Footprint confusion... - Page 1
Forum section: electronics / general

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 07, 2019, 01:10:47 pm
content: OK, I'm confused. I want to use a Bourns CM filter from digikey:https://www.digikey.com/product-detail/en/bourns-inc/SRF2012-361YA/SRF2012-361YACT-ND/2681277Firstly, the 3D render on Digikey is just... wrong. I hope. I mean, I'd rather not have the wires attached to the pad like that...digi.jpg(9.87 kB - downloaded 110 times.)But OK, it's just a picture. As it's been a while, I figured perhaps the Ultra Librarian thing might be worth another shot....So, no. Really, no. I'll just roll my own, thanks. When I added the .step file, it looked something like this:Note how a large part of the component pad is not on the actual footprint. So, I obviously figured I got the dimensions wrong (added the pad sizes):So, umh, how is a 0.46mm component pad supposed to fit on a 0.4mm footprint pad? That's even a bit out of center? The horizontal size is actually large enough but so badly off center the component is once again not on the pad. Leaving it like this is not an option as I assume the height of the solder mask will make for either the soldering to be bad or the component to rotate, or...So I guess I'll just enlarge the pads and hope for the surface tension to get it right, but seriously... WTF??

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 07, 2019, 01:32:52 pm
content: Not done yet..The "feet" of the component have different widths in the .step model. Not on the datasheet though!

author: Simon
author_rank: Unknown
timestamp: November 07, 2019, 04:28:14 pm
content: I gave up with distributor models. usually the manufacturer has a decent model but it may have some information stripped out. Often I end up creating my own models which also means they start out oriented properly. Who ever does models for any components is an idiot. To date I can't remember being given a model that even sits on the right plane (XY).I would go with the datasheet, don't trust the model.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 07, 2019, 04:31:05 pm
content: Quote from: Simon on November 07, 2019, 04:28:14 pmLol, no, I learned not to expect that.QuoteSure, the 3D model serves as sanity check, not much else. The problem, though, is that even the datasheetdoes not compute.

author: Simon
author_rank: Unknown
timestamp: November 07, 2019, 04:34:54 pm
content: To be honest I have used a choke just like that and it was fine, it was a larger one than yours but the footprint to part relationship was similar.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2019, 05:59:27 pm
content: Think I have a footprint for this somewhere but I'll have to dig around.Also think it's a part that soldered poorly. Apparently they glue or spot-weld the wires to the pads somehow, and they don't wet correctly when reflowed. Assembly wanted an alt, which I think I changed to a similar WE part.Note that their tolerance stackup shows the pads may come shorted from the factory. Not a great drawing.At nominal dimensions, the pad gap is 0.3mm, which isn't much. Just barely enough to get soldermask and two gaps between. I'd position the pad edges same as a 0.5mm pitch part, usually soldermask turns out good that way. The lengthwise pad gap (heel fillet) can be 0.0 I think. Side fillet I don't really care either way. Toe fillet >= 0.3mm I would do.3D models: I shop around for better looking models. WE equivalents usually render nicely. Check both datasheets to make sure they have the same dimensions, and further check the model of course, and if it's correct then might as well use it for layout reference, eh?Tim

author: wraper
author_rank: Unknown
timestamp: November 07, 2019, 06:05:52 pm
content: Quote from: T3sl4co1l on November 07, 2019, 05:59:27 pmProbably you should increase aperture in paste stencil, likely larger than pads themselves. Since wires add gap between PCB and pads, you need more solder to fill all of it.

author: Simon
author_rank: Unknown
timestamp: November 07, 2019, 06:17:03 pm
content: WE models are nice if on all the wrong planes. I downloaded a model for a similar bourns choke from mouser and it was a block! I ended up making my own model using outline dimensions from the drawing and some guesswork.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 08, 2019, 12:29:55 pm
content: Quote from: T3sl4co1l on November 07, 2019, 05:59:27 pmSo.. the wires *are* on the pad?? Who the hell ever thought that would be a good idea?QuoteI love WE, their footprints and their candy. But their prices through distris are just not realistic Sure, you can ask for a quote and those quotes tend to be pretty decent, but I don't want to spend an hour with a sales dude just to get the quote for a 100pc project. Ugh.

author: Simon
author_rank: Unknown
timestamp: November 08, 2019, 01:32:57 pm
content: Quote from: Ice-Tea on November 08, 2019, 12:29:55 pmHaha, yes I am being plied with food every time he comes. I like their tech support both in person and that they have things like red expert that give far more information in a parametric search than the distributors do.I was surprised to find one of their elecy caps to be the cheapest on mouser. I would go to an assembler anyway but WE will deal with them direct so I expect the pricing is not too bad.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 08, 2019, 01:49:36 pm
content: That's my experience as well. Problem is there's no way to know in advance. Unless you jump through all the hoops.