Screwing up a footprint?  - Page 1
Forum section: electronics / projects

author: workwithme
author_rank: Newbie
timestamp: July 28, 2015, 08:25:52 pm
content: I often hear how easy it is to mess up a footprint/part info when sending designs for manufacturing. I've been asking people from around the internet to hear some crazy stories.Do you remember a time where you had a simple footprint error spill out of control? How bad was it?

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 28, 2015, 08:34:20 pm
content: Not a spilled footprint directly but an inner layer in a batch of 4-layer boards that was 0.5mm off

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2015, 10:17:48 pm
content: This one time I made one that was off by 0.1mm and supposedly it caught fire when it went into the reflow oven.Kappa.Speaking of, SGDQ is on...As for "out of control", I don't know what you mean by that... at worst, your board is useless. Maybe the deadline is short and you go overbudget or don't get product out, and wind up fired or something.I think the worst one I did was a QFN with the pads arranged as if the package was 4x4 mm, but it was actually 3x3. Not even a big one, so it still worked, I think.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 29, 2015, 12:07:10 pm
content: I've had the most problems with high density SMT FFC and mezzanine connectors - inevitably I have to tweak the pad sizes from whatever is specified in the datasheet to get a good balance between solder bridging and self-centering during reflow.Most recently I had to do an awful kludge to use an ethernet PHY IC in a QFN package with ground pad and which needed the ground connection to have extremely low inductance. Putting diagonal escape traces a mere 2mm long to 0.6mm vias at each corner resulted in erratic operation so I ended up putting normal vias in each of the 4 corners of the ground pad, taking a chance that not too much solder would be sucked into them during reflowing. It worked, but I cringed at having to do it as Thou Shall Not Put Vias In Pads is one of the commandments of board design.Otherwise I've made the usual mistakes like assigning pins to the wrong pad, using a metric grid instead of imperial (or vice versa).

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2015, 12:10:44 pm
content: I once forgot to send the additional milling layer, so the DC barrel socket only had small holes instead of slots.Ended up being non-dc barrel boards because luckily it also featured a two pole terminal block for power.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 12:22:36 pm
content: Not my fault but the PCB makers fault: In those ancient days you constructed a PCB design on transparent polyester paper by hand, scale 2:1. Then the PCB maker scaled down that design on photo film for production. Although clearly documented and marked on the design itself, the PCB maker overlooked the scale. So we got the boards 2 times larger than intended. That boards were double sized VME cards.............

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 29, 2015, 12:23:25 pm
content: 1) Getting the gate the gate and source pins swapped in teeny tiny mossies.... fun to try and fix those lol.2) Getting SCK and SDA crossed and having to lift 0.1mm pitch pins on the 100 pin micro and do a bodge.3.) Cocking up the pin out for mico SD footprint.I think those are the top 3 for me so far.

author: rollatorwieltje
author_rank: Unknown
timestamp: July 29, 2015, 12:29:40 pm
content: Made the mistake to use a footprint from the Diptrace library without checking. A DPDT switch, only special thing about this switch is that it requires a drill hole slightly out of center. Guess what, the attached symbol didn't match the footprint, NO and NC were swapped. No big deal right, just place the switch the other way around. Except for that damn drill hole... And I couldn't drill a new hole myself, as there was a trace routed there...Moral of the story, don't submit boards at 2am, wait until the next day to check them first

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 29, 2015, 12:45:35 pm
content: Quote from: rollatorwieltje on July 29, 2015, 12:29:40 pmBad parts in default/common/public libraries are a problem for every board design package, it seems. People have been complaining about it since I started using software to design boards in the mid 90's. As a result, I create a library entry for every part I use, including passives that "couldn't possibly be wrong" like SMT resistors, capacitors, etc.Also your rule to wait until the next day before submitting a board is a good one. Every - and I mean every - time I've tried to beat the 3PM deadline of my favorite board vendor I've regretted it.

author: miceuz
author_rank: Frequent Contributor
timestamp: July 29, 2015, 12:49:10 pm
content: Once i've mirrored the sd card footprint, ended up glueing it and bodging the connections, no big deal as this was the first prototype.Then just last week i got a small batch of pcbs with smd leds mounted backwards, i didn't mark the first pin on the silkscreen and used wrong 3d model on a render. 50 boards 24 leds on each to resolder, oh well...

author: McBryce
author_rank: Super Contributor
timestamp: July 29, 2015, 01:11:12 pm
content: Flipped a thru-hole IC onto the bottom of the PCB while shifting parts about and then forgot to flip it back over. Wasn't obvious until the boards were delivered and the first was powered up and went bang.McBryce.

author: Rick60
author_rank: Contributor
timestamp: July 29, 2015, 01:20:32 pm
content: A decal I declared for SCART socket fitted perfectly on the solder side of the Board ,hadn't noticed the pin out was viewed from the bottom( why do they do this ! ), since then always check the view before declaring a partassuming PIC 20pin QFN parts are the same spacing there not , a PIC 18F14K50 is 0.65mm pitch a 18F14K22 is 0.5mmfor some reason the default parts library in Allegro for SOIC's doesn't have the solder paste mask declared

