PCB PRODUCTION

To uncover the tenebrous origins of a raw circuit board, and how it’s prepared for electronic components to be assembled to it, we will answer the question: what is the PCB fabrication process? To get truly acquainted, we will scrutinize what it takes to construct a high-quality circuit board fab that can be easily assembled by a contract manufacturer.

PCB FABRICATION BASIC REQUIREMENTS

PCB fabrication is the process of building the raw board that will serve as the foundation for the final printed circuit board assembly. Before the fabrication vendor can proceed with the job of building the raw board, there is a checklist of design details that must be completed. Here are some of the items that must be accomplished to begin fabrication, and what data the board vendors will be looking for:

Circuit Board Details

Before beginning a PCB layout, the design team will work with their contract manufacturer to develop the parameters and configuration of the physical board. This information is necessary for the layout of the design, and for deciding on the PCB fabrication vendor best suited for the job. Some details will include:

Board layer stackup and configuration
Controlled impedance layers
Minimum trace width and spacing
Via types and structures
Copper balancing
PCB panelization considerations

Completed PCB Design

To ensure the accuracy of PCB manufacturing, the initial design must be complete. Even a simple component update that wasn’t synchronized between the schematic and the layout could result in a changed footprint and thus a bad board build. Here are some design items to check off first so the design is fully completed and ready for manufacturing:

Circuitry has gone through a full engineering review
Schematic and layout databases are updated and synchronized
All components are placed on the board, and all net connections are routed
Circuit simulation, signal integrity, and power integrity analysis are completed
PCB design rules and constraints are checked and errors corrected
Bill of materials (BOM) is reviewed for current and available components
Design for manufacturing (DFM) rules are reviewed for error-free assembly.

Full Manufacturing Design Data

With the design complete and ready to build, a full set of fabrication and assembly data files are generated and submitted to the contract manufacturer. The PCB CM will use these files to conduct their design reviews, and then forward them to the fabrication vendor to quote the build of the raw boards. These data files will include:

Gerber or other formatted board layer image files
Manufacturing drawings for fabrication and assembly
Bill of materials (BOM)
Component XY locations (pick & place)
Test point locations
Netlist
Schematic

With all checklist items completed and ready to go, the contract manufacturer can proceed with ordering the raw circuit boards needed for the assembly. Typically the PCB CM will have a list of preferred fabricators to work with and will choose the vendor whose capabilities best align with the technical requirements of the circuit board. Another factor in choosing the fabrication vendor is whether or not the board being built is a prototype. If so, the fabrication vendor must have the capabilities to run a separate production line outside of regular processes. They’ll also need to conduct their design reviews using the Gerber data being supplied by the PCB CM to maintain the highest quality in the prototype build.

Once the fabrication vendor has been chosen, the next step is to build the raw circuit board!

WHAT IS THE PCB FABRICATION PROCESS TO BUILD A BARE BOARD?

With all the necessary design information and data in hand, the PCB fabricator will begin building the raw board. Many types of circuit boards can be fabricated including single-sided boards, high-density multi-layer designs, and flex circuits. For this example, we will look at the basic steps required in the fabrication of a standard multi-layer circuit board.

Creating the Circuitry Images

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 can be used. The image data will be transferred to the board by one of two different methods:

Photo Tooling: The standard imaging process in PCB fabrication that’s been in use for as long as circuit boards have been mass-produced. A precision photoplotter will create the circuitry images on film, which is used 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 has advantages over using 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 will have to be laser printed individually, which is a more expensive process.

The Layers of a Circuit Board

A multilayer circuit board is a composite of different layers of dielectric material and metal conductors. It’s composed of 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 there are other dielectric materials 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 the creation of a double-sided board, and laminate it together with other core structures to build the board layer stackup. Each layer must be strictly controlled for its width, copper weight, and layer to layer alignment for a quality final product.

Creating the Inner Layer Circuitry

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

The copper foil of the core is covered with a sheet of photoresist material.
The photoresist is exposed with either an 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 photoresist, which is still pliable, is chemically removed from the copper.
The copper layers of the core are etched away leaving only those areas of circuitry that are protected by the polymerized photoresist.
The photoresist is stripped off leaving only the copper circuitry.

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

Laminating the Layers Together

Together with thin layers of copper foil to cover the external surfaces of the top and bottom sides of the board, layer pairs are stacked to create a PCB “sandwich.” To facilitate the bonding of the layers, each layer pair will have a sheet of “prepreg” inserted between them. 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 together into one circuit board.

Drilling the Holes

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. The majority of the thru-holes used in a circuit board will be plated, and are usually drilled 0.005” larger than the specified finished hole size to allow for plating. If the design contains any blind and buried vias or laser-drilled microvias, those are fabricated before the lamination of the board. The extra process steps for these vias can add additional cost to the fabrication of the board but may be required for dense circuitry and/or electrical performance.

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

Creating the Top and Bottom Layer Circuitry

Now the board is ready to have its top and bottom circuitry images printed. To do this, the same photoresist will be used as with the inner layers, but this time the circuitry needs to be unprotected and plated up with additional copper:

The top and bottom surfaces of the board are completely covered with a sheet of photoresist material, including the drilled holes to be plated.
The photoresist is exposed by UV or laser, but contrary to the inner layers, all surface areas of the board are exposed except for the circuitry patterns.
Once the unexposed photoresist is chemically cleaned off, the circuitry patterns of bare copper are electrically plated with more copper to build up their metal weight.
Next, tin is plated onto all of the copper circuitry as a protective layer, 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 those areas of metal circuitry that are protected by the tin.
Finally, the tin is removed 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 the fabrication of the board.

Solder Mask, Silkscreen, and Surface Finishes

To protect the board during assembly, the solder mask material is applied using a UV exposure process similar to what was used with the photoresist. This solder mask will cover the entire surface of the board 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 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. As the board is removed from the solder bath, a high-pressure blast of hot air removes excess solder from the holes and smooths the solder on the surface metal.