Don,
I am not sure what the undercoat under your palladium is but, it is generally
nickel and that is better than over copper since it has to do with thickness
required. Another unknown is it electroless or electrolytic and if it
electroless, what is the reduciing agent.
Unknowns are (1) undercoat, (2) thickness, (3) electrolytic (4) electroless,
(5) reducing agent, (6) immersion
With the six unknowns above, here is some information that may help after you
query your supplier as to the type of palladium finish he applied to the
boards.
1. The component industry has been using a nickel-palladium electrolytic
plating on the leads of surface mount chip carriers for a number of years.
This plating replaces the tin-lead finsh and has good shelf life and solders
well with RMA fluxes and any of the tin-containing solders. If you do not
specify and receive a specifice finish, you probably have been soldering to
this finish for some time. The palladium is thin, 6-10 microinches, on some
that we could measure. This is good because when it is thicker than 20/30
microinches and it is subjected to higher temperatures, it may cause a weak
solder joint. Probably due to brittleness of some of the many Pd/Sn or Pd/Pb
intermetallic compounds that grow thick if there is enough undissolved
palladium left to continue the intermetallic growth. Others have attributed
the failures of joints with Pd platings to the codeposition of the plating
additives which occur when the thickness is above 30 microinches. Regardless,
it should be thin, 10 microinches or less.
The electrolytic nickel/palladium plating as a surface finsh works well for
circuit boards when used as an etch resist. Typically for higher temp
applications the finsh would be 200 microinches of electrolytic nickel with a
finish coat of 6-10 microinches of electrolytic palladium. Some fabricators
will follow with 6-10 microinches of electrloytic gold. These two thin
coatings over the nickel do not degrade the solder joints.
2. When electrolytic palladium is applied directly over the copper it provides
good solderability for a shorter time, but again should be keep on the
palladium on the thin side to prevent reactions between the palladium and tin
or lead as mentioned above.
3. When electroless plating is used over either nickel or copper, you must
consider the type of reducing agent. Those which use the type in which a
portion of the reducing agent is codeposited, such as DMAB or hypophosphites,
it may leave enough of these material in the solder joint after reflow or
operation at higher temperatures so that the solder joint/plating interface
becomers very weak.
When a electroless palladium that has a non-codepositing reducing agent is
used, the problem with the left-in residues should not be a problem; however,
I do not have any personal experience or know of anyone who uses it a great
deal in production.
5. I do not think the immersion palladium deposit is considered a solderable
surface for very long after it is applied. I would assume that you suppliee
did not use.
Corrosion is not a problem, but latent solder joint failure may occur if the
conditions for problems with codeposted material are present when the
operating temperature is at or above 100'C or if you use multiple or long
reflows.
Phil Hinton
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