Why are modern PCBs green in colour?

Those of you who have started in hobby electronics long back, would remember the old type of printed circuit boards (PCB) with through hole mounting of components. Inset in blue border shows the other side of those antique PCBs showing the copper strips forming the printed circuit. Most of the PCBs these days have green colour on both sides, often with surface mount devices mounted on both sides of the PCB. The green PCBs are typically made from a core of fiberglass reinforced with epoxy resin. Green colour comes from the solder mask applied over it. Solder mask is a protective polymer coating on a PCB that prevents oxidation, insulates copper traces, and stops solder bridges (shorts) between pads. This ensures components only connect where intended. It is traditionally green in colour and applied over the board, leaving openings for soldering.

FR-4 or Flame Retardant 4 is the most common, flame-retardant composite material for PCBs, made from woven fiberglass cloth bonded with epoxy resin. It offers good electrical insulation, moisture resistance, and structural strength for various dry or humid conditions. It’s the industry standard substrate for electronics, known for its reliability and cost-effectiveness, though specialized alternatives exist for extreme high-frequency or thermal needs. For high-heat applications, Metal Core PCBs (MCPCBs) or Ceramic PCBs are better. For very high-frequency signals, materials like PTFE (Teflon) offer better performance but at a higher cost.

While green is standard, modern PCBs can come in other colors (blue, red, black, etc.) by adding pigments to the solder mask, though green remains popular for its traditional look and visual benefits.