Introduction
Electroplating is a key process used during the manufacturing of printed circuit boards (PCB) to deposit metallic coatings on the substrate. It involves electrodeposition of a thin film of metal like copper, tin, nickel, gold etc. onto the PCB to build up the conductive traces and interconnects. In this article, we will learn the basics of electroplating and its role in PCB fabrication.
Why Electroplating is Used in PCBs?
Electroplating provides the following benefits for PCB manufacturing:
- Enables depositing uniform, smooth and dense metal coatings.
- Allows building up PCB copper thickness up to 1 oz/ft2 or higher.
- Deposits can be precisely controlled by process parameters.
- Conformal step coverage into small openings and vias.
- Cost-effective process with high throughput.
- Suitable for plating a wide range of metals like Cu, Sn, Ni, Au etc.
- Environmentally cleaner compared to many coating processes.
Thus, electroplating is ideally suited for metallization of PCBs during fabrication.
Electroplating Process Basics
Electroplating involves applying a voltage between two electrodes immersed in a salt solution (electrolyte) to drive metal deposition onto the cathode surface.
Key Components
- Anode – Positive electrode providing metal ions into the solution. Made of the metal being plated.
- Cathode – Negative electrode to be plated. The PCB acts as the cathode.
- Electrolyte – Contains aqueous solution of metal salts and other additives.
- Power Supply – Applies direct current between electrodes.
Basic electroplating process setup showing anode, cathode, electrolyte and power supply.
Mechanism
When a DC voltage is applied, the metal atoms of the anode dissolve into the electrolyte as positive ions.
The PCB substrate acting as cathode is charged negatively. This attracts the positively charged metal ions in the electrolyte to deposit onto the PCB surfaces forming a thin metal coating.
Electroplating in PCB Fabrication
Electroplating is utilized at multiple stages during the manufacture of a printed circuit board:
1. Copper Electroplating
- After drilling and desmear, copper is electroplated to build up traces on the PCB.
- Multiple plating cycles with increasing thickness are used to reach final copper thickness.
- Plating rates up to 1 mil/min can be achieved.
2. Tin-Lead or Tin Electroplating
- Tin-lead or pure tin plating applied on surface pads for solderability.
- Provides good solderability for SMT component assembly.
3. Nickel and Gold Plating
- Nickel plating applied over copper traces for corrosion protection.
- Immersion gold coating added as surface finish for wire bonding or enhanced connectivity.
4. Via Hole Plating
- Copper is electroplated to build up metallization inside drilled via holes.
- Ensures electrical connectivity between layers.
5. Electroless Plating
- Used before electroplating to deposit initial conductive layer on dielectric surface.
- Provides base for subsequent electroplating.
PCB Electroplating Process Steps
The PCB electroplating process involves the following key steps:
1. Cleaning
- PCBs are thoroughly cleaned to remove oils, dust particles and debris from surfaces.
- Hot alkaline cleaning agents are commonly used.
2. Microetching
- Microetching done using chemical or plasma etching.
- Removes surface contaminants and oxidized copper.
- Creates microscopic roughness to improve adhesion.
3. Electroless Plating
- Electroless copper plating applied first to make PCB surface conductive.
- Between 0.1 to 0.5 mil thickness plated.
- Provides base layer for electroplating to build upon.
4. Photoresist Coating
- Dry film or liquid photoresist coated on PCB surfaces.
- Photolithography used to create plating resist pattern.
5. Electroplating
- PCB connected as cathode and immersed in electrolytic bath.
- Desired metal deposited using DC current for required time.
- Plated metal thickness increases with plating time.
6. Resist Stripping
- Photoresist stripped away revealing conductive pattern.
- Typically removed using chemical stripping process.
This completes the PCB electroplating process. The boards may go through additional plating processes to deposit tin, nickel, gold etc.
Electroplating Process Parameters
The key process parameters that determine the electroplating results:
- Current Density – Controls plating rate. Typical range is 20 to 120 A/dm2. Higher current densities increase plating speed.
- Voltage – Typical plating voltages around 3-9 V. Higher voltages improve throwing power.
- Temperature – Typical range of 20-30°C. Higher temperature increases plating rate.
- Agitation – Solution movement using pumps or agitators improves deposit uniformity.
- Additives – Additives like brighteners refine the grain structure and brightness.
- Time – Plating time controls metal thickness. Time is adjusted to achieve target thickness.
Advantages of Electroplating
Some benefits of using electroplating for PCB fabrication:
- Produces uniform metal coatings with good dimensional control.
- Allows building up of copper thickness to over 1 oz.
- Recesses and vias can be covered through optimal throwing power.
- Reduces porosity in the metal deposit.
- Operates at close to room temperatures.
- Provides high production rate and throughput.
- Lower cost compared to some physical vapor deposition techniques.
- Simpler equipment and setup compared to CVD, PVD.
- Allows depositing a wide range of metals.
Limitations of Electroplating
Some limitations of electroplating include:
- Metal purity is lower compared to vapor deposition techniques.
- Plating thickness inside openings falls as aspect ratio increases.
- Requires additional processing steps for resist patterning.
- Stripping and cleaning steps produce liquid effluents.
- Corrosion and maintenance requirements of electrolyte bath and anodes.
- Metals like aluminum cannot be electroplated.
- Difficulty plating non-conductive substrates without seed layer.
Conclusion
Electroplating is an efficient and cost-effective process that enables metallization of printed circuit boards during fabrication. It allows depositing a wide range of metals like copper, tin, nickel to build the conductive traces and interconnects on the PCB substrate. By selecting optimal process parameters and electrolyte composition, electroplating provides an ideal solution for metallization of PCBs in a high volume manufacturing environment.
FAQs
- How is electrolytic copper different from electroless copper plating?
Electrolytic copper requires applying external current while electroless copper relies on auto-catalytic chemical process without external power.
- What are some surface finishes deposited by electroplating on PCBs?
Common PCB surface finishes plated include tin, tin-lead, nickel, gold, silver and palladium-nickel coatings.
- What defects can occur during electroplating?
Burning, pitting, nodulation, cracking, peeling, contamination, over/under-plating are some common electroplating defects.
- Why is pattern plating done for PCB fabrication?
Pattern plating allows selective electroplating only in desired conductor pattern. This eliminates need to etch away unwanted copper after plating.
- What are auxiliary anodes used for in PCB electroplating?
Auxiliary anodes placed near edges/corners compensate for low current densities in those areas and enable more uniform plating thickness.
