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Creating custom solder mask and paste mask openings for a large thermal pad on QFN

Question asked by Red5 on Sep 14, 2011
Latest reply on Sep 15, 2011 by jmatthews

QFN packages have a large thermal pad in addition to the signal pins.  I have read a number of application notes that indicate a good strategy is to do the following:

1) Create the PCB land pad 1:1 with respect to the pad on the part

2) Use vias distributed on the pad to connect it to copper on other layers for better thermal performance

3) Use tented vias to avoid having the solder wick down into them

4) Create a number of small evenly spaced apertures in the paste mask to achieve 50% solder paste coverage on the pad.

 

Up to this point, I have been using the following solution:

1) Create the part footprint, but leave off the terminal for the thermal pad.

2) In the decal editor, arrange "copper" on the solder mask layer to create an opening for the pad that has the "tents" where I plan to place vias

3) In the decal editor, arrange "copper" dots on the paste mask layer to create the openings for the solder paste

4) Once the part is placed on the board, I create a copper pour under the part and associate it with the appropriate net to make the connection for the large thermal pad.

 

Here is an example:

footprint1sa.bmp

The green is the solder mask opening, and the blue is the paste mask opening (not 50% coverage on this part).

 

This method requires some vigilance, because the copper for the pad is not built into the part, and neither is the connection/net.  I can solve the connection/net problem by adding a terminal that is the same size as one of the paste mask dots and locating it in place of one of them.

 

So here is my question.  Is there a way to create a pad in the decal editor and create custom openings for the solder and paste mask for that pad instead of just having the masks shaped like the pad?  This would be ideal, since all the design information would be included in the part.

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