Re: Green solder mask and heatsinking

On Sat, 1 Nov 2008 21:17:27 -0400, "NoSPAM" <unknown@xxxxxxxxxx>
wrote:


"krw" <krw@xxxxxxxxxxxxxxxx> wrote in message
news:MPG.2376a9442301364598a365@xxxxxxxxxxxxxxxxxxxxxx
In article <wm5Pk.7938$Dm5.1486@xxxxxxxxxxxx>, unknown@xxxxxxxxxx
says...

"Phil Hobbs" <pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote in message
news:490CD7E1.7080401@xxxxxxxxxxxxxxxxxxxxx
If you're solder-plating the copper anyway, there's no reason not to
open
the solder mask--the cost is no different, and the heat transfer will
be
very slightly better.

Large areas of unetched copper is considered bad practice, especially
with
Mil-Spec, because of the differing thermal expansion between the copper
laminate and the (typically fiberglass/epoxy) substrate. When large
areas
are needed because of shielding needs or a ground plane for a stripline,
the
etching pattern normally used is a grid or parallel traces which allow
for
thermal expansion without the copper lifting from the board during
reflow.
Unfortunately this means the loss of surface area for heat convection.

Interesting. Large pours are "normal" in some neighborhoods (I've
never done it) and certainly solid internal planes are the norm.
How do they get away with the differential thermal expansion
problem?

It depends on what you consider "large" is! I am generally talking about
areas around 35 x 35 mm, or about 2 square inches and up. It also depends
on just how reliable you want the board to be and whether rework or repair
is ever necessary. I suspect the modern designers are used to very fine
lines and more modern copper bonding technologies allowing them to ignore
differential thermal expansion. I do remember that most of the PCB CAD
programs I once used had the option of "cross hatching" large pours.

Microscopically, the copper foil contacting the substrate today is covered
with little "mushrooms" of copper grown electrolytically on the foil. When
used with a partially cured substrate (pre-preg), the high pressure curing
process locks the foil to the substrate quite tenaciously such that you
rarely see the copper lift from the substrate like you did 50 years ago.

That's called "black oxide" I think. Great for adhesion, truly rotten
for skin loss. Serious microwave boards use ultra-smooth copper, but
adhesion isn't as good.


I still have some old printed circuit boards in the junque box where every
through-hole is a small pad surrounded with a ring with only two small
traces to connect the center pad and the ring. These are quite old and I
suspect they were hand soldered.


Those are "thermals", to keep a plane from heatsinking a pad such as
to degrade a solder joint. With reflow soldering, or using a good
hand-soldering iron like a Metcal, there's no reason to do this.

But it's still commonly done, and many PCB layout programs default to
using thermal pads and vias when connecting to planes. I force "flood
over" when I do fast or thermally critical stuff, like power pads on
small chips.

John


.

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