If this request is coming from a manufacturing engineer, it's doubtful
there is any concern about heatsinking effects when the assembly is
put into use.
If the soldermask opening is much larger than the pad, and the trace
width is a large percentage of the pad size, then there could be some
concern about paste wicking away, but this has been somewhat
lessened with the surface tension of lead free paste.
There is a small concern about unbalanced trace widths on 2 lead
components, but that would have to be drastic, with other contributing
factors.
But if his explanation is the "we've always done it that way", I would
bet "always" goes back to PTH and the advent of SMT. In PTH, the
solder transfers heat to the bottom side pad and lead, and when the
heat transfers to the top side, you get solder wicking. A large trace
would pull heat away, since the pad is the initial transfer point of the
heat. It took some people a while to realize that in reflow, the entire
board and part are heated - pads, traces, laminate, parts,
terminations. Once they get hot enough, solder flows. The trace that
used to pull heat away is now heated along with the pad. If anything,
a vast difference in the trace/pad size would cause the trace to get
hotter faster than the pad. But again, pretty insignificant.
Challenge him, you both get an education. (A lesson hammered home
to me by Henry Rowan in his inimitable style back when I was a novice
bus bar designer)
Pete
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