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Re: How do we envision Pad Stacks?
In Altium (and I suspect in other commercial tools) there are two
different types of pad behavior: when creating/editing/viewing
footprints, and when working on the board.
When editing footprints, you only have the concept of top, middle, and
bottom layers. You don't know the full board stackup. So, for a PTH
pad, you can set what you want to happen on *all* inner layers, and it
can be different from the top and bottom layers if you'd like. Altium
lets you have different shapes for each of these (maybe you want a
square pad on the top for identification purposes, but round on inner
layers). Altium doesn't let you do this for SMD pads, I guess because
it doesn't make sense to pre-assign a SMD pad to a particular inner
layer in the footprint editor when you don't know on what board
stackup it will be used.
When you place a footprint or free pad on the board, you can edit that
pad in the context of the board. This means things like moving a SMD
pad to a particular inner layer, or removing the pad from one or all
inner layers on a PTH pad.
There is a toggle for stackup mode in Altium: Simple,
Top-Middle-Bottom, and Full Stack. These give you one, three, or N
sets of pad size/shape options respectively, where N is your PCB
copper layer count.
Some SMD pads require different shapes for paste layers (I've never
seen mask be anything other than an offset, positive or negative, from
the copper shape)
On Thu, Jun 11, 2020 at 10:54 AM Jeff Young <jeff@xxxxxxxxx> wrote:
> I had been assuming that you could define a separate shape for each layer. Full flexibility, but time-consuming to edit (even with commands such as “push current layer to other layers”).
> Most users are looking to route traces on inner layers between tight pads. This is commonly done with the definition of a “capture pad”. The constraint on inner layers that have connecting tracks is simply not to have a butt joint between the track and the plated hole. A capture pad is therefore a circular pad with an annulus width of the drill wander plus 1 or 2 mils; other capture pad shapes get you nothing more.
> There were also a few requests for SMD pads on a single inner layer (or perhaps multiple) — but importantly not for different shapes on different layers.
> So one could imagine allowing the pad shape to be placed on any number of layers, and a capture annulus width for any other connected layers. While it doesn’t have same flexibility, it would be a lot easier to edit.
> Another possible complication is wanting a different shape for mask (or paste) layers. Is this ever required?
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