Printed circuit boards (PCBs) form the backbone of electronics. They provide the mechanical structure to mount and interconnect electronic components using conductive copper traces etched from copper sheets laminated onto insulating substrates. PCBs come in a variety of shapes and sizes depending on the application. Some PCBs have edges that are plated with metals like gold or tin to serve critical functions. This article explores what edge plating is, why it is needed for certain PCBs, and the edge plating process.

What is Edge Plating?

Edge plating refers to the application of metal plating along the edges of a PCB. It involves depositing a metal coating along the edges of the PCB to cover the exposed copper traces. Common metals used for edge plating include:

• Gold – Provides corrosion resistance, durability, and conductivity. Most common for high-frequency applications.
• Tin – Offers solderability and corrosion resistance. Most affordable option.
• Nickel – Harder than copper. Provides wear resistance.
• Silver – Highly conductive. Provides corrosion protection.
• Palladium – Resists corrosion and tarnishing. Improves solderability.

Edge plating is applied to protect exposed copper traces along the edges of the PCB from oxidation and solder leaching effects. It also prevents shorts between closely spaced traces and improves circuit reliability. The plating extends slightly inward from the edges to coat the edges of the exposed copper traces.

Why is Edge Plating Needed?

There are several reasons why edge plating is required for certain PCBs:

• Prevent Oxidation – Copper oxidizes readily when exposed to air. This can increase contact resistance in connectors and fingers. Edge plating prevents oxidation.
• Improve Solderability – Bare copper does not solder well. Edge plating with solderable metals like tin improves solder joint reliability.
• Eliminate Copper Dissolution – Bare copper dissolves into molten solder when the PCB is reflow soldered. Edge plating prevents this solder leaching effect.
• Protect Signal Integrity – Bare copper traces can short due to solder bridging or corrosion. This affects high-speed signal traces. Edge plating eliminates this issue.
• Improve Aesthetic Appeal – Edge plating improves the look and feel of the PCB edges compared to exposed copper.

When is Edge Plating Necessary?

Edge plating is necessary for PCBs with the following characteristics:

• High density, fine pitch traces along the edges – Prone to bridging and corrosion
• Exposed copper interfaces – Connectors, fingers, testpoints etc.
• PCBs exposed to frequent soldering – Prevent leaching and oxidation
• High speed digital and RF circuits – Protect signal integrity
• High vibration or abrasion environments – Plating resists wear and tear
• Low clearance between exposed traces – Prevent shorts and leakage

Edge Plating Process and Specifications

Edge plating involves depositing metal plating layers selectively along the edges of the PCBs. Here are the key plating process steps:

1. Pattern PCB Edges

The first step is to fabricate the PCB with the required edge clearances and trace spacing. A minimum of 0.5mm clearance from edges is recommended for plating. Traces should have at least 0.2mm spacing at edges. Stress relief shapes may be etched to improve plating adhesion.

2. Mask PCB Surface

The PCB surface is masked using tape, resist or waxes to prevent plating on surface traces. The mask has to be resistant to the plating chemistry and easily removable after plating without damaging the PCB. Liquid photoimageable or dry film masks are commonly used.

3. Clean and Activate PCB Edges

The exposed edges are thoroughly cleaned to remove oils and oxides. A chemical or plasma etch treatment is used to micro-roughen the copper edges to improve adhesion. An activator promotes plating initiation sites on the copper traces.

4. Plate Edges

The PCBs are immersed in plating solution tanks with appropriate anodes to deposit the required plating uniformly onto the exposed edges. Different metal plating layers can be stacked as required. Common plating thicknesses range from 2 to 10 microns.

5. Remove Plating Resist

After plating, the resist mask on the PCB surface is removed thoroughly without damaging the plated edges. A chromic acid treatment is typically used to clean residues.

6. Test Plating Quality

The plated edges are visually inspected under microscope for coverage and thickness uniformity. Adhesion tests may be conducted using tape pull or bend tests. Resistance across plated fingers is measured to verify integrity.

Typical Edge Plating Specifications

• Plating Metals – Gold, Tin, Silver, Nickel, Palladium
• Plating Thickness – 2 to 10 microns
• Edge Clearance – 0.5mm minimum
• Trace Spacing – 0.2mm minimum
• Trace Plating – 2mm minimum inward from edge
• Adhesion – 2-4 lbs/mm minimum pull strength
• Solderability – 90% coverage minimum

Applications and Benefits of Edge Plating

Edge plating provides specialized functionality and benefits for the following PCB applications:

1. High Density Interconnect (HDI) PCBs

HDI PCBs have trace spacing and widths under 100 microns. Bare copper traces are prone to bridging, corrosion and leakage. Edge plating is essential to protect tightly spaced high speed traces.

2. RF/Microwave PCBs

Edge plating eliminates copper oxidation and maintains stable RF trace impedances. It protects signal integrity in high frequency PCBs.

3. Backplanes and Midplanes

In multi-PCB enclosures, edge fingers are used for interconnects. Gold plated edges provide reliable, low resistance connections and prevent corrosion.

4. IC Test Sockets

Test sockets have edge fingers that mate with IC pins. Hard gold or palladium edge plating withstands wear from repeated insertions.

5. Avionics and Military PCBs

Vibration and extreme temperatures accelerate trace corrosion. Edge plating is used to enhance reliability and durability.

6. Automotive PCBs

Frequent thermal cycling and soldering requires edge plating to prevent solder leaching and maintain reliability.

Conclusion

Edge plating is a critical PCB fabrication process for certain applications. It prevents copper oxidation, improves solderability, maintains signal integrity and enhances overall reliability. Common edge plating metals include gold, tin, nickel, silver and palladium. HDI, RF, avionics and automotive PCBs often require edge plating due to environmental exposures or high-density layouts. With the proper plating design rules and process controls, edge plating allows PCBs to survive extreme conditions and last the product lifetime.

Frequently Asked Questions

Q1. Why is gold used for edge plating in most RF PCBs?

Gold is commonly used for edge plating in RF PCBs due to its excellent conductivity, corrosion resistance and low contact resistance. Gold maintains the impedances of high-speed traces and ensures stable RF performance. It does not oxidize or corrode even in harsh environments. The low contact resistance of gold minimizes signal losses.

Q2. What PCB substrate materials are compatible with edge plating?

Most rigid PCB substrates like FR-4, polyimide, Rogers, ceramics etc are compatible with edge plating. The substrates must withstand the plating chemistry and allow suitable edge masking materials to be used. Flexible substrates are more challenging to edge plate due to the softness and porous nature.

Q3. Does edge plating increase the cost of PCB fabrication significantly?

Yes, edge plating increases the fabrication cost due to additional process steps of masking, surface preparation, plating and masking removal. Also, some edge plating metals like gold are inherently expensive. However, the benefits outweigh the costs for applications requiring high reliability or signal integrity.

Q4. Can multiple metal layers be edge plated – e.g. nickel followed by gold?

Yes, it is possible to deposit multiple metal plating layers by selecting compatible plating chemistries and processes. A typical stack is nickel or palladium as a base plating followed by gold as the outer plating. The nickel provides harder wear resistance while gold protects against corrosion.

Q5. How are partially plated edges specified in PCB design?

Partially plated edge requirements are defined on fabrication drawings using plating limit lines along the edges. This specifies the portions of the edges that need to be plated versus areas that are kept unplated. Careful design is needed to ensure plating uniformity.