PCB circuit board: what to specify when you order bare boards

What a printed circuit board is, how a bare board is made, the specifications on a fab order that change price and yield, what belongs in the file package, and the defects buyers can prevent on paper.

Filed under Engineering and services

Short answer: A PCB (printed circuit board) is the flat laminated board that carries the copper tracks connecting an electronic product's parts. Before any part is soldered on, it is a bare board, and it is built to the specifications on your fabrication order: layer count, material, thickness, copper weight, the smallest trace, gap and drilled hole, surface finish, solder mask, the IPC class it must meet, any controlled impedance, and the electrical test. Those choices set the price, the lead time and how many boards pass. Most delays on a new board come from a file package that leaves something out, not from the factory.

If you are buying a PCB circuit board rather than designing one, you do not need to know how to route traces. You do need to know what the fabricator reads on your order, because every line on it either costs money or prevents a problem. This note covers the bare board only: what a printed circuit board is, how one is made, and which specifications change the quote. What happens after the board is finished, when parts are soldered on, is covered in SMT PCB assembly: what happens on the line and turnkey PCB assembly.

PCB vs PCBA: the bare board and the assembled board

A PCB is the bare board: layers of glass-fiber laminate with copper tracks, pads and plated holes, covered in solder mask and printed with reference markings. A PCBA (printed circuit board assembly) is the same board after the components are soldered on. Buyers often say "circuit board" for both, and quotes go wrong when the two get mixed up. Bare-board fabrication and assembly are different processes, often in different factories, with different inspection standards. When an assembler quotes "turnkey", it is usually buying the bare boards from a fabricator for you. It still pays to know what is on that fab order, because a bare-board defect cannot be fixed at assembly.

How a bare PCB is made

The process runs in a fixed order, and each step is a place where a specification on your order comes into play.

Inner layers and lamination. On a multilayer board, the inner copper layers are imaged and etched first, then inspected. They are stacked with sheets of resin-impregnated glass cloth (prepreg) and pressed. Sierra Circuits, a US fabricator, describes the step: "Lamination is the process of bonding copper foil, prepreg, and inner layer cores under controlled temperature and pressure to form a multilayer PCB." The order of copper and dielectric in that stack is the stackup, and it decides board thickness and, on high-speed designs, impedance.

Drilling and plating. Holes are drilled through the pressed panel, then copper is plated into the hole walls so they connect the layers. How small a hole the shop can drill and plate reliably, relative to board thickness, is one of the main cost drivers.

Outer layers, solder mask and silkscreen. The outer copper is imaged and etched. Solder mask, usually green, covers everything except the pads that will be soldered. The silkscreen adds reference designators and markings.

Surface finish. Exposed copper pads need protection until assembly. Sierra puts it plainly: "The base copper is highly susceptible to oxidation without a protective coating, so applying a surface finish is essential to prevent corrosion."

Electrical test and routing. Finally, every board is tested for continuity and isolation, then cut out of the panel. Sierra again: "E-test refers to bare board electrical testing. In this step, electrical probes are used to check for shorts, opens, and netlist integrity."

The specifications that set the price

Layer count. Two-layer boards are the cheapest. Each added pair of layers adds material, another lamination cycle and more alignment risk. Four layers is a common step up when a design needs a solid ground plane.

Board material and Tg. Most boards use FR-4, a glass-fiber epoxy laminate. Tg (the glass transition temperature) is the point where the resin starts to soften, and it matters because assembly reflow is hot. According to JLCPCB, "Standard FR-4 (Tg around 130 °C) is the cheapest and is used in most consumer PCBs." Boards that see several reflow passes, or run hot in service, are often specified with a high-Tg FR-4. Radio-frequency designs are a separate category: "Most RF/5G/Wi-Fi (>2 GHz) systems instead use Rogers, Duroid, PTFE, or ceramic substrates." Those materials cost far more than FR-4 and fewer shops stock them.

