Creating a "universal" pad to fit all types - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: September 15, 2020, 02:40:37 am
content: I may be overthinking this, but I wanted to get opinions on this. I'm working on a circuit that contains diodes, specifically the 1SS427 and the BAS516. However, the two parts have different footprint sizes, with the 1SS427 having an SOD-923 footprint and the BAS516 having the larger SOD-523. I followed the recommended land patterns, but then I thought to myself: why not make a 'universal' footprint that can accommodate both sizes? Thus, this led me to my creation below.Following the recommended footprints (and playing around with silkscreen to follow along with both recommended layouts), I feel pretty confident that both diodes can fit onto this one footprint, either through hand soldering or my hot air gun. That trapezoid shape is a bit tricky to work with. This is part of a prototype board, so I guess I have the freedom to do this, but is it possible that such a strange footprint could have issues? I'm working with low voltages, and the pads are spaced 0.5 mm from each other at the inside so there's no arcing, but could there be something else that would discourage such a footprint? If such a cool looking pad (or at least mainly the use of trapezoids as a pad) allows for 'universal' part size placement, why is it not used more often? (Well, sure, there's space concerns, but I thought this is a cool way to replace parts on the fly as needed)https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/1SS427-L3M/1SS427L3MCT-ND/5298031https://www.digikey.com/product-detail/en/nexperia-usa-inc/BAS516-115/1727-2893-6-ND/1755347

author: exmadscientist
author_rank: Frequent Contributor
timestamp: September 15, 2020, 04:06:31 am
content: This sort of thing works just fine for hand-soldered one-off things. It's a bad idea for automated assembly though as the parts don't self-center as well, and generally have a reduced process window. So it's avoided in volume production.You do see it sometimes on dev boards for components expected to be populated by the user.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: September 15, 2020, 02:53:26 pm
content: Thanks for the reply. What do you mean by 'reduced process window'?

author: exmadscientist
author_rank: Frequent Contributor
timestamp: September 18, 2020, 04:36:26 am
content: "Reducedprocess window" just means that the soldering process is harder to dial in for automated assembly: the "window" in which the "process" works well is "reduced". This happens because of things like too much solder allowing parts to float around during reflow.This is no problem at all if you're soldering by hand, and accordingly these type of pads do work very well for hand soldering.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 18, 2020, 05:02:17 pm
content: Footprint optimization for very high yields and reliability in automated manufacturing processes is a difficult art, and sometimes the "recommended" footprints are no good at all.For prototyping, one-offs, or even small batches where you won't nail down the footprints perfectlyanyway, none of this matters. You can get away with very improvised ideas, and I like using "one-footprint-fits-many-parts" approach, sometimes just adding two massive plane-like pads where I can solder whatever two-terminal component I want.If you go into large-scale production, this footprint may or may not work; the same remark actually applies to the "official" footprint as well!