Silkscreening convention with passives - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: December 18, 2017, 08:40:33 pm
content: In the past I've always liked drawing a box around capacitor and resistor footprints. I've felt that it's aesthetically pleasing and can help a lot when doing manual assembly for prototypes.What are the conventions at your workplaces? Box them? Line in the center? Forego silk all together?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2017, 01:12:49 am
content: I use a line across the center, if the pad gap is small. Otherwise, an '=' between the pads, based on the body outline. This is simply my procedure for all components: I draw the body outline plus some expansion, removing silk over pads.I've seen the bounding box lots of times, it's pretty good too.For small diodes and polarized capacitors (that also fit within pads), I draw a 'C' shape where the 'spine' indicates polarity mark. Except it's usually more of a... er, lopsided serif 'I' because the '=' lines can extend slightly closer to the pad, because I use rounded rectangular pads. Then I put a solid circle near the polarity-marked pad.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2017, 08:42:36 am
content: Box is all nice and looks best I think, but it'll become the limiting factor for integration density. When you need the density, you use = or - between the pads, or nothing at all.And because mixing two styles is confusing, you'll end up never using boxes anymore after you need the density once.

author: JPortici
author_rank: Super Contributor
timestamp: December 19, 2017, 09:04:34 am
content: if the footprint allows i put the = between pads. Any component that is polarized has a line or dot or cross on the cathode / plusthen i generate a separate file which has boxes with refdes inside them to see the orientation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2017, 12:12:44 pm
content: Quote from: Siwastaja on December 19, 2017, 08:42:36 amHmm, although -- if you make and keep '_L', '_N' and '_M' versions (high, medium and low density) as needed, you could use the '=' or '|' style for the high density versions. It would even make sense to use different ones within a given design, as it's obvious an area is high density and you had to sacrifice on style to make it fit.Keep in mind, if you can't spare the space for an outline, you certainly won't have enough for individual designators. So, it's an even bigger sacrifice than just style.You might have enough area off to the side somewhere to group them, otherwise you'll need to proceed carefully with a separate assembly drawing (or have it done all by machine and not worry).Tim

author: mc172
author_rank: Frequent Contributor
timestamp: December 19, 2017, 12:44:14 pm
content: I'm one of those bastards that argues against silk and I try to omit it completly. The pick and place machine doesn't read it anyway.For fault finding they're going to have to refer to some other documentation (schematic and probably the layout as well) anyway so why not just get the positions of the components from the sheet you'd be looking at anyway?

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 19, 2017, 01:26:54 pm
content: Quote from: mc172 on December 19, 2017, 12:44:14 pmI agree for reference designators (I put them on a seperate mech layer and use it as an assembly output) but stuff like polarity and pin 1 indication is 100% mandatory, all the time.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2017, 02:15:23 pm
content: Reference designators in the silk screen just don't work out on a modern high-density board. Heck, it doesn't even need to be super modern.Silkscreen has become almost irrelevant. It was relevant when through hole designs were hand placed for wave solder. In a line of humans, each person inserting one (or a few) designators only to minimize mixups.In my designs, typically only 10-20% of the components have their designators left after the layout is finished, and I'm seldom doing extreme miniaturization, pretty basic stuff with 0402 mostly. Which makes me wonder, why do I need the designators at all, what purpose do they serve?Even if I'm able to squeeze the designators in, they are difficult to read and match to the actual components - pick&place operator either completely trusts the P&P file, or does the manual sanity check with a separate assembly drawing - for which I use component-sized boxes with the refdes inside. For repair/mods/reworking, finding a component by the designator only is a nightmare anyway - you need some kind of a drawing or a photo to assist you.So, it's a separate assembly drawing in any case that's really used. Silkscreen is mostly left for other informative purposes, like instructions, jumper configuration specifications, pinouts, manufacturer name, revision markings etc. - And for marking pin#1 or polarity! But marking the SMD passive outlines or designators is the least important purpose it serves, because it simply doesn't suit for that job in the 2000's.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 04:00:26 pm
content: Quote from: Ice-Tea on December 19, 2017, 01:26:54 pmthe space industry does not allow silkscreen, or even a mask. there are other ways of designating pin1 or polarity, for example a slightly different pad shape. but yes, for most designs, it's a good idea to indicate the correct alignment through silkscreen.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 19, 2017, 04:38:54 pm
content: Quote from: nicalitz on December 19, 2017, 04:00:26 pmNo mask? I presume so that no gas is cought in a via or something?Assembly shops don't like different pad shapes on connectors much

