SOT25 Pin numbering - Page 1
Forum section: electronics / projects

author: bolyee
author_rank: Newbie
timestamp: March 09, 2021, 12:37:41 am
content: I just got burnt using SBE807 diodes by On Semi. These come 2 in a SOT25. Their datasheet has the SOT25 with the 2 pins on the left and the numbering, as expected, starting with pin 1 in the top left corner and going anti-clockwise. I've used SOT25's many times before and didn't give it a second glance.After making a PCB that didn't work I went digging and found all the SOT25 parts I've use to date have numbering with the 3 pins on the left and that's what I used.Is there a standard for this? Or a separate name for "rotated" versions?

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 10:49:47 am
content: Are you saying this datasheet is wrong?https://www.onsemi.com/pdf/datasheet/ena1055-d.pdfI find that unlikely. Maybe a picture of your that section of your board and schematic would help.

author: tooki
author_rank: Super Contributor
timestamp: March 09, 2021, 11:02:34 am
content: Quote from: jpanhalt on March 09, 2021, 10:49:47 amRe-read the original post carefully. He didn’t say the datasheet doesn’t match the part. He’s saying that (at least for this part), ON semi started the pin numbering in a location inconsistent with everyone else. So when he chose a footprint in his EDA program, he didn’t even think to check it he pin numbering, and now the PCB is wrong.A cursory search seems to indicate that while most manufacturers seem to use the pin numbering he expected (start with leftmost pin on the 3-pin side, counterclockwise), it’s not universal. ON semi starts counting on the 2-pin side. Rohm starts on the 2-pin side and numbers themclockwise!!So the moral of the story is to verify the layout yourself, every time. :/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2021, 11:15:15 am
content: Apparently SOT-143 and relatives are the most inconsistent, offering just about every possible permutation in some part somewhere.I hadn't seen a wrong-pinned SOT-25, until now (or, didn't internalize it before), I agree I don't think they're common that way.IPC I think recommends starting on the long side, which is how it's usually done.I tend to make my symbols and footprints consistent with the most common, most standardized arrangement and orientation, and deprioritize or ignore the manufacturer's recommendation. Downside, this leads to some confusion when others are reviewing against the datasheet verbatim.Tim

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 11:31:38 am
content: Quote from: tooki on March 09, 2021, 11:02:34 amThat's what I said. There is no reason not to refer to a manufacturer's datasheet.

author: exe
author_rank: Unknown
timestamp: March 09, 2021, 11:37:32 am
content: Welcome to the club!

author: tooki
author_rank: Super Contributor
timestamp: March 09, 2021, 09:32:09 pm
content: Quote from: jpanhalt on March 09, 2021, 11:31:38 amBut that’snotwhat you said. Your reply indicated you thought the OP said the datasheet was wrong, which they did not say.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 09:42:57 pm
content: @tookiYou seem to be defending someone who didn't understand the datasheet, guessed on the pinout, was wrong, and lacked the self-respect to post a thread to that effect. If I did that, I'd certainly not be posting about it.I assumed, perhaps incorrectly, that no one would do that. So, I assumed the TS didn't understand the datasheet. You seem to think that is expected.Maybe in your field, whatever a "tooki" does, that's accepted. It certainly would not be tolerated in my field.

author: ajb
author_rank: Super Contributor
timestamp: March 10, 2021, 01:46:54 am
content: Quote from: jpanhalt on March 09, 2021, 09:42:57 pmPeople make mistakes. There's no need to be a condescending prick about it, ESPECIALLY when the person has clearly learned from it and gone out of their way to seek more information to avoid the problem in the future. Shitting on well-meaning people doesn't help anyone and it certainly isn't going to convince anyone you're a better engineer or human being.

author: tooki
author_rank: Super Contributor
timestamp: March 10, 2021, 05:52:57 am
content: Quote from: jpanhalt on March 09, 2021, 09:42:57 pmWTF you talking about? I completely agree with ajb’s reply to you: you’re just being a condescending prick.EVERYONE has been caught by a footprint error before. (I’m sure it has happened to you.) The OP didn’tmisunderstandthe datasheet, it simply didn’t occur to them that there was any need to check the way the pins were counted, so they didn’t. If one has always worked with DIP devices (or only with SMD devices from the majority of manufacturers who number them the most common way), for example, this is an eminently reasonable assumption, as DIPs are invariably numbered in exactly one way, regardless of manufacturer.And you somehow think that admitting to a mistake makes one weak or something? That’s ridiculous. I trust people who admit to mistakes. The ones who don’t, I’m skeptical of.What is it you say I think is expected? I don’t understand that statement.Quote from: jpanhalt on March 09, 2021, 09:42:57 pmPresumably we are both in electronics. Not sure what my username has to do with anything.

author: exe
author_rank: Unknown
timestamp: March 10, 2021, 09:35:52 am
content: My friends, I believe no one has bad intentions in this thread. But I feel like adding clarifications and explanations lead to further misunderstanding and strong emotions.

author: Hydron
author_rank: Super Contributor
timestamp: March 10, 2021, 11:42:31 am
content: I think all the anger should be directed instead at the manufacturers who seem to be able to settle on a footprint but fail at the basic task of numbering it consistently.