Traces without solder mask - best way to do it? - Page 1
Forum section: electronics / projects

author: hli
author_rank: Frequent Contributor
timestamp: November 12, 2015, 10:01:10 am
content: I'm working on a breakout board for the LT4320 (so I can use it easily in my prototypes). I managed to get 3 version in a 5x5cm board, one using 4 TO220 FETS and two using dual-FETs in SO8.I used 1mm wide traces - anything wider won't route with the SO8 packages. According to the KiCad calculator they are good for 3 amps (with a 20°C temperature rise).Since I would like to support higher currents, I need to put solder on the traces. Therefore I cannot have solder mask on them. I found out to do that in KiCad, but its tedious to have a mask for all traces.So my question is: is it better to skip solder mask only for the traces, or is it OK to define a larger area where no solder mask is applied? In the attached PCB picture, I don't want to have solder mask on the wide traces, but Ineedit only between the pads (and maybe on the smaller traces). For all other areas I'm unsure whether it matters that the FR4 is covered in solder mask or not.Any hints?

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2015, 10:19:51 am
content: Cant you double the power and load trace width and neck them down to get onto the SO8 pads. You'll have to get the signal traces out of the way, possibly vias under the MOSFETs and keep the necked down section as short as possible.

author: hli
author_rank: Frequent Contributor
timestamp: November 12, 2015, 10:37:21 am
content: 2mm wide traces would give 5 to 6 amps, which still might not be enough (it then depends on the internal resistance of the FETs I actually use, because they might not be able to dissipate the power). The clearance will be tough with that, and I cannot route anything inbetween the DIP pads anymore.For the TO220 version I want to handle 10 amps which would need 4mm wide traces. This would be even more problematic. Thats why I want to have additional solder on them.

author: cyr
author_rank: Frequent Contributor
timestamp: November 12, 2015, 11:08:25 am
content: Fill the available board area with copper. You don't have to use (constant width) traces for the high current paths, you can use copper pours. You get both lower resistance and better heat dissipation.That's assuming it's anything to worry about. For such short traces, does the resistance and self heating really impact anything?

author: hli
author_rank: Frequent Contributor
timestamp: November 12, 2015, 02:14:18 pm
content: I actually don't know, thats why I'm asking. I relied on the KiCad calculator. TheANIS PCB track width calculatoris a little bit more optimistic, but not much. I estimate that the TO220 version (which I want to handle at least 10A) has about 20 cm of high-current tracks. The calculator says that for 10A I need 3.79mm wide traces, and they would dissipate 1.52W. With 1 mm wide traces it would be 6W (resistance is then about 4 times as high). Thereferenced paperindicates that the track length has some influence, but it was not that much.But thats not really my question. This was more about: does having solder mask on non-copper areas buy me anything? Doing so would make defining the no-solder-mask-area easier (because otherwise it need to follow all traces), but I would like to know about the possible trade-offs.

author: Pjotr
author_rank: Frequent Contributor
timestamp: November 12, 2015, 02:41:43 pm
content: Solder has app. 8-10 times lower conductivity than copper. Putting solder on traces will not help much to get more current through them. If you need this, solder plain tinned copper wire along the traces.

author: kingofkya
author_rank: Regular Contributor
timestamp: November 12, 2015, 05:36:33 pm
content: why not Just use thicker copper?2, 4 and 6oz copper is not that bad expense wise.Or wire-links could also be an option.

author: hli
author_rank: Frequent Contributor
timestamp: November 12, 2015, 08:14:56 pm
content: For Seeedstudio (this is where I want to send the PCBs to) with 2oz copper the price increases from $9.9 to $39.8 - 4 times as much(this was the first thing I looked up).For the copper pours: I will try this too, but its difficult to get the pouring in a way so thatallparts ofallwide traces are now wide enough (especially with the TO220 board where I need something like 4mm wide traces). I just watched EEVBlog #317, where Dave measured how much the resistance decreases when adding solder to a trace (is can be as low as half the resistance).But I like the idea with adding copper wire. I will try to finalize the PCB today (and keep the solder mask off only for the traces except anybody comes up otherwise) and send it off to Seeed, so I should get them back before christmas. Then I can do some tests with how much current the traces can hold.