PCB Anatomy 101 — What a Board Is Made Of

The Layer Cake

Think of a PCB as a cake with layers. From outside in, a typical board has:

1. Silkscreen — White (sometimes yellow or black) labels printed on the surface. Letters, numbers, logos, component reference designators.
2. Solder mask — The colored coating (usually green) that covers the copper. It prevents solder bridges and protects the copper.
3. Copper layer — Thin sheets of copper foil etched into traces (the "wires"), pads, and planes.
4. Substrate / Core — The insulating material the copper sits on. Usually FR-4 (fiberglass and epoxy resin), but can be other materials for special applications.
5. Prepreg — Sheets of partially-cured fiberglass and resin used to bond layers together in multilayer boards.

A 2-layer board has copper on top and bottom of one substrate. A 4-layer board has 4 sheets of copper separated by substrate and prepreg, and so on.

Key Features You'll Hear About

• Traces — The copper "wires" that carry signals.
• Pads — Little copper landing zones where components get soldered.
• Vias — Tiny plated holes that connect copper from one layer to another. Three types: through-hole (goes all the way through), blind (from outer to inner layer), and buried (between two inner layers).
• Plated through-holes (PTH) — Holes plated with copper, used for component leads or vias.
• Drill holes — Non-plated holes for mounting screws or alignment.
• Annular ring — The ring of copper around a drilled hole.
• Edge connector / gold fingers — Gold-plated stripes along an edge for plugging into a slot (like RAM in a computer).

Common Materials

• FR-4 — Fire-retardant fiberglass. The standard for ~90% of boards. Cheap, reliable, works for most applications.
• High-Tg FR-4 — FR-4 that handles higher temperatures, used when the board sees heat (automotive engine bays, power supplies).
• Polyimide — Flexible material used for flex circuits.
• Rogers / PTFE / Ceramic-filled laminates — Specialty materials for high-frequency or RF applications (cell towers, radar).
• Aluminum / Metal core (MCPCB) — Used for LED lighting and power applications where heat must escape quickly.

Surface Finishes

The exposed copper pads need a finish to keep them from oxidizing and to help solder stick. Common ones:

• HASL (Hot Air Solder Leveling) — Cheapest. Tin-lead or lead-free.
• ENIG (Electroless Nickel Immersion Gold) — More expensive. Flat, gold-colored, best for fine-pitch parts. Industry favorite.
• OSP (Organic Solderability Preservative) — Cheap, environmentally friendly, shorter shelf life.
• Immersion silver / immersion tin — Niche uses.

Customers care about surface finish because it affects assembly yield, cost, and shelf life of the bare board.

Key Specs Customers Will Mention

• Layer count (1, 2, 4, 6, 8, 10, 12, up to 40+)
• Thickness (typical is 1.6 mm; can range from 0.4 mm to 3.2 mm+)
• Copper weight (1 oz, 2 oz, 3 oz, etc. — heavier copper carries more current)
• Trace width / spacing (how thin the wires can be — 5 mil / 5 mil is standard, 3/3 is harder, 2/2 is HDI territory)
• Minimum hole size (8 mil drilled is standard, smaller is harder/more expensive)
• Aspect ratio (ratio of board thickness to drill diameter — high aspect ratios are harder to plate)
• Tolerance / impedance control (tightly controlled trace dimensions for high-speed signals)

You don't need to memorize all of these on day one. You DO need to recognize them when an engineer says them so you can write them down accurately and pass them to your fab shop.