Manufacturer mechanical drawings... - Page 1
Forum section: electronics / general

author: westfw
author_rank: Super Contributor
timestamp: June 06, 2013, 06:19:41 pm
content: Is it my imagination and lack of experience with CAD packages, or are the mechanical drawings usually provided my manufacturers really lousy when it comes to trying to make a PCB footprint for the parts:1) Odd choices of "origin" of the part.2) and/or no origin at all, with dimensions relative to other parts of component.3) usually dimensions are specified to the edge of some pad/hole, while my CAD package wants the center of the pad/hole.Is this some "mech eng" vs "EE" disconnect, or just randomness?

author: David_AVD
author_rank: Super Contributor
timestamp: June 06, 2013, 09:14:22 pm
content: I find this frustrating too. Sometimes there's also one or more (critical) dimensions missing.Also annoying are dimensions that have been rounded and cause accumulative errors.Measuring a physical sample is often required.

author: millerb
author_rank: Regular Contributor
timestamp: June 06, 2013, 09:30:50 pm
content: I love it when a dimension you need isn't in the drawing but you can infer it by jumping through 6 hoops of calculating offsets from different areas on the diagram.

author: David_AVD
author_rank: Super Contributor
timestamp: June 06, 2013, 09:34:11 pm
content: You can, but it's somewhat fraught with danger as just one mistake can cause you grief. I quite like ones that have everything measured from one point.

author: marshallh
author_rank: Unknown
timestamp: June 06, 2013, 09:46:14 pm
content: Yes I have this problem all the time. Ex. crappy Molex drawing from 2002.The thing is that the drafters just see shapes and rectangles on paper, and they run dimensions between them.Completely backwards of what you want as opposed to putting the drawing back into autocad or footprint editor..Many times I spend 5mins "reversing" the layout and finding what would be the most convenient place to put my origin. Then i use offset measurements based off that.My PC keyboard has a calculator launch key which I use at least 20 times per footprint. After a while you can do some of this in your head

author: millerb
author_rank: Regular Contributor
timestamp: June 06, 2013, 10:28:04 pm
content: Quote from: David_AVD on June 06, 2013, 09:34:11 pmI ran into some grief a while back on a USB connector (molex brand). It was clear where the plastic pegs and shield pads were, but the signal pin pads were left floating in the diagram. It looked like they centered up in one area but they didn't. I ended up having to measure the actual part with a caliper and go through a few iterations of tweaking and printing out the design and laying it out on top of the paper printout until I got it right.

author: David_AVD
author_rank: Super Contributor
timestamp: June 06, 2013, 11:42:27 pm
content: When we start using a new part in designs I'll often do the footprint and add it to a scrap area on an unrelated prototype PCB run. That way I can test for fit ahead of placing it on the real project.

author: MacAttak
author_rank: Unknown
timestamp: June 06, 2013, 11:47:36 pm
content: Slightly off-topic, but can anyone recommend a good video that covers footprint creation (from scratch) in Eagle? I've seen a few for Altium and KiCad, but not for Eagle.Surely there must be something out there. I'm probably just looking in the wrong places.

author: marshallh
author_rank: Unknown
timestamp: June 06, 2013, 11:56:30 pm
content: Also I've run into footprints that are actually WRONG but the manufacturer's STEP file actually matched the physical part.Always order the parts to verify the footprints before you make the pcb.

author: David_AVD
author_rank: Super Contributor
timestamp: June 07, 2013, 12:47:09 am
content: Another pet peeve is the lack of "top view" or "bottom view" text. After getting caught out with a relay once (was bottom view and I assumed top view), I always pick up the part and do a quick sanity check now.

author: c4757p
author_rank: Super Contributor
timestamp: June 07, 2013, 01:16:19 am
content: Quote from: David_AVD on June 07, 2013, 12:47:09 amThat's because the ones who do top view drawings don't even consider that bottom view is an option, and the ones who do bottom view drawings are too stupid to know the difference...Seriously? Bottom view? Is it a relay or a vacuum tube??

