Printed circuit boards (PCBs) form the foundation of electronics products. Their fabrication involves various chemical processes like cleaning, etching, plating and coating to transform raw materials into robust circuit boards. This article provides an overview of key chemicals utilized across the PCB manufacturing workflow.
Overview of the PCB Manufacturing Process
The typical steps involved in making a PCB include:
- Design – Board layout is designed in CAD
- Prototyping – Initial test boards are made
- Fabrication – Full scale PCB fabrication starts
- Substrate Preparation – Cutting, cleaning, surface treatment
- Photolithography – Photoresist coating, exposure, development
- Etching – Copper etching to form traces
- Plating – Electroless and electrolytic plating processes
- Coating – Soldermask, legend/marking, edge connector coatings
- Routing and Drilling – Mechanical processes to shape board
- Testing and Inspection – QA checking for defects
- Assembly – Soldering of components to populate board
Specialized chemicals are used throughout these fabrication processes. Let’s look at the key chemicals involved in each step.
Substrate Preparation Chemicals
The PCB manufacturing process starts with the substrate – usually FR-4 glass epoxy sheets cut from large panels. This substrate is prepared using:
- Cleaning Chemicals – Alkalis like sodium hydroxide remove oils and surface residues. Solvents like acetone and isopropyl alcohol provide cleaning.
- Desmear Chemicals – Used after drilling multilayer boards to remove resin smears and debris from hole walls. Commonly permanganates.
- Etchant Resists – Masking agents like tin plating resist pre-tinning before soldermask. Protects copper during etching.
- Surface Conditioners – Promote adhesion of subsequent coatings and platings. Examples are organo-silane and poly-vinyl preparations.
Proper substrate cleaning and conditioning ensures coatings bond well in further fabrication steps.
Photolithography Chemicals
Photolithography transfers the PCB layout pattern onto the copper clad substrate using light-sensitive chemicals:
- Photoresists – Light sensitive polymers that change solubility when exposed. Two types are negative resists which become insoluble on exposure and positive resists which become more soluble, both enabling selective etching.
- Developers – Aqueous solutions, usually of sodium hydroxide or potassium borates, which dissolve the soluble photoresist portions after light exposure.
- Strippers – Used to remove cured photoresist after etching. Common strippers are dimethyl sulfoxide and solutions containing amines.
Photolithography chemicals enable cost-effective PCB patterning scaled to mass production.
Copper Etching Chemicals
Once the photoresist pattern is transferred, copper etching chemically dissolves away unwanted copper to create traces:
- Copper Clad Laminate Etchants – Usually cupric chloride solutions which rapidly dissolve copper. Iron chloride and ammonium persulfate also used.
- Tin-Lead Etchants – When etching plated tin-lead, ammonium persulfate solutions work while cupric chloride won’t. Hydrochloric/peroxide mixtures also used.
- Oxidizers – Added to replenish etchants, usually hydrogen peroxide or Bleach (Sodium hypochlorite).
- Rinse Enhancers – Sulfuric acid used to prevent undercutting during rinsing. Also improves copper surface for plating process.
Etching is the backbone of pattern transfer from photomasks to actual circuit patterns on the PCB substrate.
Plating Chemicals
Electroplating deposits conductive copper, tin, nickel, gold etc. on PCBs both to form traces and coatings:
- Electroless Copper – Deposits thin copper layer prior to electrolytic copper plating. Based on copper sulfate or chloride along with reducing agents.
- Acid Copper Plating Solution – Copper sulfate with sulfuric acid for electrolytic copper plating to desired thickness.
- Tin-Lead Plating – Immersion tin-lead plating baths protect copper traces from oxidation. Fluoroborates used.
- Electroless Nickel Immersion Gold (ENIG) – Used for corrosion resistant gold coating over nickel on exposed copper pads.
- Immersion Silver (I-Ag) – Silver coatings deposited from solutions like silver nitrate dissolved in ammonia solvent.
Plating provides solderability, corrosion resistance, and trace conductivity essential for PCBs.
Soldermask and Legend Chemicals
Soldermask coatings insulate copper traces while allowing soldering on pads. Legends help identify components:
- Soldermask – Usually epoxy-based formulations that are screen printed or photo-imageable. Provide insulation and environmental protection.
- Legend – Screen printed ink layers for component markings and identification. Require chemical and abrasion resistance.
- Conformal Coatings – Protective polymer coatings like acrylic, silicone, urethane applied on assembled PCBs. Provide insulation, durability and moisture resistance.
These coatings enhance PCB reliability and enable marking for identification.
Other Chemicals
Some additional chemicals involved:
- Fluxes – Used during soldering assembly to remove surface oxides and enable solder flow. Based on rosin, organic acids, or inorganic acids.
- Cleaning Solvents – Aqueous and semi-aqueous blends, terpenes, glycol ethers used for post-assembly PCB cleaning.
- Epoxy Resins – Used as insulation matrix for glass-reinforced FR-4 material which forms core layers. Bisphenol A derivatives commonly used.
- Dyes and Pigments – Added to photoresists, soldermasks and legends for opacity and color. Carbazole dyes and titanium dioxide pigments common.
Summary of Key Chemicals in PCB Manufacturing
| Process | Chemicals Used |
|---|---|
| Substrate Preparation | Sodium Hydroxide, Acetone, MEK, Alcohol, Conditioners |
| Photolithography | Photoresists, Developers, Strippers |
| Copper Etching | Cupric Chloride, Ammonium Persulfate, Hydrogen Peroxide |
| Plating | Electroless Copper, Acid Copper, Tin-Lead, ENIG, I-Ag |
| Soldermask | Epoxy, Acrylic formulations |
| Legend | Screen printing inks |
| Cleaning | Terpenes, Glycol Ethers, Aqueous Blends |
| Assembly | Fluxes, Cleaning Solvents |
Conclusion
A wide variety of specialized chemicals across cleaning, etching, plating, coating and assembly enable cost-effective manufacturing of printed circuit boards at scale. Responsible handling, storage and disposal of these chemicals is critical for worker safety and environmental compliance. Continued material and process innovation will enable further improving the performance, quality and manufacturability of PCBs using both existing and novel chemical solutions.
Top 5 FAQs about Chemicals Used in PCB Manufacturing
Q1: What are some of the more hazardous chemicals involved in PCB fabrication?
Copper etching chemicals, solvent cleaning agents, and stripping solutions require careful handling given potential risks from toxicity, flammability or chemical burns.
Q2: How are PCB manufacturers disposing these chemicals responsibly?
Most now have strict procedures for chemical waste disposal, neutralization, and treatment before release. Some chemicals can be recovered and recycled.
Q3: What green or environment-friendly chemicals are emerging for PCB production?
Use of solvent-free photoresists, aqueous stripping solutions, cyanide-free copper plating are examples of reducing environmental impact through eco-friendly alternatives.
Q4: How are workers protected when handling chemicals?
Use of local exhaust systems, chemical cabinets, personal protective equipment including respirators and protective apparel safeguard worker health and safety.
Q5: Does the RoHS directive restrict certain chemicals used in PCB production?
Yes, RoHS enforces limits on substances like lead, cadmium, mercury, chromium and brominated flame retardants in electronics manufacturing which also applies to PCBs.
