PCB fab cost & trace width

Thread Starter

Steve314

Joined Oct 8, 2023
4
Do boards somehow get cheaper with bigger traces? Or more reliable? Please to note that these traces are very low current and are not controlled impedance.

Im getting ready to start routing on a board. 5 mil traces would be ideal since that might make the difference between a 6 layer board and an 8 layer board.

Plenty of fab shops list 5 mil as something off of the 'standard menu'. I will order electrical test and presumably thats sufficient to ensure long term reliability.

And finally, at the end of routing there is always a temptation to go and thicken as many traces as possible. Thats fun and all, but if its not really helping I would skip that step.

Thank you!
Steve
 

dl324

Joined Mar 30, 2015
16,677
Do boards somehow get cheaper with bigger traces? Or more reliable? Please to note that these traces are very low current and are not controlled impedance.
It depends.

Removing less copper could make etching take less time and deplete the etching chemical more slowly, but the plating steps could take longer. In any case, the fab isn't likely to give you a different price and you're just paying per square inch with fees for setup and additional features.
And finally, at the end of routing there is always a temptation to go and thicken as many traces as possible. Thats fun and all, but if its not really helping I would skip that step.
It depends.

When you decrease trace separation, you increase cross capacitance to adjacent traces. You can also increase cross capacitance to traces on other layers. If there are ground planes involved, you increase capacitance on the trace you widen.
 

crutschow

Joined Mar 14, 2008
34,046
at the end of routing there is always a temptation to go and thicken as many traces as possible. Thats fun and all, but if its not really helping I would skip that step.
It may be desirable for any traces that carry higher currents, such as power traces, but likely is of no significant benefit for low-current signal traces.
 

Ian0

Joined Aug 7, 2020
9,499
Do boards somehow get cheaper with bigger traces? Or more reliable? Please to note that these traces are very low current and are not controlled impedance.
No, but they get more expensive with really thin traces and very small holes. Any holes smaller than 0.4mm cost more.

And finally, at the end of routing there is always a temptation to go and thicken as many traces as possible. Thats fun and all, but if its not really helping I would skip that step.
Make sure your power supply tracks are as thick as they can be.
A wider track is less likely to get damaged if you have to remove and re-solder components. I try to use 0.03" most of the time 0.015" to get between pins on a 0.1" connector, and 0.01" to wire up microprocessors. I never use anything smaller.
If you were lucky to get a promotional Digikey ruler, it tells you the temperature rise for pcb tracks carrying various amounts of current. If you weren't so lucky you can buy one for £1.58
https://www.digikey.co.uk/en/products/detail/digi-key-electronics/DKS-PCB-RULER-12INCH/5767550
 

Thread Starter

Steve314

Joined Oct 8, 2023
4
I looked into fab cost vs trace widths.
Many sites that just say trace width impacts cost but don’t go into detail.

Trace widths narrower than 5 mils (cost more)
https://www.sfcircuits.com/pcb-school/pcb-trace-widths

From a cost optimization standpoint, your traces and spaces should be greater than 4.25 mils.
https://www.protoexpress.com/pcb-cost-optimization/

Use a minimum track width / track spacing of 8/8 mil
https://camptechii.com/5-ways-to-lower-your-pcb-fabrication-cost/

Standard is 4mil/4mil (0.102mm) Trace width/spacing on 1/2oz material. https://pcbboardassembly.com/factors-that-increase-pcb-cost/

Enlarged trace widths will surely cost more; enlarged trace widths demand extra solder mask to the trails.

https://www.pcbbuy.com/news/PCB-manufacturing-cost-breakdown.html
 

WBahn

Joined Mar 31, 2012
29,857
Plenty of fab shops list 5 mil as something off of the 'standard menu'. I will order electrical test and presumably thats sufficient to ensure long term reliability.
Electrical test doesn't really say much regarding the long-term reliability. It says that the pads on the board that are supposed to be electrically connected actually are electrically connected (and that pads that shouldn't be connected aren't). It's there to catch significant fab errors, either in set up or in production, but it won't catch errors, such as overetching or insufficient plating thickness, that can drastically impact the life expectancy of the board as long as there is enough connectivity (or sufficient isolation) for the board to appear to be correct electrically.

Having said that, there are certainly other types of electrical tests that could be done that could get at some of these other issues, such as actually measuring the resistance of all of the traces to determine if the widths appear to be what they are supposed to be. But those are seldom done on a basic board because they are much more expensive. If you are fabbing a board that has rigorous requirements or is intended for a particularly critical application, such as life support or space, then you pay for whatever tests are needed, which may include destructively testing some fraction of the fab run.
 
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