2.3 Solder Mask, Silkscreen, Surface Finish

Once the copper etching is complete, the board is electrically functional but manufacturing-hostile. Bare copper oxidizes in seconds, and closely spaced pads act as magnets for solder bridges. The final three layers applied to a PCB—Solder Mask, Silkscreen, and Surface Finish—are not aesthetic choices. They are process control tools designed to preserve solderability and enable visual inspection. If these layers are poorly defined, the assembly line will generate defects regardless of machine precision.

Solder Mask: The Insulating Dam

Definition: The typically green polymer coating that covers the entire board, exposing only the pads required for soldering.

The Engineering Reality

Solder mask is "Liquid Photo-Imageable" (LPI). It is printed, exposed to UV light, and developed like a photograph. Its primary mechanical function is to act as a "dam" between pads. Liquid solder wants to flow together; the mask physically stops it.

• If the "solder mask web" (the strip of mask between two pads) is too thin (< 4 mil) → Then it will peel off during manufacturing, allowing solder to bridge the pins (Short Circuit).
• If the mask opening is shifted (misalignment) → Then it will cover a portion of the copper pad, preventing a solid solder joint.

Why Green?

Green solder mask has the best chemical properties and cures the fastest. It also offers the highest visual contrast for human inspection.

Pro-Tip: Avoid white or black solder mask for prototypes. They make visual trace inspection nearly impossible and reflect/absorb heat differently in the reflow oven.

Silkscreen: The Human Interface

Definition: The ink text printed on top of the solder mask. It provides the "Reference Designators" (R1, C3, U5), polarity markings, and branding.

The Engineering Reality

Silkscreen is for humans (inspectors and repair technicians). Machines do not read it. However, if applied incorrectly, it destroys solderability.

• If silkscreen ink is printed on top of a copper pad → Then the solder will not wet to the metal, causing an open circuit.
• Control: Most Fab Houses automatically "clip" (remove) silkscreen that overlaps with solder mask openings, but this can leave text illegible.
• Rule: Never place text under a component body. If the inspector cannot see the "C14" label after assembly, the label is useless for troubleshooting.

Surface Finish: The Critical Interface

Bare copper reacts with oxygen to form copper oxide, which is an electrical insulator. You cannot solder to oxide. A surface finish is a metallic or organic coating applied to the exposed copper to prevent oxidation until the moment of soldering.

Option A: HASL (Hot Air Solder Leveling)

• Process: The board is dipped in molten solder and blown flat with hot air knives.
• Result: The pads are pre-coated with a thick layer of solder.
• Pros: Extremely robust, long shelf life, cheapest option.
• Cons: Uneven Topography. The surface is bumpy.
• If you use HASL for fine-pitch parts (BGAs or QFNs) → Then the component will sit on a "hill" of solder, leading to misalignment or open circuits.

Option B: ENIG (Electroless Nickel Immersion Gold)

• Process: A chemical bath plates the copper with Nickel (barrier) and then Gold (protection).
• Result: A perfectly flat, gold-colored pad.
• Pros: Excellent planarity (flatness). Ideal for BGAs and fine-pitch components.
• Cons: Expensive.
• If you have components with legs closer than 0.5 mm → Then ENIG is the mandatory choice to ensure yield.

Final Checklist