JoAnn Amerson wrote:
> I was told that it is best to limit the number of tracks leading
> to/from a pad to one.  Two at the most and three is highly
> discouraged.  Can anyone verify this for me?  Are there any specs
> that apply?  I asked a fellow designer and he'd never heard it
> before either.

Well, I thought you would get some good answers so I wasn't gonna
jump on this one, but so far you have recieved some confusing and
conflicting messages and I kinda feel sorry for you...

I'm no expert (just a lowly layout guy) but the way I see it:

I agree that you should TRY to limit to one connection per pad for
SMT connections, but its not so important for through hole.
There are several reasons, and I'm not sure which would be most
important, but consider:

1) There is a limited amount of solder paste squeegeed (sp?) onto
each pad. As the solder melts, it will tend to "bleed out" onto
the trace that is attached to it. The more connections you have,
the less volume of solder fillet is connecting the lead to the
pad. You want all the paste to stay between the pad and the lead,
the more the better in the case of SMT, because besides being an
electrcal connection, its all that is holding the part on the
board in most cases (a mechanical connection). The soldermask
itself will help, naturally, but don't give that limited amount of
solder too many places to go, ok?

2) Imagine what the solder is going to do when it melts. It has a
"surface tension" like a drop of water. If there are NO traces on
the pad you will see a beautiful perfectly formed solder fillet,
you can see this if you look at the pins of a component that is
not connected (many ICs have unconnected pins). Pins that have one
trace coming out the end are nice too, especially if the trace is
slightly smaller than the pad width, the fillet is symmetrical. I
have been told that symetrical fillets are preferred for:

 a) mechanical strength
 b) will tend to "center" the component on the pad

Now imagine one trace coming out one side of a pad, and another
trace connecting at an angle into an opposite corner of the same
pad. Common sense seems to suggest that the solder fillet that
results will not be "ideal", right?

3) Whoever said it is thermal related, I would like to see some
data for this. From my (limited) understanding, a reflow oven
heats up the entire board and all the components to a temperature
hot enough that any solder anywhere will flow. Therefore, there is
no real "heatsinking" going on, right? Its ALL hot. But i still
agree that three traces to an SMT is BAD, I just agree on the
reason. (Please correct me if I'm wrong)

4) You also must consider the current in some cases. If you need 8
amps, and you know how much copper you need to carry 8 amps, too
bad if its hard to solder, you NEED the copper. But on the other
hand, we have done boards with 4 ground planes, and some pins have
to connect to ALL of them (through-hole pins). In this case you
DON'T use thermal wagon wheels with 25 mil spokes, you can barely
hand solder it, and I would bet you would damage the board
material integrity in the process. You be surprised how much
current four 8mil spokes can handle... just a thought.

I can't remember the other strange answers you got, so I'm done.
Don't take everything you hear on faith, TEST or ASK.
Good Luck,
                                                             Jack