author: David_AVD
author_rank: Super Contributor
timestamp: June 07, 2013, 01:27:26 am
content: In the case of the relay, I simple mounted them on the underside of the PCB for the prototype!

author: Rufus
author_rank: Super Contributor
timestamp: June 07, 2013, 01:55:30 am
content: Quote from: David_AVD on June 07, 2013, 12:47:09 amI rarely have the luxury of having all the parts in my hand before a PCB is finished. PCBs generally have the longest lead time and you might still be selecting/changing parts during layout. PCBS get ordered then there is time to order parts and one or two days later you get the chance to see if you screwed up.And yes drawings are frequently crap. I once spent 20 minutes on the phone just to determine which end of an offset SIL package was pin 1.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 07, 2013, 04:09:02 am
content: i dont quite understand the OP question. but mech drawing should provides every dimension, or at least derivable from other dimension.

author: westfw
author_rank: Super Contributor
timestamp: June 07, 2013, 04:25:15 am
content: Quote from: MacAttak on June 06, 2013, 11:47:36 pmThere's this one:http://www.instructables.com/id/ERHQQ180Y3EP286NQY/It doesn't talk about translating less-than-useful mechanical drawings.Alsohttp://www.adafruit.com/blog/2013/04/22/ktowns-ultimate-creating-parts-in-eagle-tutorial/andhttp://www.adafruit.com/blog/2012/09/14/tutorial-creating-accurate-footprints-in-eagle-the-adafruit-learning-system/(especially, importing the mechanical drawing to the silkscreen layer via ULPs.)

author: c4757p
author_rank: Super Contributor
timestamp: June 07, 2013, 04:30:02 am
content: Quote from: Mechatrommer on June 07, 2013, 04:09:02 amAnd this is the problem. If I have to bend over backwards and sacrifice a goat to my calculator to "derive" the dimensions, they're doing it wrong. Every EDA package takes dimensions the same way, why aren't they given in this way?

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 07, 2013, 12:54:38 pm
content: Quote from: c4757p on June 07, 2013, 04:30:02 amits subjective sometime you cant blame it. sometime space is too crowded, too much dimensioning will disturb viewing actual product etc. and mostly, derived dimension is just a matter of A - B.

author: westfw
author_rank: Super Contributor
timestamp: June 08, 2013, 09:40:13 am
content: Here's an example. A microUSB suggested PCB layout from a respected connector manufacturer.Try to figure out the dimensions of the big SMD pads that the housing solder to (at the bottom. They're 1.9mm "tall", but the width is ... OMG!)

author: M. András
author_rank: Super Contributor
timestamp: June 08, 2013, 09:45:05 am
content: i say 1.75 if you look at it it marks 2 as 4mm in between a 0.5mm gap

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 08, 2013, 09:47:15 am
content: I was cursing this just yesterday - Why should I need to do maths just to work out a pad size - totally dumb. These people just don't think about how the drawing will be used - you always start with a defined padd size, then decide where to put them.I don't think I've ever seen a footprint drawing showing both the dims of pads and distances between padcentres, which is what you actually need.

author: M. András
author_rank: Super Contributor
timestamp: June 08, 2013, 10:16:49 am
content: yeah i was looking at that drawing for minutes to figure it out. i cant imagine why is that so hard to give measurements for indivual componens both lenght and width and respect to 1 of the outer outlines

author: Rufus
author_rank: Super Contributor
timestamp: June 08, 2013, 10:19:48 am
content: Quote from: westfw on June 08, 2013, 09:40:13 am(9.8-6.2)/2 and (4.3-0.5)/2 ain't rocket science. What is abysmal about that drawing is that 4.3 and the 4.0 locating peg dimension are almost coincident and they chose to place the peg dimension close to the pads and the pad dimension close to the pegs.