PCB mounting screws - is there a common standard? - Page 1
Forum section: electronics / general

author: eTobey
author_rank: Super Contributor
timestamp: January 23, 2025, 12:47:05 pm
content: Hi,i just found out, that a PCB is mounted with a - what looks to me rather exotic - screw size. It is a M3, BUT it has a pitch of 0.6mm! There arent much of this kind (screws in general), and most often it is only statet "M3" without the detail about the pitch.Now i am wondering, if this is a standart in Japan, or other countries?Its especially bad, because taking another screw for it, will "screw" up that thread. And i almost did that, since i have to make a modification.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 23, 2025, 02:10:21 pm
content: A 4-40 imperial screw (USA) is close to that.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 23, 2025, 02:19:35 pm
content: How old is the assmbly? B.A. sizes were common in UK instruments a while ago. 4 B.A.= 3.6mm dia. 0.66mm pitch, 5 B.A. = 3.2 x 0.59, 6 B.A.=2.8 x 0.53.

author: eTobey
author_rank: Super Contributor
timestamp: January 23, 2025, 09:42:34 pm
content: I measure a diameter of 2.98mm, and the pitch was roughly measured. I have to to a proper measure of that pitch.It also feels, like the screw (thread) is getting tight when deeper screwed in. They may have used the wrong tool it seems.Its a recent product, about 2016 to 2023.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 23, 2025, 10:37:14 pm
content: M3x0.6 is a specialty thread you can get taps, dies, screws, and nuts for; they're used for indexed machine tooling to hold the bit.M2.5, #4-40, and M3 are so close in size that I tend to use a known-size screw or bolt to test.The nearest UTF is #4-40, at 2.85mm outer diameter and 0.635mm pitch. It's quite commonly used in enclosures, especially in PC/server motherboard standoffs, although the standard brass and plastic standoffs are also available in M2 (M2x0.4), M2.5 (M2.5x0.35), and M3 (M3x0.5). I use all four, #4-40, M2, M2.5, and M3. Most metric machine screws I have have outer diameters somewhat under the nominal.If I were to guess, I'd say it was a #4-40 standoff where someone cross-threaded an M3 screw into, or vice versa.

author: Someone
author_rank: Super Contributor
timestamp: January 23, 2025, 10:47:51 pm
content: Quote from: eTobey on January 23, 2025, 12:47:05 pmYes:https://www.classicfasteners.com.au/jis-zinc-pre-iso-coarse-pitch/Quote

author: wraper
author_rank: Unknown
timestamp: January 23, 2025, 11:00:23 pm
content: If it was made in US or designed/sold mainly in US, I assume those are imperial screws. I used to repair some US made laboratory equipment full of screws which beside being imperial were some oddball large pitch type, virtually nonexistent for purchase in Europe.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 23, 2025, 11:24:32 pm
content: The 4-40 screw is considered "coarse" but is by far more common than the fine version that is 4-48. That does not hold true in larger sizes. You will also see rarely a 5-40 UNC pitch thread. When looking for unusual imperial threads, look for "gun screws."Edit: corrected UNF size.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 23, 2025, 11:48:26 pm
content: Quote from: eTobey on January 23, 2025, 09:42:34 pmOne of the things I find amazing is the precision needed for a screw to thread into a tapped hole easily. They are made in huge numbers, but if they are just a little off, threading into a deep hole relative to diameter will tell you. And, the tighter the fit tolerance value, the more important that becomes.

author: eTobey
author_rank: Super Contributor
timestamp: January 24, 2025, 05:40:04 am
content: Quote from: Someone on January 23, 2025, 10:47:51 pmI did a google search for it: i didnt find much of this. I would have never found this. Thanks a lot!Did you search for this? What words did you use then?

author: Smokey
author_rank: Super Contributor
timestamp: January 24, 2025, 07:20:19 am
content: Here is another thing to consider when picking a mounting screw size.If you are going to be using PEM standoffs in a metal enclosure, make sure they have the type and size you are looking for.https://www.mcmaster.com/products/pem-style-standoffs/standoffs~/I almost always use magnetic steel 4-40 Socket Head Cap Screws these days. Great for production. Much better than philips.

author: jfiresto
author_rank: Frequent Contributor
timestamp: January 24, 2025, 09:13:59 am
content: Pozidriv (Phillips v2) is not bad and is popular outside the U.S.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 24, 2025, 11:17:55 am
content: @SmokeyI also prefer the hex drive but with a larger diameter head, like button, flanged button, or flanged socket. The first two being my favorite.

author: Someone
author_rank: Super Contributor
timestamp: January 24, 2025, 10:11:59 pm
content: Quote from: eTobey on January 24, 2025, 05:40:04 am"JIS thread pitch"

author: Smokey
author_rank: Super Contributor
timestamp: January 24, 2025, 10:59:59 pm
content: Quote from: jpanhalt on January 24, 2025, 11:17:55 amFor holding down circuit boards, larger heads means larger land patterns on the boards which means less routing area. If you have the space, go for it. But I don't feel like having more head area has much benefit and it takes up space.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 24, 2025, 11:12:39 pm
content: I'm just a hobbyist. I don't design smartphones.

author: Smokey
author_rank: Super Contributor
timestamp: January 25, 2025, 12:01:17 am
content: Quote from: jpanhalt on January 24, 2025, 11:12:39 pm... especially not with that attitude!!I kid.. I kid...