PCB Manufacture and assembly question

[Pax Writer]

Hello all

I'm designing a high power PCB, and I am considering having it made with 2oz/70my gold plating, but I have some questions:
1. Can I solder directly on gold plating?
2. Will it pay in terms of lower resistance and better heatsinking over common copper?
3. Are there any other things, I should take into consideration over normal PCBs (not counting the higher price)?

Fire at will

/Pax

 

[dknguyen]

If you're worried about corrosion, that's what a solder mask is for.

If you're worried about resistance, make the traces thicker/wider and this will also lower inductance (which is more important in power circuits anyways). And if you're still worried about resistance, just remember you're using gold plating and not gold traces so there's not a very big difference. Plating over the copper traces with a very thing layer of gold is going to do squat for your resistance and even less for your inductance.

Gold is not that much better a conductor or heatsink than copper, especially for the price (it's so close it might as well be the same, wiki actually has copper as being lower resistivity than gold). And if you need a heatsink the plate material of the traces won't make a difference- use a heatsink.

You're also not working in the GHz range so the gold's advantage over copper when the skin effect kicks in does not apply here. And if it did, you'd be using microstrip and other much more expensive PCB fabrication methods.

You could probably solder to gold if you used special solder, but why bother with all the stuff mentioned above?

And it's a power board- would you rather fry a tin-plated board or a gold-plated board?

No, it doesn't pay. Spend your extra money on a 4oz thick copper PCB.

 

[Pax Writer]

Hello dknguyen

As you might remember, I am designing a PWM-motorcontroller (I think you already helped me with some other threads). Thing is, the board will be made as small as possible, so I was trying to figure out how to make the best of the board, considering its size.
Originally I was thinking about an all-gold-board, but seeing your comments, I now wonder if plating the solder terminals for the wires. What do you think about that? Will I be able to solder on the gold?

Others are welcome to share their thought too

 

[dknguyen]

Your money is still better spent on getting thicker copper PCB than gold plating it. It effectively won't increase the size of the board (like making wider traces will).

Just stay away from gold...if you want lower resistance solder 12AWG wire point-to-point on the PCB for power traces. Much cheaper and way more effective than gold plating.

 

[felis]

Will I be able to solder on the gold?

I've never soldered gold plated PCB; gold plated RF connectors solder just fine.

 

[Pax Writer]

Okay, thanks both, your advice is greatly appreciated.

 

[Pax Writer]

@dknguyen: I just saw your edited post, and it pretty much answers all my questions. Thanks a lot for the quick reply

 

[mneary]

A thin layer of gold is about the best treatment I know for preventing corrosion and extending bareboard shelf life. It is extremely easy to solder, especially since it won't be corroded. Gold plated boards do NOT require special solder.

Solder mask prevents corrosion in the areas you DON'T intend to solder. No comparison.

Gold is NOT a better conductor than copper; therefore it doesn't have any advantage re: skin effect, resistance, or heat sinking. Perhaps you were thinking of silver. Consider the copper your only conductor; the gold is just an anti-corrosion treatment.

Gold isn't very expensive in the thickness applied to a board, so it's probably worth it if you don't plan to assemble it within a few months.

Places that used to plate with solder had to stop using lead (Pb). Some have switched to gold; others are using tin. Unfortunately tin corrodes more quickly.

 

[Pax Writer]

Hello mneary

Thanks for clearing up those points about gold plating. Nice to not be completely in the dark concerning this matter.

Cheers

 

[ericgibbs]

hi Pax,
This is quite a useful tutorial for pcb design.

**broken link removed**

 

[OutToLunch]

i would suggest a 4 layer board with 2oz copper on all layers (you could likely get away with 1oz in the middle layers, bu 2oz is better).

second layer from top should be dedicated to GROUND - no other signals, traces, etc. all ground connections can then be brought to the ground plane either directly from the pad or through a very short trace from the pad to a via.

the third layer from the top should be a power plane layer. divide it up into input, output and any other intermediate power planes necessary.

top and bottom layers are for routing nice fat gate traces, small signal stuff and more power. don't use traces for power - always use shapes/polygons.

