PCB mounting screws - is there a common standard?

[eTobey]

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.

 

[jpanhalt]

A 4-40 imperial screw (USA) is close to that.

[Andy Watson]

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.

[eTobey]

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.

[Nominal Animal]

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.

[Someone]

Now i am wondering, if this is a standart in Japan, or other countries?

Yes:
https://www.classicfasteners.com.au/jis-zinc-pre-iso-coarse-pitch/

In April 1967 the JIS (Japanese Industrial Standard) was changed to match the screw pitches of the ISO standard.
A little chart is below comparing pre April 1967 JIS screw pitches with post April 1967 JIS screw pitches.
It may help explain why some metric screws don't seem to fit on your motorcycle or car.
 
Pre Apr 1967 JIS
3mm.....0.6
4mm.....0.75
5mm.....0.9
6mm….. 1.0
8mm.....1.00
10mm...1.25
12mm...1.5
 
Post Apr 1967 ISO
3mm.....0.5
4mm.....0.7
5mm.....0.8
6mm… ..1.0
8mm.....1.25
10mm...1.5
12mm...1.25

[wraper]

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.

[jpanhalt]

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.

[jpanhalt]

It also feels, like the screw (thread) is getting tight when deeper screwed in. They may have used the wrong tool it seems.

One 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.

[eTobey]

Now i am wondering, if this is a standart in Japan, or other countries?

Yes:
https://www.classicfasteners.com.au/jis-zinc-pre-iso-coarse-pitch/

In April 1967 the JIS (Japanese Industrial Standard) was changed to match the screw pitches of the ISO standard.
A little chart is below comparing pre April 1967 JIS screw pitches with post April 1967 JIS screw pitches.
It may help explain why some metric screws don't seem to fit on your motorcycle or car.

I 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?

[Smokey]

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.

[jfiresto]

Pozidriv (Phillips v2) is not bad and is popular outside the U.S.

[jpanhalt]

@Smokey

I also prefer the hex drive but with a larger diameter head, like button, flanged button, or flanged socket.  The first two being my favorite.

[Someone]

Now i am wondering, if this is a standart in Japan, or other countries?

Yes:
https://www.classicfasteners.com.au/jis-zinc-pre-iso-coarse-pitch/

In April 1967 the JIS (Japanese Industrial Standard) was changed to match the screw pitches of the ISO standard.
A little chart is below comparing pre April 1967 JIS screw pitches with post April 1967 JIS screw pitches.
It may help explain why some metric screws don't seem to fit on your motorcycle or car.

I 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?

"JIS thread pitch"

[Smokey]

@Smokey

I also prefer the hex drive but with a larger diameter head, like button, flanged button, or flanged socket.  The first two being my favorite.

For 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.

[jpanhalt]

I'm just a hobbyist.  I don't design smartphones.

[Smokey]

I'm just a hobbyist.  I don't design smartphones.

... especially not with that attitude!!  I kid.. I kid...