Component layout on PCB's - Page 1
Forum section: electronics / beginners

author: cksa
author_rank: Regular Contributor
timestamp: July 07, 2010, 12:07:56 am
content: I've been doing my component layout for my PCB in Eagle on 1mm grids. I now notice that Sparkfun uses 0.05 inch grids.Is there a difference? Will board houses readily accept designs made on 1mm grids (I can't see why not, but I'd like to make sure)?

author: EEVblog
author_rank: Unknown
timestamp: July 07, 2010, 12:32:27 am
content: It does not matter, you can use no grid at all if you want (bad idea though), it makes no difference to the board maker.What grid you use will depend on what type of components you are using and the design requirements.Sparkfun would use an imperial grid because they design a lot with through hole components and connectors which are mostly still 0.1" pitch or multiples of.Dave.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 07, 2010, 05:32:59 am
content: Yep, imperial grids are far more common due to a lot of components still conforming to sub multiples of inch spacings. eg 0.1 for DIP, 0.05 for SOIC etc.It is always amusing to have your data sheet tell you the pin spacing is 1.27mm, which is really 50thou (mil).The imperial measurement is of course missing on the datasheet.It is always worth converting oddball metric dimensions, they may be nice imperial values in disguise.

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2010, 09:21:44 am
content: In my experience, through hole is generally imperial and SMT is more often metric so choose a grid which fits most of the components on the board.I once used 1.25mm pin spacing for DILs until I needed to fit a 40 pin socket which gave me a few problems, fortunately it was fixed by drilling the holes slightly larger.

author: jahonen
author_rank: Super Contributor
timestamp: July 07, 2010, 01:30:27 pm
content: Whatever coordinate system is used in PCB design, one must ensure that it is maintained through the process. That means if you use mils, then your gerbers and drill files should be in inches. I use grid when placing the components but PADS router works best without grid.All newer (TQFP, BGA, QFN etc.) component cases are in metric dimensions, so I usually use millimeters (I can't remember when I made the transition from mils to millimeters). Mils convert exactly to millimeters (with hefty number of decimals but anyway) but not other way around.Regards,Janne

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2010, 05:33:30 pm
content: I find mils confusing because many people abbreviate millimetre to mil. The correct term is thou, I think mil is an Americanism.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 07, 2010, 09:35:08 pm
content: mil is standard terminology within the PCB industry, and yes it is a thou, not a millimetre.I learnt this from the early days of Protel, and that was when they were a small fledging company in Hobart, Tasmania, well before the Americans bought them out, and Australia is allegedly a metric country (just try getting any metric bolt though at your small hardware outlets)As for a mil being an exact multiple of millimetres, what drugs are you on? Decimal places to billy oh do not countI do agree that you should stick to the one type of grid throughout your design as it sets the expected track clearances consistently.When it comes to connecting to an off grid component (eg TQFP to a 0.1" header), Protel at least allows you to pick the unrouted net at the off grid component and will snap to the centre of the pad. You can then radiate from the pad and eventually step onto the usual grid with a 45 deg transition without too much hassle.Amother odd bal spacing involves DB9 DB25 etc connectors. They don't match either grid nicelyAs for using a metric or imperial grid, well it really is determined by the bulk of your components footprints.If you only have one or two oddball spacings, use the other grid as it matches with the majority of the components much better and you'll have less hassle pushing tracks between pads.

author: Unknown
author_rank: Unknown
timestamp: July 07, 2010, 09:46:16 pm
content: Quote from: RayJones on July 07, 2010, 09:35:08 pmThe inch is defined as exactly 25.4mm, so a mil/thou is exactly 25.4um (=.0254mm). It's not possible to represent 1/25.4 in a reasonable number of decimals. He didn't claim it was an exact multiple, but that you can exactly represent it in the decimal system (in just three digits). Although you could argue that an exact binary fraction is more important, and neither of the conversion factors are nice binary fractions.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 07, 2010, 10:19:45 pm
content: Quote from: alm on July 07, 2010, 09:46:16 pmIf you think that is workable you too must be on drugs25.4 um - get off the grass, were designing PCBS, not microchips!As I said, use the grid most appropriate for your design.To battle along with a metric grid with predominantly imperial component spacings is foolish, as is doing the opposite.Only the designer knows what components they will be using.Each has their purpose when used correctly.

