PCB soldermask question

[semaphore]

Many times I see people leave out the soldermask on the relay or power supply traces. The rest of the board is cover up with soldermask. What is the reason for this? Only thing I could think of is because it help with heat transfer. Or maybe it has something to do with high voltage? why? Why? WHY?

http://www.sure-electronics.net/mcu,display/DE-DD210_3_b.jpg

http://cgi.ebay.com/Two-Relay-Module-Board-8051-AVR-PIC-Project-12V-/130505865568?pt=LH_DefaultDomain_0&hash=item1e62c17960

http://www.sure-electronics.net/mcu,display/PS-SP12118_3_b.jpg

[tnt]

My guess would be that leaving out the soldermask allow the tinning processes to cover those tracks and this increases their thickness (and thus their current carrying capacity).
As you said, this would also help heat transfer.

[mikeselectricstuff]

Yes - it's to allow capacity to be increased by covering with  solder, typically when wave-soldering through-hole parts - not uncommon in switchmode PSU boards.
It also lets you neatly solder a length of copper wire along the track for beefier applications.

[neoone]

yep, the main reason is higher thickness so you can send higher current. Look at any switch mode power supply PCB if you can and you'll see that usually they have not remove the whole solder mask but rather some stripes. In that case my guess is that it cases tin to form thicker structures due to surface tension, thicker than it would be if whole solder mask was removed.

[semaphore]

Ah... so just the cheap way of getting more for out of the PCB. To me it is more bad than good. Leaving soldermask open on those high current traces are just too dangerous. They could easily be shorted out by accident. I would just pay a little more and go with heavier ounce of copper.

What about heat transfer benefits? Would leaving soldermask out on the trace help moving heat out to the air faster?

[RayJones]

It's a solder mask, not an insulation mask. Yes it may provide a small degree of insulation BUT.

A far better proposition to protect against accidental contact would be the use of Presspahn.

[neoone]

leaving out solder mask helps a bit in heat terms. Making board with thicker copper is not always a good way to go as new issues start to appear.

[semaphore]

What are the issues with thicker copper beside price?

[allanw]

Once you get to 2 oz it costs a lot more to go higher, and I don't think the current dissipation ability scales linearly either. For cheapo low cost things, it's just more effective to let the solder do some of the work. And soldermask shouldn't be absolutely relied on for isolation .

[neoone]

with thick copper like 105um there are for example problems with maintaining proper track width and spaces between tracks especially with those under 0.5mm in width. Also sometimes the solder mask starts to "splashes" so it looks strange between tracks and may not be so reliable. So if there is a need of having both strong currents on the board and many thin signal tracks sometimes it is better to have thinner copper like 35 or 70 and some uncovered mask regions.

[RobK]

It's a solder mask, not an insulation mask. Yes it may provide a small degree of insulation BUT.

A far better proposition to protect against accidental contact would be the use of Presspahn.

I agree with what RayJones said. I once made the mistake of referring to solder mask as an insulator, and was quickly corrected by one of my former bosses who told me that solder mask should not be considered as a proper insulating material.

[semaphore]

It's a solder mask, not an insulation mask. Yes it may provide a small degree of insulation BUT.

A far better proposition to protect against accidental contact would be the use of Presspahn.

I agree with what RayJones said. I once made the mistake of referring to solder mask as an insulator, and was quickly corrected by one of my former bosses who told me that solder mask should not be considered as a proper insulating material.

I agree that shouldn’t be the primary insulator. But it offers some level safety in case of accident. For example, someone drop a screw on the PCB or touching a live circuit board traces.

[Zero999]

Clumsy people ahouldn't be poking around with live PCBs.

Yes, the solder resist shouldn't be relied on for insulation. I've seen high voltage on a CFL power supply arct right through solder resist like it wasn't there.

[semaphore]

It happen. I'm sure many of us have play with the live circuit board on the bench or the table that have solder flake or cutted component leads. With solder mask it can help to tolerate some short  Or another instant would be intalling live wires in the screw terminal block one hole at a time. While you have on wire in and another wire just flying and short on traces.

I'm not totally rely on the solder mask, but it give me some degree of comfort.

[d_n]

yep, the main reason is higher thickness so you can send higher current. Look at any switch mode power supply PCB if you can and you'll see that usually they have not remove the whole solder mask but rather some stripes. In that case my guess is that it cases tin to form thicker structures due to surface tension, thicker than it would be if whole solder mask was removed.

How can the stripes or the dashed soldering pattern help in increasing the current carrying capacity?? and also there are some star shaped pattern to mount high current carrying components... why??

[Rerouter]

the greater the cross sectional area of a trace the greater the current capacity (less voltage drop) and to an extent the greater the cooling capacity (more surface area)

in this case they are using the solder as a cheap way to increase the cross sectional area, and the ripples have the added bonus of increasing the surface area for the same trace width,

As for the star shaped mounting, i'm pretty sure your talking about thermal reliefs, without them it is near impossible to get enough heat into a joint to cleanly relfow, essentially it acts like a thermal resistance, reducing how much of the heat from your iron goes into heating up a power plane vs heating up the joint,

And for the original OP, if your using very cheap PCB manufacturers, your Gerber's can be edited without notice so your solder mask may not follow the exact shape you had specified, (may be thicker or thinner, or removed on points of fine detail) so you cant even rely on it as a physical separator between 2 close pads,

[d_n]

Is the concept of surface tension or cohesion also related with the above..  these are the pics of the striped tracks and star pattern..