PCB Manufacturer - Page 1
Forum section: electronics / projects

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 01:26:06 am
content: Hello, my attempt to get suggestions for my BLDC servo controller failedso I'll resort to pinpoint questions.I have a project that requires high currents going through the traces, and probably needs a higher copper thickness than the 1oz often found in less demanding applications. Given that and that I want reasonable turnaround times, what PCB manufacturer would be a cost-effective choice? OSH Park I've used before, but they only offer 1oz copper on their 4 layer boards. Some others (e.g. Sunstone Circuits) charge around $400 for a single board of my size (2.165x2.165") shipped in a week, which is outside of my budget. Was hoping for something like $50-100 with the thicker copper, shipped in around a week. This is a prototype and I'm expecting errors to come up and multiple revisions to be necessary, so the cost can't be too high.Does anyone have a recommendation as to a good PCB manufacturer that fits or comes close to these criteria? Thanks in advance.

author: floobydust
author_rank: Super Contributor
timestamp: August 18, 2018, 04:21:35 am
content: PCB Shoppercompares pricing on many sites, including shipping.Finished copper thickness is usually at the IPC bottom end (out of china) i.e. 1oz I think 30um is the minimum.Some fabs take 1oz base copper and plate up, others will use 2oz base laminate. The approach affects how much copper your vias will have.I've seen APC put little test strips on their UPS PCB's (microhm resistance) to ensure the copper was actually as thick as specified. The boards would otherwise overheat.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 04:34:21 am
content: Thanks, that site is really helpful! I found several manufacturers with a reasonable cost for some samples with 2oz plating, although higher than that they are pretty expensive (>$100). I'll start looking through them.My current may be high enough to vaporize a small trace (around 30A), so I need to be pretty careful.You mean testing exposed copper with standard thickness test strips, or using the resistance to confirm the thickness of the copper?

author: DaJMasta
author_rank: Super Contributor
timestamp: August 18, 2018, 04:37:53 am
content: It's pretty common for inner layers to be thinner, even 1/2oz, and because they are not exposed to the air, they will heat faster than external layers. I would run some calculations with a pcb trace calculator to make sure you're within a reasonable margin, but keep the main drive traces on the outermost layer and make them wide - build in some extra safety margin for transients or slight defects in the manufacturing.

author: floobydust
author_rank: Super Contributor
timestamp: August 18, 2018, 04:41:56 am
content: APC has a lone thin zig-zag trace between two test-points with PCB silkscreen marked "14mohm". I remember seeing that when I was repairing one and took a while to figure what it was about. I guess it's a factory check of the PCB copper.You could consider adding solder to the traces to increase current handling. Solder has about 10X the resistance of copper, but it's easier to build up. Or add copper bus wires soldered on top of the traces.I did 32A with a 4-layer board from PCBWAY and it was good. I think you can't get thick copper on inner layers, just the outside layers so it was 1oz+0.5oz+0.5oz+1oz for around 3oz total.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 05:00:05 am
content: The drive traces will be on the outer layers.I tried using a trace width calculator, and am getting a trace width close to 6mm (with 2oz copper) in order to ensure a temperature rise of only 50 Celsius. The issue is that the source pad on the dual channel mosfet I'm looking at is only about 1mm wide, which prevents me from making a very wide trace. Interestingly, the drain pad is huge, but the source pad is pretty small... Should I perhaps go with single channel mosfets with a larger footprint?I've heard of using the solder on the trace trick. Bus wires sound good too, just not sure how appropriate either method will be if I ever need to scale up the production of these boards.Almost tempted to take apart one of my ESCs to see how they coped with that problem. If they can get away with a PCB for 30A or 60A BLDC motors, they might just have the solution.

author: Rerouter
author_rank: Super Contributor
timestamp: August 18, 2018, 05:20:44 am
content: The ESC PCB's use the wires to take away a large amount of there heat, and are built as compact as possible to reduce the total amount of heat,Remember that a trace that is half as long creates the same amount of heat as a trace that is twice as thick.So if its a fat wire, a few mm to a mosfet then a few mm to the output wire, almost all of that heat will run out the wires.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 05:32:01 am
content: Thanks, that makes sense. I did make the traces going to the mosfets and current sense resistors as short as possible so that should help.I did take apart that ESC but stopped short of desoldering pieces off of it (it is a stack soldered together). It is still a mystery to me, as the top layer seems to house 2 single channel mosfets while the bottom layer (and the layer to which the wires go) has 12 ICs whose purpose I don't know. There is a massive heatsink though on that bottom board, so I'm tempted to think that's where the mosfets actually are.edit: what I thought were mosfets are actually 2 LDO voltage regulators. That means the mosfets are indeed on the bottom part of the sandwhich, but I don't know why there are 12 ICs total there.

author: swglabs
author_rank: Newbie
timestamp: August 18, 2018, 06:15:26 am
content: +1 for removing the soldermask and adding solder/soldering a solid-core wire on top of the trace if they're on the outer layers, since budget is your driving requirement

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 06:19:22 am
content: Yep, for the prototype that sounds very lucrative. I just don't know how it'll scale up if I need to increase production of these later.

author: Rerouter
author_rank: Super Contributor
timestamp: August 18, 2018, 09:26:36 am
content: 6 of those IC's would be mosfets, as there would be 3 pairs of H bridges, The other 6 unknown, atleast 1 IC would be the controlling IC doing current measuring and phasing,I would recommend keeping the soldermask on, Again,only a tiny fraction of the heat will be in the traces. Airs thermal resistance is still the problem, Lucky for most applications there strapped to something that literally fan force cools them.Download the saturn PCB program, and pull up the conductor tab, and pull up the "Conductor Properties" tab, a 6mm wide trace for 2oz plating, can handle 24A so long as your OK with it getting to 90C if nothing else pulled heat from the board. (Dissipating about 0.5W for a 10mm long trace)But you have a wire rated for 24A soldered within mm of it, he will be pulling that heat away and raising your maximum theoretical current higher, For that you would just need to work through the thermal resistance into the wire core, but I imagine it would barely push over 45C due to the surface area and amount of mass that that wire has. (Wire insulation is electrical insulation, It still lets heat through pretty well)

author: anvoice
author_rank: Frequent Contributor
timestamp: August 18, 2018, 08:55:27 pm
content: Yep I realized 6 of those are mosfets, what I don't know is the purpose of the other 6. Perhaps current-sense resistors? The control IC is on the top of the sandwich, conveniently away from the heat-generating mosfets.Ok, I get the idea. Basically design as compact as I can and keep the power wires close to absorb the heat. Should that fail, I can always strip the solder mask and add more conductor. I think I'll also choose 6 single N-channel mosfets to drive the motor, that way I get a larger footprint for the source portion of the transistor than when using a dual channel version. A few tweaks and I should be able to get a quote from one of those manufacturers floobydust's website suggested.