On Jun 7, 4:51�pm, Mike Silva wrote:
On Jun 7, 12:41�pm, Erich wrote:

One of the biggest problems I have with the lead free solder is that
over time it tends to grow tin whiskers. �Then it shorts out to adjacent
pads. �Current reliability requirements are to encapsulate all the
solder joints with epoxy. �This is painful and really causes problems
with rework.

No-lead; a feel-good solution to a non-problem.

Erich N6FD

http://nepp.nasa.gov/WHISKER/(check out the photo gallery)

Wonder if the EU is going to pick up the eventual world-wide tab?

EU doesn't have to. RoHS (Reduction of Hazardous Substances) is an
in-country (European) specification that covers a LOT of different
substances. In one way RoHS is no different than the FCC
specification
that forbids MARKETING of receivers than can listen to cellular
telephone
frequencies...or the way older specification that stated TV sets sold
in the
USA must be capable of tuning in UHF TV channels...or the FCC
specification about linear RF amplifiers that can cover CB that aren't
allowed to be sold IN the USA.

RoHS is NOT an international specification. Manufacturers who sell
ONLY in the USA and don't care to enter the European (or RoHS-member)
markets aren't obliged in any way to obey RoHS standards. USA DoD
contracts on electronics can use lead-bearing solders, no problem.

NASA used CONFORMAL COATINGS or encapsulation on spacecraft
electronics in the beginning of the 1960s. NOT for any sort of 'tin-
whisker'
problems but for the fact that microgravity environments all all kinds
of small
or large junk to float around freely. It MIGHT be that some teeny
conductive
junk could short out traces on a PCB in space. Hardly likely in an
earth
environment with gravity all the time. Those same conformal coatings
were
later found to inhibit the growth of CERTAIN metal whiskers (tin is
not the only
one). Except for one potting compound allowed back in the 1960s, such
coatings and pottings were not epoxoids. The others were closer to
offshoots
of silicone elastomers and could be reworked.

There are all sorts of coating compounds available now. Emerson &
Cuming
built their long-running business of providing all kinds of polymer
compounds for the electronics industry. Those aren't cheap. No such
products are.

"Tin whisker" problems take a LONG time to show up. Usually it takes
a half year for the first such whiskers' start to become visible under
100x magnification. Repeated heating-cooling tends to hasten such
growth (of a
sort of crystaline form of tin) but such studies aren't quite complete
yet. But,
once started, the growth can accelerate. Even that is not speedy.

Thanks to MARKETING folks, there has been a lot of hysteria about this
"tin-
whisker" thing generated and picked up by all who have no experience
in such
things. A conformal coating will inhibt whisker growth. Even an
ordinary varnish used on wood surfaces. I'd avoid the green 'solder-
mask' stuff found
on PCB assemblies which are used to prevent excess solder during wave-
and reflow-soldering; it is tough stuff and much harder to remove if
put on
soldered pads/lands via additional coatings. I'd avoid lacquers
because they
leave a lot of stubborn ash behind if one MUST 'rework'
something...despite
their easy application and quick drying time and nice appearance after
application.

73, Len AF6AY