Weird pattern on the copper track - Page 1
Forum section: electronics / beginners

author: Juntendo
author_rank: Newbie
timestamp: February 24, 2021, 06:54:21 am
content: Greetings everyone!I took apart an old ATX power supply and it had an interesting square pattern on the copper tracks.Is this really meant to improve the current capability or is this used for other purposes such as ESD protection, shielding, or improve thermal conductivity?Cheers!

author: Jwillis
author_rank: Super Contributor
timestamp: February 24, 2021, 07:00:33 am
content: Heat dissipation ? Just a guess .

author: Berni
author_rank: Super Contributor
timestamp: February 24, 2021, 07:16:58 am
content: Probably to increase the current carrying capability of the tracks.They can't have large open areas because the solder from the wave soldering process would form big blobs that might short out things.But this is certainly an unusual pattern, usually they make thick lines in the solder mask to create thick solder "tracks" in the direction the current flows.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 24, 2021, 08:13:04 am
content: Quote from: Juntendo on February 24, 2021, 06:54:21 amOnly for more current.

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 24, 2021, 08:21:45 am
content: it doesn't increase current. why? if you have only 0.1mm serial track with no square soldered, the current is limited by the track.so current 'increase' is ZEROheat? yes, improved a littlejust some 'Eng.' toying with the mouseto increase current capability, you need ALL the crt path made thicker, not some squares, the remaining pcb track just became a fuse

author: RoGeorge
author_rank: Super Contributor
timestamp: February 24, 2021, 08:39:31 am
content: It lowers the resistance of the traces, therefore a trace with the same area (but thicker here and there) can now carry more current for a given max voltage drop constraint. In two words, more current.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2021, 02:52:30 pm
content: Not (significantly) more current, as the current is defined by the source and load. More current CAPABILITY. Try to communicate clearly.Anyway, you gain maybe some 20%. It works when the design is otherwise marginal but "almost there" and you don't have space to widen the track.In this case, it seems the designer has done this trick "everywhere" with probably no real reason to do so. Usually you see only certain traces like this.

author: golden_labels
author_rank: Super Contributor
timestamp: February 24, 2021, 04:03:14 pm
content: Quote from: perieanuo on February 24, 2021, 08:21:45 amExcept it is not.Your statement is true only if power dissipation is the only limiting factor. Even then it can be directly applied only to average power. But that’s not the only limit one may face and, for a typical traces used to power chips on a PCB, it’s not even among the important ones. The two more important ones are in this case:Voltage dropResponse to fast changes in loadThe model here is: Vdrop= I · (Runplated+ Rplateable)The two possible scenarios are:Vdrop@unplated= I · (Runplated+ Runplated)Vdrop@plated= I · (Runplated+ Rplated)Since Rplated< Runplated, Vdrop@plated< Vdrop@unplated∎As for the second part: while maximum average power loss in the thinnest link limits the maximum continuous current, it does not limit instantenous current. You may have short spikes of relatively high currents and it’s not a problem. That is because what causes damage is not power, but energy accumulated in a piece of track in a form of heat. That is: integral of power over time. On top of that, heat is being lost to the surrounding areas. If sections of your track can run cooler due to higher current capacity, it means they are slightly contributing to cooling down thinner fragments.I am highly doubting heat dissipation is better due to those solder areas. They do not significally increase surface area, which would contribute to cooling by convection. At the same time they may be less porous than soldermask, which would decrease radiative cooling. But in this case it is merely my guess.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: February 24, 2021, 04:18:53 pm
content: Quote from: Siwastaja on February 24, 2021, 02:52:30 pmI would agree with this. In particular since the resistivity of solder is about 8.5 times that of copper.A much better approach is to just spec thicker copper. It's also cheaper and wastes fewer resources.

author: golden_labels
author_rank: Super Contributor
timestamp: February 24, 2021, 04:27:08 pm
content: Quote from: KrudyZ on February 24, 2021, 04:18:53 pmUnless you are not billed for amount of solder used, at which point it becomes gratis — compared to more expensive PCBs with thicker copper layer. Half of which is lost during etching.

