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Green solder mask and heatsinking

P

pimpom

Jan 1, 1970
0
Does anyone know something or point me to a web resource about
the heat radiating/insulating property of green solder mask?

I'm making two PCBs for a custom-built project and want the
finished boards to have a professional look. I've done the screen
printing and etching. One of the PCBs has a large (~8 sq.in.)
plain copper area to supplement the separate finned heatsink. All
soldering will be done by hand and from experience, I find it
difficult to get a nice finish covering even a couple of sq.in.
with a thin film of solder by hand (for corrosion protection).

The alternative would be to cover the heatsink area with green
solder mask along with the rest of the PCB. But I would like to
be able to estimate the loss of heat radiation. I'm not really
expecting precise figures as there are many variables, but I'll
be grateful for some indication other than a wild guess.

(Note: Without going into details at this time, let me assure you
that I don't have the alternative of ordering the PCBs from a
manufacturer. I'm doing everything myself).

Thanks in advance for any help.
 
Does anyone know something or point me to a web resource about
the heat radiating/insulating property of green solder mask?

I'm making two PCBs for a custom-built project and want the
finished boards to have a professional look. I've done the screen
printing and etching. One of the PCBs has a large (~8 sq.in.)
plain copper area to supplement the separate finned heatsink. All
soldering will be done by hand and from experience, I find it
difficult to get a nice finish covering even a couple of sq.in.
with a thin film of solder by hand (for corrosion protection).

The alternative would be to cover the heatsink area with green
solder mask along with the rest of the PCB. But I would like to
be able to estimate the loss of heat radiation. I'm not really
expecting precise figures as there are many variables, but I'll
be grateful for some indication other than a wild guess.

(Note: Without going into details at this time, let me assure you
that I don't have the alternative of ordering the PCBs from a
manufacturer. I'm doing everything myself).

Thanks in advance for any help.

Loss of radiation will be miniscule. I always cover my gound planes/
heat blocks with solder mask.
 
P

pimpom

Jan 1, 1970
0
Loss of radiation will be miniscule. I always cover my gound
planes/
heat blocks with solder mask.

Thanks for the info and for the quick reply.
 
P

pimpom

Jan 1, 1970
0
Phil Hobbs said:
Heat *radiation* will be greatly improved, because the organic
solder mask is highly absorbing in the thermal IR (say 5-15
microns wavelength) and therefore is an efficient emitter.
(The second law of thermodynamics requires that the
absorptivity and emissivity of any surface be the same at all
wavelengths, because otherwise you could have heat
spontaneously flowing from cold to hot, which can't happen.)
Shiny metal is a very poor infrared radiator.

Heat transfer by convection, i.e. conduction to the air
followed by mass motion of the heated air, won't be changed
much, just as pcw1.cad says. (Puke one cad? What kind of name
is that?) The reason is that natural convection is so lousy
that a mil or two of plastic won't hurt it much.
Thanks for the confirmation. In that case, I wonder why many
manufacturers leave solder mask off ground planes that double as
heatsinks even when there's neither a clutter of solder points
nor a need to reinforce conductor cross-section with solder.

(The HS is for two 7812 regulators that together will dissipate
about 10W max. The copper plane will be thermally coupled to the
ICs via their ground leads and to a blackened extruded Al
heatsink via mounting bolts. I considered using a switched-mode
PSU but decided against it).
 
P

pimpom

Jan 1, 1970
0
Phil Hobbs said:
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.


I'd want to calculate the expected thermal drop before I used
lateral conduction in a thin foil to connect with a heat sink.
You might get an unpleasant surprise.
The regulators are mounted on the Al heatsink. The ground plane
is also thermally coupled to the heatsink by the bolts simply
because 1) it is mechanically convenient and 2) it supplements
the IC leads as a heat conduction path to the copper.
 
N

NoSPAM

Jan 1, 1970
0
Phil Hobbs said:
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.

