The solder mask opening is the exposed area of a pad or via left uncovered by the solder mask. Solder mask is a thin coating that protects the board and prevents solder from bridging the wrong connections. Fusion creates the opening by expanding outward from the pad area by the offset value. Solder mask geometry appears on layer 29 (SolderMaskTop) and layer 30 (SolderMaskBottom).

In Fusion, the solder mask opening is a design rule. When a pad or via does not meet the rule, Fusion either adjusts the opening or reports a violation, depending on how the opening is defined and the Auto Resize option.

Set the solder mask opening

The Parameter Value determines how Fusion sizes the opening. Select one of the following:

Option	Description
Absolute	Defines the opening offset as a single minimum value.
Calculated	Calculates the offset as a percentage of pad or via size, within an allowable minimum and maximum range. For non-circular pads, Fusion uses the smaller of width or height as the diameter.

Resize openings automatically with Auto Resize

The Auto Resize checkbox controls what happens when a solder mask offset falls outside the range the rule allows:

State	Behavior
On	Fusion adjusts the offset to the nearest valid value and reports the change in the Warnings tab of the DRC panel.
Off	Fusion keeps the offset you entered and reports the violation in the Errors tab of the DRC panel.

Tent small vias automatically with Auto Tent

The Auto Tent checkbox controls whether Fusion covers small vias with solder mask automatically. It applies to vias only, not to other pads:

State	Behavior
On	Fusion tents a via when its solder mask is set to Auto and its drill size is below the limit defined by the rule.
Off	Fusion does not tent any via automatically.

Set the solder mask on individual vias and pads

You can override the rule for a single via or pad in its command dialog or in the Inspector. Choose one of the following:

Option	Description
Auto	Uses the design rule to set the opening size, or tents small vias automatically.
Offset	Sets a custom opening size for the object.
Tented	Always covers the via or pad with solder mask.

How Fusion applies solder mask to pads and vias

Behavior depends on the solder mask property you set:

• Tented: Fusion does not generate solder mask for that object.
• Auto: Fusion calculates mask size from the rule. It first checks Auto Tent. When auto tenting is on and the drill diameter is below the rule limit, Fusion tents the via and does not generate solder mask. Otherwise, Absolute mode uses the rule Minimum size, and Calculated mode uses a percentage of pad or via size clamped to Minimum Offset and Maximum Offset. No DRC error or warning appears when Fusion calculates the size.
• Offset with a specific value: Fusion compares your value to the rule. Auto tenting does not apply. When the value meets the rule, no issue appears. When it violates the rule and Auto Resize is on, Fusion adjusts the value and reports a warning. When Auto Resize is off, Fusion keeps your value and reports an error.

Resolve solder mask violations

Fusion reports solder mask issues in the DRC panel:

• A warning appears when a design rule adjusts an opening beyond your specified value.
• A warning appears when a via is tented automatically.
• An error appears when an opening violates the rule and Auto Resize is off.

Select an entry to see its details and to locate the affected object. You can resolve a violation by changing the object's opening, editing the solder mask rule, or turning on Auto Resize.