Basic Design of Solder Mask

Common double-sided PCBs (Printed Circuit Boards) consist of three layers extending from the substrate surface: the copper layer, the soldermask layer, and the silkscreen layer. These layers are interconnected through plated through holes (PTH) in the drilling layer to achieve electrical connectivity between top and bottom layers.

Purpose of Solder Mask

1. Prevents moisture and the intrusion of various chemicals and electrolytes, which can cause oxidation and corrosion of the copper traces, compromising electrical performance.

2. Guards against external mechanical scratches, thus maintaining insulation between copper traces and preventing short circuits.

3. Prevents unintended solder connections during component soldering, avoiding short circuits.

4. Reduces the consumption of pad surface finish (e.g., ENIG, HASL) in non-solderable areas.

5. Enhances the aesthetics of the board by giving it various colors.

Solder Mask Design

Solder Mask, as the name implies, is not about preventing all soldering. Some novice engineers may mistakenly believe that patterns drawn on the solder mask layer causes areas to be not solderable. This understanding is incorrect. Solder Mask refers to the areas on the board where solder resist ink is applied. As it is a negative pattern, the areas with patterns on the solder mask layer are not coated with ink. To facilitate understanding, let's use a snowy landscape analogy:

Imagine a pavilion (A) as the solde mask layer. After a heavy snowfall, the ground below the pavilion (B) will not have snow (solder resist ink), while the area not covered by the pavilion (C) will be entirely covered in snow (solder resist ink). Using this analogy, we return to PCB design for solder mask:

1. Patterns on the copper layer represent the copper traces.

2. Patterns on the solder mask layer remove ink coverage.

3. Areas where the copper layer and solder mask layer overlap on the same side create exposed copper (solder-coated or gold-plated) regions.

Solder Mask Manufacturing

In the actual production process, after drilling and copper plating the board, unwanted copper areas are removed, leaving behind the required copper areas (the traces). The process of soldermask production then begins:

1. The etched copper traces undergo processes such as board scrubbing and acid washing to remove oxide and impurities, roughen the copper surface for better adhesion with solder resist ink.

2. The entire board is coated with solder resist ink, dried, and a solder mask film is placed over the board. Ultraviolet (UV) light exposure causes the solder resist ink to solidify where soldermask patterns are present, protecting the designated areas.

3. Subsequent development and cleaning remove any uncured ink, revealing the original copper surface. Tin or gold plating can then be applied in later steps.

Solder mask windows need to be larger than their associated pad sizes to tolerate possible alignment errors (generally about 0.1–0.2 mm larger overall, equivalent to 0.05–0.1 mm expansion on each side). This can lead to a change in the shape of some pads due to solder mask window placement, mainly in the following ways:

1. Areas where the relevant pad is stand-alone and has no connected traces: no change in pad shape.

2. Areas where the relevant pad is connected to traces: a short segment of the traces are exposed in addition to the pad.

3. Areas where the relevant pad is formed of a soldermask opening on a large copper plane: the pad shape is defined by the soldermask opening, and will therefore be slightly larger than designed due to soldermask expansion.

Important note: For special requirements demanding "exactly the same pad shape and size as designed," please design the soldermask size considering our manufacturing capabilities. When placing an order, make sure to specify: "Do not modify original solder mask size." Additionally, ensure to carefully review the production files. We adhere to IPC standards to control pad sizes within a range of ±20%.

Exciting News: To meet customer demands, we've incorporated advanced equipment for use in multilayer boards. The solder mask windows for multilayer boards are now 1:1 with the pads.

Difference Between Solder Paste and Solder Mask Layers

Solder Paste Layer: Used by stencil manufacturers to create stencils, ensuring precise application of solder paste onto component pads for subsequent SMT processing.

Solder Mask Layer: Used in PCB manufacturing, areas with patterns on the solder mask layer remain uncoated by solder resist, while areas without patterns are covered in solder resist ink.

Emphasis:

To have specific traces, copper areas, or pads remain uncoated by solder resist ink and coated with solder (or gold), you must add a soldermask layer pattern. Only the areas of the copper geometry overlapping with this soldermask pattern will have exposed copper and be treated with the selected surface finish (e.g., HASL, ENIG).

In contrast, paste layers are exclusively for stencil creation and have no relevance to PCB production. PCB review engineers do not handle or provide paste layer files.

Solder Mask Webs

For densely packed IC pads, to minimize the risk of solder bridging between adjacent IC pins during soldering, you can design solder mask webs (i.e., applying a layer of solder resist ink between two adjacent IC pads). In conjunction with our manufacturing process, solder mask webs can be designed under the following conditions:

Conclusion

The solder mask is applied as a negative pattern, meaning that areas with patterns on the solder mask layer are not coated with the protective ink. This allows for the desired copper traces and pads to be exposed for soldering.

During PCB manufacturing, the solder mask is applied to the board and solidified using UV light. It helps prevent moisture and chemicals from damaging the copper traces and provides insulation between them to avoid short circuits.

It's important to design the solder mask with consideration for the size of solder mask windows, which should be slightly larger than the associated pads to account for any alignment errors. This may slightly change the shape of some pads.

To summarize, the solder mask is a protective layer that prevents solder from adhering to unwanted areas on the PCB. It helps maintain the integrity and reliability of the circuit by protecting the copper traces and preventing short circuits.