Introduction to PCB Copper Plating
PCB copper plating refers to the process of electroplating a thin layer of copper onto a printed circuit board (PCB) substrate. The copper layer serves as the conductive pathways or traces for the electrical current to flow.
Copper is the metal of choice for PCB fabrication for several reasons:
- Excellent electrical conductivity. Copper has a very low resistivity, allowing current to flow efficiently.
- Corrosion resistance. Copper forms a protective patina when exposed to air that prevents further corrosion.
- Solderability. Copper readily forms intermetallic compounds with solder, creating strong solder joints.
- Cost. Copper is affordable compared to other conductive metals like gold or silver.
The copper thickness on a PCB can vary depending on the current flow requirements. Still, typical copper weights (thickness) are 1 oz (35 μm) and 2 oz (70 μm) for outer layers and 0.5 oz (17 μm) for inner layers.
Plating refers to the electrochemical process of depositing a metal coating onto a conductive surface. To electroplate copper onto a PCB substrate, the board is immersed in a copper electrolyte solution and electrically charged to attract copper ions onto its surface.
So in summary, PCB copper plating is the process of electroplating a copper film onto a PCB substrate to create the conductive traces or circuitry. Proper plating thickness and quality are essential for a functioning high-reliability PCB.
Why Copper Plating is Used in PCBs?
There are several key reasons why copper is the preferred metal used for plating PCBs:
1. Excellent Electrical Conductivity
Copper has the highest electrical conductivity rating among commercial metals. With a conductivity of 5.96×107 S/m, copper allows electrical current to flow with little resistance. This enables PCBs to operate at higher frequencies and speeds.
2. Corrosion Resistance
Copper forms a patina oxide layer when exposed to oxygen that protects it from further corrosion. This property allows copper traces to withstand oxidation and endure long-term use in electronics.
3. Solderability
Copper readily alloys with tin-lead solder to form high-strength solder joints. The intermetallic compounds create an excellent bonding interface between copper PCB traces and component leads.
4. Thermal Conductivity
Copper has excellent thermal conductivity, allowing it to dissipate heat efficiently from high power PCB components. This helps avoid excessive thermal buildup and component failure.
5. Ductility
Copper is highly ductile, allowing it to be rolled or pressed into thin sheets or foils. This thin foil can be laminated onto PCB substrates.
6. Cost
Copper is much more affordable compared to other highly conductive metals like gold, silver, or palladium. This lower cost makes it commercially viable for all types of PCB manufacturing.
In summary, copper’s blend of electrical and thermal conductivity, corrosion resistance, ductility, and cost-effectiveness make it universally adopted as the conductor of choice for PCB plating.
PCB Copper Plating Methods
There are several techniques used in the PCB fabrication industry to deposit copper onto substrates. The most common plating methods include:
Electroless Copper Plating
Electroless copper plating is an auto-catalytic process used to deposit an initial thin copper layer onto non-conductive surfaces. It does not require any external power source. The PCB substrate is immersed in a copper solution which plates copper onto the board through a chemical reducing reaction.
Electroless copper allows plating on dielectric materials like FR-4, Flex, ceramics etc. It provides an initial conductive layer which allows subsequent electrolytic plating. The thickness of electroless copper is typically 0.1-0.5 μm.
Electrolytic Copper Plating
Electrolytic plating requires the substrate to be electrically conductive. It uses electrical current to deposit copper ions onto the PCB from a copper electrolyte solution. Electrolytic plating forms the bulk of copper thickness on a PCB due to its faster deposition rate.
Thickness can be precisely controlled by adjusting the electrical current and plating time. High electrical current densities allow thickness of up to 70 μm to be achieved. PCB panels are plated using either vertical plating or horizontal plating processes.
Panel Plating
In panel plating, the PCB substrate panels are arranged vertically in a plating tank with the copper anode plates. The parallel arrangement allows higher current densities resulting in faster plating. Vertical panel plating is ideal for high-volume PCB production.
