Gold-Plated PCB-mount Sockets?

[[email protected]]

I've been reading about how two different types of metal, when in
contact as an electrical connection, can cause bad things to happen.
Specifically, for my current interests: gold and tin (or tin/lead)
connector-contacts should not be mated. So, for example, it would be
a bad idea to use DIP IC sockets that have gold plating, unless the IC
pins are also gold. So far so good. I can (not) do that.

I've also been reading that gold-plated pins should not be soldered
with tin/lead solder, IIRC. Is that correct? I've think I've read
that doing so will cause the solder joints to be more brittle and have
higher resistance, and possibly get worse with time.

But then what about all of those nice gold-plated Molex PCB headers
and sockets? What should I use to solder them to a PCB? Or should I
just switch back to tin?

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

 

[John Larkin]

I've been reading about how two different types of metal, when in
contact as an electrical connection, can cause bad things to happen.
Specifically, for my current interests: gold and tin (or tin/lead)
connector-contacts should not be mated. So, for example, it would be
a bad idea to use DIP IC sockets that have gold plating, unless the IC
pins are also gold. So far so good. I can (not) do that.

We do solder or tin coated pins in gold sockets all the time. No
problems in the last 20 years.

I've also been reading that gold-plated pins should not be soldered
with tin/lead solder, IIRC. Is that correct? I've think I've read
that doing so will cause the solder joints to be more brittle and have
higher resistance, and possibly get worse with time.

Not a problem either. The gold on most pins is so thin, microinches,
that it totally dissolves in the solder.

John

 

[[email protected]]

We do solder or tin coated pins in gold sockets all the time. No
problems in the last 20 years.




Not a problem either. The gold on most pins is so thin, microinches,
that it totally dissolves in the solder.

John

Thanks, John.

It's reassuring to hear the results of your experiences. From
everything else I had read, mating tin and gold contacts sounded like
a very bad idea. But I couldn't really tell HOW bad. Maybe it's not
as much of a problem as I was beginning to think it was.

I have just read some threads about mating tin and gold connector
contacts, in the Google Groups archive, that imply that the "leaf-
spring"-contact types of gold-plated IC sockets are much worse to use
with tin or tin/lead IC pins than are the "turned" or machined types
of gold-plated sockets, at least as far as galvanic corrosion of the
tin- or solder-plated IC pins is concerned.

There's also the thermocouple effect of the two dissimilar metals in
contact, creating a small series voltage which might be a problem in
high-precision circuits (but probably only something like a 1 uV
offset).

And, FWIW, I just found a short Kester article about soldering gold-
plated metals, which mentions the sometimes-major effects of different
thicknesses of the gold plating, which can be downloaded at:

http://www.kester.com/en-us/technical/kb_detail.aspx?KnowledgeID=1

I also read somewhere that using silver-bearing 62/36/2 solder helps
to alleviate some of the problems associated with soldering to gold-
plated metals, by preventing most of the leaching of the gold during
soldering.

All in all, if possible, it seems best to try to mate contacts of the
same metal. But in the case of ICs and sockets, since almost all
plastic DIP through-hole ICs have tin or solder-plated pins, is there
any advantage in using sockets with gold-plated contacts, to make it
worth facing the possibility of eventual degradation from galvanic
corrosion (assuming the use of sockets, at all, is a given)?

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

-

 

[Eeyore]

I've been reading about how two different types of metal, when in
contact as an electrical connection, can cause bad things to happen.
Specifically, for my current interests: gold and tin (or tin/lead)
connector-contacts should not be mated. So, for example, it would be
a bad idea to use DIP IC sockets that have gold plating, unless the IC
pins are also gold. So far so good. I can (not) do that.

A classic error is to use the popular 0.1" pitch IDC connectors with gold plated
pins and tin plated sockets. The trouble is that it's difficult to see there's a
problem until it fails.

I've also been reading that gold-plated pins should not be soldered
with tin/lead solder, IIRC. Is that correct? I've think I've read
that doing so will cause the solder joints to be more brittle and have
higher resistance, and possibly get worse with time.

What would you use instead of tin-lead or even lead-free solder ?

Graham

 

[Eeyore]

From everything else I had read, mating tin and gold contacts sounded like
a very bad idea.

It is

But I couldn't really tell HOW bad.

It's bad !

Maybe it's not as much of a problem as I was beginning to think it was.

Oh it is but that has nothing to do with soldering gold plated leads.

Graham

 

[krw]

We generally use tin/solder plated cheap, spring-type ic sockets, and
the gold ones are the round socket machined types.

I suppose in a very humid environment you could see some corrosion
effects form mixing metals, but I haven't personally. For bad
environments, we tend to avoid sockets and conformally cost.

