Gold plating is commonly applied to the edge connector fingers on printed circuit boards (PCBs) to provide a highly conductive and corrosion resistant contact surface. Determining the optimal gold thickness for an application requires balancing cost against performance needs like durability, contact resistance, and insertion lifetime. This guide examines typical gold finger plating specifications, cost tradeoffs, and key factors in selecting appropriate PCB edge connector plating.
What Are Gold Fingers on PCBs?
Gold fingers refer to the gold plated copper pads or traces along the edge of a PCB that make electrical contact when inserted into a connector. Also known as edge card connectors or finger connectors, these gold fingers allow boards to be easily installed and removed from electronic devices and backplanes.
Gold plated edge card connector fingers on a PCB
The gold plating serves several purposes:
- Provides excellent conductivity and low contact resistance
- Resists corrosion and oxidation over long-term use
- Allows many insertions and removals without degradation
- Provides lubricity for smooth insertion into mating connectors
- Protects against fretting wear during vibration or shock
- Ensures solderability for terminations and connections
Proper specification of the gold thickness is essential to maximize performance while controlling plating cost.
Typical Gold Finger Thickness Ranges
Common gold plating thicknesses used on PCB connector fingers range from around 0.05 μm to 5 μm (micrometers). Here are typical plating ranges for various applications:
| Gold Thickness | Applications |
|---|---|
| 0.05 – 0.15 μm | Consumer electronics |
| 0.15 – 0.3 μm | Industrial electronics |
| 0.3 – 1 μm | Military/aerospace electronics |
| 1 – 5 μm | High durability connectors |
Thicker gold plating is required when the application demands extended insertion lifetime over tens of thousands of cycles. Thinner gold, down to 0.1 μm, may be suitable for lower duty consumer products.
Many standards like MIL-G-45204 and IPC-4782 specify minimum gold thicknesses on PCB fingers for various classes of hardware. Typical minimum values are in the range of 0.127 μm to 0.762 μm (5 to 30 microinches).
Military and industrial standards for minimum gold plating thickness
Let’s look at the key considerations when selecting gold finger plating thickness for a particular application.
Durability and Insertion Lifetime
One of the main reasons for using thicker gold plating is to increase durability over repeated insertions into the mating connector. Thinner gold below around 0.15 μm will show visible wear and edge cuts after a few hundred insertions. At 0.05 μm almost all the gold can wear off after less than 100 cycles.
For consumer devices that only see occasional connections, this may be acceptable. But industrial controls, telecom hardware, and military systems often require thousands of insertions over decades of use.
Here is how plating thickness impacts insertion lifetime:
| Gold Thickness | Insertion Lifetime |
|---|---|
| 0.05 μm | < 100 cycles |
| 0.1 μm | < 500 cycles |
| 0.3 μm | 1,000 – 5,000 cycles |
| 1 μm | 10,000 – 50,000 cycles |
| 3 μm | > 100,000 cycles |
Thicker gold above 2-3 μm would be specified for extremely high duty connectors requiring hundreds of thousands of mating cycles without deterioration.
The substrate material also affects wear life. Harder nickel underplating prolongs gold pad lifetime compared to softer copper or brass bases.
Contact Resistance Stability
Thicker gold plating maintains lower and more stable contact resistance as the connector is repeatedly mated over its lifetime. This provides better electrical performance and reduces variation.
Contact resistance tends to rise as gold wears down, exposing the base metal which oxidizes. Excessively thin gold sees resistance spikes after fewer mating cycles.
Minimum gold thickness targets found in industrial and military specifications aim to ensure stable contact resistance over the required lifetime, beyond just providing environmental protection.
Solderability
Sufficient gold thickness must remain at the end of the product’s insertion life to allow soldering to PCB connector pads. This ensures damaged connectors can be repaired and individual fingers replaced if needed.
Areas with visible base metal exposure may be difficult to solder. A minimum of 0.1 μm gold remaining is recommended for consistent solder connections.
Lubricity
Thicker gold plating above 0.3 μm provides lubricity that allows smoother insertion and removal from mating connectors. Thinner gold has higher friction that can lead to sticking.
The lubricity effect reduces abrasive wear during mating cycles. But separate lubricants are often used on gold fingers to further improveInsertion and extraction forces must be considered to avoid damage, especially for high density connectors.
Other Plating Material Options
While gold is generally the standard for PCB finger connectors, other platings like silver, tin, and palladium are sometimes used:
- Silver – Lower cost but prone to tarnishing that increases contact resistance. Requires gold flash coating.
- Tin – Least expensive plating but wears quickly. Not suitable for high durability needs.
- Palladium – More affordable than gold but may not withstand harsh environments.
- Alloys – Combinations like tin-lead, tin-bismuth, or silver-nickel provide cost savings over pure gold.
Evaluate tradeoffs between cost and performance when assessing plating materials. Mixing gold with other metals can offer an optimal balance for some cost-sensitive applications.
Factors That Determine Gold Finger Plating Thickness
- Insertion cycles – Expected lifetime number of mate/demate cycles. More cycles need thicker gold.
- Contact resistance – Lower and more consistent resistance requires thicker gold.
