PC Board Fabrication

Mouse Bites

Taken from this mentor graphics blog post.

0.5mm diameter holes with 1mm between holes gives 0.5mm of board between holes to hold the board within the panel. Having the centerline of the mouse bite holes 0.25mm above the milled board edge helps minimize cleanup after the boards are depanelized.

Placement of Breakaway Tabs

The space between the breakaway tabs shouldn’t exceed 10cm to help keep the boards rigid in the panel during assembly.

Plane Pull Back

The planes should be pulled back at least 1mm from the mouse bites. For best EMI/EMC performance all signals should have an unbroken plane underneath them, so any signal traces should also be at least 1mm away from the mouse bites.

When depanelizing there is a fair amount of flex stress that the boards go through. Components (especially chip resistors and capacitors) should be kept at least 2-3mm away from the mouse bites.

Paste Stencils

Paste stencils are something I do not have a lot of experience with and because of this I tend to follow the advice of others.

In general, you would want your paste stencils apertures to have rounded holes to help keep the paste from sticking to the stencil. Also, the specific size of the apertures, in particular the ratio of the aperture to the pad they are allowing paste for is a science unto itself. I was pointed to the SMTA for specifics. At the time of writing this (Dec 21, 2014) I have been using the general rules given by this document (copied locally for posterity) as a starting point.

Basic rules of thumb:

  • Round the corners of the paste apertures.
  • Reduce the size of the paste apertures by 1-2mil to prevent excess paste from being applied and causing shorting problems.

Eagle does not easily allow you to round the edges of the paste (“cream” in Eagle parlance) layers. I’ve submitted a feature request to them. Eagle’s DRC rules allow you to round pads but not the stop mask or cream layers.

Eagle does, however, allow you to easily adjust the stencil aperture size in the same DRC menu. Look under the “Masks” tab:

Here you can see that I’ve asked Eagle to make the cream apertures 95% of the pads they are covering, up to a maximum of 2mil. (I’m not really sure how to interpret the minimum/maximum but I find with those settings that my stencil apertures shrink back from the pads by 2mil on all sides.

Eagle doesn’t allow for some of the fancier stencil adjustments like reducing different amounts for the toe/heel of the pins or adjusting differently based on the size of the pad, but this seems to be good enough for most people.

Regardless of what you do, the stencil maker usually performs additional adjustments based on the stencil thickness and other factors.

picture58-1.png (21 KB) Andrew Kohlsmith, 12/21/2014 02:07 PM

picture58-2.png (7.37 KB) Andrew Kohlsmith, 12/21/2014 02:07 PM

picture58-3.png (13.4 KB) Andrew Kohlsmith, 12/21/2014 02:07 PM

picture858-1.png (44.1 KB) Andrew Kohlsmith, 12/21/2014 02:19 PM

carolinas_chapter_smta_feb11a.pdf (311 KB) Andrew Kohlsmith, 12/21/2014 02:20 PM

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