Printed circuit boards (PCB) need protective coatings on the copper traces to prevent oxidation and improve solderability. Gold is commonly used as PCB surface finish due to its high conductivity, corrosion resistance and ability to form reliable solder joints. The two main gold finishing processes used are immersion gold and electroplated gold.
This article explains immersion gold and gold plating for PCBs, compares their properties, process differences, reliability and costs to help understand their pros and cons. It also provides guidance on selecting the right gold finish for a PCB application based on considerations like cost, durability, complexity etc.
Introduction to PCB Gold Finishing
PCBs have exposed copper pads and traces that are prone to oxidation and corrosion when left unprotected. This negatively affects soldering performance. Gold finishing deposits a thin layer of gold on the copper conductors which provides excellent protection.
Gold is used due to its unique properties:
- Highly conductive
- Does not oxidize or corrode
- Bio-compatible and lead-free
- Good solderability and wire bondability
- Surface hardness
Gold finishing is typically applied over nickel underplating on PCBs. The nickel layer protects copper from leaching into the solder during high temperature soldering processes.
What is PCB Immersion Gold?
Immersion gold or electroless gold involves depositing a layer of gold on the PCB conductors by immersing the boards in an aqueous gold solution containing ions of a reducing agent like sodium borohydride.
The electrons from the reducing agent provide energy to reduce gold ions in the solution to metallic gold which deposits on the PCB surface.
The thickness of the gold layer can be controlled by immersion time, solution parameters and temperature typically ranging from 3 to 8 micro-inches. The deposition occurs uniformly on the entire PCB surface.
What is Gold Plating on PCBs?
Gold plating refers to the electrolytic deposition of gold on PCBs using electrical current. The PCB acts as the cathode and gold ions in solution deposit on the PCB surface in the presence of an electrical potential.
Thickness can be precisely controlled ranging from 10 to 100 micro-inches. Only selected areas like fingers, pads, traces that are connected to the current get plated.
Comparing Immersion Gold vs Gold Plating
|Auto-catalytic chemical process. No external electrons supplied.
|Electrolytic process requires external DC current to drive deposition.
|Uniform deposition on all surfaces.
|Selective deposition only on surfaces connected to current.
|Lower process control and thickness uniformity.
|More process control and thickness uniformity.
|Suitable for simple board geometries.
|Suitable for complex board geometries.
|No power supplies or process control needed.
|Power supplies and process control instrumentation required.
|Lower (~100 Knoop)
|Higher (130-160 Knoop)
|Moderate (~1-2 years)
|Very high (5-10 years)
|Lower equipment costs.
|Higher equipment costs.
|Lower process control costs.
|Additional process control instrumentation needed.
|Higher solution costs due to lack of solution recovery.
|Lower solution costs due to >99% solution recovery.
|Overall lower cost process.
|Overall higher cost process.
PCB Applications of Immersion Gold vs Gold Plating
Immersion Gold is Typically Used for:
- Double sided and multilayer boards
- High density interconnect (HDI) boards
- For finer features below 100 microns
- For board features with spacing <150 microns
- Budget consumer electronics assemblies
- Shorter service life products
Gold Plating is Ideal for:
- High reliability electronics – aerospace, medical, automotive
- Mission critical electronics like servers
- High current power electronics
- Low voltage digital logic boards
- Complex and dense board geometries
- Long field life products
Pros and Cons of Immersion Gold vs Gold Plating
Advantages of Immersion Gold
- Simple process with no electrical instrumentation
- Lower equipment and processing costs
- Uniform coverage on all surfaces
- Faster processing time
- Suitable for fine features and PCBs with dense spacing
Disadvantages of Immersion Gold
- Lesser control on thickness uniformity and reproducibility
- No possibility of selective plating
- Lower purity, hardness and bondability
- Less wear resistance and durability
Advantages of Gold Plating
- Excellent thickness process control and uniformity
- High purity, bondability, hardness and wear resistance
- Selective plating ability
- Superior solderability and corrosion resistance
- High durability with thickness up to 100 micro-inches
Disadvantages of Gold Plating
- Requires current supply and monitoring instrumentation
- Slower than immersion with higher cost
- Complex for boards with dense, small features
- Higher initial capital investment
Choosing Between Immersion Gold vs Gold Plating
Factors Favoring Immersion Gold
- Cost sensitive products
- Consumer electronics with shorter life cycles
- Very fine feature PCBs below 100 microns
- Quick turnaround boards
- Simpler board geometries
Factors Favoring Gold Plating
- High reliability products like medical, defense, aerospace
- Mission critical enterprise server boards
- Complex, dense board geometries with various feature sizes
- Very flat gold finish needed for wire bonding
- Long field operational life expectancy
In summary, immersion gold provides a faster and cheaper process while electrolytic gold plating offers superior durability, solderability and selective plating capability. For low cost consumer PCBs with fine features, immersion gold usually suffices. But for complex, high performance boards with long service lives, gold plating may be the optimal choice.
