Gold plating provides a highly conductive, corrosion resistant surface finish for printed circuit boards. However, thick gold plating is expensive, so PCB designers must use it judiciously. This article examines how thick gold can be plated on a PCB based on process limitations, reliability factors, use cases and cost considerations. Recommended gold thickness guidelines are provided.
Purposes of Gold Plating on PCBs
The key benefits of gold plating include:
- Corrosion prevention for long-term reliability
- High conductivity with low contact resistance
- Solderability for component terminations
- Wire bonding adhesion for ICs
- Conductive die attach surface
- Enhanced wear and abrasion resistance
Gold is applied as a PCB surface finish or component lead, pad, and via plating to leverage these characteristics.
PCB Gold Plating processes
Gold is applied onto PCB surfaces using electrodeposition. The typical process steps are:
- Copper circuits are formed by etching away undesired copper
- Electroless nickel plating applied first as a diffusion barrier
- Electroless gold rapidly deposits initial gold layer
- Thicker gold is built up using electrolytic gold plating
- Plating thickness is controlled by current density and time
Plating equipment consists of solution tanks, anode baskets, cathode work holders, rectifiers, and precision process controls.
Maximum Gold Thickness Capability
In theory, gold can be plated to any thickness, but in practice, there are limitations:
- Process Capability – Thickness uniformity and repeatability degrades as maximum plating thicknesses are approached.
- Stress – Internal stresses can distort a PCB or cause bond lift if plated too thick. Annealing relieves stress.
- Hardness – The PCB can become very rigid and brittle with thick gold, risking cracks.
- Solderability – Excessive gold inhibits intermetallic bonding during soldering.
- ** peel strength** – Thick gold resists adhesion, potentially causing delamination or lifted traces.
- Creep – Gold slowly deforms under pressure which can open contacts.
- Machining – Thicker gold requires more machining allowance during fabrication for weight considerations.
- Density – The high density of gold adds substantial weight as thickness increases.
Most PCB manufacturers limit gold plating thickness to 25-50 microns maximum. Thicker gold up to 150 microns may be used for niche applications if designed properly.
Design Rules for Thick Gold Plating
For PCBs requiring thicker gold, designers should:
- Use larger trace/spacing/via sizes to allow for plating buildup
- Account for increased weight in vibration, shock and torque limits
- Specify annealed gold to prevent embrittlement issues
- Allow for thicker gold finish on pad/land patterns
- Derate adhesion peel strength in mechanical stress models
- Reduce risk of solder leaching with soldermask defined pads
- Expect to pay exponentially higher plating costs
- Qualify plating adhesion, solderability, and flexibility
- Allow very tight thickness tolerance to ensure uniformity
- Verify assembly process integrity for bonding, soldering, etc.
Thick gold plating on PCBs demands special considerations in design, fabrication, and assembly.
Typical Gold Thickness by Application
Here are some typical PCB gold thickness conventions:
Wire Bonding – 0.5 to 2 microns
Connector Contacts – 0.5 to 1.5 microns
General Solderability – 0.05 to 0.3 microns
Corrosion Resistance – 0.3 to 2 microns
Printed Gold – Around 5 microns
Slide/Wiping Contacts – Up to 25 microns
Die Attach – 150 microns (requires annealing)
Applications dictate necessary gold thickness. Use the minimum amount suitable for the function.
Factors Impacting Maximum Gold Thickness
Key considerations determining max gold thickness include:
PCB Thickness
Thinner boards flex more, limiting very thick gold before cracking issues arise. 1.6mm boards support more gold than 0.8mm boards.
PCB Technology
Standard FR-4 can hold less gold than high Tg materials before becoming too brittle. Metal-based boards allow greater thickness.
Trace Width
Narrow traces restrict gold thickness due to increased risk of undercutting during deposition.
Plating Adhesion
Maximum thickness is limited by the weakest adherence point like bare through-hole barrels or edges.
Soldermask Type
Photoimageable mask can withstand thicker gold than liquid types before delaminating.
Annealing
Heating finished boards relieves stresses allowing thicker gold than non-annealed PCBs.
Environmental Exposure
Boards in benign environments can utilize thicker gold than highly corrosive exposures.
Failsafe Requirement
Mission-critical boards necessitate thinner gold for redundancy versus consumer boards.
Accounting for these factors ensures reliable performance when plating thicker gold.
Cost Considerations for Thick Gold Plating
As gold thickness increases, cost rises exponentially due to:
- Much more gold material consumed
- Longer plating cycles
- Tighter process controls
- Lower plating yields
- Increased amounts of chemical solutions
- Greater power and facility overhead
- More frequent solution filtration/maintenance
- Extra waste treatment fees
- Declining line capacity throughput
- Higher risk of scrap and rework
Gold plating costs typically double with each additional micron. Thick gold adds substantial expense.
Risk Analysis for Maximum Gold Thickness
The maximum acceptable gold thickness is found by analyzing:
- Process capability charts identifying range limits
- Measured plating distribution uniformity
- Thermal stress simulations assessing deformation
- Thermal cycling reliability tests checking failures
- Solderability tests verifying joint formation
- Adhesion tests rating bonding peel strength
- Material analysis and microsections of grain structure
- Flexibility testing quantifying stiffness
- Solder leach resistance testing
- Fatigue predictions under environmental conditions
This comprehensive analysis defines the optimum gold thickness balancing performance, reliability, and cost.
Summary
- Gold offers excellent conductivity, corrosion resistance and solderability but is expensive.
- PCB gold plating is applied using carefully controlled electroplating processes.
- Excessive thickness causes issues with brittleness, weight, adherence, soldering and cost.
- Most PCB applications are served with less than 2 microns gold thickness.
- Thicker gold up to 25 microns may be utilized when designed properly.
- Multi-faceted testing and analysis determines maximum workable gold thickness.
Understanding gold plating thickness design rules and process limitations allows engineers to make informed tradeoffs for their application.
Frequently Asked Questions
What is the thickest possible gold plating available on a PCB?
The maximum gold thickness is limited by process capability, material performance factors, and reliability risks to around 25-50 microns typically. Some specialty applications may plate up to 150 microns gold with proper design and testing.
What gold thickness is recommended for connector contacts?
A gold thickness of 0.5 to 1.5 microns offers the ideal balance of conductivity, corrosion resistance, wear resistance and cost for connector contacts. Thinner gold risks corrosion while thicker gold is excessive.
How does thicker gold plating impact the flexibility and rigidity of PCBs?
Excessively thick gold plating can embrittle PCBs and components, making them crack prone during bending or vibration. Annealing helps relieve stresses but only allows some increase versus non-annealed boards.
Why is gold thickness uniformity important in PCB plating?
Non-uniform plated gold risks issues like weak adherence in thin areas or brittle gold in thick areas. Tight tolerances on thickness range ensures reliability. Measurement sampling must represent the whole board area.
How much does gold plating thickness affect weight and density of PCBs?
With gold’s high density, increasing thickness substantially increases board weight. For example, 1 micron of gold adds over 30 grams weight per square meter. This impacts vibration resistance and inertia.