The main issue mixing sockets is the contact pressure. Gold is soft
and requires (withstands) little pressure. Tin is harder and needs
some contact pressure to penetrate the surface. A connector designed
to mate with gold may not have enough pressure to mate[*] with a tin
contact. A contact designed to mate with tin may scratch through
gold.

[*] though the machined gold contacts are quite good.

 

[John Larkin]

Thanks, John.

It's reassuring to hear the results of your experiences. From
everything else I had read, mating tin and gold contacts sounded like
a very bad idea. But I couldn't really tell HOW bad. Maybe it's not
as much of a problem as I was beginning to think it was.

I have just read some threads about mating tin and gold connector
contacts, in the Google Groups archive, that imply that the "leaf-
spring"-contact types of gold-plated IC sockets are much worse to use
with tin or tin/lead IC pins than are the "turned" or machined types
of gold-plated sockets, at least as far as galvanic corrosion of the
tin- or solder-plated IC pins is concerned.

We generally use tin/solder plated cheap, spring-type ic sockets, and
the gold ones are the round socket machined types.

I suppose in a very humid environment you could see some corrosion
effects form mixing metals, but I haven't personally. For bad
environments, we tend to avoid sockets and conformally cost.

There's also the thermocouple effect of the two dissimilar metals in
contact, creating a small series voltage which might be a problem in
high-precision circuits (but probably only something like a 1 uV
offset).

That can be corrected by proper layout, making sure all such
connections are isothermal. See the pcb pic I recently posted to
a.b.s.e, on the subject of slit ground planes. The lower-right region
is thermocouple stuff.

John

 

[Joerg]

We generally use tin/solder plated cheap, spring-type ic sockets, and
the gold ones are the round socket machined types.

I suppose in a very humid environment you could see some corrosion
effects form mixing metals, but I haven't personally. For bad
environments, we tend to avoid sockets and conformally cost.

The main issue mixing sockets is the contact pressure. Gold is soft
and requires (withstands) little pressure. Tin is harder and needs
some contact pressure to penetrate the surface. A connector designed
to mate with gold may not have enough pressure to mate[*] with a tin
contact. A contact designed to mate with tin may scratch through
gold.

As had been evidenced by the old HP4191 here in the lab. One sunny day I
couldn't calibrate it anymore. Turned out they had pushed fancy
gold-plated mil-type PROMs into el-cheapo spring type sockets. Like
Porsche Design leather seats in a Yugo. It did work for 20 years but
then quit.

[*] though the machined gold contacts are quite good.

Still, if it isn't absolutely necessary to use sockets, don't.

 

[Eeyore]

The main issue mixing sockets is the contact pressure.

No, it's fretting corrosion.

Graham

 

[Rich Grise]

It is


It's bad !

Not necessarily. With the cheapie leaf-spring contacts, that make contact
surface-to-surface, as it were, for some reason the tin and gold gall -
I think maybe some gold dissolves into the tin and they make some kind of
alloy that separates into nonconductive flakes; the gold-plated machine
screw sockets don't have this problem, because the inside of the pins is
shaped kind of like a funnel - like inside-out of a barbed hook, which
actually dig into the pin, scraping the gold of the socket and the tin off
the lead, so you get at least four gas-tight contact points that are base
metal to base metal, and the rest of the assembly is corrosion-resistant.

Oh it is but that has nothing to do with soldering gold plated leads.

He was worried about soldering gold-plated pins of gold-plated sockets,
and that's never been a problem in most people's experience.

Cheers!
Rich

 

[Eeyore]

Not necessarily. With the cheapie leaf-spring contacts, that make contact
surface-to-surface, as it were

For an IC socket it may be fine but for connectors generally it most certainly
isn't.

Graham

 

[krw]

No, it's fretting corrosion.

Wrong. You can easily see it happening with gold/tin SIMMs.

 

[Eeyore]

[email protected] says...

Wrong. You can easily see it happening with gold/tin SIMMs.

The problem that happens with classic ribbon cable type conectors is entirely a
fretting corrosion issue. It's caught loads of ppl out.

Graham

 

[[email protected]]

It is


It's bad !


Oh it is but that has nothing to do with soldering gold plated leads.

Right. I was asking about two separate problems. One was about
putting DIP ICs with tin- or solder-plated pins into sockets that had
gold plating. The other was about soldering the gold-plated pins, of
Molex gold-plated wire-to-board sockets, to PCBs.

Thanks.

- Tom Gootee

 

[[email protected]]

A classic error is to use the popular 0.1" pitch IDC connectors with gold plated
pins and tin plated sockets. The trouble is that it's difficult to see there's a
problem until it fails.


What would you use instead of tin-lead or even lead-free solder ?

