PCB Thickness: What to Expect - Page 1
Forum section: electronics / projects

author: EPAIII
author_rank: Super Contributor
timestamp: September 27, 2023, 07:13:59 am
content: I am working on a design where a combination of the normal electronic concerns as well as some geometric/mechanical factors must be combined. And I want to use a couple of PCBs in such a combined role. I am trying to work my way through the various design factors that the PCB fab houses publish and I am finding that it is somewhat difficult to put them all together to arrive at the numbers that I need.The design is only in the very early stages so I do not have full details. I will try to provide what I can. At this point I am thinking of single layer PCBs about 1" or 25mm in diameter with a number of NPTHs in it. There are no plated holes and no active components. Until I see a reason for another board thickness, I am thinking about 1/16" or 1.6mm. And the design could easily evolve into the need for a larger diameter. That is not hard and fast.One thing that I am considering is pressing some 4mm, ball bearing balls into several of these NPTHs. They would reach almost to the opposite side which has no copper and then be soldered to the copper donuts surrounding those holes. These balls would serve as the contacts of several switches. A problem arises because at least some fab houses require a copper free ring surrounding a NPTH before the copper of the pad begins. For instance, JLCPCB says they will leave a ring of bare FR-4 between 0.2 and 0.25 wide. I worry about getting the solder to jump that gap.Where things get hard to design is knowing the thickness of the PCB that I can expect so I can determine just how much of that copper less ring the ball would need to force itself into in order to reach the copper donut pad.I do understand tolerances. What I do not know is what the thickness dimension of a PCB actually refers to when they say, for instance, 1.6mm +/-10%. That translates to a range of 1.44 to 1.76 mm. Fine and good. But what is included in those numbers. The FR-4? Certainly. The copper? Probably. But what about the plating? Is that included? I see no specs for the thickness of the plating. And is there anything else to consider?And then that 10%. Is that much really encountered often? Or would 99.9% of all boards made be a lot tighter than that? And the 1.6 mm base number. Is that really at the center of the tolerance range? Or do most boards measure near one end or the other of that range? Or do they actually use the original 1/16" number for the target value? And of course, there is a tolerance on the hole sizes which also figures into this. I really need to figure the worst case situation.In other words, what can I reasonably expect?Any enlightenment here would be greatly appreciated.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: September 27, 2023, 12:10:49 pm
content: When I order 1.0mm thick boards, thickness is always 1.0mm (with copper on both sides) and 0.8mm for no copper areas. That's what my caliper says. It's quite strange, because I doubt that copper is 3oz (0.1mm) by default for my orders, it is possible probably thinner. I'll look into this next time I order some PCBsI like 1.6mm boards. 1.0mm boards are quite rigid even for sizes around 50x50mm.. I use 0.6mm and 1.0mm for microwave circuits, because traces become too wide for the same impedance and may behave strangely.For your project, experimentation would probably be the best option. I think that it is better to use plated holes and add circles made of VIAs around them. It may improve mechanical strength, especially if solder fills the VIAs too. So why do you try to avoid plated holes?

author: CountChocula
author_rank: Unknown
timestamp: September 27, 2023, 01:54:02 pm
content: My suggestion would also be to try and figure this out experimentally. You could design a test board that all of the elements you need—in fact, ideally, you'd want severalvariationsof each element, so that you can see how the fab treats different setups. Pretty much any of the popular DYI services will make a board for a nominal fee, and so this feels like an inexpensive way to make sure that you're going to get what you need before you commit to the time it takes to design a final board.—CC

author: lo_volt
author_rank: Newbie
timestamp: September 27, 2023, 02:47:03 pm
content: I'll suggest that you contact the board house and talk to them about tolerances and what to expect on a finished board. JLCPCB, OSHPark and most Asian suppliers sell boards cheaply because they use a standard and very common board spec. This allows them to group different boards for different customers when they fab the boards. There are domestic board houses that will work with your requirements to build the boards you need. They will most assuredly cost more but if you need something out of the ordinary, that may be your best option.

