10 Jul
2016
10 Jul
'16
4:14 a.m.
Stated Tothwolf tothwolf at concentric.net:
"Both contact surfaces must also be the same material or tin oxide will form
on the surface of the gold plating and cause a major headache. This was a
serious problem with 486 and earlier Pentium PCs with 30 and 72 pin SIMMs
and it led to a number of lawsuits."
Almost every DEC System Unit ("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
How does that situation jibe with the SIMM issue/experience?
-----
paul
10 Jul
10 Jul
9:07 a.m.
On Sun, 10 Jul 2016, Paul Birkel wrote:
Stated Tothwolf tothwolf at concentric.net:
I'm not familiar with them used in DEC systems in that way, but the
problems with mixing tin and gold plated connectors is well documented.
Even the connector manufacturers warn against mixing different platings.
Another problem that can occur is because a gold surface is softer than
tin oxide, if the header pin is tarnished, mating the gold plated contact
with the header will not break the surface of the tin oxide. Tin oxide is
not a good conductor.
"Both contact surfaces must also be the same
material or tin oxide will
form on the surface of the gold plating and cause a major headache.
This was a serious problem with 486 and earlier Pentium PCs with 30 and
72 pin SIMMs and it led to a number of lawsuits."
Almost every DEC System Unit ("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers. How does that situation jibe with the SIMM
issue/experience?
Somewhere around here I still have some 4MB 30 pin SIMMs with gold plated
contacts that were used in tin plated sockets for several years. The
damage/contamination of the gold plated SIMM contacts is significant.
1:07 p.m.
On Jul 10, 2016, at 9:07 AM, Tothwolf <tothwolf at
concentric.net> wrote:
On Sun, 10 Jul 2016, Paul Birkel wrote:
While "don't mix contact surfaces" is sufficient, it isn't necessary.
What matters is the "anodic index" of the metal, or rather, the difference
between those two values for the two metals in contact. If that difference is large, you
have a problem; if it's small enough, you do not. "Small enough" depends on
the environment; aboard an oceangoing ship the number has to be smaller than in an office
setting. I remember looking into this topic for an investigation of what types of contact
platings are acceptable for lithium coin cell battery holders in IT equipment.
https://en.wikipedia.org/wiki/Galvanic_corrosion has some information.
paul
Stated Tothwolf tothwolf at concentric.net:
I'm not familiar with them used in DEC systems in that way, but the problems with
mixing tin and gold plated connectors is well documented. Even the connector manufacturers
warn against mixing different platings. "Both contact surfaces must also be the same
material or tin oxide will form on the surface of the gold plating and cause a major
headache. This was a serious problem with 486 and earlier Pentium PCs with 30 and 72 pin
SIMMs and it led to a number of lawsuits."
Almost every DEC System Unit ("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers. 
2:07 p.m.
On 07/10/2016 12:07 PM, Paul Koning wrote:
This applies to bolted contact for structural things. Gold
connectors usually have light contact pressure to preserve
the soft gold plating. Tin contacts usually have higher
contact force to scrape the oxide off the tin surface. When
they are mixed, the tin can wipe onto the gold and then
allow oxides to form due to the lower contact force. Tin
contacts are supposed to provide enough pressure to form
gas-tight contact areas.
And, of course, when exposed to salty air, then everything
goes downhill REAL fast, corrosion galore.
In a salt environment, I'd use semi-hermetically sealed
connectors, and still expect lots of problems.
The Navy probably knows a LOT about these things.
Jon
On Jul 10,
2016, at 9:07 AM, Tothwolf <tothwolf at concentric.net> wrote:
On Sun, 10 Jul 2016, Paul Birkel wrote:
While "don't mix contact surfaces" is sufficient,
it isn't necessary. What matters is the "anodic index" of the metal, or
rather, the difference between those two values for the two metals in contact. If that
difference is large, you have a problem; if it's small enough, you do not.
"Small enough" depends on the environment; aboard an oceangoing ship the number
has to be smaller than in an office setting. I remember looking into this topic for an
investigation of what types of contact platings are acceptable for lithium coin cell
battery holders in IT equipment.
Stated Tothwolf tothwolf at concentric.net:
I'm not familiar
with them used in DEC systems in that way, but the problems with mixing tin and gold
plated connectors is well documented. Even the connector manufacturers warn against mixing
different platings. "Both contact surfaces must also be the same
material or tin oxide will form on the surface of the gold plating and cause a major
headache. This was a serious problem with 486 and earlier Pentium PCs with 30 and 72 pin
SIMMs and it led to a number of lawsuits."
Almost every DEC System Unit
("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
3:29 p.m.
On Sun, 10 Jul 2016, Jon Elson wrote:
On 07/10/2016 12:07 PM, Paul Koning wrote:
Another thing to keep in mind here is that electrical current is being
passed through the junction. Mixed metals greatly increases the potential
for electromigration.
