Footprint (VQFN) with thermal vias in library

[noeldev]

Hello,

I am creating a new library with MCUs in both VQFN28 and VQFN32 packages. I've initially created a 3D package with a SMD thermal pad in the Package Generator before I noticed in the datasheet that thermal vias were required.

So here is what I've done:

- Open the footprint in the Footprint Editor
- Select the Group tool
- Select the large SMD for the thermal pad
- Click Delete an object under Quick Actions
- Select the the Top layer (1)
- Select the Polygon tool
- Click Solid option
- Set Grid Size to 1.1mm and polygon Width to 0.25mm (for a 2.45 x 2.45 center pad, 4x4x1 VQFN28)
- Set Grid Size to 1.5mm and polygon Width to 0.25mm (for a 3.25 x 3.25 center pad, 5x5x1 VQFN32)
- Draw the boundaries of the polygon (it is automatically filled)
- Select the tStop layer (29)
- Draw a similar polygon
- Select the Pad tool
- Select the Round shape
- Set Drill Width to 0.33mm as per datasheet data
- Set Grid Size to 0.6mm (for a 2.45 x 2.45 center pad, 4x4x1 VQFN28)
- Set Grid Size to 1.2mm (for a 3.25 x 3.25 center pad, 5x5x1 VQFN32)
- Place pads at the grid crossings within the boundaries of the polygon
- Place four (2 x 2) pads (for a 2.45 x 2.45 center pad, 4x4x1 VQFN28)
- Place nine (3 x 3) pads (for a 3.25 x 3.25 center pad, 5x5x1 VQFN32)
- the footprint is complete

- Open the Device Editor
- Click Connect
- Select pads P$1 to P$4 (or P$9 for VQFN32) in the Pad column
- Select GND in the Connection column
- Click Append
- Click OK to close

- Save changes

Questions:

1. Are the steps listed above correct and enough?
2. In the Footprint Editor, the Stop and Cream properties were checked (but not Thermals) for the original large SMD thermal pad. For the vias, the Thermals and Stop properties are checked for both Top and tStop layers, I cannot find any Cream properties for the polygons, is this expected?
3. The hatched rectangle on the tStop layer was larger than the red rectangle on the Top layer with the original large SMD thermal pad, should I make the polygon larger on the tStop layer as well? If yes, by how much?
4. Regarding the Silk screen, tDocu traces overlap the tPlace marks, does it matter or should I enlarge the marks on the tPlace layer (and move the dot that marks pad #1 as well)?

Thanks for reading and for any help that you can provide.

-Noël

[noeldev]

I've enlarged the polygons on layer (29) tStop by 0.2mm to fit the "circles" around the thermal vias.

I did notice that 0.2mm was the gap between the thermal pad on the layer (1) Top (datasheet dimensions) and the one on layer (29) tStop for footprints created by the Package Generator.

I still have the issue with the Cream property: Gerber soldermask looks correct:

But solderpaste is missing on the center thermal pads for both VQFN28 and VQFN32:

Your help would be appreciated. Thanks,

[noeldev]

What I've done is draw a polygon over the thermal center pad on layer 31 tCream, the same size as the one on layer 1 Top. The center pad has to be exposed so I'm still wondering if this is OK and if the drills will do the thermal job.

Could anyone please confirm if what I've done so far is correct? Thanks,