author: Unknown
author_rank: Unknown
timestamp: July 07, 2010, 10:29:18 pm
content: Quote from: RayJones on July 07, 2010, 10:19:45 pmWhat is it with you and drugs,projection? But I'll get of your lawn with the other kids and leave you alone with your drugs.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 07, 2010, 10:47:57 pm
content: LOL, it is a common Aussie phrase for "you can't be serious?", as is "get of the grass", and if you failed to notice, the comments were associated with grin. ie just a bit of gentle ribbing.I was simply stating practical facts and also offering alternatives, perhaps you could try offering some constructive, practical comment on the actual topic?ie to use an absurdly high grid resolution defeats the purpose of using a grid in the first place, which is to quickly and efficient place tracks which sufficient clearances without needing to stop and think all the time.

author: Unknown
author_rank: Unknown
timestamp: July 07, 2010, 11:55:43 pm
content: I know you were joking, but I felt it did get a bit repetitive, you should vary those expressions a bit more.I wasn't suggesting that someone should use a .2um grid or something like that, but metric vs. imperial is a related choice. For example, things like Gerber and drill files can be both in mm and inch. In that case, being able to express both exactly in just a few digits is useful. I do agree that choosing the grid that corresponds to the pitch of most of your components makes the most sense (whether it's 50mil or 1.27mm), but I don't usually think that adding AOL-style me-too posts add much to the discussion.

author: jahonen
author_rank: Super Contributor
timestamp: July 08, 2010, 02:17:52 pm
content: Yes, indeed I meant that 1 mil = 0.0254 mm, which is accurately represented by "reasonable" number of decimals, whereas other way around is not, 1 mm = ~39.37008 mil. Or it is exact if using fractions, 1 mm = 5000/127 milI did not mean that would be good choice as a grid pitch, as it would not be even fine enough as demonstrated by the following example:There is a similar problem for using imperial grid even with imperially dimensioned components if the pitch is small enough. Take a IC with 0.635 mm pitch (or 25 mils if you prefer) and even number of pins per row, now if you have the component origin at the geometrical center of the package (for the pick and place machine), then you would need a 0.3175 mm (12.5 mils) grid to route the pins using the grid. So one would fallback to decimals anyway at some point. PCB Matrix IPC-7351 PCB land pattern calculator defaults to millimeters, so "go figure".Mentioned "optimum grid"-hassle is one of the reasons why I personally won't use a grid at all when routing, since the software I use will push away the neighborhood traces anyway according the design rules (unless I protect them, in which case it just won't let me route too close) when I route too near which would otherwise violate the rules, and traces will automatically snap into pads regardless which end I start the traces. It might be necessary to fiddle with grid with less advanced PCB software which can't do the aforementioned "plowing".I believe the "professional" layout guys at my work do this similarly, or at least the grid is most usually completely off whenever I open the layout file for checkingOTOH, I use something like 0.5 or 0.2 mm grid when placing the components, it is then easier to make them in nice rows. I never have had any problems doing that.Regards,Janne

author: RayJones
author_rank: Frequent Contributor
timestamp: July 08, 2010, 11:13:45 pm
content: That is interesting about pick and place machines.I have always set the origin of the device to be the centre of pin 1, thus always keeping the pads on grid. I'm pretty sure this is the recommended standard for the Protel component libraries (or at least it was).Are the pick and place machines that fussy?I always thought the use of feducial markers was far more important as you cannot rely on the board's outer dimensions for referencing the machine.The Protel packages I have used tend to use a 20mil component placement grid by default (though I must admit have not tried their latest). This is pretty much equivalent to 0.5mm - but not exactly

