"Wetting" current not enough for connector?

[Flyback]

Hi,
We are dividing down 390Vdc to 2.5V, then feeding this to a PFC control chip (UCC28070A) which is on a daughter PCB, through the below header and socket housing.......there is no current going through the header pin.....is this OK?....or should we put the lower divider resistor on the daughter PCB, so that at least a few 100 microamps flows through the header connector? (to "wet" the contacts).

Internet re-searching this reveals a huge number of different opinions, without concensus, eg...
https://www.eng-tips.com/viewthread.cfm?qid=107221

Pin header
https://uk.farnell.com/multicomp/mc34767/header-2-row-r-angle-8way/dp/1593450

Socket for pin header
https://uk.farnell.com/harwin/m20-7830446/socket-pcb-0-1-4-4way/dp/7991991

 

[For The Popcorn]

The contact material of the header and socket should to the same.

The header is gold plated, the socket tin.

 

[Flyback]

The header is gold plated, the socket tin.

Thanks, now that is a v good point...does it really make a difference here?...i mean..there'll just be some "thevenin voltage" or whatever they call it?

Seriously, i am thinking we are going to have to change these sub-bords to horizontal mount and solder them in.........inconvenient though that will be in terms of space.

Is this really a problem...this "wetting" thing...the internet is 50% for it and 50% saying its nonsense.

 

[rjenkinsgb]

Just use tin for both side of the connector. It flows under pressure and forms a gastight connection; no wetting voltage needed.

(No good quality gastight connector should need wetting, that's normally a thing with switching contacts - but there may be offsets from different materials).

 

[Flyback]

Thanks very much, i think thats our issue, we dont know if the connector system of the top post is quality and gas-tight.

 

[For The Popcorn]

You need to get out in the world. Dual-row 0.1" header connectors have been used successfully for years. Until recent years, computers were loaded with them for hard drives, CD ROMs, floppy drives, I/O for switches, audio, RS-232 and parallel ports. Problems with these connectors are extremely rare.

Add to computers, any rack-mount instrument is full of them too.

 

[Flyback]

Thanks, so even say just a signal voltage, where no current flows through the mating junction, and then goes into say a comparator or opamp input, would be OK?
When you say 0.1" has been used sucessfully for years...what plating was used on both mating connector surfaces?...and what quality of plating?.......abd did they treat the surfaces in some process before the mating was done?...as such, there are still grey areas here, or seem to be.

"Gas-tightness" seems to be the requirement for definte success with mating surfaces...and that definetely involves more expense than we can budget for this.

 

[For The Popcorn]

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[Nigel Goodwin]

Just use tin for both side of the connector. It flows under pressure and forms a gastight connection; no wetting voltage needed.

(No good quality gastight connector should need wetting, that's normally a thing with switching contacts - but there may be offsets from different materials).

I agree, he's over-thinking things again - wetting is for switch (and particularity relay) contacts - a connector is likely to be plugged in, and possibly never disturbed again, and even if it is, connectors tend to have a 'rubbing' action as they go in place.

 

[Flyback]

Thanks, about 10 or so years ago, i went for an interview with Ultra Electronics in UK.....The Chief engineer brought up the subject of wetting in switches during that interview...and stated that it was a real issue to be extremely concerned about.....so admittedly he was talkign about "switches".

Most of the PSU's ive reverse engineered, in fact all of them, have the daughter boards soldered in...not with connectors.....this could be for cost...but also, could be because of wetting issues.

We are only doing a prototype, but if we get sometimes_intermittent joints for the signals. it will really screw up everything..specially since eg a feedback signal being absent could cause mayhem.

Even if there is the benefit of "rubbing"...what degree of subsequent vibration may undo this?

 

[rjenkinsgb]

Even if there is the benefit of "rubbing"...what degree of subsequent vibration may undo this?

Enough so that the parts wear away to the point there is no contact pressure??

Frequent vibration at reasonable levels will likely improve contact connectivity. The problems with both connectors and switches are when they are not disturbed for long periods, so the non-contact exposed areas tarnish or corrode etc., then something disturbs the contact position and it moves to an area of tarnish.

That is unlikely if not impossible with tin plated connector contacts, as the metal flows with any movement.
It is quite common with silver switch contacts, and consumer "shiny" connectors that are hardly gas tight.

Note that "gas tight" is nothing to do with sealed connectors, it just means the contact design and mating point pressures are high enough to prevent oxygen penetration, so no tarnish or corrosion.

A spray with a lubricating contact cleaner will also prevent oxygen ingress and make just about any type corrosion proof for long periods.

 

[Dick Cappels]

Think about CMOS logic chips plugging into 10¢ dual-wipe sockets.

As far as I know I have never had a socket fail because a CMOS input did not draw enough current.

 

[Flyback]

Thanks yes, though i think those sockets are very tight clasping...compared to the flimsy header socket....

https://cdn.harwin.com/pdfs/M20-783.pdf

 

[rjenkinsgb]

compared to the flimsy header socket.

Nothing flimsy about them! But not particularly relevant anyway.

The critical value is the force per unit area within the actual metal-to-metal contact points, which may be microscopic and be under a pressure of anything from hundreds to thousands of PSI.

Header socket contact designs are generally no different to those in IDC ribbon connectors, and they (assuming a named brand) are incredibly reliable, as others have said, and have been used for decades throughout industry and consumer goods.

I just cannot understand why you are so picky with things for what you have already said is a prototype of a hobbyist design... It's not a final product, or from what I understand, anything that will be used in a product.

These things have been used for decades and problems are incredibly rare.