The printed circuit boards (PCBs) that are ubiquitous today were originally called printed wiring boards (PWBs) and began entering commercial devices in the 1950s.  Back then, engineers would be lucky if they could do away with the point-to-point wiring that was common at the time and get even a 1-layer board.  Today, Automated Test Equipment (ATE) customers order 96-layer boards as a regular part of their design process.

Printed Circuit Board Engineers can combine additive and subtractive manufacturing techniques to create boards of varying layer counts.  As a rule of thumb, the fewer process steps and bits of raw material needed, the cheaper the PCB.  However, some savings are only seen at extremely large scales.  For example, in small scale and prototype manufacturing, a single-panel 1-layer PCB will have the same non-recurring engineering and process steps as a 2-layer PCB so that no cost savings can be realized.  But at very large scales, single-layer PCBs can be made in etch-only processes with punches that make all through-holes at once.  If you intend on ramping up from a few boards to a few million, be sure to speak to the engineers at your fabrication house and learn everything you can.  A $0.10 saving per board across a million board order can save $100k.

1 Layer Boards

Single-layer boards have a single layer of copper above a dielectric that may or may not be fiberglass reinforced.  They work best with through-hole components placed on the non-copper side of the board.  The copper traces connect the various pins, and when a net collision is unavoidable, a jumper wire can connect the traces from the component side of the board, providing a second layer for connections.  These boards sometimes even forgo solder mask and coat everything with hot-air surface-level (HASL).

This image shows a small section of the solder side of a power control board.  This is the only side of the pcb with any copper features on it

1.5 Layer Boards

Devices that use silicone rubber with a graphite pill as a keypad (remote controls, calculators, toys, cheap consumer electronics, etc.) will usually have a layer of conductive carbon ink on the board on top of the solder mask layer.  The graphite connects to copper pads at the surface of the board.  Combined with wire jumpers, this can provide another conductive layer for your board – the result is four routing layers composed of one copper layer, one carbon ink layer (on top of the solder mask), and up to two jumper wire layers.  Due to the poorer electrical performance of the graphite in comparison to a jumper wire, it’s unusual to see graphite jumpers on anything other than single-layer PCBs.  It’s also unusual to see graphite jumpers on two sides of a PCB unless it’s part of a two-sided remote control.

This image from a calculator keypad shows interwoven fingers that make each individual key as well as rectangular jumpers made from the same material. 

2 Layer Boards

Two-layer boards begin life as a copper-clad core from the factory.  PCB fabricators drill, electroplate, and then etch boards to produce vias, traces, and pads.  With solder mask protecting the copper traces, it’s now possible to place jumper wires on both sides of the PCB, providing 4 routing layers.  Keep in mind though, that jumpers need to be straight-line connections and will incur additional assembly cost if all the other components on your board are surface mount.

3 Layer boards (and other odd-number layers)

While some three-layer boards exist, there’s no compelling reason for them to exist.  A 3-layer board is constructed by laminating a sheet of copper and dielectric to a 2-layer core.  And a fabricator would go through all the same steps they would go through to make a 4-layer board.  You’d be left with a board that costs as much as a 4 layer board but with one fewer routing layers.  Additionally, the dielectric layers would likely be of uneven separation distance and likely have differential coefficients of thermal expansion, which means your board might warp during the assembly process.

4 Layer boards

Four layer boards can be made with one of two construction techniques called “Cap” and “Foil.”

In foil construction, a single pre-laminated core is sandwiched between layers of dielectric and outer layers of copper.  In cap construction, two pre-laminated cores are separated by a layer of dielectric and pressed together.  Foil is always preferred by fabricators since cap construction has traces on the outer layers that can become damaged during the pressing process.  Additionally, it’s sometimes difficult to keep the two cores aligned during the pressing process.

Adding two layers of jumper wires provides a total of 6 routing layers.

6-18 layer boards

Boards with 6 or more layers are made with the same techniques used for four-layer boards.  Traces are electroplated & etched on interior layers, dielectric, and copper foil are laminated to the outside, and more drilling, electroplating,  and etching take place.

18+ layer boards

It’s possible to make boards of almost any layer count.  But at some point, you’ve got to stop putting your project through additional lamination cycles.  For high layer count boards, it usually makes sense to make a series of lower layer count sub-stacks, and adhere them together with transient liquid phase sintering paste.  The TLPSP permanently bonds the substacks together to make a board of any arbitrary thickness.

Some engineers are forced to make 96-layer boards out of six 16-layer sub-stacks to test and qualify new IC designs, but the lead time and fabrication cost usually leaves these as one-offs.  You don’t often see 96-layer boards in commercial designs.

We hope this helps!  If you have any questions, please contact our engineering team for clarification – they can help advise you on your next design!