author: EEVblog
author_rank: Unknown
timestamp: July 09, 2010, 07:41:02 am
content: Quote from: RayJones on July 07, 2010, 09:35:08 pmThe Yanks didn't buy out Protel. I can assure you the original Tasmanian founder still owns and runs the show. But now they have a new name, a stock market listing, and had a habbit of gobbling up smaller overseas companies.Dave.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 09, 2010, 08:40:08 am
content: LOL, OK I got the American bit wrong.But if I recall correctly they certainly set up shop in the US at some stage in the 90's so they could acquire a bigger exposure?At some stage I certainly got tired of needing to keep shelling out thousands more for an upgrade (with fresh bugs).It then became better to skip versions as the incremental costs were more palatable.I'm not ashamed to admit I still use Protel99SE as it does all I need.This includes an 8 layer board I designed, but most are only DS.I have used a later version at times to check a design file, and they appear to be nicer, but based on past experience the user interface / concepts have probably shifted again and another learning curve would be required.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2010, 09:36:18 am
content: It does annoy me when software vendors expect their users to upgrade very often.I still use Windows XP, I don't see any point with Windows 7 which will just slow my PC down more. Heck, Windows 2000 is still fine for most applications.

author: EEVblog
author_rank: Unknown
timestamp: July 09, 2010, 12:46:25 pm
content: Quote from: RayJones on July 09, 2010, 08:40:08 amYes, and they still have a big US office. But it's just sales and support, the US was and still is the biggest market, so a presence there was necessary to look legit to the Yanks.Most development is done here in Sydney, with some in the Netherlands. There is a still a small token contingent in Tasmania who work on the libraries.QuoteMuch of the industry is still using 99SE. It's the defacto industry standard in China for example, most unpaid of course!Dave.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2010, 10:08:26 pm
content: I didn't know Farnel owned Eagle now.I've tried Eagle before but didn't get on with it. Now, I've downloaded and am having another go at and it's just as horrible as ever. I've used other CAD and SIM packages before and none of them are this horrible to use - I really find it hard not to swear! One needs to toolbar to do everything from deleting an object to moving objects and I've still not figured out how to move multiple objects simultaneously.How long have Farnel had Eagle for?I hope they finally do something about this, there again, if they do there'll probably be loads of people who complain because they prefer the horrible user interface.

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 09, 2010, 10:10:26 pm
content: The interface has been cleaned up slowly. I don't think they'll ever change the odd group copy procedure, though.

author: charliex
author_rank: Frequent Contributor
timestamp: July 09, 2010, 10:17:55 pm
content: i rarely ever use the toolbar in eagle, just type the commands. the cut n paste thing is just different to how normal cut n paste is, its definitely whacky.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2010, 10:21:46 pm
content: Yes, I've just figured it out but I shouldn't have had to, it should have been obvious. It's still far from perfect, I'm now trying to stretch a group of wires, I'm sure I'll do it eventually.One should draw a box to select objects and drag to move them in the same way most drawing programs made over the last 20 or so years have worked.

author: David
author_rank: Frequent Contributor
timestamp: July 10, 2010, 09:53:49 am
content: Does anybody here use Cadence Allegro? I've not really seen any talk of it on the forums...

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2010, 10:53:44 am
content: No, I've never heard of it.On the Eagle front: I've got further than I had when I last used it, I've managed to draw an astable multivibrator!Hopefully I'll manage to do the PCB.EDIT:Eagle has just told me that my schematic is wrong because there are missing net junctions, which I've found out are those little dots where wires join, which every other schematic editing program I know of places automatically! WTF?

author: nessatse
author_rank: Regular Contributor
timestamp: July 10, 2010, 12:42:39 pm
content: I am always astounded at how popular Eagle is. Over the years I have played or worked with most of the 'free' as well as some of the $$$ professional packages like Altium and Orcad (Altium still beats all others IMHO!). Eagle fails probably in every aspect when compared to all the others.The UI sucks - it looks like they purposely tried to come up with the most un-intuitive design possible.The library design is fundamentally flawed - combining footprints and schematic symbols as one is simply sillyEditing PCB tracks - well to be honest most PCB packages have their quirks here, but Eagle simply outdoes them all. Its just bad.The list goes on and on, but why bother? There are far better free packages out there:- KiCad, Diptrace are two that come to mind immediately, and if you are brave - gEDA :-)