author: Berni
author_rank: Super Contributor
timestamp: February 24, 2021, 06:24:03 pm
content: The reason why solder can make a difference is because while its conductivity is fairly low, it does make a significantly thicker layer than copper. Increased thermal conductivity also helps because it can shift the power dissipated in narrow areas of the track onto wider areas while also helping to pull away heat from components soldered to it. This helps reduce hotspots caused by heat generating components (often these components dissipate more than the track itself) or conduct heat into nonheat dissipating components to help cool the track.That being said the difference is not going to be very big in any case, especially for this lazy dot approach. As pointed out thicker copper makes a MUCH larger difference. But the thing is that thicker copper costs a good deal of money, both because copper itself is a fairly expensive metal and because the plating process used to put the thick copper on it is slow and energy intensive. If they had that sort of money to spend on the PCB they would have likely rather went for a double-sided PCB rather than single-sided. These products are scraping the bottom of the barrel for every last cent. But putting solder on areas of the board is incredibly cheep. The boards already go through a wave soldering machine, so tinning traces with solder costs 0 extra time and energy. The only extra cost is the little bit of extra tin they had to throw into the wave soldering machines melting pot. So even if the gain is next to nothing, they do it anyway because of how cheap it is to do in production.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2021, 06:35:40 pm
content: Quote from: KrudyZ on February 24, 2021, 04:18:53 pmThat's true, it's more resistive, but it's also thicker. The thickness of the solder blob from the wave solder or reflow is about 5 times that of 1oz (35um) copper. You can't fill the whole track (molten solder would flow to one end of the track due to gravity) so you need to add it in regions separated by soldermask like in the example, so fill rate ends up somewhere around 80% maximum.All of this results in practical resistance drop of some 20-30%.It doesn't matter if it "feels" the inferior way. The facts are, it's free of cost (unless the CM charges separately for the reflow solder / paste used, in which case it's still very cheap), it improves current handling by some 20%, maybe 30%. If that is all that is required, there is no reason not to do it.I do it time-to-time. Just don't expect miracles.When prototyping, such solder mask / paste mask openings easily allow you to actually solder in copper reinforcement, or leave it as-is, and measure how it performs.Of course, better strategies include: primarily shortening the distances by component placement; widening the tracks; going for thicker (like 2oz) copper. Sometimes component placement can't be improved, there is no more room for widening the tracks, and 2oz copper costs extra, quite significant cost difference in low-cost production actually!

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2021, 07:29:48 pm
content: Dave did a video on this nearly a decade ago:https://www.eevblog.com/forum/blog/eevblog-317-pcb-tinning-myth-busting/

author: KrudyZ
author_rank: Frequent Contributor
timestamp: February 25, 2021, 04:57:49 pm
content: Regarding the cost aspect:A metric ton of copper is ~$7000, lead is $2000, tin is $18500 and lead free solder alloys will be even more, so adding solder vs copper is definitely more expensive from the raw material's cost perspective. Electroplating or etching losses / chemical disposal might change the overall cost in favor of tin/lead, but I don't have the numbers to make a call on this.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2021, 05:27:40 pm
content: Quote from: KrudyZ on February 25, 2021, 04:57:49 pmYou miss the fact that this trick is normally only played on a single or a few important tracks. Typically it's like 1-5% of the total area of the PCB. The OP's photo seems like an exception, I bet the designer had just heard about this or has a strange habit of doing that by default. At some point their CM may give a comment about it.Plating is slow, process time is expensive. For one "thicker" trace, the whole PCB needs to sit in plating tank for half an hour extra, then you get thick copper everywhere need it or not. Thick copper increases etching time and reduces resolution (minimum track width / clearance specs and/or yield). 2oz copper option is always a cost increase, and it doesn't even double the current capability.In small and medium scale manufacturing (thousands, not millions units), you pay for a process step, not for the raw materials so the cost is amortized over all customers of that CM.If you cover 50% of the PCB with solder mask openings and order 100 000 units I'm sure the CM will factor the cost for that because of the raw material cost getting very real as you explained with the numbers.

author: golden_labels
author_rank: Super Contributor
timestamp: February 25, 2021, 11:01:41 pm
content: Quote from: KrudyZ on February 25, 2021, 04:57:49 pmYou don’t need to compare raw materials cost. Just see the offers of PCB manufacturers. This is what the client is going to pay, not the theoretical costs. And those prices are clear. Thicker copper = higher price. More solder = no additional cost at all.

author: Berni
author_rank: Super Contributor
timestamp: February 26, 2021, 06:19:32 am
content: Quote from: KrudyZ on February 25, 2021, 04:57:49 pmOh wow didn't realize tin was that expensive. Copper tends to be seen as the valuable metal. Guess that gives the chinese even more motivation to keep using the good ol 60/40 solder.But yeah i take it most of the expensive cost of thick PCB copper layers comes from the slow and nasty chemical process used to put it on there. While the process of putting solder on there is just simply tossing the board into a molten pot of the stuff.

author: Ian.M
author_rank: Super Contributor
timestamp: February 26, 2021, 11:43:32 am
content: Selective heavy copper plating is even more expensive. If you need to ask, you probably can't afford it! See:https://www.epectec.com/pcb/powerlink-technology.html

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2021, 12:20:03 pm
content: For small batches, paying someone to cut some 0.254mm or 0.3mm copper sheet (goes with plain old kitchen scissors) in shape, then reflow these reinforcements on tracks works. This scales well; moderate-sized batches of copper reinforcements can be stamped, then it's only matter of picking&placing them prior to reflow. Even if it involves manual human P&P work, it's still likely cheaper than very thick plating processes.

author: Berni
author_rank: Super Contributor
timestamp: February 26, 2021, 01:12:42 pm
content: Quote from: Siwastaja on February 26, 2021, 12:20:03 pmYep done that one last time i had to carry 100A trough a PCB. Works really well and is actually pretty cheep. We already have a company that does some sheet metal work for us, so we had then cut a shape from a DXF file into some copper sheet with the laser cutter. When we asked them if they can do it they actually said they did it before and even had a guy run it trough nickel plating to make them nice and shiny, keeping it oxide free for the soldering step.