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.

• Use Separate Solder Mask Expansion - enable this option to specify separate expansions for the top and bottom sides of the board. Leave this option disabled to specify a single value for expansion, that is applied to both sides of the board.
  • Expansion – this constraint is presented when the Use Separate Solder Mask Expansion option is disabled. Use it to specify the value applied to the initial pad/via shape (or hole) to obtain the final shape on the solder mask layer (on both sides of the board).
  • Expansion Top - this constraint is presented when the Use Separate Solder Mask Expansion option is enabled. Use it 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 presented when the Use Separate Solder Mask Expansion option is enabled. Use it to specify the value applied to the initial pad/via shape (or hole) to obtain the final shape on the bottom solder mask layer.

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. The reference for the calculated mask expansion can either be the perimeter of the object - the copper land edge for a pad or via - or the perimeter of the pad/via hole. This is determined through use of the Solder Mask From The Hole Edge option, which is part of the configuration properties for pads and vias. 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)).
2. The significance of the Solder Mask From The Hole Edge option is that when selected, the Solder Mask opening will follow the shape of the pad or via hole. The mask is therefore independent of pad shape and size, and is scaled from both the hole size and shape. So for example, a pad/via with a square hole will create a square mask opening that matches the hole dimensions, plus the assigned expansion value – as applied by the assigned pad/via expansion design rule. Also note that a pad or via's expansion mask opening size will track any changes in the hole size.
3. Note that the applicable Solder Mask Expansion Rule will be applied from the pad/via hole perimeter rather than its land pattern perimeter, so an existing rule may lead to unexpected results. To cater for this, an additional Solder Mask Expansion rule can be created using the query expression HasSolderMaskFromHoleEdge = True. If the expression HasSolderMaskFromHoleEdge = False is also added to the existing rule, separate expansion values will be applied to the two Solder Mask Expansion configurations (expansion from pad/via land edge, and expansion from pad/via hole edge).
4. 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.
5. Solder mask expansion can be defined for pads and vias on an individual basis, in the associated properties dialog. 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. Options are also available to force complete tenting of the pad/via on the top and/or bottom.
6. Solder mask expansion can also be defined at the individual level for the following objects (through their associated properties dialogs): 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.