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2015, 03:50:45 pm
content: Quote from: MagicSmoker on July 29, 2015, 12:07:10 pmMeh. QFNs are *made* to have via-in-pad. That's just one of those things you have to shrug at, I think.Most footprints even indicate where to put the vias, and what size.I normally assume that the pad itself needs slightly less paste thickness (say, 0.05 mm) than the mask thickness would normally provide, and that the excess gets wicked down the vias. Which sets the via size and count, for a given board thickness.Also, for vias 12 mil (0.3 mm) or less (i.d.), solder doesn't wick in very much, especially for lead free.The biggest problem is plugging one side or the other, which traps gas and makes voids in the solder joint. If you're going to use a filled/plugged via fab, make sure they are fully plugged, to the soldering surface. Tenting just one side supposedly has bad results. Open vias are much less of a hazard, having less solder (due to wicking) versus voids due to gas.TLDR: uncapped, small, via-in-pad is fairly normal.Tim

author: workwithme
author_rank: Newbie
timestamp: July 29, 2015, 04:36:45 pm
content: Quote from: T3sl4co1l on July 28, 2015, 10:17:48 pmKappa indeed. That's terrible!Respect to everyone in the thread. Thanks for sharing.

author: technix
author_rank: Super Contributor
timestamp: July 29, 2015, 04:50:58 pm
content: I have once designed the board with 74LVC1G04 in SC70-5 (original intention was SOT23-5 from the get-go) but ordered SOT23-5...Good to know that that chip in the two packages have compatible pinout, and SOT-23 part can somehow made to make contact with SC70-5 pads...

author: dmills
author_rank: Super Contributor
timestamp: July 29, 2015, 05:21:33 pm
content: SFP connector pinning 180 degrees outright side of the board, just completely the wrong way around.Also, that whole bottom view thing (Snarl).I would note that when you are using SPI, do NOT call the nets SDI & SDO, call them MOSI and MISO or such, otherwise somebody will end up connecting SDI to the SDI pin on both the master and slave (Not me, I just had to fix it).Regards, Dan.

author: Corporate666
author_rank: Unknown
timestamp: July 29, 2015, 06:37:13 pm
content: Not necessarily a strict footprint error, but we sent a couple of thousand boards to our contract manufacturer at the time who was supposed to source some of the bog-standard parts for us. I got a call that they were having trouble sourcing the linear regulator we had specified, so I had a hurried look on Digikey and found a replacement and had a reel sent overnight to the CM so they could keep going.We get the boards back and not only do they not work... they immediately blow out the MCU when powered on. That was fun to discover after a bunch of them were already toasted.Turned out the pins on the regulator were not the same... IIRC it was an SOT-223 package and the original had the tab connected to output and the ground pin in the center, with input and output on the sides. The replacement part had the tab connected to ground, but ground was on one end and input was in the middle and output on the side. So the net effect was connecting Vin went to the ground pin which was connected to the tab and put input power to the whole 3.3V section.Expensive and time consuming lesson - always check your footprints and your pinouts.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 07:35:30 pm
content: Quote from: MagicSmoker on July 29, 2015, 12:45:35 pmWrong! Your boss will not accept that spilling your time on double checking saves a lot of time in the end anyway

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: July 29, 2015, 08:59:29 pm
content: Anyone else find it a bit nerve wrecking when you're in charge of ordering a few thousands pounds/dollars worth of assembled boards when it's not your cash?? Getting them in and hoping to hell you haven't got something wrong and ruined a batch of prototypes, or worse, production boards.!! Most of my prototypes have involved some minor bug hunting and modding prior to confirming a production batch. But, knowing you are "solely" responsible if it goes tits up...

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 29, 2015, 10:00:13 pm
content: I always end up reversing the pinout on parts, especially big clunky ones. WHY DO MANUFACTURERS NEVER WRITE THE VIEW ON THEIR DIAGRAMS? How much effort does it take to put "view from bottom" or "view from top" on their pinouts? Usually it's just a slight difference in the look of the part, some 3D difference like a cutout that they show on one view but not the other, and somehow you're magically supposed to know that cutout is on the bottom and therefore the diagram showing a cutout is a view from the bottom...even though there's no 3D picture of the part on the datasheet from which you could glean that information.Do they expect everybody to buy the part first, then hold it in their hand while looking at the datasheet, before they know which view is which so they can design the board for it?

