Paste mask

[Omicron]

When building prototyping boards using a stencil and reflow oven I'm still having a lot of bridges when I use fine pitch components (say 0.5mm pitch). One of the things I've realised I need to do is to make the openings in the stencil smaller for these footprints.

Normally in Altium they are the same size as the pads. I understand that most production houses prefer it that way as they post process the paste mask themselves. So that really means I can't change my library footprints because then I wouldn't be able to use them in case I ever need to send a board to such a production house. Fortunately Altium allows to shrink the paste mask via a rule. I'm a bit confused however on how to set this up.

My idea was to somehow "tag" the footprints in my library that I feel need shrinking and then set up paste mask expansion rules to only affect the footprints with a matching tag, that would allow me to define different categories of "shrinkage". If I need to send the board to a production house I'd simply have to disable these rules and it would all revert to normal. The question is how to actually do this, i.e. is there a good way I can "tag" my footprints in the library so I can filter a rule on the tag. It could be a naming convention or the use of a parameter etc.

Anyone ever done something similar?

[DerekG]

When building prototyping boards using a stencil and reflow oven I'm still having a lot of bridges when I use fine pitch components (say 0.5mm pitch). One of the things I've realised I need to do is to make the openings in the stencil smaller for these footprints.

Simply ask the stencil shop to close up the required pad openings by a chosen percentage.

Normally in Altium they are the same size as the pads.

Yes & no. Altium gives you the ability to change the paste mask & often the default is to make the openings a bit smaller than the pad dimensions. This prevents any pooling of solder paste around the edges of the pad.

Altium treats the paste layers as layers, just like your top & bottom copper layers. Just turn on the top solder paste layer (press "L" on your keyboard) & take a look.

If you wish to change the paste expansion (or contraction), right click on the pad (you may have to unlock it first) & adjust the paste expansion to what you desire. You can select multiple pads & make a "global edit" of all the pads that are selected. Do not do an update from the library after you make these changes, otherwise the footprints will be changed back to the default library footprints.

I understand that most production houses prefer it that way as they post process the paste mask themselves.

It may be that some production shops do this, but in my experience they simply reproduce what's on the Gerber that you supply them.

[IconicPCB]

I generally speaking reduce apertures in 0.1mm stainless shim to 90%, in 0.2mm shim to 70% for a consistent volume of applied paste.

0.2mm stencil if not particularly good for anything other than 0805 size component.

[T3sl4co1l]

Query the problem pads by size.  Ex.:

Rule: Paste Mask Expansion
Query: (AsMils(PadSizeX) < 10) AND (AsMils(PadSizeY) < 10)
Expansion -4 mils

(I forget what pad size fields are actually called, this is lucky if it works verbatim )

Tim

[T3sl4co1l]

Also, you can modify the pads on the PCB itself, but this leaves them different from the library so if you ever update the footprints, your change goes away.  Use this as a last resort kind of change.

Tim

[Omicron]

Query: (AsMils(PadSizeX) < 10) AND (AsMils(PadSizeY) < 10)
Expansion -4 mils

Cool! That is just what I was looking for! I'll give this a try...

[zapta]

When building prototyping boards using a stencil and reflow oven I'm still having a lot of bridges when I use fine pitch components (say 0.5mm pitch). One of the things I've realised I need to do is to make the openings in the stencil smaller for these footprints.

Another option is to use thinner stencils. OshStencils for example offer 3mil and 5mil.

[marshallh]

The secret is to only squeeze the paste down the stencil 1 time. You need to load up way more on paste to do this.

But the second time you squeegee over the same area, it smears the paste that was already there. (Even if the paste is shorted, as long as the volume is correct, it will reflow without shorts)