How Much PCB Mask Expansion is Too Much?

 

What is Solder Mask Expansion?

Solder mask expansion refers to the enlargement of pad openings in the solder mask layer of a printed circuit board (PCB) compared to the actual copper pad size underneath. It creates a clearance between the solder mask edge and pad, exposing more of the copper pad.

<p align="center"> <img src="https://i.imgur.com/PhA6vJX.png" width="400"> </p>

The expansion distance or margin is the difference between the pad opening diameter in solder mask and the actual pad diameter in the copper layer. Typical expansion margins are 50-100 μm (0.002”-0.004”).

Why Use Mask Expansion?

Solder mask expansion has several benefits for PCB manufacturing and assembly:

• Increased solderable area - More pad area is exposed for soldering, allowing larger solder fillets and joint strength. This improves soldering yield.
• Reduced mask slivers - The separation from pad edges reduces risk of solder mask slivers, which can impair solder wetting.
• Relaxed alignment tolerances - The extra margin allows some misalignment between layers during PCB fabrication.
• Prevents wicking and shorts - The mask clearance from pads prevents solder wicking that could cause shorts.
• Improves inspection - The exposed copper pad area provides better optical contrast for automated inspection.
• Higher assembly yields - For reasons above, mask expansion improves assembly yields and reduces rework costs.

The right amount of expansion balances the benefits above with risks of excessive expansion.

Maximum Recommended Expansion

How much expansion is too much? Industry guidelines provide maximum recommended values:

• IPC-2221B - Up to 50% of minimum annular ring width
• IPC-7351B - Up to 50% of pad width or diameter

This limits expansion to avoid:

• Excessive pad reduction - Excessive expansion shrinks the mask-defined pad area too much.
• Risk of shorts - High expansion brings pad openings too close together, increasing shorting risk.
• Solder beading - Large openings relative to pads can lead to solder beading.
• Reduced masking protection - Exposing too much copper reduces the coverage benefits of solder mask.

As a rule of thumb, maximum expansion should be:

• Through-hole pads - 100 μm (0.004 inches)
• SMT pads - 50 μm (0.002 inches)

Factors Affecting Ideal Expansion

Several factors determine the ideal mask expansion settings:

Board Complexity

• High density designs - Allow less expansion due to tighter pad spacing. Expansion risks shorts.
• RF/microwave boards - Tighter impedance control limits expansion allowance.
• HDI boards - Very fine features restrict expansion margin.
• 6-layer or more - Via-in-pad density limits pad expansion allowance.
• 4-layer or less - Simpler designs have more space for expansion.

Board Aspect Ratio

• Thick boards - Allow more mask expansion since drilling smear is reduced.
• Thin boards - Smearing risks mean less expansion margin is advisable.

Solder Mask Type

• Liquid photoimageable - Allows the most expansion due to resolution capability.
• Dry film - Limited resolution means less expansion range.
• Inkjet solder mask - Expansion flexibility depends on materials and process used.

Pad Design

• Small pads - Require less expansion due to close spacing.
• Large pads - Enable greater expansion margins.
• Fine pitch pads - Must limit expansion to prevent bridging.

Component Types

• Fine pitch SMT - Require minimal expansion to control bridging risks.
• High pin count SMT - Enable larger expansions given pad spacing.
• Through-hole - Allow substantial expansion around pad edges.

Implementation Guidelines

To optimize expansion for a design:

• Specify expansions per IPC guidelines - Up to 50% of minimum annular ring width.
• Increase expansion on large through-hole pads versus SMT pads.
• Allow 50-100 μm expansion for most pad sizes and pitches.
• Use smaller expansions on designs with high-density routing or fine pitch pads.
• Work with the PCB vendor to simulate the effects of different expansion values based on your board design constraints. Determine the maximum workable values through prototyping.
• Check expansion settings along with solder mask type to ensure adequate openings for inspection points.
• Perform solderability testing to confirm sufficient exposed pad area for wetting with specified expansion values.
• Be consistent in applying expansion settings to the same pad definitions across the board layout.

With controlled experiments and standards as a guide, PCB designers can determine the optimal mask expansions for robust manufacturing and assembly.

