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Material World
Object

Printed circuit board

Copper foil on woven glass in epoxy — the substrate every electronic thing is built on, and the reason electronics could be manufactured rather than wired.

A printed circuit board is a sandwich: sheets of woven glass fibre cloth impregnated with epoxy resin, with copper foil bonded to the outside, the unwanted copper etched away, and holes drilled and plated through to connect the layers.

The laminate grade is called FR-4 and the name is worth unpacking, because it is the whole specification. FR is flame retardant, and the retardancy comes from bromine chemically bound into the epoxy. 4 is the woven glass reinforcement. Together they give a board that is stiff, dimensionally stable when heated, electrically insulating, and self-extinguishing — which matters in an object full of things that can overheat.

Before it, electronics were assembled by hand-wiring components to tag strips, which is why a 1940s radio chassis looks like it does. The board turned assembly from a wiring job into a printing job, and it is the change that made mass-produced electronics possible.

Processing

Subtractive, in the main, and the name is a fossil of it: the pattern is printed and the rest is removed.

Copper-clad laminate is coated with photoresist, exposed through a mask, developed, and etched in ferric chloride or cupric chloride so that only the protected copper survives. Holes are drilled and their walls made conductive — electroless copper, then electroplated — so that a track on one layer reaches a track on another. Multilayer boards repeat the whole sequence and press the layers together with more resin.

The finish on the exposed pads is a small decision with large consequences. Bare copper oxidises and will not solder after storage, so pads are given a coating: immersion gold over nickel where reliability matters, tin or silver where cost does, and hot-air-levelled solder where the board is old-fashioned. The gold is a fraction of a micrometre and is one of the reasons a tonne of boards assays richer than a tonne of ore.

Solder mask — the green — is a polymer coating that stops solder bridging between tracks. It is green because the earliest formulations were, and it has stayed green because inspection equipment and inspectors are calibrated on it.

Environmental impact

Two problems, one solved and one not.

Lead was solved, more or less. Tin-lead solder was the standard for a century, and the RoHS directive from 2006 pushed the industry to tin-silver-copper alloys across almost all consumer electronics. Those melt about 30 °C hotter, wet less readily, and grow tin whiskers under some conditions — a genuine reliability change that aerospace and medical electronics were exempted from for exactly that reason.

The board itself was not. A cured epoxy is a thermoset: it cannot be melted, so it cannot be reprocessed into anything. Recycling a board means shredding it, recovering the metals, and burning or landfilling the remaining resin-and-glass fraction. The brominated flame retardants make burning it a controlled operation, and informal recovery — open burning to get at the copper, which is how a great deal of the world's e-waste is processed — releases them into the air and the people doing it.

So the substrate that made electronics manufacturable is also the part of electronics that most reliably becomes waste, and no substitute at scale has emerged in seventy years.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is made of

  • Copper — element · the foil bonded to the laminate and etched down to the tracks, and the plating through every hole that joins one layer to another
  • Glass fibre composite — material · the FR-4 laminate itself: woven glass cloth in flame-retardant epoxy, stiff, dimensionally stable when heated, and self-extinguishing
  • Epoxy resin — material · the matrix of the laminate, with bromine bound into it for the flame retardancy the grade is named for
  • Glass fibre — material · the woven reinforcement, which is what stops the board moving as it is heated through a reflow oven
  • Solder — alloy · every joint on it, and tin-silver-copper since RoHS pushed the lead out from 2006
  • Gold — element · a fraction of a micrometre over nickel on the pads, because bare copper oxidises and will not solder after storage
  • Nickel — element · under the gold, as the barrier that stops it diffusing into the copper

is made using

  • Glass fibre composite — material · which is by a wide margin the largest use of glass fibre composite by number of parts, and the one nobody counts as a composite application

is used in

  • Electronics manufacture — industry · and it is the change that made electronics manufacturable: assembly stopped being a wiring job and became a printing one

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Printed circuit board back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Printed circuit board → is made of (the foil bonded to the laminate and etched down to the tracks, and the plating through every hole that joins one layer to another) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Printed circuit board → is made of (a fraction of a micrometre over nickel on the pads, because bare copper oxidises and will not solder after storage) → Gold → is sourced from (a significant share of world gold arrives this way, recovered from copper anode slimes at no mining cost of its own) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Printed circuit board → is made of (under the gold, as the barrier that stops it diffusing into the copper) → Nickel → is produced by (reduced from roasted pentlandite concentrate, with the platinum-group metals following into the refinery) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Printed circuit board → is made of (every joint on it, and tin-silver-copper since RoHS pushed the lead out from 2006) → Solder → is composed of (under a per cent, to slow the rate at which the joint dissolves the copper it is sitting on) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Printed circuit board → is made of (the matrix of the laminate, with bromine bound into it for the flame retardancy the grade is named for) → Epoxy resin → is produced by (the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Printed circuit board → is made of (the woven reinforcement, which is what stops the board moving as it is heated through a reflow oven) → Glass fibre → is produced by (drawn through a bushing of hundreds of platinum-alloy holes, at a diameter of a few micrometres and a speed of tens of metres a second) → Glass melting → takes as input (as the stabiliser, and without it a soda-silica glass would slowly dissolve in water) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water

These are the most distinct paths back. Printed circuit board can be traced through others besides.

Downstream — what it becomes

  • Printed circuit board → is used in (and it is the change that made electronics manufacturable: assembly stopped being a wiring job and became a printing one) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain