PCB edge tolerances and edge "finish" - Page 1
Forum section: electronics / general

author: derGoldstein
author_rank: Regular Contributor
timestamp: January 03, 2018, 10:59:41 pm
content: I've been working on some PCB designs that have to fit into very specific, accurately machined enclosures. The PCBs that I've ordered so far have had some very loose tolerances, and the edges range from very smooth (fresh carbide bit) to extremely fuzzy (dull bit and probably a slightly vibrating machine).Are there standards for PCB perimeter accuracy and finish?Also, for the most part it seems like the placement of holes will be relatively accurate (the distance between centers), as well as hole diameter, but the distance between a hole and the edge of the PCB can vary quite a bit. I assume that this is because the holes are drilled in one shot, whereas you have to travel the milling bit along the edges. Are there separate standards here as well?

author: AndyC_772
author_rank: Super Contributor
timestamp: January 04, 2018, 07:45:12 am
content: The position of the board edge with respect to the pattern of copper and holes normally has quite a wide tolerance. In most applications, it's just not something that matters much.If, as in this case, the board edge is important, you may need to agree a tolerance and finish with your supplier. Whether it's something for which there's an IPC standard or not, you're much more likely to get a straight answer - and usable product - from the PCB fabricator if you explain to them exactly what you need, and put it in your acceptability criteria.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2018, 09:13:45 am
content: I would be embarrassed to see worse than 10 mils (0.25 mm), but if you want work with absolute bargain basement suppliers, caveat emptor...Board edge tolerance and finish should be specified on the board / mech / drill drawing. Although don't be surprised if it adds a little extra cost (or is ignored outright).One method I like to employ from time to time: v-grooved breakaway tabs. Most of the board perimeter is routed smooth, including any curves or rounded corners you have (as long as the side has a single outermost edge that can be v-grooved), and what's left is not very ragged (as mouse bites would be), but a little fuzzy (loose fibers/burrs). The residue can be quickly sanded or cut away, or left alone.In a tight metal enclosure, it may happen that simply inserting it is enough to deal with the burrs.Mind that the board itself will expand and contract due to thermal expansion, as well as moisture absorption. Confining the board rigidly inside a frame may not be the best strategy. (If you do, make sure to use enough fixed points so that the board is truly captive, and can't bow or twist itself, which could damage components.) Expansion scales with size, so this would be a concern for larger boards (20cm or more, maybe..?).Tim

author: Rerouter
author_rank: Super Contributor
timestamp: January 04, 2018, 11:30:39 am
content: V-scored board edges in the past from itead, a batch of 200 boards, while fuzzy due to being v-scored where all exactly the size i specified, for my application it clipped in to a plastic holder, I determined my tolerances and specified exactly in the middle, and all the boards i measured where within 2 microns after de-burring,another batch of 25 boards i ordered from all-pcb a few weeks ago did have a fully routed edge, this generally needs a vacuum table to accomplish, the tolerance did vary about 18 microns between boards, but the edges where always parallel, implying it was some offset from there tool changer.

author: DerekG
author_rank: Frequent Contributor
timestamp: January 04, 2018, 11:55:25 am
content: Quote from: Rerouter on January 04, 2018, 11:30:39 amYou were very lucky. Most board manufacturers won't guarantee anything better than plus/minus 0.25mm for V-grooving with plus/minus 0.5mm being the norm.V-grooving is a fairly imprecise method of determining board size, whereas using a milling tool on an NC machine is much more accurate.As I said, you were just lucky. This is not repeatable batch after batch.

author: Rerouter
author_rank: Super Contributor
timestamp: January 04, 2018, 12:06:28 pm
content: I have to question why to that end? a v-score by nature is centered no matter how deep, there is cutting on half the surface so it will tend to pull in the cutting direction, however I assume the machine should be far more rigid than 250 micron tolerance, and a normal v-score bit has very short unsupported overhang due to the board being so thin,whereas on an edge mill, the final tolerance is down to how well the board is held while milling, and any material flex will show up.I can understand the machine ops rounding the number they use on the machine, if its any difference I have been specifying in metric, and almost exclusively use rounded numbers for my edge sizes, using Kicad.I did later get another batch of the same order of those 200 boards, however i did not measure them that time, however my tolerance meant it was at least within 55 micron.