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2010, 04:21:05 pm
content: We used to use A!tium where I used to work.Eagle makes me swear but I know lots of companies use it, so after being unemployed for nearly a year I've decided to teach myself how to use it so I can add it to my CV. I've finally got to doing the PCB, after having to partly redraw the schematic because I didn't realise that they both files had to be open simultaneously to update which A!tium does perfectly regardless of whether all the files are open or not.I know what you mean, using Eagle just makes me long for A!tium which I don't have at home. Now I've nearly done the PCB but I'm having problems with not being able to connect some of the components because the pads are off grid which is unacceptable because I'm using the default grid and libraries which should use the same grid, or at least a compatible one and this shouldn't be a problem anyway because it should be possible to work off-grid.Eagle's 'help' file is a joke and makes the documentation provided with most open source software look world class. Nevertheless I'm going to persevere with Eagle for now because it will look good on my CV and hopefully an employer will be impressed that I had the patience to learn it.EDIT:I've finished the PCB. I didn't know how to put pads in so I used vias, which I hope will be fine. I don't like the way half of the text is vertical and half is horizontal and there doesn't seem to be a way to move the text separately.I got round the grid problem by changing the components to ones that use a 0.05" grid, which involved going back to the schematic and replacing the transistors and the capacitors, what a pain?Next I'll try a circuit with logic gates or a quad op-amp to see how well/poorly it handles swapping pins or gates/op-amps so it fits nicely on the board. Don't worry, I'll start a new thread - I've derailed this one enough.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 10, 2010, 10:28:50 pm
content: Your PCB design looks good, and is a classic follow the schematic approach (nothing wrong with that).Not so sure though about the extra details in the component overlay, eg bumps on resistors, swirls in LEDs, cap symbol with small +.Best cap overlay I've seen is where they have about 30-40% shaded on the negative side (ala the stripe on the cap) Failing that a prominent + is good too.I still use Protel99SE, and it too is a perpetual battle getting schematic descripitions to align with the PCB footprints.Perhaps the newer versions are better and allow you to design the schematic symbol AND the PCB footprint in one place.....One especially frustrating aspect is you place a NPN from the standard schematic library, and associate it with a TO-92A in the standard PCB library (or even TO-220)When you place the matching PCB footprint, the pads are numbered 1, 2, 3 BUT the schematic is (more logically) numbered B, C, E. Arrrggggh

author: RayJones
author_rank: Frequent Contributor
timestamp: July 11, 2010, 12:30:54 am
content: Argghhh - just been caught again with Protel.A standard diode in the schematic uses pins 1,2. PCB uses A,K - the exact opposite of my earlier transistor example

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 11, 2010, 01:59:40 am
content: Quote from: Hero999 on July 10, 2010, 10:53:44 amIt puts them down for me. I have no idea how you were able to draw that schematic with incorrect connections.Quote from: Hero999 on July 10, 2010, 04:21:05 pmIf you start the trace from the off grid pad it will centre properly.QuoteYou use the 'Smash' tool to break the item into it's components and you can move the labels around how you want.

author: charliex
author_rank: Frequent Contributor
timestamp: July 11, 2010, 07:37:37 pm
content: for pad's i just add pad in the schematic, with a library of pad parts. i don't know if theres a better way.the off grid junction thing i came across a couple of days ago myself when i edited someone elses schematic, it wouldn't place a junction or connect the wires since the grids were off. So this is just a matter of standardisng the grid before anything is created, the default grid i find is too coarse.you can write a script that will realign the packages to the grid, which is useful for this sort of thing, there's probably something already on their ftp site too.i did find eagle fairly intuitive to pick up, but then i like the scripting and command line aspect of it, so i think its just what you're used too. we use protel99 at work and i did find it a steeper learning curve.Didn't really find the ui that bad, if i'm making a quick circuit i'll usually do it in eagle, you just have to learn its odd quirks, which i think they all have in some form. i think if you're used to the bigger pcb packages you'll struggle with eagle, but for the first timers it seems easier to pick up, but then you'll hit its growing curve , which is why i always recommend A!tium to newbs, but they'll balk at the cost and go with eagle, epecially since most open source hardware stuff seems to use it too.A!tium just feels like the 600lb corporate gorilla and not as friendy an enviroment to get into, eagle gives the impression of beign a bit more 'hacker' friendly. It's that mob mentality, you see other people using it, so you do too. I'm sure A!tium aren't strapped for customers, but I reckon it couldn't hurt them to look at this enviroment like Ti tried to do with the MSP430, but only if they can give that same feel.its also good to use the vector font option, sparkfun have a nice introduction to setting up eagle for the first time.