Pattern Plating
This method selectively deposits copper only onto the circuit regions on a panel. A photoresist dry film is laminated and imaged to expose only the desired copper pattern. This allows plating copper traces directly without any etching. It eliminates copper waste and is a cleaner process.
Through-Hole Plating
Boards with plated through-holes require copper to be plated onto the inner walls of the drilled holes. This electrically connects the conducting layers to allow current flow between layers. Electroless copper first seeds the hole walls, followed by electrolytic copper plating to build up thickness.
PCB Copper Plating Process Steps
The typical sequence of PCB copper plating steps is as follows:
- Drilling – Through-holes are mechanically drilled as per circuit design. Panels may also be slotted or routed if required.
- Deburring – The holes are deburred using abrasive media to remove rough edges and drill debris.
- Cleaning – Alkaline cleaners remove drilling oils, resins, and debris from the panels.
- Activation – Panels are microetched and treated with a predip chemical to remove oxides and activate surfaces.
- Electroless Copper – An initial thin layer of electroless copper is deposited on non-conducting substrate.
- Panel Dry Film – A photoresist dry film laminate is applied on panels and imaged to expose the plating areas.
- Electrolytic Copper Plating – Bulk copper is electrolytically plated onto boards to the specified thickness.
- Strip Resist – The dry film is stripped away, leaving only the desired copper pattern behind.
- HAL & OSP – Plated boards are treated with hot air leveling (HAL) and organic surface protectant (OSP).
- Etching – Unwanted copper is chemically etched away leaving only the intended copper traces and pads.
- Soldermask & Silkscreen – A soldermask insulates the copper surfaces, while silkscreen provides printed labels.
- Hot Air Solder Leveling – Reflows and levels the plated copper to obtain a smooth solderable surface.
- Final Finish – Additional surface finishes like ENIG, Immersion Tin, or HASL can be applied as per PCB requirements.
- Electrical Testing – Plated boards are electrically tested to ensure correct function before assembly.
Importance of Good Copper Distribution
The distribution of copper across the PCB layer has a major impact on reliable functionality. Here are some key considerations for good copper distribution:
- Ensure copper fills are adequate in board areas with high current flow to avoid overheating.
- Strategically distribute ground planes to provide noise shielding and controlled impedance paths.
- Allow sufficient clearances between adjacent copper paths to prevent shorting or crosstalk.
- Balance copper weights on layers to prevent warping from internal stresses.
- Don’t block air flow under BGA packages which can cause solder joint failure during reflow.
- Reduce large accumulated local copper areas which may lead to pre/post-etch copper wicking.
- Verify global current density is within limits to avoid potential plating burn while maximizing plating rate.
A well-designed copper layout is vital for thermal management, EMI control, signal integrity and overall reliability of the PCB. Simulation and modeling tools help optimize copper area fill, trace widths, and clearances.
PCB Copper Plating Defects
Some common defects that can occur during PCB electrolytic copper plating include:
1. Pitting
Tiny holes and pits are formed in the copper deposit due to impurities or additive depletion in the plating solution. This roughens the copper surface.
2. Nodules/Protrusions
Small bumps or growths are formed on the copper surface due to high current density areas.
3. Orange peel
The copper surface exhibits a rough wavy texture resembling an orange peel. This is caused by poor solution agitation.
4. Plating voids
No copper deposit occurs in certain regions due to lack of solution access or air bubbles shielding the surface.
5. Dull/Burned deposits
The copper has a dark and rough deposit with poor reflectivity due to very high plating rates.
6. Cross panel plating variation
The copper thickness differs substantially between various areas of the panel due to non-uniform current distribution.
Good process control, solution maintenance, optimized plating parameters and racks/tooling design help minimize plating defects.
Pad Plating in PCBs
The copper plating on component contact pads also greatly impacts PCB assembly quality. Some key aspects are:
- The pad surface should be smooth, bright and lacking defects for good solder wetting.
- A uniform thickness across the pad length provides consistent soldering.
