Working with the Solder Mask Expansion Design Rule on a PCB in Altium Designer

Rule category: Mask

Rule classification: Unary

Summary

The shape that is created on the solder mask layer at each pad and via site is the pad or via shape (or hole), expanded or contracted radially by the amount specified by this rule.

Constraints

Default constraints for the Solder Mask Expansion rule.

• Expansion top - this constraint is used to specify the value applied to the initial pad/via shape (or hole) to obtain the final shape on the top solder mask layer.
• Expansion bottom - this constraint is used to specify the value applied to the initial pad/via shape (or hole) to obtain the final shape on the bottom solder mask layer.

Use the  button to toggle between single, or separate expansions for the top and bottom sides of the board. When linked ( ), specify a value for the expansion in the Expansion top field. The Expansion bottom field will automatically inherit that same value. When unlinked ( ), you are free to specify different values for expansion, since the Expansion bottom field will become available to you for editing.

• Solder Mask From The Hole Edge - use this constraint to determine the reference for the calculated mask expansion. When disabled, the perimeter of the object is used (the copper land edge for a pad or via). When enabled, the perimeter of the pad/via hole is used. For example, a 5mil Solder Mask Expansion applied to a 60mil diameter round pad will create a mask opening of 70mil (pad diameter + (2 x expansion)). If the reference is the hole edge, and the same pad had a hole diameter of 30mil, then the 70mil mask opening would be achieved by a 20mil expansion (hole diameter + (2 x expansion)).

How Duplicate Rule Contentions are Resolved

All rules are resolved by the priority setting. The system goes through the rules from highest to lowest priority and picks the first one whose scope expression matches the object(s) being checked.

Rule Application

During output generation.

Tips

1. Partial and complete tenting of pads and vias can be achieved by defining the appropriate value for the Expansion constraint:
   1. To partially tent a pad/via (covering the land area only) – if the expansion is from the land pattern perimeter, set the Expansion to a negative value that will close the mask right up to the pad/via hole. If the expansion is from the hole edge, simply set the Expansion to be 0.
   2. To completely tent a pad/via (covering the land and hole) – if the expansion is from the land pattern perimeter, set the Expansion to a negative value equal to, or greater than, the pad/via radius. If the expansion is from the hole edge, simply set the Expansion to be a negative value equal to, or greater than, the pad/via hole radius.
   3. To tent all pads/vias on a single layer, set the appropriate Expansion value and ensure that the scope of the rule (the Full Query) targets all pads/vias on the required layer.
   4. To completely tent all pads/vias in a design, in which varying pad/via sizes are defined, set the Expansion to a negative value equal to, or greater than, the largest pad/via radius.
2. Solder mask expansion can be defined for pads and vias on an individual basis. While browsing properties for a pad or via through the Properties panel, options are available to follow the expansion defined in the applicable design rule, or to override the rule and apply a specified expansion directly to the individual pad or via in question. You can also force complete tenting of the pad/via on the top and/or bottom, using the Tented options in the Solder Mask Expansion section of the panel. When these options are disabled, the pad/via has no solder mask opening on the top/bottom of the board, and is therefore tented.

3. Solder mask expansion can also be defined at the individual level for the following objects (through the Properties panel, when browsing the properties of a selected object): Track, Region, Fill, Arc. Options are available to follow the expansion defined in the applicable design rule, to override the rule and apply a specified expansion directly to the individual object in question, or to have no mask at all.