author: ajb
author_rank: Super Contributor
timestamp: September 27, 2023, 03:36:53 pm
content: Quote from: EPAIII on September 27, 2023, 07:13:59 amCNC touch probe?QuoteSometimes it's helpful to tape a pair of calipers and set them to a dimension and look at it -- or if you have one of those PCB rulers with sample trace/space widths, that would work too -- for an intuitive sense of how something will work. 0.2-0.25mm is pretty small, I don't think you need to worry too much about bridging the gap, especially since the side of the ball and the top surface of the board form a slightly acute angle, which should make for a fairly nice fillet situation. If you're planning to stencil paste on, then you probably need to design the stencil with apertures right up to (or even slightly crossing into) the hole to ensure the paste is in contact with the ball before reflow, and you might also have to adjust your process to account for the thermal mass of the balls.QuoteI don't think pressing the balls into the PCB is likely to work. Normally things that are press-fit into FR4 have sharp edges that can dig into the material, while the gaps between the edges provide space for the displaced material to flow into. The balls don't have that feature. Per my quick sketch here, the wedge angle of the ball at the top surface is 14-33° for 1.44-1.76mm of PCB thickness, which I suspect is too large for the ball to actually be retained once pressed in but too small to prevent upsetting the edge of the FR4, which will mess with your pad and make things harder to solder. You could try asking the fabricator to leave copper right up to the edge of the hole--often fabricators will do things outside of their normal spec as long as you accept that the results may suck and/or pay them extra to deal with it.QuoteIf by "plating" you mean the copper buildup, that's accounted for in the copper thickness. If you mean the finish (HASL, ENIG, or whatever), that varies with the specific finish. HASL has more significant variation in thickness over the board, so presumably has a much larger overall thickness tolerance as well. ENIG is much more consistent, which is why it's preferred for leadless parts. OSP I guess would be effectively zero thickness after soldering.QuoteGenerally, you're going to tune any process to hit the middle of your tolerance band, so that you have maximum margin for error in both directions. I would expect any decent, established PCB fabricator to do the same, and furnish a board that's pretty close to the nominal thickness pretty much every time. Generally the final deviation is assumed to have a normal statistical distribution, with more parts closer to nominal and fewer parts further away. Analyzing tolerance stackups can get fairly complicated, but here's a basic overview of the math:https://www.smlease.com/entries/tolerance/tolerance-stackup-analysis/(Found in a cursory google search, there are many more resources out there on the subject).Depending on your application, it may be better to allow for final adjustment of the overall assembly to compensate for PCB/assembly tolerances, rather than trying to control all of those upstream tolerances. If you are doing something like a CNC touch probe, the standard practice for those is to include centering adjustment screws in the probe base, which compensate for any deviation in the probe body/mechanism as well as for runout in the probe tip itself whenever that's replaced.

author: jonpaul
author_rank: Super Contributor
timestamp: September 27, 2023, 04:19:22 pm
content: In USA stock FR-4 G-10 Fiberglass stock is fabricated in 0.062 (62 mil) thickness.We used 0.032 (32 mil) for a special HV application.Plating and solder mask add to the bare fiberglass thickness.Non standard thickness boards are usually special order, MOQ/MPQ appliesJon

author: EPAIII
author_rank: Super Contributor
timestamp: September 28, 2023, 06:43:47 am
content: Why am I avoiding plated holes? Because the board will be mounted on a metal part - flat against that metal part. So I don't want a bottom layer that would short out. And I don't want to add an insulator. But that's just where I am now and, as I said, things are very preliminary. Perhaps that will change as the design progresses.Quote from: Georgy.Moshkin on September 27, 2023, 12:10:49 pm

author: wraper
author_rank: Unknown
timestamp: September 28, 2023, 06:52:42 am
content: Quote from: EPAIII on September 28, 2023, 06:43:47 amMakes no sense. Nothing forces you to connect them to any traces on the board. I'm not saying you need them, just that reasoning is strange.

