Copper traces need to be protected from oxidation and corrosion due to environmental exposure. The common method of protection is to cover as much of the board as possible in a solder mask.

Solder mask serves other functions as well: 

• Helps stop solder from bridging between pads 

• Stops the growth of metal whiskers sticking out of the PCB

• Reduces the amount of solder paste used

• Increases the breakdown voltage of the board dielectric material.

• Protects the circuit board from contaminants during handling

• Increases the shelf-life of the PCB.

The solder mask is mainly Liquid UV Photo-imageable Ink (LPI) with green, red, blue, yellow, white, black and purple colors, green should be the most commonly used one.

The cost for green, blue, red, blue, yellow, white and black are the same and don’t cost extra. Purple, matte green, matte black mask cost extra and sometimes there is a longer lead time. 

In fact, different colors have different specs. 

For standard PCBs:

For 1oz outer copper thickness, the spacing between IC pads is minimum 0.2 mm (8mil) for green color. The other colors require a minimum spacing of 0.22 mm (9mil). You have to pay extra to achieve 7mil and 9 mil respectively. 

For 2oz outer copper thickness, the spacing between IC pads is minimum 0.25 mm (9.8mil) for green color. The other colors require a minimum spacing of 0.27 mm (10.6mil). You have to pay extra for 8mil and 10mil. 

Types of solder mask DFM failures

Positive solder mask expansion is when the solder mask doesn’t cover adjacent copper traces or power planes due to misalignment or not enough overlap. 

Zero solder mask expansion is when there is no spacing or gap between the solder mask and the pad.

Negative solder mask expansion is when soldermask is covering a pad and shouldn’t be. 

Solder Mask Registrations: what we allow and what not

Soldermask registration on SMD pad

FYI: Soldermask may encroach on lands as long as the minimum annular ring requirements are maintained. On SMT-pads with a 1.25 mm pitch, encroachment is permitted on one side of land only and does not exceed 50 micron (2 mil). On SMT-pads with < 1,25 mm pitch, encroachment is permitted on one side of land only and does not exceed 25 micron (1 mil).

Sometimes negative mask registration over component landing pads is intentional. This is called solder mask defined pads and is common in BGA layouts. 

Solder mask defined pads (SMDP) are those where the mask opening is smaller than the copper pad such that the solder mask will define the size of the pad used in BGAs. Non-solder mask defined pads (NSMDP) include a gap between the pad and the mask. Component datasheets will usually call out SMDP dimensions if required. 

Other times when negative mask registration is required is when you’re using a solder mask to tent over vias. This is called ‘tented vias’. 

Tented vias is when the via is covered with solder masks. Unlike via filling, here only the annular ring is covered by the solder mask. Plugging or filling the vias is an extra step and can be conductive or nonconductive plugged.

Tenting vias is popular. The purpose of via tenting is to leave fewer exposed conductive pads on the PCB surface that could cause shorts. Also, tenting minimizes the likelihood of a via being damaged through exposure to the operating environment.

When designing a PCB, take a moment to inspect the solder mask layer and ensure that the mask is pulled away from SMT pads and through holes by 2-4mils. No more, no less, unless your reasons are deliberate.