Thankyou to everyone for their replies so far!
To answer both Bev's and Dave's questions together;
These solar cells are used in concentrating systems, and so see up to 100
suns of concentrated sunlight, with operating temperatures between 80 and
100 degrees C [175-210F]. Our aim is to achieve 20 year reliability.
If anyone would like to see some photos of the solar cells and tabs, you can
go to http://solar.anu.edu.au/ and follow the links - Research,
Photovoltaic, Cells and Receivers.
A better description of my application - The front of our silicon solar
cells are plated with a network of fine silver tracks to collect the
electrons from the surface of the cell as they are generated by the PV
effect. The silver is used for two reasons. The first is that it is
difficult to bond metals directly to silicon, so the plating is built up
using several layers. Chromium bonds to silicon, palladium bonds to
chromium, silver bonds to palladium and solder bonds to silver. The second
reason is that the shinier the plating, the more light we can reflect onto
the solar cell. This extra light contributes a non- trivial amount to the
efficiency of the system.
Down two parallel edges of the front of the cell are silver busbars, to
which are soldered comb-like copper tabs. The tip of each finger of the comb
is reflow soldered using 62/36/2Ag paste.
The back of the cell is entirely plated with silver because it will not see
any sun and is soldered with tabs in a similar manner.
Whenever possible, the cells are stored in nitrogen. This slows the
tarnishing down, but does not stop the problem. After the final assembly the
cells are potted in an optically clear encapsulant. Tarnishing continues to
occur after encapsulation.
At this stage I have had no unusual soldering problems, and usually get good
looking joints (under 10X to 70X magnification). My problem arises from
tarnishing of the silver. As Dave suggests, we believe that it is due to
sulphur compounds in the air. The tarnishing poses a problem for the
reflectivity of the silver surfaces as mentioned earlier, and also the
layers of silver involved are by necessity very thin so any corrosion at all
will significantly decrease the conductivity of the current collecting
tracks. Gold plating is intended to alleviate these problems, and I am
trying to avoid it creating problems for the soldering process!
I am also open to any other anti-tarnishing approaches that anyone would
like to suggest.
On a slightly different note, I understood that silver loaded solder would
greatly reduce silver scavenging, but the replies that have seen suggest
that the joints will still be adversely affected. Would silver loaded solder
reduce the solubility of the gold by any chance?
Thanks again for your assistance!
Regards
Jeff Brown
Centre for Sustainable Energy Systems
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