author: Psi
author_rank: Super Contributor
timestamp: September 28, 2023, 06:56:35 am
content: I've had 0.8mm PCBs that were 0.72mm full stack, etc.. So you do see error in PCB width.You should be prepared to deal with 0.1mm over/undersize.But check what spec they give for thickness tolerance

author: EPAIII
author_rank: Super Contributor
timestamp: September 28, 2023, 07:46:57 am
content: Ajb,CNC touch probe? No, a Z axis calibrator. Kind of the same thing, but upside down. I am trying to design it with unique features and come up with an attractive price. Not an easy thing to do in the US. I need all the help I can get. Perhaps I am giving away too much here, but then, this is not a simple project. Anyway, talking to some of your points:I know very well what 0.2mm looks like. Stack two sheets of common, copier/printer paper and look at the combined edges. That's just about 0.2mm. Also it is about 0.008" if you are more comfortable with the British king's finger. I work back and forth between both systems. But YES, it is small. On the other hand I have never tried to solder anything like this before so it is a point of concern for me. I have been looking into soft soldering stainless steel because it is corrosion resistant. I need to do some experiments with different stainless alloys. But it does not look good. Ideally a paste that is made for that would be great, but how to find it? Another web search to start. And perhaps some visits to local welding supply houses. And I know I may need to find another alloy that is easier to soft solder. Fortunately, balls are made of many alloys. Unfortunately many of those alloys are prone to rust and would normally have a grease or oil bath retained with them in a BB raceway to prevent that.Pressing the balls into the PCB? Obviously this is something that is not often done. I have no idea if it will work. My first idea was to cut the balls in half and surface mount them. That would be fun! And in a moment of sanity I thought of just pressing them into a hole and soldering them. The angle does not worry me. They would only need to stay put until I can melt the solder. I don't know if that would upset the copper pad and surrounding area to a degree where it would not be viable. Another area where I need to experiment.By plating yes, I did mean the finish applied to the copper traces. I have made and have had made PCBs for many, many years now, going back to pasting donuts and black tape on plastic drafting medium. IIRC, one fab that I used did a tin and lead plating through a mask and used that as the resist when etching the copper. Then they heated the etched board to combine them into a solder plating which was noticeably thick. I am not up to date on all of today's possibilities. So I am now studying the available finishes. Thanks for poking me in that direction.And yes, there will be adjustment screws.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: September 28, 2023, 08:13:45 am
content: One way is to solder copper washer to cover the gap, but it is probably not the best option. Find copper washer with best outer and inner diameter and thickness. Order thick stencil with ring openings (divided in sectors to hold ring in the center), apply solder paste to all rings at once, put washers on top of each solder paste ring, and do reflow soldering using hot plate or some better reflow machine.

author: jbb
author_rank: Super Contributor
timestamp: September 28, 2023, 08:30:58 am
content: On thickness tolerance: you absolutely WILL see the +-10% occur. You are also likely to get different distributions from batch to batch.On Cu and plating: you might have 35 um of Cu (1 oz/in2) and… err… 15um of ENIG? That’s 50 um all up, so the tolerance of Cu etc can get lost in the washOnce upon a time we tried to get tighter thickness tolerances and it just never went well.

author: Someone
author_rank: Super Contributor
timestamp: September 28, 2023, 08:39:06 am
content: Quote from: jbb on September 28, 2023, 08:30:58 amIt can be done if you have a co-operative board house, but they need to run some wastage through the presses to get the process under control (same method is used for tight impedance control) so its expensive in time and resources.

author: EPAIII
author_rank: Super Contributor
timestamp: September 28, 2023, 10:03:34 am
content: Nothing forces me to connect them to any traces? I am trying to build a very precise switch. One that will make and break contact at a given position that remains the same over many cycles of use and over long periods of time (years). The balls are there as conductive contacts. They ARE the circuit on the board. So YES, I must connect them to the traces.But thanks for asking. Thinking about this has given me some additional ideas.Quote from: wraper on September 28, 2023, 06:52:42 am

author: EPAIII
author_rank: Super Contributor
timestamp: September 28, 2023, 10:05:17 am
content: Thanks! That's the kind of real world info I was looking for.Quote from: Psi on September 28, 2023, 06:56:35 am

author: EPAIII
author_rank: Super Contributor
timestamp: September 28, 2023, 10:07:39 am
content: More good real world info. Thanks!Quote from: jbb on September 28, 2023, 08:30:58 am