Dr. Barry L. Ornitz WA4VZQ
 
K

krw

Jan 1, 1970
0
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?
 
Does anyone know something or point me to a web resource about
the heat radiating/insulating property of green solder mask?

I'm making two PCBs for a custom-built project and want the
finished boards to have a professional look. I've done the screen
printing and etching. One of the PCBs has a large (~8 sq.in.)
plain copper area to supplement the separate finned heatsink. All
soldering will be done by hand and from experience, I find it
difficult to get a nice finish covering even a couple of sq.in.
with a thin film of solder by hand (for corrosion protection).

The alternative would be to cover the heatsink area with green
solder mask along with the rest of the PCB. But I would like to
be able to estimate the loss of heat radiation. I'm not really
expecting precise figures as there are many variables, but I'll
be grateful for some indication other than a wild guess.

(Note: Without going into details at this time, let me assure you
that I don't have the alternative of ordering the PCBs from a
manufacturer. I'm doing everything myself).


CONFORMAL COATING WORKS FINE
 
N

NoSPAM

Jan 1, 1970
0
krw said:
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.

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.

Dr. Barry L. Ornitz
 
D

David L. Jones

Jan 1, 1970
0
pimpom said:
Does anyone know something or point me to a web resource about the heat
radiating/insulating property of green solder mask?

I'm making two PCBs for a custom-built project and want the finished
boards to have a professional look. I've done the screen printing and
etching. One of the PCBs has a large (~8 sq.in.) plain copper area to
supplement the separate finned heatsink. All soldering will be done by
hand and from experience, I find it difficult to get a nice finish
covering even a couple of sq.in. with a thin film of solder by hand (for
corrosion protection).

The alternative would be to cover the heatsink area with green solder mask
along with the rest of the PCB. But I would like to be able to estimate
the loss of heat radiation. I'm not really expecting precise figures as
there are many variables, but I'll be grateful for some indication other
than a wild guess.

(Note: Without going into details at this time, let me assure you that I
don't have the alternative of ordering the PCBs from a manufacturer. I'm
doing everything myself).

Thanks in advance for any help.

Get gold plating for your entire board. Many PCB fabs don't even charge you
extra for that.
Green solder mask is passe, there are many other colours that are much
better looking. Gloss black is nice.
A good cheap place to get your boards made is www.pcbcart.com

Dave.
 
D

dalai lamah

Jan 1, 1970
0
Un bel giorno pimpom digitò:
The alternative would be to cover the heatsink area with green
solder mask along with the rest of the PCB. But I would like to
be able to estimate the loss of heat radiation. I'm not really
expecting precise figures as there are many variables, but I'll
be grateful for some indication other than a wild guess.

For the typical PCB temperatures, heat radiation gives a very little
contribution anyway, it's almost all about transmission and convection. For
the heatsinks in free convection, there is a difference extimation in
thermal resistance less than 10% between the best radiation case (heatsink
painted in black) and the worst case (bare metal). In forced convection,
the difference is practically zero.

Probably if you use a dark solder mask you will get a very small, almost
unnoticeable improvement.
 
K

krw

Jan 1, 1970
0
It depends on what you consider "large" is!

In the case of planes, the size of a panel is fairly normal.
I am generally talking about
areas around 35 x 35 mm, or about 2 square inches and up.

That's tiny to be a hard restriction.
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.

How is "cross hatching" any different? I'd think you still be
pulling at the corners of the "hatch". In any case, planes aren't
"hatched". I *have* seen boards that are somewhat balanced on
top/bottom (equal number of planes, etc) to reduce warping.
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.

50 years? No, I didn't see much of that 50 years ago either. ;-)
A *lot* of things have changed in 50 years.
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.

Wasn't that to prevent delamination?
 
Well, it seems to me that talking about how good of a
heatsink soldermask makes is like talking about how
good of a parachute an umbrella makes.