The PCB application, cost targets, reliability needs and product life cycle should be evaluated to decide between immersion gold vs gold plating finish. With new technological advances, hybrid techniques can also potentially combine the advantages of both processes.
Q1. Is it possible to first use immersion gold and then gold plate?
Yes, it is possible to deposit initial immersion gold layer followed by gold plating to thicken critical areas needing more gold like contact fingers while minimizing costs.
Q2. Does solder mask affect choice of gold finishing process?
Yes, solder mask clearance constraints like 75-100 microns favor using immersion gold which can coat beneath tight spaces. Gold plating requires more spacing.
Q3. Which gold finishing is suitable for ENEPIG surface finish?
ENEPIG refers to electroless Ni, electroless Pd and immersion Au finish. It already uses immersion gold and provides excellent solderability. Gold plating over ENEPIG is generally not required.
Q4. Can immersion gold withstand multiple reflow cycles?
Immersion gold can withstand around 1-3 reflow cycles as compared to 4-6 for hard gold. Recommended thickness is 50-100 microinches for multiple reflow capability.
Q5. Does porosity of copper affect gold finishing?
Yes, highly porous copper can trap process chemicals causing issues. A nickel barrier underlayer helps mitigate porosity issues for both immersion gold and gold plating processes.
Gold is applied to the surface treatment of PCB board because gold has strong conductivity, good oxidation resistance and long life. Generally, it is applied to key board, gold finger PCB, etc. The most fundamental difference between ENIG and gold plated is that gold plating is hard Gold (abrasion resistant), immersion gold is soft gold (not wear resistant).The detail as follows:
1. The crystal structure formed by immersion gold and gold plating is different. The thickness of gold is much thicker than that of gold plating. The gold is golden yellow, which is more yellow than gold plating. This is the method of distinguishing gold plating and gold immersion, gold plating will be slightly white (nickel color).
2. The crystal structure formed by immersion gold and gold plating is different. The immersion gold is easier to weld than gold plating, and will not cause poor welding. The stress of the immersion gold plate is easier to control, and it is more conducive to the processing of the bond for the bonding products. At the same time, because immersion gold is softer than gold plating, the gold is not wearable (the disadvantage of immersion gold).
3. The immersion gold board only has nickel gold on the pads. The signal transmission in the conductor skin effect is not affected by the signal in the copper layer.
4. Compared with gold plating, immersion gold has a denser crystal structure and it is not easy to be oxidized.
5. With the higher precision requirement of circuit board, the line width and spacing have reached 0.1mm or less. Gold plating is prone to shorts in gold wire. The immersion gold has only nickel gold on the pad, so it is not easy to produce a gold wire short circuit.
6. The immersion gold board has only nickel gold on the pad, so the soldering on the line is more firmly combined with the copper layer. The project does not affect the spacing when making compensation.
7. For the board with higher requirements, the flatness requirement is better. Generally, the immersion gold is used. The immersion gold generally does not appear as a black mat after prototype pcb assembly. The flatness and service life of the immersion gold board is better than that of the gold plating PCB.