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2023, 10:04:27 pm
content: Also be aware some of the cheaper PCB places have larger allowable thickness tolerance so it's easier for them to stock less raw material options. eg, they might choose to save money by not carrying all pre-preg/core thicknesses. But as a result they cannot actually make pcbs exactly 0.8mm for example, even if they wanted to. Based on what core/prepreg they have they might be able to do 0.75 or 0.89mm So they will have spec'ed their tolerance so that is ok for someone ordering a 0.8mm pcb (as an example).Basically what i'm saying is... Just because you spec a thickness doesn't mean the normal distribution over all batches you order will center around the thickness you spec'ed, it often doesn't due to reason above.

author: EPAIII
author_rank: Super Contributor
timestamp: October 01, 2023, 11:48:06 pm
content: Well, that's one I wasn't thinking about. But, now that you mention it, it makes a lot of sense. For them, of course.This is kind of like the resistor values for a given tolerance. They are set up so that any resistor that a company makes, regardless of it's actual, measured value, can be marked as one size or another in the range of values and fall within tolerance. So when a 5% resistor that was supposed to be X Ohms actually measures X plus 5.0001% Ohms, it is just marked the next size up. Ditto for the other tolerance ranges. And NO WASTE. No parts ever need to be trashed.Smart business logic. Thanks for pointing it out here.Quote from: Psi on September 30, 2023, 10:04:27 pm

author: Microdoser
author_rank: Frequent Contributor
timestamp: October 02, 2023, 02:05:29 pm
content: I still don't understand why you don't want plated holes, the balls will protrude aboveandbelow the board, so will touch whatever is above and below far sooner than the bare copper around the hole they are sat in.You say you want to mount the board flush against the case/backplate/whatever it will be mounted to and don't want to use insulation but don't give any design reasons why except you 'don't want to'.If you could explain why the board must be flush against the case, and why the case must be electrically conductive, then IMO we could supply more meaningful and helpful answers.

author: Microdoser
author_rank: Frequent Contributor
timestamp: October 02, 2023, 02:15:01 pm
content: Quote from: EPAIII on September 28, 2023, 10:03:34 amPersonally, I would not reinvent the wheel. Very precise and reliable switches exist that are rated for hundreds of thousands, if not millions, of operations and they are very cheap. They are in many sizes from the very small to the very large.

author: EPAIII
author_rank: Super Contributor
timestamp: November 17, 2023, 06:34:23 am
content: Still working hard on this and reviewing some of the basics.OK, I have decided to use plated holes. And I am going to have some additional "PCBs" made with absolutely no copper on their outer sides. They will act as insulators and will be bonded to the active boards with epoxy or something even better.The balls will NOT protrude both ways, only above on the lower PCB and below on the upper PCB. At present I see them being dead even with the other surfaces of those boards and after they are bonded, with the insulation boards which will also act as mechanical surfaces they rest on to prevent any movement from the many cycles of contact they will make during the life of the device. So the insulation boards will have a dual purpose.And nothing is intentionally flush against the body of the device which will be at battery-negative potential. But the clearances are small (thousandths of an inch/tenths of a mm) and shorts would easily be possible. Disassembly to remove them would be an undesirable "feature".The body WILL be metal, aluminum or stainless steel, I haven't completely decided.And all of this is totally irrelevant to my question about board thickness tolerances and the actual deviations to be expected.Quote from: Microdoser on October 02, 2023, 02:05:29 pm

author: EPAIII
author_rank: Super Contributor
timestamp: November 17, 2023, 11:04:15 am
content: Show me just one switch where the manufacturer GUARANTEES activation AND de-activation (connect and disconnect) within a stated mechanical range that is under 0.001"/0.025mm and at a cost under, say 5 USD each (qty of 100). And that is guaranteed to maintain that same positional accuracy over thousands (preferably tens of thousands) of cycles and periods of months (preferably years). Heck, most switches deliberately incorporate some form of hysteresis. And I am trying to avoid any amount of that.Actually, one thousandth of an inch is the absolute LARGEST range I hope to achieve. I really want ten-thousandths of an inch (microns) over those conditions, if at all possible. And these numbers come from the basic purpose of the device I am trying to make.I don't think there is a single manufacturer in the world that even publishes the specs for that, much less having a spec that meets my needs. And if there is, the price is likely 100 times or more than what I could ever budget for this.In short, I really DO need to reinvent the wheel.OK, after writing the above I got curious. I looked at switch specs. As I suspected, I could not find any specifications on the accuracy of the ON and OFF points for any of them. So I found some used micro switches in my stockpile and tested three of them. All three had very consistent ON points: +/-0.0005" or less. That really surprised me. But the first one had an OFF point that was about 0.010" above the ON point and the second one, a very similar model by the same manufacturer, had it at about 0.015" higher. No where near what I wanted. The third one was within about 0.0005" for the two points. But how do I know which are which? Am I supposed to just buy different ones and test them until I find one that works - and hope the next production run is the same? As I said, there are NO SPECS on this.Quote from: Microdoser on October 02, 2023, 02:15:01 pm