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 05:40:29 pm
content: there's a lot of reasons to avoid materials that outgas in vacuum. most notably to prevent condensation on/inside optical elements.interesting. what exactly is their concern with different pad shapes? i've never had any issues. but that being said, im used to dealing with shops that are familiar with our requirements

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2017, 05:51:36 pm
content: Non-issue. If you can't have silkscreen, then you just don't; then the assembly house needs to rely on the two standard ways:1) P&P file, which is the primary machine readable (CAM) document in a standardized format (however, the standards are long, nonintuitive, not well known about, and poorly followed)2) Assembly drawing, the purpose of which is to provide a human operator a way to verify the intention shown in the P&P fileSilkscreen orientation marking would be the tertiary confirmation. Nice to have, but if you can't have it, then you just don't. Might be due to outgassing of the ink, or just the miniaturization level preventing any meaningful silkscreening. (You know, if you mark the important detail too far away from the relevant location, or if the detail is near multiple potentially relevant locations such as different components, then it's going to confuse instead of helping.) If the assembly house has a problem with this, then it's best to find another one.

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2017, 11:41:20 pm
content: I put reference designations on silkscreen whenever it makes practical sense to do so. Much of what I make is hand assembled and if it's not, I'd still like people to be able to service it. Normally for stuff like resistors I don't silkscreen the outline but occasionally for larger parts I'll draw an outline on the board and use a dot to denote pin 1 on ICs and connectors. Really it depends on the design.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 20, 2017, 09:55:59 am
content: Quote from: nicalitz on December 19, 2017, 05:40:29 pmhttps://www.smta.org/chapters/files/wisconsin_dfx-root_cause_failure_analysis_final.pdfPage 41. So, not really about different pad shapes but square in general. Memory must be a bit rusty. Even though I remeber reading about mixing it up, too. Something to do with uneven heating or some suchMore thermal mass for one pad, process variation perhaps? Dunno...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2017, 12:14:01 pm
content: I was slightly confused for a moment, but then aha, Plexus, that explains it. Probably the biggest assembler in WI...Tim

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 20, 2017, 01:47:57 pm
content: Meaning... what? Better forget about it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2017, 01:51:02 pm
content: As in, there isn't a bigger player than Plexus in all of WI, AFAIK. Sorry to confuse.Tim

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 20, 2017, 02:21:02 pm
content: Quote from: T3sl4co1l on December 20, 2017, 01:51:02 pmI'm guessing it means it holds some value then? Because I have no idea if 'biggest player in WI' means anything. Wisconsin could have the electronics industry of a peanut for all I know.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2017, 02:31:43 pm
content: Actually on closer inspection, WI is one of the bigger states for electronic assembly.But that doesn't really mean much as so little assembly is done in the US to begin with (10k's employed per state vs. millions in China).Tim

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: December 21, 2017, 07:06:37 am
content: I'm also in the camp of thinking silk screens should be limited to the absolute minimum. The internet is a thing and if you publish designators on a downloadable pdf then you and others can view them on a huge computer monitor and just cross reference.I have done boards where I put boundary lines around modular sections on the PCB such as "power supply", "VCO", "Second Stage", "Input Amp 1" that reference the schematic and they can be handy.Also you can screen IO because that can be very handy for quick debugging. Things such as RX/TX, MISO/MOSI, GND, 3V3 on the pads that are for external interfacing.Lastly would be the things I consider most essential, company name, board name, revision number but even those I have been etching on the copper and exposing the mask. My thought here is that it will last longer and then I don't need to worry about the silk screen quality from manufacture. I realise it does not 'pop' out as much.What I never do is put designators for passives. They are way too small, and yes I see a post above saying 0402's are MASSIVE but that has to be a joke. If you need 0402's or smaller then one of you aims is probably high density and in that case you want to pack those things as close together as you can and a silk screen is just going to look like a mess.So I say skip the passives and use a nice PDF as a guide if you need it. I'm also hand placing components with the aid of the computer so it's no big deal to have a board with no silk screen on it.