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 10:13:20 pm
content: Quote from: suicidaleggroll on July 29, 2015, 10:00:13 pmI suppose you are living on the mirror side of the globe?Manufacturers never number the pins clockwise as seen from the top, always anticlockwise.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 29, 2015, 10:23:54 pm
content: Quote from: Pjotr on July 29, 2015, 10:13:20 pmDoesn't help when there aren't pin numberseg:http://www.acopian.com/dcdc-pcb-d.html

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 10:39:42 pm
content: Then don't design with virtual parts, have real parts at handBut I must admit, pin headers for IDC ribbon cable connectors are an exception on the anticlockwise rule

author: langwadt
author_rank: Super Contributor
timestamp: July 29, 2015, 10:50:28 pm
content: Quote from: suicidaleggroll on July 29, 2015, 10:00:13 pm"viewed from top/bottom" isn't that uncommon in datasheets

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 29, 2015, 10:59:53 pm
content: Quote from: langwadt on July 29, 2015, 10:50:28 pmNone of the ones I seem to run into have it. Maybe I'm just unlucky.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 29, 2015, 11:42:59 pm
content: Quote from: MagicSmoker on July 29, 2015, 12:07:10 pmNot really. There's no other way to build many boards these days.

author: McBryce
author_rank: Super Contributor
timestamp: July 30, 2015, 08:13:42 am
content: Quote from: suicidaleggroll on July 29, 2015, 10:23:54 pmIt's obviously viewed from the bottom in that diagram, if that was a "viewed from top" view it would have to be on the other side of the side view drawing. That's basic draftsmanship.McBryce.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2015, 12:26:57 pm
content: Quote from: suicidaleggroll on July 29, 2015, 10:23:54 pmThat of course is a bottom view (you can see the pins). They don't have a projection note, but they should. I suppose they don't need numbered pins if they don't use numbers anyway, but IPC would prefer that they be numbered, not named.Projection is the biggest difference between conventional US and world standard drawings. There's a little conical section (a frustrum) in the title block which shows a side and profile view. The end view is either solid (both edges visible) or one circle dashed (smaller side away from view).There's no need to spell out the projection side if the projection type is known, so pay attention to this.To the newbies: do take the time to read about drafting! No, the drawings aren't always self-evident and intuitive, and yes all those random marks do mean something. Don't ignore it as noise -- if you see a shape you don't recognize, look it up!Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 30, 2015, 12:37:42 pm
content: Quote from: suicidaleggroll on July 29, 2015, 10:00:13 pmThat's what I do... it's usually a lot cheaper to buy the part than it is to reorder boards. Whenever I create a new package entry in my library I print it out and place the part on it to make sure everything lines up and that the pads look workable. I almost always end up tweaking the pad size to improve solderability and/or to make visual inspection easier (particularly important with leadless parts like DFN and QFN).Quote from: Alex Eisenhut on July 29, 2015, 11:42:59 pmThat some parts have exposed pads on the bottom which need to be connected for electrical (and, often, thermal) reasons using vias does not suddenly mean that solder won't wick into said vias, potentially causing voids or outright joint failure. When you do have to resort to putting vias in pads and you don't want to pay extra for tenting/filling them (*if* the board house even offers such services) then you should expect a higher defect/rework rate for that component.

author: McBryce
author_rank: Super Contributor
timestamp: July 30, 2015, 01:08:14 pm
content: Quote from: T3sl4co1l on July 30, 2015, 12:26:57 pmI think that's one of the failings of modern courses. Back when I learnt, draftsmanship was part of the course. I understand that the manual drawing part isn't required today, but how to read a drawing should still be taught.McBryce.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 30, 2015, 01:33:28 pm
content: Quote from: McBryce on July 30, 2015, 01:08:14 pmSame applies to schematic drawings these days. More and more these come almost unreadable thanks to modern CAD packages with a clumsy user interface. They only seem to be intended for the designer to get a PCB ready as fast as possible. Service engineers later on should find their own way in the schematic jungle of the device.Recently I came across a schematic of a machine controller that consisted of 20 A4 sheets with mainly net references and page references. In the old fashioned way of drawing that schematic fitted easily on one or two A3 sheets. People seem to forget nowadays that a schematic also has the purpose of showing to other people like service engineers, how the the components interact. It is simply not educated any more.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2015, 01:48:20 pm
content: Oh, don't worry. Service schematics have been awful since forever. I've seen C sheets loaded with digital logic with all individual wires drawn. Good luck following the one wire in a bus of 20+ wires, oh and they cross over randomly as the bus goes around corners and such...Tim

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 30, 2015, 01:54:57 pm
content: Yes dumboo's are from all timesBut modern CAD packages and the lack of education these days make only it worse.

author: rs20
author_rank: Super Contributor
timestamp: July 31, 2015, 04:21:24 am
content: Quote from: T3sl4co1l on July 30, 2015, 12:26:57 pm+1. Although, am I the only one who learned about 1st vs 3rd angle projection at high school? And then again doing engineering at university?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 31, 2015, 06:42:00 am
content: Quote from: Jeroen3 on July 29, 2015, 12:10:44 pmHad the same happen to me on a board I designed at work. Fortunately, it was a small run (two boards) and it didn't take long to cut the slots with a knife. Against all odds, once assembled, both boards worked just fine!

author: McBryce
author_rank: Super Contributor
timestamp: July 31, 2015, 07:27:24 am
content: Quote from: rs20 on July 31, 2015, 04:21:24 amIt depends what classes you took. I took "Technical Drawing" so you learnt about 1st / 3rd angle, isometric and oblique projections etc. But if you didn't take that course it wasn't covered anywehere else.McBryce.