author: Microdoser
author_rank: Frequent Contributor
timestamp: November 17, 2023, 03:53:53 pm
content: Sounds like you need a long and in depth conversation with a number of fab houses about specific requirements rather than an open discussion on a forum.

author: donlisms
author_rank: Frequent Contributor
timestamp: November 17, 2023, 05:55:22 pm
content: I still can't get over the idea of trying to solder ball bearings. You've already tried and succeeded, yes?

author: EPAIII
author_rank: Super Contributor
timestamp: November 18, 2023, 12:58:30 am
content: Perhaps so. Thanks for your replies. I do appreciate it.Quote from: Microdoser on November 17, 2023, 03:53:53 pm

author: EPAIII
author_rank: Super Contributor
timestamp: November 18, 2023, 01:17:42 am
content: Tried and succeeded? Well, NO.I am working on it and it may be possible. But there are difficulties. I probably could solder ball bearing balls made of other steel alloys, but those would be prone to rust and I do not want that on switch contacts. So, stainless steel seem to be the way to go. The soldering fluxes that are in common use in electronics just will not work on stainless steel. You can use all the flux you want and all the heat and you are still left with a ball of solder that did not adhere to the stainless steel.There is a flux that is said to work, but it is a strong acid. So, I would not want that anywhere near a PCB. Nor would I want it on the area of the SS ball that will be making contact with the opposite SS ball. So, how do I use that acid flux on about half of a 4mm SS ball to allow me to tin that half while preventing the acid flux to the other half. Oh, and I would not want any solder on the other half, just on the half that will bond to the PCB.So, I am experimenting with techniques. So far, nothing that I can actually use. I purchased a solder pot and a couple pounds of solder. What I need is a way of coating 1/3 to 1/2 of the balls with something to keep the acid flux and solder off. Then dip in flux, followed by a dip in the solder pot, cleaning ALL the flux off, soldering them to the PCB (technique unknown), and finally removing that flux and solder resistant coating, perhaps with a solvent of some sort.I am also looking at conductive epoxy. But I haven't purchased any to try yet. All around, it may be a much easier way to do this. And a few extra Ohms will not hurt my circuit.Quote from: donlisms on November 17, 2023, 05:55:22 pm

author: amyk
author_rank: Super Contributor
timestamp: November 18, 2023, 03:43:18 am
content: Quote from: EPAIII on November 17, 2023, 11:04:15 amA digital caliper ASIC with some modifications to switch on specific readings is probably within the range of precision you're after, and 5 USD is definitely more than what some cost (in qty 1).

author: Microdoser
author_rank: Frequent Contributor
timestamp: November 19, 2023, 03:30:56 pm
content: Quote from: donlisms on November 17, 2023, 05:55:22 pmNot only that, but to within those tolerances too. This would introduce yet another tolerance issue (and tolerance issues stack up rather than cancel out except in rare situations) of finding ball bearings with finer tolerances than you need, and soldering them in accurately enough that their final resting place is still within those tolerances.I can't help but think this physical electrical engineering problem is one that might be better fixed by using a non-contact detection method and some tweaking in software.

