Maybe 40 years ago it was considered bad practice [1], but boards and epoxies are far better now. We do big outer-layer pours all the time and I've never seen a problem. It's *hard* to peel a big copper pour off the surface of a board. If you're really worried, stitch it with vias.
I haven't seen any serious curling lately, either. Boards seem to come out of multilayer lams pretty flat, regardless of the copper arrangement.
John
[1] "Bad practice" often refers to techno-urban legends.
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.
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
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.
John
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
David L. Jones schrieb:
AFAIR even pcbcart charges extra for non-green boards.
Philipp
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.
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 ( snipped-for-privacy@misty.com)
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.
I have found it is often significant - especially in cramped areas where the PCB faces something other than bare metal.
- Don Klipstein ( snipped-for-privacy@misty.com)
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 ( snipped-for-privacy@misty.com)
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 ( snipped-for-privacy@misty.com)
Yup, I was referring to gold plate more in regards to the "professional look" the OP is after.
Dave.
Yes they do. On a 100x100mm board 100qty I just priced 13% more for black solder mask compared to green which is not hugely significant if you are after a more professional look. Immersion gold finish was another 6% extra.
This would look pretty sucky in Green for instance:
Dave.
Gold boards, smobc+gold flash, sure are pretty. They look like jewelry. And solder just leaps onto them.
John
If this is critical, I'd do the experiment and measure the effect. I am assuming you know how to measure temperature with a forward biased diode on the chip in question. I know when we characterized "copper" heatsinks at Maxim, it was without soldermask simply because board grinders were used.
(I DID get the joke but..)
Is there no consideration for eye strain?
RL
If this is critical, I'd do the experiment and measure the effect. I am assuming you know how to measure temperature with a forward biased diode on the chip in question. I know when we characterized "copper" heatsinks at Maxim, it was without soldermask simply because board grinders were used.
Yes, I know how to measure temperature with a diode, and I occasionally also use Natsem's LM35. But the issue is not really critical. I'm mounting the ICs on a 3.5 degC/W extruded Al heatsink which alone should provide adequate cooling for the 10W (max.) loss. The copper plane is just extra insurance, but I wanted the info for future applications too.
This thread has received more response than I expected and it has been enlightening. Looking back, I realise that I should have been more careful in my choice of terms. When I spoke about radiation, I meant getting rid of the heat in general. It should have been obvious even to me that solder mask would enhance heat radiation rather than hinder it. What I was really concerned about was the effect of insulating the copper from contact with circulating air. It has been clearly pointed out that the thin layer of solder mask won't make much difference. Thanks, everyone.
I've read the responses, and am not 100% sure the solder mask increases radiation. Granted, some good arguments were made. The mask is an insulator. When you wear a jacket, does your body radiate heat better. Theory is fine, but theory always needs to be tested. Once you make your board, you could order one board without the mask. I've only done PCBs for prototyping and building a few units of something, so I can't vouch for how the real world does it. A bit OT, but I recall a PCB vendor telling me that some customers say the solder mask causes measurable leakage.
Anyway, theory is good, you test it, then when it doesn't work like you thought, you refine the theory. Newton thought he was pretty smart, but Einstein proved him wrong.
No, Newton is right in the limiting case where v->0 and very approximately right in the case where v
Here's one...
ftp://66.117.156.8/GoldT550.JPG
which is awfully cute in person.
John
mask
you
Hmm, that gives me an idea. That's awfully close to orange and navy (Auburn colors). ;-)
Have something to add? Share your thoughts — no account required.
Ask the community — no account required