PCB Fabrication Process - Building Bare Boards

A Successful PCB Assembly Starts with a High-Quality Bare Board Fabrication

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Manufacturing a printed circuit board is a complex process that begins with a product concept and ends with a fully functional PCB assembly. Along the way, a schematic captures the net connectivity, and a PCB designer lays out the circuitry to optimize electrical characteristics within the design constraints. After soldering the components onto the board and completing assembly testing and verification, the circuit board integrates into the system.

Taking a circuit board from start to finish has many intricate details that demand adherence. One area in this process that doesn’t get the recognition it deserves is the fabrication of the bare board. The design of a PCB receives a lot of attention due to the complex software tools used in its creation. Manufacturing will get equally lauded in the satisfaction of completing the final product. But somewhere in the middle of this lies the mysterious process of circuit board fabrication, often unknown outside of the design world.

To uncover the tenebrous origins of a raw circuit board and its preparation for electronic component assembly, we will answer the question: what is the PCB fabrication process? To get acquainted, we will scrutinize what it takes to construct a high-quality circuit board fab that a contract manufacturer can easily assemble.

PCB Fabrication Basic Requirements

PCB fabrication is building the raw board as the foundation for the final printed circuit board assembly. A checklist of design details is necessary before the fabrication vendor can make the unpopulated board. Here are some of the items that require addressing before beginning 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 design layout and 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

The initial design must be complete to ensure the accuracy of PCB manufacturing. Even a simple component update without synchronization between the schematic and the layout could result in a changed footprint and, thus, a potential need for rework or revision. Here are some design items to check off before finalizing the design and preparing for manufacturing:

Circuitry has gone through a full engineering review
Schematic and layout databases are updated and synchronized
All components are on the board, and no unrouted connections in the netlist
Optimization via circuit simulation, signal integrity, and power integrity analysis
PCB DRC with error correction (i.e., DRC should return no errors)
Bill of materials (BOM) review for current and available components
Design for manufacturing (DFM) rules employed for error-free assembly

Full Manufacturing Design Data

With the design complete and ready to build, 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, the contract manufacturer can order 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 selecting the fabrication vendor is whether or not the board is a prototype. If so, the fabrication vendor must have the capability to run a separate production line outside of regular processes. They’ll also need to conduct their design reviews using the Gerber data supplied by the PCB CM to maintain the highest quality in the prototype build.

After selecting a fabrication vendor, 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, the PCB fabricator will begin building the raw board. Many circuit fabrications, including single-sided boards, high-density multi-layer designs, and flex circuits, are available. Consider the fabrication steps for 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 are available. Two methods can transfer the image data to the board:

Photo Tooling: The standard imaging process in PCB fabrication that’s been in use as long as mass-produced circuit boards exist. A precision photoplotter will create the circuitry images on film, which acts as a template in the fabrication process for board image printing.
Direct imaging: A laser prints the circuitry images directly onto the circuit board, bypassing the need for photo tools. This approach 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.

The Layers of a Circuit Board

A multilayer circuit board is a composite of different layers of dielectric material and metal conductors. It comprises 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. Each layer requires strict control for 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:

A sheet of photoresist material covers the copper foil of the core.
Exposure of the photoresist via 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.
Etching of the copper layers of the core leaves only those circuitry areas protected by the polymerized photoresist.
Stripping of the photoresist, leaving behind only the copper circuitry.

When this process concludes, the core layers need inspection by an AOI system (automated optical inspection) 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.

Laminating the Layers Together

Layer pairs are stacked to create a PCB “sandwich.” Each layer pair will have a sheet of “prepreg” inserted 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.

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. Most of the through-holes used in a circuit board require plating for electrical conductivity; these drilled holes are 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 require drill-and-laminate cycles before the ultimate lamination of the board. The extra process steps for these vias add additional cost to the board’s fabrication but may be necessary for HDI or electrical performance.

After drilling, technicians clean holes using chemical and mechanical processes 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 conductive surface creates a metallic base for electroplating additional copper into the holes and onto the outer layers.

Making the Top and Bottom Layer Circuitry

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

A sheet of photoresist material covers the top and bottom surfaces of the board, including the drilled holes awaiting plating.
Photoresist exposure by UV or laser occurs, but contrary to the inner layers, all board surface areas lack photoresist coverage (except for the circuitry patterns).
After chemically cleaning the unexposed photoresist, the circuitry patterns of bare copper are electrically plated with more copper to build up their metal weight.
Next, tin plates onto the copper circuitry as an oxidative protective layer, and the photoresist is stripped off the remainder of the board in preparation for etching.
Etching removes all copper except for those areas of metal circuitry underneath the tin.
Removal of the tin leaves behind the finished plated copper pads, traces, and thru-holes; the printed circuitry is complete.

Solder Mask, Silkscreen, and Surface Finishes

A solder mask application is necessary to protect the board during assembly using a UV exposure process similar to 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. 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 process 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 leaves the solder bath, a high-pressure blast of hot air removes excess solder from the holes and smooths the solder on the surface metal.

Assembly Prep, Inspection, and Test

The final step of the PCB fabrication process is preparing the circuit board for assembly. If necessary, routing removes the circuit boards from their manufacturing panels, or they undergo breakout after assembly. Depanelization is done by 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.

PCB Fabrication and Assembly, Working Together to Build Your Circuit Board

Successfully manufacturing a high-quality circuit board requires a seamless partnership with your PCB contract manufacturer and the fabrication vendor they choose to build the bare boards. To ensure the expected levels of fabrication quality, the PCB CM has a list of criteria a fabrication vendor must pass to qualify. Here are some desirable attributes of a fabrication vendor:

They hold an ISO 9001 certification.
They are experienced in and have demonstrated proficiency with the type of PCB technology that is required.
They consistently deliver PCBs that measure up to the expected specifications and tolerances.
They can verify the design by providing cross-sections and a detailed first article inspection (FAI) report that measures all attributes of the board.
They are competitive on cost while achieving 100% on-time deliveries.

Local PCB contract manufacturers typically build a lot of quick-turn assemblies, so they need equally quick turn-around times on the bare boards they order. They’ll want to cultivate partnerships with local PCB fabricators that respond quickly and have a reputation for building quality prototype boards. PCB CMs will also have overseas fabricators for larger orders that are more favorable on cost but less flexible on their delivery schedule.

VSE Understands Circuit Board Fabrication

Building you a superior circuit board is our goal at VSE. We understand the pressures you face in electronics markets, and we’re prepared to develop your prototype, NPI, or production circuit boards on time and at the highest levels of quality. To that end, we have an advanced engineering team ready to help you on your project with the following:

Component sourcing and procurement.
Design engineering and PCB layout capabilities.
Bill of materials review with recommended alternatives.
Design for manufacturability review and suggested corrections.
Mechanical design with box build, wire harness, and cable assembly capabilities.

We also understand the importance of starting your PCB assembly with the highest quality bare board fabrications. We have an established group of local and overseas PCB fabricators to ensure we have the capacity for any project.

A Skilled and Experienced PCB Contract Manufacturer

We understand the PCB fabrication business because we have worked with many vendors worldwide. Over 40 years of experience working with a wide range of vendors has uniquely positioned us to help work through any concerns you may have about the fabrication side of PCB manufacturing and answer all your questions. We have the knowledge and experience to get your next board manufactured. Call us, and let us show you how we can partner to make your next PCB assembly successful.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.