I just found one more reference on the reliability of exposed copper. It
comes from the Clyde F. Coombs book; "Printed Circuits Handbook".

In chapter 37 (Reliability of Printed Circuit Assemblies), paragraph 37.5.1.3
Metal Finishes; "The metal finish on the SMT and TH pads can have an impact
on PTH reliabilty and on the reliability of the solder joints made to these
pads. Common metal finishes for solder-mask-over-bare-copper (SMOBC) boards
include hot-air solder leveling (HASL or HAL), organic-coated copper (OCC),
and electroless NiAu. Galvanically plated CuNiAu and CuNiSn made by another
processing route are also available. These finishes provide a solderable
finish for later printed circuit assembly. The pros and cons of the various
finishes are discussed in turn.

Of the most common metal finishes, HASL is the only one which can directly
reduce reliabilty of the board. In a typical HASL process, the board receives
a severe thermal shock when it is dunked into a bath of molten eutectic Sn-Pb
solder. The PTH's can survive only a certain number of solder shocks without
failure; this process step uses up one of these thermal cycles before the
board leaves the fabricator.

Organic-coated copper provides a consistent, flat, solderable metal finish.
Exposed copper after printed circuit assembly has been a persistent
reliability concern, because it is generally not permitted on HASL boards.
While exposed Cu on HASL boards is associated with poor solderability, which
may be due to contaminates that were not removed before the HASL process,
there is little evidence that exposed Cu on a properly processed OCC board
causes reliability problems. Surface insulation resistance (SIR) testing
shows that OCC boards have comparable or better perfomance than HASL boards
in high-temperature, high-humidity storage tests."

-Steve Gregory-

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