- Contact density – Higher density connectors experience faster wear requiring thicker plating.
- Environment – Harsh conditions like humidity, temperature, and vibration demand thicker gold protection.
- Cost constraints – Minimal thickness based on performance needs to control plating cost.
- Reliability requirements – Mission critical systems justify thicker gold than consumer hardware.
- PCB material – Softer copper needs more gold than hard nickel underplating.
- Solderability – Sufficient gold must remain for repair soldering after usage.
Analyze all factors above to select the optimal gold finger plating thickness for each application’s needs. Over-spending on unneeded thickness wastes money. But inadequate thickness risks premature failure.
Gold Finger Plating Process
Gold is applied to PCB connector fingers through electroplating. The PCB panel goes through a series of plating baths to deposit various metal layers:
1. Copper PCB Traces – Base metal fingers are formed by copper etching
2. Electroless Nickel – Thin nickel layer enhances adhesion
3. Electrolytic Nickel – Thicker harder nickel to prolong wear life
4. Gold – Final gold plating applied to specified thickness
The gold bath contains gold potassium cyanide or gold sulfite dissolved in solution. Applying current to the PCB deposits gold ions onto the nickel underplating.
Typical PCB gold finger plating sequence
After plating, a glassivation step may apply solder mask to protect from solder bridging between fingers. A lubricant coating also improves insertion.
Cost Impact of Thicker Gold Plating
Adding plating thickness requires longer plating time and more gold material, increasing process costs. Here is an approximate cost comparison of gold thicknesses:
| Gold Thickness | Relative Plating Cost |
|---|---|
| 0.05 μm | 1X |
| 0.15 μm | 1.5X |
| 0.75 μm | 2X |
| 1.5 μm | 3X |
| 3 μm | 5X |
Doubling thickness adds around 50% to the base cost. Going from 0.05 μm to 1 μm increases cost by 2-3X. The gold material alone represents over 90% of this added cost.
For a moderately complex PCB, going from 0.15 μm to 0.75 μm gold might add $5 to $10 to the overall board fabrication cost. On boards with high edge connector density, the incremental cost for thick gold is higher.
Plating cost rises non-linearly with thickness due to gold material usage
Work with your PCB manufacturer to determine optimum gold finger thickness for your budget and connector requirements.
Design Rules for Gold Fingers
Follow these guidelines when laying out gold fingers on the PCB edge:
- Minimum gold finger width: 0.2mm
- Minimum space between fingers: 0.2mm
- Avoid acute angles on fingers
- Rounded corners ease insertion
- Extend gold 0.5mm from edge
- Align with via grids for stiffness
- Limit unsupported span length
- Solder mask web between fingers
Well-designed gold fingers withstand repeated insertions and minimize risk of pad lift, solder fractures, or PCB warping.
Gold Finger PCB Connectors
Many standard connector footprints are designed for gold fingers including:
- PCI/PCIe
- Memory – DIMM, SODIMM
- Board-to-board – DIN41612, Nano-D
- Backplanes – VME64x, CompactPCI
- Storage – SAS, SATA
Common PCB connectors using gold plated edge fingers
Most PCB connector series offer optional gold plating on housing and pins to match gold fingers on the mating board.
Alternative Finger Materials and Platings
While gold is the norm, other finger plating and base material options exist:
- BeCu copper alloy for increased hardness and wear resistance
- Hardened steel or beryllium copper pins
- Tin, tin-lead, and tin-bismuth platings for cost savings
- Nickel/palladium to replace gold in some harsh environments
- Selective gold plating only on contact wiping areas
- Dual plating with gold only on mating tip of fingers
- Gold dot flash coating over silver or palladium base
Evaluate overall cost savings from any gold reduction techniques against possible tradeoffs in reliability.
FQA about Gold Fingers on PCBs
Why is gold used for edge connector fingers?
Gold provides excellent conductivity, corrosion resistance, and lubricity. It allows many insertion cycles without degrading electrical or mechanical performance.
How thick should gold plating be for automotive electronics?
0.3 to 1 μm provides good durability for automotive environments seeing thousands of insertion cycles under vibration and temperature swings.
What is the cheapest plating that can be used on fingers?
Pure tin is the least expensive at less than 10% the cost of gold. But it oxidizes quickly and wears out after a few dozen insertions.
What is the hardest metal used for PCB edge fingers?
Beryllium copper alloy provides the greatest hardness and wear resistance. But beryllium is toxic requiring careful handling. Steel fingers are more common for high durability needs.
Is it possible to reduce gold thickness on non-contact areas of fingers?
Yes, selective gold plating only on the mating wiping surfaces can save cost with minimal impact on performance. The ends and sides of fingers can have reduced gold.
Conclusion
Specifying the optimum gold plating thickness for PCB connector fingers requires balancing cost versus required lifetime durability, contact resistance stability, solderability, and other factors. More insertion cycles, higher densities, and harsher environments demand thicker gold over 0.5 μm to prevent premature failure. But excessive margins waste cost. Work closely with your PCB partner to validate the best gold finger plating for your application’s functionality, reliability, and budget needs.