author: ajb
author_rank: Super Contributor
timestamp: November 19, 2023, 09:23:38 pm
content: There's no need for the positions (or sizes) of the balls to be so accurate. The three-pins-on-three-pairs-of-balls arrangement is kinematic or critically constrained: there is exactly one position that will satisfy the system regardless of how accurately the contact components are positioned (as long as the errors aren't so egregious that they can't make contact at all). Reasonable errors in positioning the balls or pins relative to each other will cause an error in the position of the stylus relative to the probe body, but again: that is what the adjustment screws are for. The probe kinematics just need to put the stylus within a millimeter or so, and then you use the adjustment screws and a dial test indicator to adjust the runout (on a work probe) or perpendicularity (on a tool setter) of the stylus tip to as many decimal places as you want.Quote from: EPAIII on November 18, 2023, 01:17:42 amCarbide is typical. Rigidity of the contact elements as well as the rest of the kinematic chain is more important to the performance of the probe than initial accuracy of the contact positions. Imagine jacking up a car: the wheel doesn't immediately leave the ground with the first crank, because the weight of the car has compressed the suspension and flattened the bottom of the tire against the road. So as you crank the jack up, you have to relieve that initial strain in the system -- the suspension has to fully expand, and the tire has to relax into its normal round form -- before it breaks contact. Switch between front and rear wheels, and it will probably take a different number of cranks, because the weight distribution and suspension characteristics (and maybe even the tires) are different, so there is more or less strain in the tire/suspension to relieve before the wheel leaves the ground. Whether the car is initially sitting on a flat surface matters a lot less (as long as it's not going to roll away on you...).The same thing happens in a probe like this, although at much smaller scales. Under preload from the spring the mating surfaces of the pins and balls will conform to each other just like the tire conforms to the road, and the pins, probe stylus, body, etc all deflect a bit just like the suspension does. If there is more strain at one of the contact sets than the others, because of differences in spring pressure, or nonuniform rigidity in the system, this will result in different actuation travels in different directions. Ideally, the probe mechanism, not to mention the machine, would be perfectly rigid and any amount of deflection at the stylus tip would separate the probe contacts and be detected. That's not possible in the real world, but a hard material like carbide that resists deformation under point contact is preferable for the balls/pins because it means less overall strain, and less variation in strain with load/position changes. The rest of the system should be reasonably rigid as well, and most importantly it should beuniformlyrigid so that the amount of deflection required at the stylus tip to part the contacts is consistent in all directions.It is possible to null out some residual errors via the chosen probing strategy or calibration method. During a measurement, a longer actuation travel in one direction makes the stylus appear smaller in that direction, so a strategy that compensates for runout compensates for that as well. A lot of that is baked into the standard probe/setter calibration routines from Renishaw et al, but getting the probe right to begin with so the error is consistent in all directions is of course better.Quote from: Microdoser on November 19, 2023, 03:30:56 pmPhysical contact is the simplest way to achieve these levels of precision and repeatability in tool and work probing. There are laser-based tool setters, but the optical and optoelectronic aspects required to get into the range of precision and repeatability that's required here are not trivial. They have advantages for very small tools (<1mm), but there's a good reason that 99% of tool and work probes use the basic mechanical contact mechanism the OP is trying to replicate. Work and CMM probes that can do surface profiling use strain gauges or capacitive sensors to provide an analog output, but they still use a stylus to physically contact the surface. Anything else is too vulnerable to confounding factors external to the probe. A work probe has to deal with all kinds of shapes and a wide range of materials -- steel, aluminum, plastic, composites, etc. A tool setter can count on dealing with things that are roughly radially symmetric, but still a range of materials -- tool steel, carbide, bonded abrasive, diamond, etc -- and it needs to accurately detect the very outermost edge of a thin, sharp cutting surface, often with complex facets. So all in all would be hard to come up something that would be simpler than mechanical contact unless you can constrain the material or shape quite a bit.

author: ajb
author_rank: Super Contributor
timestamp: November 20, 2023, 12:10:13 am
content: Oh, and as far as soldering: if the balls are a suitably corrosion-resistant material to make for a decent switch contact with the pin, then why not use that to your advantage and remove the PCB from the mechanical equation entirely? Fit the balls directly to a mechanical frame and use spring contacts on the PCB to make contact with them. The frame could be something nonconductive (glass reinforced plastic, ceramic if you want to get fancy?) but a metal frame wouldn't be hard either: just use a nonconductive epoxy loaded with glass spheres (to enforce a minimum bond thickness) to bond them in. Or heck, if the frame is aluminum, get it anodized.