Drilling and milling tolerances - Page 1
Forum section: electronics / general

author: ebastler
author_rank: Super Contributor
timestamp: December 19, 2020, 09:01:21 am
content: I have a design in mind which requires a little stack of PCBs, some of which are "abused" as spacers and optical masks. I need various plated through-holes to register precisely between the PCBs (i.e. end up directly on top of each other) when the stack is assembled. Mounting holes (preferably non-plated) need to register at the same time. And for cosmetic reasons it would be nice if the outer contours of the PCBs also match, without much sanding after assembly.This has triggered some questions about manufacturing tolerances and the PCB manufacturing process:Am I right to assume that the to-be-plated holes are drilled early in the process, while the non-plated holes are drilled in a separate, later step? Hence, would I be well-advised to make my mounting holes plated too -- to ensure they are drilled in the same step as the critical plated "mask" holes, and hence positioned precisely relative to the mask holes?I assume that contour milling is done in yet another step. Hence, for separate PCBs, there is no guarantee that the contours register in precisely the same way with the plated or non-plated through-holes. Right? (Or do at least non-plated holes and contours register well, since they are done on the same machine in directly successive steps?)If I combine all the PCBs into one panel, I would assume that the relative position of plated and non-plated holes, and of the milling contours, is at least consistent across all PCBs. (I.e. while all the contours may be displaced by, say, 0.2 mm vs. the drill positions, they are consistently displaced by the same amount on all PCBs.) Is that the way it works?Any other ideas for getting good positional consistency between the outer contours and the plated holes across multiple PCBs?Many thanks for your comments!

author: phil from seattle
author_rank: Super Contributor
timestamp: December 19, 2020, 06:08:01 pm
content: A lot depends on your vendor. They should specify tolerances. But I think you will be fine.I have never taken a micrometer to every board I received in a batch. I have stacked them up and have never seen an obvious issue with sizes within a single batch. I just checked boards from 3 different batches from OSHPark cut to the same outside dimensions and they are fairly uniform - a stack of them looks very close. A micrometer shows a bit less than .2mm difference (well less than one mil). My fingers can't feel the difference.On plated through holes. Some vendors are pretty heavy with the plating and others are stingy but that is like 2 or 3 mils difference. I doubt it will make a difference for your plated mounting holes. The only issue I can think of with plating mounting holes is if you use a conductive bolt/screw.

author: ebastler
author_rank: Super Contributor
timestamp: December 19, 2020, 08:30:23 pm
content: I plan to use JLCPCB. (This is just a hobby project, but I'd like to give a few PCBs away, and hence want to make sure the gadget is straightforward to build.) If I go by JLCPCB's stated tolerances, it's a lost cause anyway.E.g. they reserve the right to let non-plated hole diameters deviate from the spec by +- 0.2 mm (80 mil), regardless of the hole size...But you are right, I am probably overthinking this, and it will be good enough in practice. I certainly am not worried about the toleranceswithinone processing step (e.g. the relative position of plated holes within a PCB vs. the other plated holes.) What I was a bit concerned about are relativevariations between steps which happen on different machines, and require re-mounting of the panel. I have had some "outliers" where these were off by several tenths of a mm (100 mil and more), but normally they are better.I will proceed as outlined in my original post: Combine the position-critical boards in one panel, and specify all holes as plated to ensure they are all drilled in one go. Should be alright...

author: Harjit
author_rank: Regular Contributor
timestamp: December 19, 2020, 09:12:46 pm
content: I've made structures using milled PCBs and it works very well.One of the constraints is that slots have rounded edges. To get around that, I have put non-plated drill holes in the corners (the corners look like Mickey Mouse ears). This way, the slot length matches the "tab" on the other PCB that fits in and this has worked very well too.An alternate I tried was not doing the holes in the corner but then I used a very sharp hobby knife (Xacto) and cut the round edges into a right angle. I was careful to not make the slot longer. This worked fine.I also didn't have solder mask around the tab and the slot and I used 30 minute epoxy. I don't know if the solder mask matters or not. I was always careful to clean everything with isopropyl alcohol to remove any oils from when I was handling the boards.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 19, 2020, 09:42:41 pm
content: Quote from: ebastler on February 05, 1975, 02:03:02 pm0.2/25.4 = .00787 or 8 mil. Not tiny but not close to 1/12 inch.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 19, 2020, 09:51:36 pm
content: Quote from: Harjit on December 19, 2020, 09:12:46 pmWell, rounded inside corners, anyway. The PCB house should tell you how big a bit they use and you can dog-bone your corners to the radius if you need to fit it on something square. I looked at JLCPCB'scapabilities pagebut didn't see it.

author: ebastler
author_rank: Super Contributor
timestamp: December 19, 2020, 09:57:21 pm
content: Quote from: phil from seattle on December 19, 2020, 09:42:41 pmOops... You are right of course. I better stick with metric units for mechanical stuff...