Choosing PCB specs: layers, thickness, copper, surface finish and solder mask

A Gerber file describes the shapes. It does not, by itself, tell the fabrication partner how thick the board is, how heavy the copper is, or whether the pads should be tinned or plated with gold. Those are specifications you choose, and they belong in the same message as the zip. If you leave them out, someone will assume a common stack, and that assumption may not match your enclosure or your fine-pitch IC.

Bare boards are made through our manufacturing partners. We source and deliver the parts when you need parts as well, and the board you approved is what the quote describes. You get a single GST invoice. The price you accept has no hidden charges. Start from PCB fabrication or the instant quote page, and read the fabrication terms for what happens when a file changes.

Layers

Count copper layers, not Gerber files. Mask, silk, paste and outline are not copper.

One copper layer is enough when the circuit is simple and you do not need a ground plane. If the layout only closes with jumper wires, a second layer is usually the cleaner board. Two layers suit most microcontroller boards: parts on top, a ground pour on the bottom, as long as the routing finishes and the signals do not need a solid plane under every track.

Four layers is the step when two layers have become a puzzle, or when you want a continuous ground and a power plane. Know which inner layer is ground before you mention impedance. Six and above are for boards that have run out of room. Say the count in the first line. Delete unused inner files from the zip, or a two-layer design can be built as four.

Thickness

Finished thickness of 1.6 mm is the ordinary board. Connectors, card guides and standoffs are often drawn around that number. Choose it unless you have a reason not to.

Use 1.0 mm or 0.8 mm when the enclosure or a stacking connector demands it. Thin boards flex, so a connector or a heavy inductor needs mechanical support. Say whether your number is finished thickness or only the core.

Go thicker than 1.6 mm when the board is structural or carries heavy connectors. A small drill in a thick board is a fabrication question. Mention both numbers. If you need a target impedance, write it and name the tracks. Otherwise say "standard stackup" and do not copy an impedance note from another project.

Copper weight

One ounce outer copper is the usual start. It is what most footprints and track-width calculators assume until you change them. Choose heavier copper when a track carries current that a normal pour will not handle at the temperature rise you can accept. When you do, tell us the current and the track width you used, so the person reviewing the file can see the same problem you saw.

Inner layers on a multilayer board are often a lighter copper than the outers. If your power path depends on a heavy inner layer, write that down. Do not assume the inner plane is one ounce because the outer is.

Very heavy copper changes the way fine gaps etch. A board that is both a power board and a fine-pitch digital board is a compromise. If you need both, say so at the start. It may be a four-layer stack with the heavy current on its own layer, rather than two ounces everywhere under a 0.4 mm pitch package.

The Gerber does not store copper weight reliably. The text file in the zip does, if you write it there. See how to prepare the Gerber pack.

Surface finish: HASL and ENIG

The surface finish is the coating on the exposed pads. It is what you solder to. It is not the solder mask.

HASL covers the pad with solder and levels it. The surface is slightly domed. That is fine for ordinary parts and less kind to fine-pitch ICs or pads that must sit flat. Name lead-free HASL when the product should not use leaded solder and flatness does not matter.

ENIG is a flat gold-over-nickel finish. Ask for it for fine-pitch packages, pads that must be planar, or boards that will sit before assembly. ENIG is not hard gold. Edge fingers that mate thousands of times need a finish named for that wear. Say "gold fingers" if that is what you mean.

If a datasheet demands another finish by name, copy that name. Otherwise pick HASL or ENIG. Ask for both prices if you are undecided, then approve one before fabrication. There are no hidden charges for the finish you approved, and it will not be swapped afterwards without a question to you. Board, stencil and assembly are a separate choice: PCB vs PCBA vs SMT stencil.

Solder mask and silkscreen

Solder mask keeps solder off the tracks. Colour does not change the copper. Green is the ordinary choice. Other colours are cosmetic. They do not make the circuit better.

Openings do matter. Mask should open on pads you will solder and cover tracks you will not. A sliver between fine-pitch pins reduces bridges. A missing opening means that pad will not solder. Check the finest IC before you send the file.

Vias can be tented or left open. Tented vias stay out of the paste. Open vias are easier to probe. Say which you want. Via-in-pad and vias tight against a fine-pitch part are special and should be named on the quote. The terms list several of these.

Silkscreen is ink for designators, polarity and the revision. Keep it off the pads. Put the revision on the silk or the copper so a failed board can be identified later.

What to write in the order message

Copy this list, fill it in, and put the same words in a text file inside the zip:

• Layers (1, 2, 4, 6, or whatever you designed)
• Finished thickness in millimetres
• Outer copper, and inner copper if it is not a normal stack
• Surface finish: lead-free HASL, leaded HASL, or ENIG
• Solder mask colour, and whether vias are tented
• Silkscreen colour, and the revision mark
• Quantity of single boards, or a panel you have already agreed
• Anything special: impedance, castellated edges, gold fingers, via-in-pad

Then say bare boards, boards plus stencil, or PCBA. Attach the BOM and centroid if parts will be fitted. A footprint change from an out-of-stock swap is a new Gerber. See alternatives for out-of-stock components.

If the stack cannot be built as drawn, you get an engineering question before production. Answer it in writing. The board is not changed quietly.

Common questions

Is 1.6 mm and one ounce always the right default?

Only when your drawing and your current agree. Write the thickness and copper you designed, including a thinner enclosure or a power track you already calculated.

Should I always choose ENIG?

No. Use ENIG for flat or fine-pitch pads, or when boards will sit before assembly. Use lead-free HASL for ordinary boards that do not need a flat finish. Approve one. That finish is what the single GST invoice covers. There are no hidden charges for a finish nobody mentioned.

Does mask colour change the circuit?

No. If a colour is a different quote line, you will see it before you pay. If colour does not matter, choose green.

Can you pick the specs if I only send Gerber?

We can quote a normal stack if you accept it in writing. We will not invent impedance, heavy copper or ENIG. Fill in the list above, even where a line says "your normal choice". The manufacturing partner builds that reply.