Choosing the Right Solder Mask Clearance for Your PCB

Tempo Automation

June 26, 2018 , in Blog

I don’t know if I’d call myself a Trekkie, but I thoroughly enjoyed exploring the final frontier with Captain Kirk, Spock and the rest of the Enterprise crew. Watching Star Trek motivated me to learn more about Astronomy and Classical Physics and taught me about the importance of space. Lack of adequate space between celestial bodies, for instance, can lead to catastrophe. Although the consequences aren’t quite as severe with PCB layouts, adequately spacing elements on a PCB is very important.

Most of the space on a PCB is primarily occupied by the solder mask. Solder mask is essentially a resin-based covering that protects your PCB from contamination and oxidation from external sources, such as manual handling, manufacturing processing and environmental effects. Ideally, the solder mask would cover all areas of your board except for drill holes and footprint pads. But in practice, there exists a tolerance that limits how close the solder mask can be to surface elements. This tolerance is known as solder mask clearance and determining the right tolerance specifications for your PCB layout can significantly impact your board’s manufacturability. Let’s take a closer look at solder mask clearance, especially with respect to its impact on the PCB assembly process, to assist you in making the right choices for your PCB layout.

Solder Mask Clearance

PCB assembly, the process during which components are attached to your board, is probably the most important aspect of PCB manufacturing. During assembly, the components are secured by a soldering method, such as wave or laserjet. To ensure successful soldering, areas of the board that require solder, like component pads and vias, must be isolated from the areas that do not. This isolation is achieved with solder masking. The solder mask protects the board from corrosion and oxidation, and without it, the likelihood of premature PCB failure is significantly higher.

In some situations, it’s not possible or advisable to apply solder masking; for example, where the separation between pads is very small, where heat sinks are used, and when the pitch from a component is very fine or too close to drill holes. Areas where solder mask should not be applied are defined by solder mask clearance restrictions, which are defined by your contract manufacturer (CM). The primary reason for solder mask clearance restrictions is to provide adequate spacing or solder dams between surface elements that receive solder to prevent the formation of solder bridges. Solder bridges form when solder links conductive elements of the board that were not meant to be electrically connected. Solder bridges can result in short circuits or burned components.

Solder mask clearance is typically defined as a general separation recommendation, along with particular specifications defined according to the type of surface element(s) being isolated. Particular specifications apply to pads, which may be solder mask defined or non-solder mask defined, and drill holes, which may be plated through hole or non-plated through hole.

Types of Surface Mask Clearance Specification

  Drill Holes

  • Plated through hole (NTH) is a via that typically carries a signal or provides ground return.
  • Non-plated through hole (NPTH) is usually a mounting hole that does not carry current.

  Pads

  • Solder mask defined pads are pads where the solder mask opening is smaller than the pad itself.
  • Non-solder mask defined pads involve a gap between the pad and the solder mask. The gap is the solder mask clearance or relief. Less solder paste is required for non-solder mask defined pad connections.

Solder Mask Expansion

Most PCB design software packages allow you to set the distance between the solder mask and surface elements universally (for the entire board) or individually (for elements). It may also be possible to set both of these design parameters and allow the program to apply rules to determine the specification to apply to each case. This parameter is often referred to as the solder mask expansion and may be positive, zero or negative.

  • Positive solder mask expansion means there exists a distance between the pad outer perimeter and the end of the solder mask around it that is uncovered.
  • Zero solder mask expansion involves no space or “gap” between where the pad ends and the solder mask starts.
  • Negative solder mask expansion exists when the solder mask covers a portion of the pad. The value is the distance between the pad perimeter inward to where the solder mask stops.

In most cases, it would be ideal to have zero solder mask expansion, which provides the required solder dam and maximum board protection. However, there are cases where it may be better to have negative expansion, like when implementing via tenting, which involves covering a via to prevent solder from being drawn into the hole. Practically, it is best to specify either a positive or negative solder mask expansion that is manufacturable based on your design needs and aligns with your CM’s equipment capabilities.

How to Choose the Right Solder Mask Clearance for Your PCB

As with other tolerances for your PCB layout, your solder mask clearance should be tailored to your CM’s specifications. As we have seen, solder mask clearance is important to PCB assembly and, if inadequate, may lead to short circuits or even component destruction. Another important parameter that should be used as necessary to facilitate the PCB assembly process is solder mask expansion, which defines the separation between the solder mask and surface elements. In order to choose the right solder mask clearance for your PCB, you should consult your CM to ensure that your specifications fall within their equipment capabilities and requirements. It is also a good idea to consider the following:

Additional Considerations for
Choosing the Right Solder Mask Clearance for Your PCB
Solder Mask Clearance Considerations Why it is Important
Solder mask width If it’s too narrow, the solder mask may pop off during PCB assembly.
Solder mask color Different colors have different tolerance requirements.
Device sensitivity Highly sensitive devices can be disrupted by leakage currents through the solder mask.

By instituting these considerations as well as solar mask expansion into the DFM for your CM, you can optimize your usage of the space on your PCB.

Tempo‘s Custom PCB Manufacturing Service

  • Performs entire turnkey process in as fast as 3 days.
  • Emphasizes DFM to eliminate time-consuming back-and-forth design corrections.
  • Performs BOM verifications to reduce possibility of component obsolescence issues.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time.
  • Performs multiple automated inspections during assembly to ensure PCB quality for prototyping.
  • Provides support throughout the PCB manufacturing process, beginning with design.
  • Smooth transition from prototyping to production.

Tempo Automation is the industry leader in fast turnkey PCB manufacturing for custom PCB design. Tempo employs an open and transparent “white box” approach to PCB manufacturing where the designer and CM form a symbiotic relationship to minimize the turnaround time and maximize the quality of your boards.

An essential aspect of this relationship is making our capabilities transparent and accessible; this includes solder mask specifications that will comprise the basis for your DFM. You can also easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on choosing the right solder mask clearance for your PCB or incorporating that choice into your design, contact us.

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