Robison Michael R CNIN wrote:

> hello,
>
> well, thats a pretty dumb question, isn't it?  hahaha.
> but i'm serious.  i'm trying to reproduce a board which
> has smt top and bottom, and it appears that some of the
> smt pads overlap from top to bottom.
>
> so how do smt pads work?  on a thru-hole, they obviously
> go thru the entire board.  how far does a pad go thru as
> an smt?  obviously it has to at least burrow thru to any
> layer it runs on, but do top and bottom overlapping smt
> pads necessarily net together?  i'm confused.

No, they don't.  If you want any SMT pad to be connected
to anything, it needs either a trace on that outside copper
layer, or a via within the pad to connect to another layer.
(A trace connected to a via will also do it.)  Some solder reflow
processes can't handle the thermal sink of a via within the
pad, or the solder paste will be sucked into the via, depleting the
amount of solder for the component.  So, amny people do
not plave vias within the pads for manufacturing reasons.

So, you can pretty much treat the two sides of the board
as if they really were separate PC boards as far as SMT
components are concerned.  The only link between the
layers is through vias.

Jon

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