On Jul 10, 2016, at 9:07 AM, Tothwolf
<tothwolf at concentric.net> wrote:
This applies to bolted contact for structural things. Gold connectors
usually have light contact pressure to preserve the soft gold plating.
Tin contacts usually have higher contact force to scrape the oxide off
the tin surface. When they are mixed, the tin can wipe onto the gold
and then allow oxides to form due to the lower contact force. Tin
contacts are supposed to provide enough pressure to form gas-tight
contact areas. On Sun, 10 Jul 2016, Paul Birkel wrote:
While "don't mix contact surfaces" is
sufficient, it isn't necessary.
What matters is the "anodic index" of the metal, or rather, the
difference between those two values for the two metals in contact. If
that difference is large, you have a problem; if it's small enough, you
do not. "Small enough" depends on the environment; aboard an
oceangoing ship the number has to be smaller than in an office setting.
I remember looking into this topic for an investigation of what types
of contact platings are acceptable for lithium coin cell battery
holders in IT equipment. Stated Tothwolf tothwolf at concentric.net:
> "Both contact surfaces must also be the same material or tin oxide
> will form on the surface of the gold plating and cause a major
> headache. This was a serious problem with 486 and earlier Pentium
> PCs with 30 and 72 pin SIMMs and it led to a number of lawsuits."
Almost every DEC System Unit ("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
I'm not familiar
with them used in DEC systems in that way, but the
problems with mixing tin and gold plated connectors is well
documented. Even the connector manufacturers warn against mixing
different platings. And, of course, when exposed to salty air, then
everything goes downhill
REAL fast, corrosion galore. In a salt environment, I'd use
semi-hermetically sealed connectors, and still expect lots of problems.
The Navy probably knows a LOT about these things.
Even in a reasonably good atmospheric environment weird issues can crop
up. I once evaluated an air handler controller which had worked perfectly
in product testing, but once field deployed, had a very high failure rate.
It was made up of two pc boards with a pair of .100" pin and socket board
to board interconnects. The two boards were physically held together with
4 nylon snap-in standoffs. The lower board contained terminal blocks,
modular connectors, and the power supply circuitry and the upper board
contained the microcontroller, network circuitry, etc.
The cause of the failures turned out to be fretting corrosion of the board
to board connectors caused by vibration. Another contributing factor was
that many installers were not installing all 4 mounting screws when
mounting the controller inside the unit (these were field retrofitted
controllers) but were instead only installing 2 screws in opposite
corners.
The fix was to replace all of the existing board to board interconnects,
both the header and socket with parts that had 30 microinches of hard gold
over nickel (the original parts had 15 microinches of gold) and to use a
contact lubricant during assembly. Repaired boards were also to be
installed using all 4 mounting screws. The vendor later redesigned the
controller so it was all on a single board (while still admitting no
fault, of course).
2:58 p.m.
On 7/10/16 1:14 AM, Paul Birkel wrote:
Almost every DEC System Unit ("backplane")
that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
I'm not near one right now, but there should be gold plating on the finger in the DEC
connector block at the point of contact with the pcb edge connector. It's easier to
see on the VAX era blocks.
9:46 p.m.
On 07/10/2016 01:58 PM, Al Kossow wrote:
On 7/10/16 1:14 AM, Paul Birkel wrote:
Absolutely. The WW pins were tinned, but there was
selective gold plating on the card edge contact fingers.
It would take a strong light and magnifier to see it down
inside the connector, but you can rest assured the contact
was gold-gold.
Jon
Almost every DEC System Unit
("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
I'm not near one right now, but there should be gold plating on the finger in
the DEC
connector block at the point of contact with the pcb edge connector. It's easier to
see on the VAX era blocks.
11 Jul
11 Jul
3:19 a.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Jon Elson
Sent: Sunday, July 10, 2016 9:46 PM
To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic and Off-Topic
Posts
Subject: Re: Connectors: Both contact surfaces must also be the same
material?
On 07/10/2016 01:58 PM, Al Kossow wrote:
On 7/10/16 1:14 AM, Paul Birkel wrote:
Absolutely. The WW pins were tinned, but there was selective gold plating
on the card edge contact fingers.
It would take a strong light and magnifier to see it down inside the
connector, but you can rest assured the contact was gold-gold.
Jon
-----
Evidently a strong light *and* a strong magnifier (at least for me!). Off
to find one ... well, two.
paul
-----
Almost every DEC System Unit
("backplane") that I've ever seen uses
tinned-contacts, yet the Modules all use gold-plated fingers.
I'm not near one right now, but there should be gold plating on the
finger in the DEC connector block at the point of contact with the pcb
edge connector. It's easier to see on the VAX era blocks.
2934
days inactive
2935
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7 comments
5 participants
participants (5)
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aek＠bitsavers.org -
elson＠pico-systems.com -
paulkoning＠comcast.net -
pbirkel＠gmail.com -
tothwolf＠concentric.net