Effects of Excessive Expansion

What happens when solder mask expansions exceed recommended limits? Several issues can occur:

Solder Mask Slivers

Excessive enlargement of pad openings raises risk of solder mask slivers along pad edges:

<p align="center"> <img src="https://i.imgur.com/Vnr3Vxl.png" width="300"> </p>

Slivers can impair solderability. They occur more frequently with thicker stencils due to wider openings.

Reduced Masking Protection

Too much expansion removes the protective masking around pads:

<p align="center"> <img src="https://i.imgur.com/gWBX6lY.png" width="250"> </p>

This exposes the PCB to contamination and oxidation, reducing assembly and field reliability.

Solder Beading

Over-expanded pad openings relative to actual pads can lead to solder beading:

<p align="center"> <img src="https://i.imgur.com/UFcS8FZ.png" width="300"> </p>

Excess solder volume concentrates around pad edges instead of spreading evenly.

Bridging Risks

Substantial expansions brings pad openings too close together:

<p align="center"> <img src="https://i.imgur.com/dVDJJNE.png" width="350"> </p>

Minimal spacing raises the likelihood of solder bridging between neighbouring pads during assembly.

Insufficient Annuular Rings

Large expansions shrink the annular ring width around plated through holes below acceptable limits:

<p align="center"> <img src="https://i.imgur.com/2J6TZ6T.png" width="300"> </p>

This risks reduced solder fillet strength and reliability.

Troubleshooting Assembly Issues

If assembly problems arise, examine mask expansion as a possible cause:

Solderability defects - Insufficient fillets may indicate openings are undersized. Increase expansions.

Solder beading - Overexpanded openings relative to pads cause beading. Reduce expansion.

Bridging defects - Excess expansion causes shorts between neighboring pads. Decrease to provide more spacing.

Mask slivers - Slivers along pad edges imply expansion is too great. Target more conservative values.

Solder wicking - Wicking between pads signals that mask insulation between pads is inadequate due to overexpansion.

Measuring expansion margins and correlating them to defect locations can pinpoint the problem areas. Adjust the expansion settings to prevent those defects in future board builds.

Solder Mask Expansion Process

During PCB fabrication, solder mask expansion is implemented as follows:

1. PCB layout - Pad and track geometries are defined in CAD, with nominal pad sizes. No expansion is applied at this stage.
2. Expansion settings - The designer defines global mask expansion settings for pad types in the CAM tool. These will enlarge pad sizes in solder mask openings.
3. Solder mask layout - The CAM tool automatically enlarges pad geometries per the expansion settings when creating the solder mask artwork.
4. Panel fabrication - Board fabrication builds up layers, including final solder mask patterning using the expanded pad sizes.

This achieves mask openings larger than actual pad sizes, providing the desired expansion margin.

Summary

• Solder mask expansion enhances solderability and reliability when used appropriately.
• Up to 50% of annular ring width or pad width/diameter is recommended.
• Allowable expansion varies based on board density, aspect ratio, mask type, and other factors.
• Excessive expansion can cause shorts, slivers, beading, and other defects.
• Work with your PCB vendor to prototype different settings and determine optimal expansion values.

With controlled experiments and adhering to standards, PCB designers can confidently apply mask expansions for robust, high-yielding PCB assemblies.

FAQs

What is the typical mask expansion value used?

A mask expansion of 50-100 μm (0.002-0.004 inches) is common for most PCB designs. Through-holes often use 100 μm while SMT pads employ 50 μm expansion.

Does greater expansion always improve soldering yield?

No, excessive expansion beyond recommended limits can actually increase risks of defects like solder beading, bridging, and mask slivers which reduce yield. Moderate expansion enhances yield.

Can different expansions be used on different pad types?

Yes, unique mask expansions can be specified for individual pad definitions in the PCB layout software. For example, setting higher expansion on through-hole versus SMT pads.

How does mask expansion affect annular ring size?

By enlarging the pad openings, expansion reduces the annular ring widths. Excessive expansion can shrink annular rings below IPC acceptable limits.

Can solder mask expansion correct registration errors?

Up to a point, the extra margin helps compensate for small layer-to-layer registration errors during PCB fabrication. But substantial misalignments still risk causing shorts.