Partial side plating reduces costs when only selective shielding or layer transitions are needed. Full side plating maximizes EMI isolation and thermal dissipation across the entire board perimeter.

How does side plating affect PCB fabrication?

Side plating requires additional steps of coating, imaging, and plating the side walls. This increases complexity and cost while also introducing potential risks of plating adhesion issues or via cracking.

What special considerations are needed when designing a side plated board?

Use thick reference planes, allow edge spacing around components, minimize side wall vias, include stress relief features, model impedance carefully, and plan trace routing utilizing the side walls.

How is side plating quality tested?

Inspection techniques include visual checking, microscopic examination, cross-sectioning, x-ray imaging, continuity testing, impedance measurement, and EMI testing. This ensures plating integrity, uniformity, and performance.