Additive Process in PCB Manufacturing: Design Notes
Subtractive processes have defined printed circuit board fabrication for decades: copper is etched away and solder mask is applied, exposed and developed. An additive process builds the coating where it is needed instead of removing what is not. That change sounds like a manufacturing detail, but it alters several rules the designer has been applying by habit, particularly around the solder mask, and it is worth understanding before the first board is released to a shop that uses it.
What Changes for the Designer
In an additive process the material is deposited only at the locations that require it. The practical consequence is that the boundaries the design tool assumes are no longer the boundaries the process produces. Solder mask, in particular, has always been treated as a subtractive layer: the tool exports a mask artwork that assumes a continuous surface, and the shop works out the deviations. With inkjet deposition, the shape and the thickness of each mask feature are produced directly, which means the geometry the designer specifies is closer to the geometry that exists on the finished board. It also means the designer has more influence over the result than the artwork convention implies, and correspondingly more responsibility for specifying it.
Mask Boundaries Are Not What the Tool Assumes
An EDA tool exports a mask layer that represents openings, not the coating. Where the coating is applied by jetting, the design intent has to be interpreted in terms of deposited material rather than removed material, and a computer aided manufacturing vector file describes the deviation from an assumed continuous surface rather than the actual deposition. The consequence for the designer is that the mask layer must be reviewed as a set of features to be created, not only as a set of openings to be cleared, and any assumption inherited from a subtractive flow should be re-checked against the process being used.
Advantages of an Inkjet Solder Mask
The most immediate advantage is that no hole or via has to be filled or masked off. In conventional processing a via may be covered by mask even when nothing in the design asked for it, and the shop then has to deal with ink deposited on the printing table, either by scraping it away or by changing the table or its liner. Neither remedy is free, because both interrupt production. A related benefit is that leaving the via open is generally considered better for reliability, so a process that does not force mask into the via supports a design choice that was previously awkward to make. Because the coating is deposited only where required, the material consumption is lower, and the coating can be built as a mesh or an array rather than as a solid area, which reduces the ink further while still providing the support that a stencil frame needs. Finally, an inkjet process does not use developer chemistry, which simplifies the waste stream.
Declare Every Hole
Because an unmarked hole may still receive mask by default, every hole should be declared explicitly in the design data with its requirement. If a hole is intended to remain open, that should be stated; if it is intended to be covered, that should be stated too. Leaving the question to a default is what produces ink on the table and an unplanned interruption in the shop, and it also produces a board whose vias behave differently from the design intent. This is a small documentation habit with a measurable effect on delivery, and it belongs in the same set of habits as the general manufacturability rules described in this article on PCB design guidelines that keep a board manufacturable.
Solder Mask Dam Rules and the Numbers
The most demanding feature of any mask layer is the dam that separates two adjacent pads, because it is the narrowest structure the process has to form reliably. The dam geometry also expresses a choice between pad designs, and the traditional NSMD pad, where the mask opening is larger than the pad, is what the conventional dam rule assumes. The numbers are instructive. Two pads separated by 200 micrometres are unremarkable. The largest mask dam that can be produced between them depends on the imaging or deposition method: with a laser direct imaging process, the useful width is reduced by the beam width plus twice the alignment tolerance, which can bring an apparently generous 200 micrometre space down to about 100 micrometres of usable dam. Where conventional techniques are used, dams of 50 micrometres or less are difficult, which is why the rule exists and why it constrains fine-pitch layouts. An additive process attacks the problem differently, because the material is deposited by volume rather than defined by an exposure, and filling a narrow gap becomes a possibility rather than a limitation. That in turn questions the original choice between mask-defined and non-mask-defined pads and leads toward a pad-defined solder mask, in which the coating conforms to the pad geometry instead of being defined by an aperture.
Thickness, Conformality and Feature Size
Deposition also changes the relationship between thickness and shape. A thin coating can follow the surface closely while remaining flat where it matters, and a thinner mask allows smaller features to be produced. Conversely, a thicker mask or thicker copper raises the minimum feature size the process can hold. The designer therefore has a real trade-off to make rather than a fixed rule to obey, and the choice should follow the function the mask performs. That function is narrower than it first appears. Copper without mask will oxidize, while the laminate beneath it will not, and a high-voltage trace without mask may short where a bare laminate would not, so the laminate itself does not require mask. What the coating adds is convenience during processing and a surface that can support the stencil. Neither of those requirements demands a uniform maximum thickness, and an additive process can vary it where the design benefits.
The Profile of a Narrow Dam
The cross-section of a narrow deposited feature is rounded rather than rectangular, which is a change from the conventional expectation. A rounded profile has no sharp edge and no cavity, and both of those features improve mechanical and chemical stability: there is no thin edge to lift and no recess to trap process chemistry. The change also affects the way the mask behaves at the edges of a pad, where a rounded transition is generally easier to clean and to coat than a square step. For the designer this is mostly a change in expectation, since the artwork does not describe the profile and the shop controls it.
Layer-Based Mask Structures
Because the coating is built by deposition, additional geometry can be placed on top of the mask itself. The most useful applications are mechanical: a raised feature can support a component, and a defined wall can limit where a dispensed material flows. Both are structures that would otherwise require a separate part or a change to the stencil. Where a design uses dispensing or potting, a mask feature that contains the material can replace a dam that would otherwise have to be added in the mechanical design, and the materials considerations are described in this article on solder mask ink thixotropy. Where a via must remain open and unplated, the same freedom applies, and the comparison between the available via constructions is set out in this article on via in pad versus plated through.
Design Rule Check Items
Several checks belong to the fabrication side of the design rule check rather than to the electrical side, and they are usually performed in the computer aided manufacturing step. They still need to be understood by the designer, because they explain why a layout that looks acceptable may be queried. The minimum radius of a mask feature depends on the deposition equipment being used. The minimum gap that can be produced between two mask features is smaller than the minimum mask feature that can be printed, which is the opposite of the conventional assumption. Thinner mask permits smaller features, while thicker copper or thicker mask increases the minimum. Finally, the placement of the mask features relative to the pads, rather than the placement of the openings, is what determines whether the finished board matches the intent. The best outcome is achieved when the tool and the manufacturing software agree on where the mask is required at component level, because that is the information needed to control thickness locally and to avoid defects such as tombstoning.
FAQ
Why does an additive process change the solder mask rules? Because the coating is deposited where it is needed rather than defined by an aperture, so the geometry is created directly instead of being the residue of an exposure and development step.
Should a via be left open or covered? Open vias are generally considered better for reliability, and an additive process makes that choice easier because it does not require the hole to be filled or masked before coating.
What is the main documentation requirement? Declare every hole and every mask feature explicitly. Defaults that were harmless in a subtractive flow can produce ink on the machine table and an unplanned stoppage.