author: andri
author_rank: Contributor
timestamp: July 11, 2010, 08:21:40 pm
content: Quote from: Hero999 on July 10, 2010, 04:21:05 pmYou can use thesmashcommand for that. This separates labels from the element and you can move them separately.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2010, 05:16:52 pm
content: Thanks didn't know about the smash command, then there's the unsmash, which does the opposite.I suppose I can understand people finding Eagle easier to use than Altium because of the command line interface, especially if they're used to AutoCAD.I've put-off learning Eagle or any other PCB package apart from Altium for too long. I've made plenty of PCBs at home which I drew using OpenOffice.org draw rather than a proper program. I wish I had taken the time to learn Eagle before.I'm still finding new things, the delete function doesn't work for tracks, one has to use the ripup command which is silly.

author: charliex
author_rank: Frequent Contributor
timestamp: July 12, 2010, 06:55:05 pm
content: delete is mostly for schematics, ripup for pcb's, i can see that it might work if delete implied rip on tracks on the board. I find the polygon processing a bit annoying, i don't want to flood fill to happen on every ratsnest, so i turn it off, but then i want it sometimes. I haven't found an easy way of doing that yet, i hope there is one, that and a proper undo/redo would be great. Can't count the number of times i've accidently run an autoroute when i was doing manual routing and it won't step back, though no doubt you can use the b?d files to do it, but seriously. I also dont like if i turn off a layer and when i'm routing it turns it back on again, especially after i explictly give it a layer command.It'd be nice if you could add stuff in the pcb and it'd appear in the schematic the program probably has too much of a wall between the schematic and pcb to implement it. i can see it being some design choice.ahh well, if only everything were perfect!

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2010, 08:28:50 pm
content: Quote from: charliex on July 12, 2010, 06:55:05 pmIt often seems like Eagle is designed to confuse people.Why not just have one delete command which works for both schematic and PCB?I discovered the ripup command when Googling for something else, there's no other way I would've found it.QuoteThat's annoying too.Once I've learned enough Eagle to get by I think I'll write a tutorial as I've yet to see a good one. The only ones I've seen are either too long winded and packed full of superfluous information or are too brief and skip the basic things which most nubes seem to struggle with.

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 12, 2010, 08:35:04 pm
content: Because using delete applies to the net and using ripup applies to the routing. Because it's different doesn't make it wrong.

author: RayJones
author_rank: Frequent Contributor
timestamp: July 13, 2010, 08:10:23 am
content: Most of my work with Protel involves simple keystrokes to do the work. eg PT = Place Track, PP = Place Pad, MD = Move Drag end, and many many more.In a recent post Dave stated the latest Altium version also still honour these keyboard shorts cuts.I suppose it helps if you actually started out with AutoTrax!

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2010, 11:07:21 pm
content: Quote from: TheDirty on July 12, 2010, 08:35:04 pmIt's inconsistent which is poor UI design - all half decent GUIs use the same functions to accomplish similar tasks.

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 14, 2010, 01:47:39 am
content: Quote from: Hero999 on July 13, 2010, 11:07:21 pmI'm sorry, but your constant complaining about these UI choices is just tiresome now. It's not inconsistent it's a UI choice of distinguishing deleting which would apply to the net itself or ripping up, which applies to the routing. It's logical to distinguish between the two things.

author: EEVblog
author_rank: Unknown
timestamp: July 14, 2010, 06:43:05 am
content: Quote from: charliex on July 11, 2010, 07:37:37 pmThey dropped the low end of the market a long time ago.Not too long ago tried to reclaim it in a way with massive price reductions, but still out of the reach of the low end user.Frustratingly for many, the "low end" product does NOT include PCB, you have to buy the full package to get basic PCB and schematic functionality.The "low end" focus is on FPGA development.Dave.