Hi everybody,
I have been reading the e mails on the above subject and have been assuming
that wiser councils will answer but nobody has so I will have a go.
In the early days of surface mount, late 70's early 80's, chip components
were made with silver palladium end metallisation which wetted the ceramic
thus holding it onto the end of the component and also boded metallurgically
with the silver interlayers, forming the all important conductive path to
the outside world.
The drawback was that when we went from hand soldering to mass soldering we
tended to want more time and temperature to form all of the joints. Silver
palladium dissolved very readily into the 63/37 solder. But if the whole of
the end metallisation dissolved into the solder then the molten solder would
not adhere to the ceramic and you lost the joint. We called it 'leaching'.
It was found that if you add 2% silver you get two main advantages. The
first is a joint that has superior fatigue strength (anybody remember
thermal fatigue? I lived off that for a few years.) The other is that the
silver palladium dissolves into the silver containing alloy at a much slower
rate, thus opening the available process window.
Of course we now have nickel barrier layers that stop the leaching but we
continue the 2% silver because it provides the extra strength and we are
creatures of habit.
Hope this helps.
Regards
Eric Dawson MIM

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