Printed circuit boards use a lacquer-like solder mask layer to cover base copper from oxidation while leaving metal pads and traces exposed at soldering points and interfaces. This solder mask expansion helps ensure a robust, reliable board.
But how much should pads/traces expand under the solder mask? The answer involves balancing manufacturing tolerances against solder bridging risks. We will analyze appropriate expansion margins and when too much overlap under solder mask jeopardizes reliability.
The Purpose and Function of Solder Mask
Before analyzing the nuances of mask openings relative to metal features underneath, we should understand why PCB fabrication applies solder mask in the first place.
Oxidation Protection - Solder mask coats copper traces, planes, and non-soldered surfaces to prevent air exposure leading to corroion over time. This maintains electrical connectivity and function.
Insulation - The mask layer also provides critical insulation between adjacent conductive features to prevent short circuits. Clearances are defined relative to solder mask boundaries.
Solder Control - The mask openings precisely define solderable regions limiting flow only where required for component pads/terminals. This avoids bridging solder between points intended to remain isolated.
Balancing all three of these core functions involves proper solder mask dimensional definition relative to board features below - exactly what mask expansions help control.
What is Solder Mask Expansion?
Solder mask expansion refers to how much the dielectric solder mask opening is intentionally made smaller than the metal pad or trace beneath it to ensure complete coverage even with process variance.
Ideally alignment would be exact between layers, but in reality fabrication stack up tolerances, registration errors, and etch variances prevent this. Expansion margins mitigate the risks of any metal exposure while providing leeway for mask opening size.
Where Does the Expansion Value Come From?
There are a few factors that influence appropriate solder mask expansion values when hatching board fabrication. First we must consider planned solder mask thickness relative to metal below:
1. Solder Mask Thickness - Typical 30-60um mask deposit creates a vertical wall relative to copper. Expansion accounts for wall spacing around pads/traces.
2. Copper Etching Tolerances - Uncertainty in Cu etch consistency leadings to trace width/pad diameter variances requires added expansion to cover potential undercut areas.
3. Registration Variance - Alignment between layers is not exact - typical +/- 50um registration error must be covered by expansions.
When balancing all elements of fabrication capability, common default expansion settings range from 50-100um extra solder mask clearance around board features. This ensures robust coverage.
Issues from Insufficient or Excessive Expansion
If expansions are too small, solder mask coverage fails leading to brittle, unreliable PCBs over time in operation. On the other hand, expansions that are too large infringe on design rules leading to manufacturing escapes.
Problems with Too Little Expansion
Exposed Copper - Mask openings become smaller than copper below leading to oxidation, corrosion, insulation failures, and breaches over time degrading board reliability:
Violating Design Rules - Spacing rules defined from solder mask edges no longer reflect actual distances between metal conductors without adequate built-in expansion margins in openings. Electrical constraints may be silently violated risking failure.
Dangers with Excessive Expansion
On the other hand, overly generous mask expansions begin blocking areas that must remain exposed for component terminations according to the reference design rules:
Bridging Risk - Huge mask expansions can fully bridge the gap between adjacent pads creating what functionally acts as larger solder pools shorting together points that should remain isolated according to nominal clearance design rules:
Component Fit Issues - Many electrical specs like pitch, lateral spacing, etc are defined from metal edges. Mask expansions shrinking opening diameters down smaller than expected metal landing areas impacts component seating, footprint tolerances and attachments.
Real-World Example of Expansion Gone Wrong
A cautionary tale of excess mask expansion interference comes from Nintendo's recent troubles manufacturing the analog joysticks on their Switch gaming systems.
Joystick contacts relied on precise openings to solder ribbon cable leads, but masking agent crept into the gaps:
This lead to immediate production line failures and long term fragile connections prone to failure after a short usage lifespan for customers.
Clearly for ultra high volume manufacturing, insufficient planning for mask expansions risks tremendous unnecessary fallout!
Finding the Optimal Expansion Balance
We need enough margin to absorb fabrication variances in alignment, layer thickness, and registration without fully bridging design rule gaps. How do we balance this?
1. Compute Minimum Clearances
First, calculate the tightest clearance gaps between metal features based on voltage isolation constraints, insulation needs and sizing for the manufacturing process capabilities selected. This may require multiple design rules for different areas.
2. Define Expansion Values
With minimum clearances set appropriately, define a solder mask expansion rule that maintains at least 75-80% of that clearance gap to leave some safety margin for registration errors and randomness avoiding bridging risks.
3. Simulation and Testing
Model the stackup in a field solver at higher precision to visualize electric field intensities, flux linkages, and termination behaviors with the chosen expansions to validate insulation reliability over environmental ranges for the application.
Through these best practices, optimal expansions can help boards pass testing while averting dangerous manufacturing defect escapes!
Intelligent Solder Mask Expansion Settings by Area
One size does not necessarily fit all cases on a complex PCB with diverse components and density requirements. Intelligent assignment of expansion rules aids reliability.
Higher Expansion for Dense Regions
In ultra fine pitch high density sections, larger expansions help avoid bridging:
Lower Expansion for Component Terminations
But around component pads/leads/balls, only small expansions maintain attachment area:
Other Factors Influencing Expansion
- Via tube gap risks
- Connector spacing
- Test point clearances
- Conformal coating
- Thermal dissipation needs
In summary, an intelligent designer defines multiple solder mask expansion rules adapted across the PCB layout zones balancing clearance reliability and manufacturability.
FAQ
Q: What is the main purpose of solder mask expansion?
A: Absorbing fabrication process variances and registration errors to prevent exposed copper while avoiding bridging adjacent pads together.
Q: How much expansion margin is typically used?
A: A 50 to 100 um expansion range is common, maintaining 75%+ of underlying clearance gaps.
Q: When does too little expansion cause issues?
A: Insufficient expansion leads to oxidation, corrosion, insulation failures over time plus silently violates electrical spacing rules.
Q: What problems can happen with excessive expansion?
A: Huge expansions can fully bridge component landing areas intended to be isolated, shrinking attachment pads.
Conclusion
Carefully crafted solder mask expansions which balance tolerance absorption against spacing risks enable robust, reliable PCB fabrication tolerant of process variances while averting dangerous defect escapes through both insufficient and excessive masking.
Intelligent selective application of expansion rules across layout zones playing off design clearances unlocks optimized manufacturability.
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