If you have heat to dispose of, and the heatsink is
adequate, that's all there is to it. You should not be
depending on soldermask to do the job. The only
time we use on-board copper as a heatsink is in
cases where no heatsink is used and the device
generates only very minimal heat.

In those cases, the soldermask is opened, to allow
a direct metal-to-metal contact.

Jean

www.pcb-now.com
 
P

Philipp Klaus Krause

Jan 1, 1970
0
David said:
[...]
Green solder mask is passe, there are many other colours that are much
better looking. Gloss black is nice.
A good cheap place to get your boards made is www.pcbcart.com

AFAIR even pcbcart charges extra for non-green boards.

Philipp
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
Ridiculous you worry about corrosion on a copper square but not about IC pins. believe me they go first.
 
K

krw

Jan 1, 1970
0
Gold is about as bad as copper an an IR emitter, with emissivity as
close to zero as doesn't matter. It's generally microinches thick
anyhow. But radiation isn't usually a serious pcb cooling mechanism
anyhow.




We like yellow silk over blue mask.

ftp://66.117.156.8/DSC01786.JPG


We have a customer who decided to use red mask on "esd sensitive"
boards. Then their QC people argued that all boards are esd sensitive,
so now they are all red.

The manufacturing manager at my CPoE decided to change the solder
mask each board revision to make them easy to spot. White is
*ugly*. I think he gave up after that bad idea because they all
seem to be (not quite as ugly) green again.
 
D

Don Klipstein

Jan 1, 1970
0
Does anyone know something or point me to a web resource about
the heat radiating/insulating property of green solder mask?

I'm making two PCBs for a custom-built project and want the
finished boards to have a professional look. I've done the screen
printing and etching. One of the PCBs has a large (~8 sq.in.)
plain copper area to supplement the separate finned heatsink. All
soldering will be done by hand and from experience, I find it
difficult to get a nice finish covering even a couple of sq.in.
with a thin film of solder by hand (for corrosion protection).

The alternative would be to cover the heatsink area with green
solder mask along with the rest of the PCB. But I would like to
be able to estimate the loss of heat radiation.

I think solder mask over bare metal would be an improvement over bare
metal. So far in my experience, heat transfer from very warm bare metal
to the ambient is improved by having the metal covered by solder mask,
paint or tape. Bare metal radiates longwave infrared poorly, and coating
the metal with something else achieves a radiation improvement that
outweighs the thermal resistance of the coating.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
Gold is about as bad as copper an an IR emitter, with emissivity as
close to zero as doesn't matter.

Gold does indeed have less emissivity than copper, Gold is more
reflective of IR than copper - gold is used for IR mirrors, including
ones for thermal IR.
It's generally microinches thick anyhow. But radiation isn't usually a
serious pcb cooling mechanism anyhow.

I have found it is often significant - especially in cramped areas where
the PCB faces something other than bare metal.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
In <[email protected]m>,
Well, it seems to me that talking about how good of a
heatsink soldermask makes is like talking about how
good of a parachute an umbrella makes.

If you have heat to dispose of, and the heatsink is
adequate, that's all there is to it. You should not be
depending on soldermask to do the job. The only
time we use on-board copper as a heatsink is in
cases where no heatsink is used and the device
generates only very minimal heat.

In those cases, the soldermask is opened, to allow
a direct metal-to-metal contact.

I would agree with having the heat source touch the copper without
solder mask in between. But copper or tinned copper that is getting rid
of heat does that better with solder mask over it.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
Un bel giorno pimpom digitò:


For the typical PCB temperatures, heat radiation gives a very little
contribution anyway, it's almost all about transmission and convection. For
the heatsinks in free convection, there is a difference extimation in
thermal resistance less than 10% between the best radiation case (heatsink
painted in black) and the worst case (bare metal). In forced convection,
the difference is practically zero.

And when the PCB is in a cramped area where convection is poor and not
surrounded by bare metal that reflects the radiated heat back, the
difference gets to be more than 10%.

- Don Klipstein ([email protected])
 
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