PCB Fabrication: An In-Depth, Step-by-Step Guide

The first step of circuit board fabrication is to transfer the PCB design circuitry image data from the manufacturing files supplied by the CM to the board. Usually, data arrives in a file format known as Gerber, although other formats and databases are available. Transfer of the image data to the board occurs by one of two different methods:

• Photo Tooling: The standard imaging process in PCB fabrication that’s been in use for as long as PCB mass production. A precision photoplotter will create the circuitry images on film, which assists in the fabrication process as a template to print the images onto the board.
• Direct imaging: A laser prints the circuitry images directly onto the circuit board, bypassing the need for photo tools. This technique has advantages over film because it’s more precise, there aren’t alignment issues, and photo tooling won’t require periodic recreation to replace worn-out films. Conversely, each layer must be laser printed individually, which is more expensive.

A multilayer circuit board is a composite of different layers of dielectric material and metal conductors. It contains layer pairs with a dielectric core material of epoxy resin and glass fiber, more commonly known as FR-4, sandwiched between two layers of copper foil. While other dielectric materials are available, FR-4 is the most common core material used in PCB fabrication.

Multilayer boards will take a thinner version of the same core structure used in creating a double-sided board and laminate it with other core structures to build the board layer stackup. Quality processes strictly control for width, copper weight, and layer-to-layer alignment for a quality final product.

The first step in PCB fabrication is to print circuitry images onto the inner layer cores:

• Technicians cover the copper foil of the core with a sheet of photoresist material.
• Technicians expose the photoresist with either ultraviolet light through the photo tooling or by direct imaging with a laser.
• Only the areas of copper circuitry, such as pads and traces, are exposed, which polymerizes or hardens the photoresist over the circuitry patterns.
• The unexposed, still pliable photoresist separates from the copper chemically.
• The core’s copper layers are etched away, leaving only the areas of circuitry protected by the polymerized photoresist.
• Technicians strip the photoresist, leaving only the copper circuitry.

Once complete, an AOI system (automated optical inspection) checks the core layers for defects. Once each inner layer pair of the board has gone through this process, they’ll be ready to be laminated into one complete circuit board.

Layer pairs are stacked to create a PCB “sandwich.” Each layer pair will have a sheet of prepreg between them to facilitate the bonding of the layers. Prepreg is a fiberglass material impregnated with epoxy resin that will melt during the heat and pressure of the lamination process. As the prepreg cools, it will bond the layer pairs together.

Compositing the board together during this phase requires a lot of attention to detail to maintain the correct alignment of the circuitry on the different layers. Once the stackup is complete, the sandwiched layers are laminated, and the heat and pressure of the lamination process will fuse the layers into one circuit board.

The next step in PCB fabrication is to drill holes in the board for component mounting, thru-hole vias, and the non-plated holes of mechanical features. Most of the through-holes in a circuit board are holes drilled 0.005” larger than the specified finished hole size to allow for plating. If the design contains blind and buried vias or laser-drilled microvias, their fabrication occurs before the board lamination. The extra process steps for these vias can add additional cost to the board’s fabrication but may be necessary for dense circuitry or electrical performance.

After drilling, chemical and mechanical processes clean holes to remove resin smears and debris caused by drilling. The entire exposed surface of the board, including the holes’ interior, is then chemically coated with a thin layer of copper. This process creates a metallic base for electroplating additional copper into the holes and onto the surface in the next step.

The same photoresist application as with the inner layers occurs, but this time, the circuitry needs to be unprotected and plated up with additional copper:

• Technicians cover the top and bottom board surfaces with a sheet of photoresist material, including the drilled holes for plating.
• The photoresist undergoes UV or laser exposure, but contrary to the inner layers, all board surface areas except for the circuitry patterns receive it.
• After removing the unexposed photoresist chemically, the circuitry patterns of bare copper are electrically plated with more copper to increase their metal weight.
• Next, a tin plating process provides a protective layer to the copper circuitry, and the photoresist is stripped off the remainder of the board in preparation for etching.
• The board is etched to remove all copper except for the areas of metal circuitry protected by the tin.
• A final step removes the tin, leaving the plated copper pads, traces, and thru-holes.

At this point, the board’s circuitry is complete, but there are still a couple of more steps to finish fabricating it.

The solder mask material is applied using a UV exposure process similar to what was used with the photoresist to protect the board during assembly. This solder mask covers the entire board surface except for the metal pads and features that will be soldered. In addition to the solder mask, component reference designators and other board markings are silk-screened onto the board. Both the solder mask and the silkscreen ink get cured by baking the circuit board in an oven.

The circuit board will also have a surface finish applied to its exposed metal surfaces. This finish helps to protect the exposed metal and assists in the soldering operation during assembly. One example of a surface finish is hot air solder leveling (HASL). The board is first coated with flux to prepare it for the solder and then dipped into a bath of molten solder. After removing the board from a solder bath, a high-pressure blast of hot air removes excess solder from the holes and smooths the solder on the surface metal.

The final step of the PCB fabrication process prepares the circuit board for assembly. If necessary, a router cuts the circuit boards out of their manufacturing panels, or board depanelization occurs after assembly. The depanelization uses scoring a V-cut on the board outline or routing out the board except for small breakout tabs.

The finished board goes through continuity testing with automated test equipment such as a bed of nails test fixture, or a flying probe test system. Tests look for any unintentional shorts between nets which would invalidate the board. Once the testing is complete and the board passes inspections, it’s shipped back to the PCB contract manufacturer for component assembly.