DFM - Soldermask Coverage

There are no design rules in Altium to identify the soldermask coverage problem described below. 

Note the soldermask shown below is too close to a different net. Standard soldermask registration accuracy is +/- 3mils (75um).

You need to use fabrication CAM tools to identify these errors or set your copper to copper rules with clearances that account for soldermask registration accuracy.

Source: High Volume Overseas Contract Manufacturer.

"Soldermask alignment capability exposes traces and features."

"Bridging during the assembly process may occur if the circuit features that pass close to pads are not fully covered by soldermask or potential corrosion problem if lack of coverage exposes copper."

Suggested Solution from Contract Manufacturer:

"Move the circuit features away from the pad or move the pad away from the circuit features. And if the soldermask clearance is large enough then reduce the soldermask clearance. If all of them can not be modified, soldermask encroach on pad should be allowed in lieu of allowing exposed copper feature."

My Comments and Solutions:

The DFM tolerance in the CAM tool for this violation was 2.953mil.  

In Altium the minimum copper to copper clearance for 'ALL'  different nets was 0.15mm (5.905mil). And the soldermask swell was set for 4mils. so the math works out to:

5.095 minus 4 = 1.905  (Copper to Copper minus Soldermask Swell = Coverage)

Copper to Copper Rule minus soldermask swell = soldermask coverage with perfect soldermask registration.

However the soldermask registration accuracy (+/- 3mil) needs to be considered for the outer layers which are covered with soldermask. 

To allow for soldermask registration accuracy of +/- 3mil we would need a minimum copper to copper clearance rule of 7mil.

7 minus 4 = 3mils (Copper to Copper minus Soldermask Swell = Coverage).

For standard LPI soldermask processes the copper to copper rules for the outer layers should be set to least 7mils, preferably 8mils.

For fine pitch parts with pin to pin clearances equal to or less than the minimum copper to copper rule clearance you can gang the soldermask and create rooms with appropriate clearance rules.

That's It.