Using Wedges of Solder Paste for better manual reflow @scalarelectric #Manufacturing

cj at @scalarelectric posts on Twitter about using wedge-shaped solder-paste cutouts in their stencils:

For fine-pitch leaded components, I’m seeing tremendous success with Wedges™ for manual-paste/place reflow.

• Increased gap between paste shapes
• 50% reduction in total paste application
• Occupies same rectangular envelope as default paste shape
• Huge improvement in yield

I can see that it can increase the space between fine pitch pads. cj continues:

To get 50% coverage by way of EAGLE’s (design-wide) pad-shrinking rules would give you what’s shown on the right. Compared with 50% coverage by Wedges™ left. I do use pad-shrink almost everywhere, but for pads with high aspect ratios I think I prefer this bulkier shape.

If there was a way to set pad-shrink on a per-package basis (instead of per-design), I might do exactly that. But once you gotta go in and draw the art manually, the pad-shrink feature goes out the window.

cj continues: I tend not to have much of an issue with leadless packages using simpler paste art. The issue I see with leaded packages is that the high-aspect-ratio recommended footprints result in way too much solder going down, and all that extra solder ends up in undesirable places.

And I came up with the idea after building a proto with the default stencil artwork and getting an outrageous number of bridges.

This is that DF11, .5mm w/ Wedges, no mask between. No bridges, no opens.

cj later says: In my limited use of this technique so far (~150 joints), I’ve had zero shorts or other cause for rework. Compared to a previous attempt at assembling this design with the standard stencil artwork, where 5-10% of joints had issues.

I use it almost everywhere, but EAGLE’s “Perimeter Shave” pad-shrink technique struggles with high-aspect-ratio pads. To get (e.g.) 50% paste coverage you end up with paste apertures that are basically as long as your pad, but ~half as wide. Sometimes *very* narrow.

A final interesting discussion:

Boldport

Replying to @ScalarElectric

The circles are also interesting. Do you feel that you get better application of hand pasting application with the circles compared to rectangles? Intuitively I’d say that it should be more even…

cj @ scalar‏ @ScalarElectric

I’m not really sure, and in many cases I don’t think it really matters. I know in this particular case that EP is not doing a ton of heavy lifting. But theoretically, maybe circles are better? A smooshed circle stays a circle. But a smooshed rectangle?

cj @ scalar‏ @ScalarElectric

Also maybe corners something something solder paste something something stencil release?

Here’s probably the real reason: My CAD tool defines rectangles by 2 vertices, which is inconvenient for “center shape here, make it a size” thinking. The (X, Y, r) definition for circles aligns better with my design intent than (X1, Y1, X2, Y2) for rects so that’s what I used.

See the whole thread here.