Thickness and copper weight. 1.6 mm (about 0.062 in) is the most common finished thickness. Copper is specified in ounces per square foot, and 1 oz is the most common weight. Heavier copper carries more current but needs wider gaps between tracks, and it costs more.

Smallest trace, gap and hole. The finest line width, the tightest spacing and the smallest drilled hole on the board decide which processes and which shops can build it. Blind and buried vias, microvias and via-in-pad each add process steps. If one corner of the layout needs a tight feature, the whole board is priced for it.

Surface finish. The finish affects solderability, flatness and how long boards can sit on a shelf before assembly:

• HASL (hot-air solder leveling), leaded or lead-free. Cheap and robust, but not flat. JLCPCB notes: "Uneven Surface Planarity: Solder thickness varies (typically 1–25 μm), making it unsuitable for fine-pitch components (e.g., BGA, QFN < 0.4mm pitch)."
• ENIG (electroless nickel immersion gold). Flat and long-lasting, the usual choice for fine-pitch parts: "ENIG provides an ultra-flat surface that prevents solder bridging, supports 3–4 reflow cycles, and offers a longer shelf life (12+ months) than HASL." It needs a well-controlled process, because the same guide warns of "Black Pad Risk: Potential nickel hyper-corrosion during immersion gold reaction if chemical bath parameters are not strictly maintained."
• OSP (organic solderability preservative). Flat and low-cost, but "OSP boards have a short shelf life of 3 to 6 months."
• Immersion silver, immersion tin and ENEPIG fill specific needs, such as wire bonding with ENEPIG.

Choose the finish for the parts on the board and for when it will be assembled. If bare boards will sit in a warehouse for months, the shelf life matters as much as the price.

Solder mask and silkscreen. Green is standard and usually the fastest. Other colors are possible but can add time. Fine-pitch parts may need solder mask "dams" between pads, which some low-cost processes cannot hold.

Controlled impedance. High-speed and RF traces need a set impedance, which the fabricator meets by adjusting trace width and dielectric thickness. It is verified on test coupons built into the panel. Sierra Circuits states: "Standard tolerance is plus or minus 10% ohms. If tighter tolerance is needed, we can deliver plus or minus 5% ohms." Put impedance requirements on the order. Do not leave them to be inferred from the layout.

IPC class: 2 or 3, and why it matters

The fabrication standard most buyers cite is IPC-6012 (performance of rigid boards), with IPC-A-600 showing what acceptable and rejectable boards look like. Both have been revised recently. The current IPC-A-600M listing says: "The document synchronizes to the acceptability requirements expressed in IPC-6012F and IPC-6013E. Supersedes IPC-A-600K." Name the revision on your order, for example "fabricate to IPC-6012 Class 2", instead of writing "IPC standard".

Class 2 covers most commercial and industrial products. Class 3 is for products where failure is not acceptable, such as medical, aerospace and some industrial controls. Sierra Circuits sums up the difference: "Class 3 allows no visual or functional defects, while class 2 permits limited cosmetic imperfections." It applies to hole and pad geometry too. Altium notes that "In the newer IPC-6012 standards, breakout is allowed for all but class 3 products." Class 3 costs more because the shop holds tighter process limits, inspects more, and rejects more boards. And it has to be specified at fabrication. As Sierra puts it, "That means a class 2 bare board cannot be deemed class 3 even after a perfect assembly and testing process."

What goes in the fab package

A fabricator builds from data, not from a picture of the board. A complete package usually includes:

• Gerber files (or ODB++ / IPC-2581) for every copper, mask, silkscreen and paste layer, plus the board outline.
• Drill files with a tool table, with plated and non-plated holes separated. NCAB Group, a PCB supplier, points out a common gap: "Contrary to popular belief, Gerber data does not include drilling files."
• A fabrication drawing. Advanced Circuits advises: "Provide a PDF fabrication drawing to communicate layer stackup, controlled impedance, tolerances, materials, and surface finish for your gerber pcb design."
• The stackup, the IPC class, impedance requirements, quantity, panel requirements and the test requirement.

