Re: Another scope 7854

Bruce Griffiths

No its just the usual formation of gold-tin intermetallics:

However bonding of aluminium wires to gold pads can result in Aluminium gold intermetallics

One of these intermetallics (AuAl2) is purple (hence the name purple plague) and another (Au5Al2) is white in color.


On 05 May 2017 at 16:41 "David @DWH [TekScopes]" <TekScopes@...> wrote:

You reminded me that I know of another failure in these vertical
plug-ins. Some of the early ones have a heavy gold plating on the
printed circuit board traces and the tin-lead solder can eventually
suffer from gold embrittlement breaking the solder connection.

If this happens, resoldering the connection will reveal what I suspect
is Purple of Cassius under the solder.


The solution I found was to use a soldering iron on low heat to
repeatedly add and remove solder until the Purple of Cassius and gold
is removed. The tin in the solder dissolves the gold until the base
metal is reached. This is actually the recommended soldering
technique for heavy gold plating if tin solder is used.

Modern flash gold plating is deliberately thin enough to completely
dissolve into the solder preventing this problem.

On Thu, 4 May 2017 19:23:10 +0000, you wrote:

>I had a 7A26 with intermittent vertical instability in channel 2 only. I found that alongside the attenuator plug-in modules, there are discrete 1/8 watt resistors. They are also plugged in and one had developed high contact resistance. I replugged it about 4 times after using a q-tip to swab its leads with contact cleaner, being careful not to get it anywhere except on the ends of the leads. The problem never came back.
>Cliff Carrie

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