- Inner layer pad plating should not exhibit cracks or separation from base copper.
- Plating foldover at the pad edge should be minimized to prevent solder wicking.
- Pad coplanarity should be maintained across the PCB panel.
- Pad surfaces must be free of contamination to enable soldering.
Boards intended for lead-free soldering require special pad plating treatments like immersion tin or OSP coating to provide leach resistance and wettability.
Environmental Considerations
The PCB copper plating process uses substantial quantities of water and creates metal-laden wastewater. Facilities are required to actively treat their effluent to avoid polluting water bodies:
- Counterflow rinsing recycles water and reduces intake volume.
- Copper is recovered from rinse waters using ion exchange resins.
- Plating baths are purified using electrolytic extraction or filters.
- Chemical precipitation converts metal ions into metal hydroxides which are filtered out.
- Anaerobic or aerobic bacterial breakdown of organic contaminants.
- Ozone destructive oxidation, activated carbon, and other tertiary treatment methods.
- Zero liquid discharge can be achieved using evaporation and crystallization techniques.
By implementing rigorous wastewater treatment with high heavy metal removal rates, PCB copper plating can be an environmentally responsible process.
Trends in PCB Copper Plating
Some emerging trends in PCB copper plating technology include:
- Direct metallization techniques like inkjet printing metals onto substrates or aerosol jet printing to replace plating.
- Pulse plating or pulsed reverse plating to deposit copper more uniformly and reduce internal stress.
- Panel obstruction sensors to improve plating distribution across panels.
- Conformal copper plating into High Density Interconnects (HDI) microvias to build reliability.
- Incorporating organic additives like suppressors, accelerators, and levelers for better copper interface.
- Microetching pre-treatments to improve copper adhesion.
- Co-deposition of particles like CrO2 or SiC with copper to provide better wear and abrasion resistance.
- Tin-silver and tin-bismuth alloy plating for enhanced lead-free solderability.
Frequently Asked Questions
Q1. Why is plating required in PCB fabrication?
Plating is required in order to deposit conductive copper onto the insulating dielectric PCB substrate. Electroless copper provides initial conductivity, while thicker electrolytic copper forms the traces and pads. Plating allows reliable and efficient functioning of a PCB.
Q2. What PCB substrate materials are typically plated?
The most common materials plated are rigid and flexible FR-4, polyimide, CEM, PTFE composites, ceramic substrates, and Rogers laminates. Both organic and inorganic substrates can be plated.
Q3. What are the different PCB copper finishes?
Common finishing on top of copper plating includes Hot Air Solder Leveling (HASL), Electroless Nickel Immersion Gold (ENIG), Immersion Silver, Immersion Tin, and Organic Solderability Preservative (OSP).
Q4. How are specifications like 1 oz, 2 oz copper defined?
Oz refers to the plated copper weight in ounces per square foot (oz/ft2). An 1 oz copper foil is 1.4 mils (35 μm) thick. 2 oz copper is twice the thickness at 2.8 mils (70 μm).
Q5. What are typical PCB trace/space widths?
High density boards have trace widths between 4-6 mils (0.1 – 0.15 mm) and spacing of 4-8 mils between traces. Medium density boards have 8-10 mil trace/space while low density boards have >10 mil traces.
Conclusion
In summary, PCB copper plating is the vital process of electrodepositing a thin copper film onto the insulating substrate to construct the conducting circuitry. Electroless copper provides initial conductivity for electrolytic plating to build up thickness.
Copper is universally used due to its high conductivity, corrosion resistance, cost and excellent soldering properties. Uniform copper distribution is essential for thermal management, EMI shielding and signal quality. Plating parameters and pre-treatments must be optimized to obtain good copper adhesion and minimize defects like pitting or dull deposits.
Environmental compliance via wastewater treatment and recovery is critical for sustainable PCB copper plating. Emerging trends include direct printing, pulse plating, and plating into HDI microvias to meet demands of higher density and reliability in electronics.