Missing information does not stop the job outright. It produces an engineering query (EQ): the factory's CAM engineers send questions and wait for answers before they will build. NCAB reports how often that happens: "More than 30% of the data packs we receive contain elements or features that needs clarification." Each question waits on your answer, and with the time difference to Asia, one unanswered question can cost a working day. Advanced Circuits lists the usual culprits: "Common problems include missing board outline, incorrect film polarity, mixed units, unreferenced apertures, and drill files without tool tables."

For the assembly side of the package (BOM, centroid file, approved alternates), see PCBA: what to ask before you send Gerbers.

What goes wrong, and how to prevent it

Warpage. A bowed board will not sit flat in the assembly line's paste printer or oven. IPC-6012 sets limits on bow and twist, tighter for surface-mount boards (Eurocircuits explains the numbers). The usual cause is in the design: "Warping happens due to an imbalance or asymmetry of materials when the stack is heated up and cooled down" Balanced copper and a symmetrical stackup prevent most of it.

Finish past its shelf life. Boards with OSP or immersion tin that wait too long before assembly can solder poorly. Order quantities that will be assembled within the finish's shelf life, or choose ENIG.

Plating and hole defects. Thin or voided copper in holes can pass a quick check and fail later in heat cycling. This is where the IPC class and the fabricator's cross-section reports matter. For a new board, ask for a microsection on the first lot.

Untested boards. Always require 100% electrical test of bare boards against your netlist. A flying-probe tester needs no fixture and suits prototypes and small runs. A dedicated fixture tests faster but is a one-time cost, the kind of charge explained in what is NRE cost.

A first article nobody checks. Before releasing volume, measure the first boards against the drawing: thickness, hole sizes, outline, finish and markings. Our first article inspection note covers what that report should hold.

Where bare boards come from now

Most bare boards for US products are made in Asia, and the mix is shifting. China tariffs for US importers](/notes/offshore-sourcing-cost-down-what-moves-landed-cost/) walks through how they stack.

Whichever country builds your board, the order decides the result. A complete fab package, a named IPC class and a required electrical test do more for yield than any choice of supplier. If you want a second set of eyes on a board before it goes out for quotes, the electronics and PCBA steps page shows how JCH handles board programs.

Straight answers

What is a PCB circuit board?

A PCB (printed circuit board) is a flat board of layered glass-fiber laminate with copper tracks, pads and plated holes that connect an electronic product's components. Before components are soldered on it is a bare board; after assembly it is called a PCBA (printed circuit board assembly).

What is the difference between a PCB and a PCBA?

A PCB is the bare board made by a fabricator. A PCBA is the same board after an assembler has soldered the components onto it. They are different processes, often in different factories, with different inspection standards.

What should I specify when ordering bare PCBs?

Layer count, board material and Tg, finished thickness, copper weight, minimum trace, gap and hole size, surface finish, solder mask color, the IPC class (for example IPC-6012 Class 2), any controlled impedance, panel requirements, quantity and 100% electrical test. Put them on a fabrication drawing with the Gerber and drill files.

Which PCB surface finish should I choose?

Lead-free HASL is low-cost and robust but not flat, so it is a poor fit for fine-pitch parts. ENIG is flat and has a longer shelf life, and is the usual choice for fine-pitch and BGA parts. OSP is flat and cheap but has a short shelf life of a few months. Choose by the parts on the board and by how long boards will wait before assembly.

What is the difference between IPC Class 2 and Class 3?

Class 2 covers most commercial and industrial products and allows limited cosmetic imperfections. Class 3 is for products that must not fail, such as medical and aerospace electronics, and has tighter limits on holes, pads and plating. Class 3 costs more and must be specified at fabrication; a Class 2 bare board cannot be upgraded later.

How this note was made: researched and drafted with AI assistance, checked against the sources listed below and edited at JCH Design & Manufacturing.