Thanks for the response, Graham.

From my second message in this thread:

"I also read somewhere that using silver-bearing 62/36/2 solder helps
to alleviate some of the problems associated with soldering to gold-
plated metals, by preventing most of the leaching of the gold during
soldering."

- Tom Gootee

 

[[email protected]]

The main issue mixing sockets is the contact pressure. Gold is soft
and requires (withstands) little pressure. Tin is harder and needs
some contact pressure to penetrate the surface. A connector designed
to mate with gold may not have enough pressure to mate[*] with a tin
contact. A contact designed to mate with tin may scratch through
gold.

That is yet-another problem with it.

Thanks.

- Tom Gootee

[*] though the machined gold contacts are quite good.

 

[[email protected]]

As had been evidenced by the old HP4191 here in the lab. One sunny day I
couldn't calibrate it anymore. Turned out they had pushed fancy
gold-plated mil-type PROMs into el-cheapo spring type sockets. Like
Porsche Design leather seats in a Yugo. It did work for 20 years but
then quit.

Twenty years? Not too bad! Better than what I'd been led to believe
from what I've read about the problem. (But probably not good-enough
as an actual lifespan design goal, in my case.)

[*] though the machined gold contacts are quite good.

Still, if it isn't absolutely necessary to use sockets, don't.

That is, no doubt, the best solution.

 

[[email protected]]

Not necessarily. With the cheapie leaf-spring contacts, that make contact
surface-to-surface, as it were, for some reason the tin and gold gall -
I think maybe some gold dissolves into the tin and they make some kind of
alloy that separates into nonconductive flakes; the gold-plated machine
screw sockets don't have this problem, because the inside of the pins is
shaped kind of like a funnel - like inside-out of a barbed hook, which
actually dig into the pin, scraping the gold of the socket and the tin off
the lead, so you get at least four gas-tight contact points that are base
metal to base metal, and the rest of the assembly is corrosion-resistant.

Thanks, Rich.

That agrees with almost everything else I've read about that problem.

- Tom Gootee

He was worried about soldering gold-plated pins of gold-plated sockets,
and that's never been a problem in most people's experience.

That's good to hear, again. The usenet archive posts I've read seemed
to imply that it was a problem, because the gold that was dissolved
into the solder would form intermetallics that were relatively
brittle, and (IIRC) less conductive.

 

[[email protected]]

I've been reading about how two different types of metal, when in
contact as an electrical connection, can cause bad things to happen.
Specifically, for my current interests: gold and tin (or tin/lead)
connector-contacts should not be mated. So, for example, it would be
a bad idea to use DIP IC sockets that have gold plating, unless the IC
pins are also gold. So far so good. I can (not) do that.

I've also been reading that gold-plated pins should not be soldered
with tin/lead solder, IIRC. Is that correct? I've think I've read
that doing so will cause the solder joints to be more brittle and have
higher resistance, and possibly get worse with time.

But then what about all of those nice gold-plated Molex PCB headers
and sockets? What should I use to solder them to a PCB? Or should I
just switch back to tin?

- TomGootee

http://www.fullnet.com/~tomg/index.html

From what I can see, the consensuses seem to be:

1. For almost all receptacle-to-pin-mating situations, both socket and
pin surfaces should be made of the same metal. One exception seems to
be that acceptable performance can be gotten when putting a tin or tin/
lead DIP IC pin into a (round) machined or turned gold-plated socket
(but NOT a gold-plated "flat contact" or "leaf-spring" type of DIP
socket), although, if practical, no IC socket at all would be best.

2. Soldering a gold-plated pin into a PCB, using tin/lead solder,
"should" be OK. But using 62/36/2 tin/lead/silver solder should be
better.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

-

 

[Joerg]

On Mar 23, 1:00 pm, Joerg <[email protected]>
wrote:




Twenty years? Not too bad! Better than what I'd been led to believe
from what I've read about the problem. (But probably not good-enough
as an actual lifespan design goal, in my case.)

When I found out what the problem had been I was also surprised that it
hung on for 20 years. But from such professional equipment I'd expect
the same life span as people expect from aircraft. Something along the
lines of 30+ years. IMHO this socket/PROM combination was a design flaw.
Oh well, we all make mistakes, nobody is perfect.

[*] though the machined gold contacts are quite good.

Still, if it isn't absolutely necessary to use sockets, don't.

That is, no doubt, the best solution.

Doing that right now. But we have something HP didn't have back in those
days: EEPROMs. It's amazing, I just spec'd in a 256k EEPROM for under
$2. Nicely SPI bussed. Heck, it even has pretty good over-write
protection where you can lock up a part of the address range and leave
the rest open for writes.