 

[dknguyen]

I don't mean to hijack, but about how the ground plane should always be as unbroken as possible and the power plane may be split up to different voltages, in a single supply PCB, is there really any difference between the ground plane or power plane? Each is a reference to the other really...but in a multi-voltage PCB I can understand since there really is only one reference (ground plane) since the multiple voltages are actually all referenced to ground.

 

[Pax Writer]

Hello OutToLunch: Thanks for your advice, however, I don't think the size and complexity of the PCB justifies 4 layers. The PCB has just been completed and sent to production, and its only 3x3.5cm and contains only around 15 nets.
The power nets are routed on both sides of the print, each side being plated with 2oz of copper. The nets are made as planes/polygons as you suggested, and the total length of each net is less than 1cm with plenty of plane copper and vias to connect the two layers.
The prototype was made on a protytype board with only 3mm wide tracks (completely filled with tin upon component soldering), and it seemed to work relatively well during the small tests, I subjected it to.
What I hope to achieve with the manufactured PCB apart from ease of manufacture, is lower resistance in the PCB, as the whole circuit is basically inserted serially between the battery and the motor, so I want the lowest possible voltage drop over the PCB.
Let's see

Thanks all, again for your answers - This thread got longer and more interesting than I thought it would - not least due to the linkage to the guidelines further up. Very interesting.
Thanx

 

[mneary]

Tin coating on the traces looks impressive, but tin's conductivity is 6x worse than copper. This doesn't make it harmful, but you need a .012 pile of tin to cut the resistance of 0.002 copper in half.

The extra mass may help reduce peak trace temperatures if the current is in surges.

 

[Pax Writer]

Hey mneary

Well, that was just the prototype, to check out if the circuit would work in the field. I expect my new and hopefully much superior PCBs to arrive in 14 days time, and then the proto will become another proto-museum-piece in my Pile O' Protos Museum in my bottom drawer

 

[ericgibbs]

hi Pax,

Did you get the chance to look thru that pcb tutorial I posted?

Like your opinion.

 

[Pax Writer]

Hi Eric

I downloaded it, but only skimmed it for now. Did you write it? And would you like to get constructive criticism and suggestions if I have any?
My first impression is good

/Pax

 

[OutToLunch]

I don't mean to hijack, but about how the ground plane should always be as unbroken as possible and the power plane may be split up to different voltages, in a single supply PCB, is there really any difference between the ground plane or power plane? Each is a reference to the other really...but in a multi-voltage PCB I can understand since there really is only one reference (ground plane) since the multiple voltages are actually all referenced to ground.

The ground plane can carry a lot of current and you want this to be as low impedance as possible - meaning don't break it up and try not to swiss cheese it up TOO much with a plethora of vias. just try to keep current flow through the ground plane in mind. Everything in a power supply is referenced to ground, so by having the entire second layer dedicated to ground, every component has a very short, low impedance path to it. i'm not a proponent of separating signal and power grounds on a power supply, there really is no need for it with a good ground plane. if more precision is required in the regulation, then one can use regulator ICs that have differential sensing that effectively removes voltage differentials that can occur across a ground plane.

since the ground plane is what everything is referenced to globally, it is placed on the layer directly under the controller ic. in this way, any noise that may be, say, on the bottom layer, can be effectively blocked by the ground plane.

the power plane, on the other hand, can be split up into input, output, phase and ground nodes. this is because these areas are only required locally. input caps, control FETs, etc only need to tie into the vin copper, etc.

 

[ericgibbs]

Hi Eric

I downloaded it, but only skimmed it for now. Did you write it? And would you like to get constructive criticism and suggestions if I have any?
My first impression is good

/Pax

hi Pax,
No, its not my work, although I used similair methods when laying out artwork.
The reason I asked for your findings was because we get a number of students asking for guidance with artwork and would useful if we could recommend a readily available tutorial document.
As you probably already know, there are lots of crappy tutorials on the web, bit like a mine field for a 'newbie'...

 

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