author: Kean
author_rank: Unknown
timestamp: January 04, 2018, 12:37:07 pm
content: Quote from: Rerouter on January 04, 2018, 12:06:28 pmV-grooves are cut as an entirely different operation from routing, on a machine a bit like a table saw and using a circular blade. Accuracy is usually not that great as the V-groove position may actually be manually set by the operator for each job, and the PCB is generally held in a sliding guide or against a fence just like on a table saw. It is likely that higher cost/quality fabricators use more accurate & automated machines, but even then the tolerance is always going to be worse than a routed PCB due to the extra machining operation. V-grooving just isn't meant for use when you need good dimensional tolerance, especially as it leaves you with a rough edge.Another note - if you're doing smallish PCBs (e.g. 25mm per side or less), then to get the highest accuracy of the routed outline you are best advised to panelise with tabs & mouse bites in the most approprate locations for your mechanical design. Small PCBs just won't stay in place that well on the vacuum table, and could shift as the cut finishes. For some really small PCBs like a sensor I make that uses a 7mm diameter PCB, I get them made up in as a larger PCB or panel with registration holes and then I route the final size myself with small breakout tabs. Quite handy as these are for a low volume product range and I sometimes need to route them with different sized outlines. The first batch I ever got of these were routed as 7mm diameter. It was a pretty good attempt by the fab, but they were not actually round and they all needed manualy filing to size which was really fiddly.

author: Corporate666
author_rank: Unknown
timestamp: January 05, 2018, 03:36:57 am
content: Quote from: Rerouter on January 04, 2018, 11:30:39 amI'm calling bullshit on this. There's no way you can measure a board to "within 2 microns". Even a Mitutoyo micrometer has a stated accuracy of only +/- 1.3 micron, and when you are taking readings at that level, there are specific processes you have to follow to get an accurate reading. There's just no way to achieve that with an FR4 circuit board. It's inherently an uneven surface and it's not hard - the variation would be much more than 2 microns.Maybe there's some different terminology in Oz where micron means something other than millionth of a meter?

author: sleemanj
author_rank: Super Contributor
timestamp: January 05, 2018, 03:55:52 am
content: I'd wager that Rerouter flubbed the factor in his mental conversion and his measurement was actually within 0.02 mm (20 micron), which is certainly measurable with some good calipers.

author: Corporate666
author_rank: Unknown
timestamp: January 05, 2018, 11:10:58 pm
content: Quote from: sleemanj on January 05, 2018, 03:55:52 amNo it isn't.Very good calipers (let's say a set of Mitutoyo calipers in good condition and within calibration) are specified with an accuracy of +/- 0.001" which is +/- 0.0254mm, or 25.4 microns. So whatever reading you think you're getting, you are only up to 25.4 microns of the actual reading, plus or minus. The repeatability is +/- 0.005" or +/- 0.01mm aka 10 microns, so even if you got the exact same measurement every time on the display, all you "know" is that your readings were within 20 microns of each other.But there is absolutely zero chance of getting that sort of measurement on an FR4 circuit board. A nice milled finish in metal is going to have 15 to 25 microns of height variation, and a routed finish on an FR4 PCB with, essentially, a carbide burr is going to be *significantly* rougher. Not to mention that measuring a soft and layered material like FR4 on a routed edge just doesn't allow for a measurement precision anywhere near 20 micron.Sorry if I sound like a stickler on this issue, but I deal a lot in precision CNC machining and manufacturing and measuring the parts we're making goes hand in hand with that. It's a huge pet peeve of mine when people (not you, just in general) toss around numbers like microns and claim to be measuring them or achieving them dimensionally as if it's no big thing. I chase 25 microns daily - not that it's so hard, but it requires diligence and knowledge and the right tools and materials to get those results, and the right measurement equipment to know you're getting it. Then some Kickstarter $99 3D printer maker claims they can position to 10 microns and make parts accurate to 20 microns.The high quality PCB fabs I've used have been able to produce boards where >90% fall within 0.005" (127 micron) of each other. Shittier quality/cheaper fabs I've easily seen more like 250 microns of variation in board dimensions. Most board fabs will provide a dimensional tolerance so IMO if it's important, it should be worked out with the manufacturer ahead of time.</stickler mode off>

author: sleemanj
author_rank: Super Contributor
timestamp: January 05, 2018, 11:19:43 pm
content: Quotehttp://ecatalog.mitutoyo.com/ABSOLUTE-Digimatic-Caliper-Series-500-with-Exclusive-ABSOLUTE-Encode-Technology-C1381.aspxScroll to table at bottom, select metric, note accuracy, you will find it is specified as +/- 20 micronBeing a Japanese brand (all be it I have linked to the American website), I would expect that they use metric in their design and QA processes.To be fair, if we dig a little further we can find in their JP supplied catalog specificationshttps://www.mitutoyo.co.jp/eng/useful/catalog-2016/html5.html#page=225which show that their stated accuracy does not include the quantization error, so it's actually +/- a count as well.

author: derGoldstein
author_rank: Regular Contributor
timestamp: January 06, 2018, 02:00:51 am
content: Thanks for all the replies, I'll likely have to e-mail several fabs and see what I can get. At least now I know I have to ask about "edge to edge" and "hole to edge" separately.I checked several boards that I got, and the worst ones were just over +0.2mm off on an 80mm board. None were smaller than specified, they were all larger, so I could file them down.If I can get them within 0.05mm that'll be fine (~0.025mm would be great, but that seems rare).The finish on the edge doesn't seem to be mentioned specifically anywhere (that I could find). That's frustrating because sometimes the overall dimensions were spot on but the edge was jagged enough to make measuring with calipers difficult to rely on (because the "fuzzy" edge would allow some compression when measuring).

author: DerekG
author_rank: Frequent Contributor
timestamp: January 06, 2018, 03:49:09 am
content: Quote from: derGoldstein on January 06, 2018, 02:00:51 amThis is usual & to be expected. As I mentioned before, V-grooving is an imprecise way of separating boards. It is only offered as a cheap method of separation for volume manufacturers who want the cheapest boards possible. It holds all the boards together for SMD loading, easy handling through the infrared/vapour phase oven & wave soldering.If you need boards of an accurate size, donotuse V-grooving. Use a specialised milling bit & discuss the finished size you desire with the manufacturer. Let the manufacturer panelise the boards. Just supply the manufacturer your Gerber files for a single board.QuoteWhy would it? The board manufacturer simply uses a scoring wheel (like a pizza slicer) on both sides of the board. The finish you end up with depends entirely on how the remaining part of the board breaks apart when you bend it.QuoteWhen V-scoring, each board is placed (panelised) exactly side by side. No space is left between the boards as no milling bit is used. The V-scoring wheel cuts the "V" by applying a lot of pressure. This "squeezes" the board which is why its size grows (generally) a tiny bit.So, now you understand it fully, all you can do is ask the board manufacturer what their tolerance is when V-scoring. Either accept that or discuss the more precise method of using a specialised milling bit.

author: Someone
author_rank: Super Contributor
timestamp: January 06, 2018, 07:13:43 am
content: Quote from: derGoldstein on January 06, 2018, 02:00:51 amIf you need smooth edges go straight for gold plating, as above the routing done while still panellised can be very accurate if care is taken so for boards that needed tight mechanical fits I've gone with plated outlines and mouse bitesinsidethe board outline so they are beneath the mating edges.

author: derGoldstein
author_rank: Regular Contributor
timestamp: January 06, 2018, 04:39:45 pm
content: Quote from: DerekG on January 06, 2018, 03:49:09 amNone of the boards were separated using v-scoring, they were all milled. I wouldn't expect v-groove separation to produce more than maybe 0.5mm accuracy.