Good data and information. Thank you Keith.
Dewey 

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Tuesday, October 04, 2005 10:30 PM
To: [log in to unmask]
Subject: Re: [LF] Pb contamination in Pb-free solder

Jack,

Yes, the fact that copper is an alloying element in most lead-free
solders as well as a contaminant makes it difficult to deal with the it
in standards.

There is a data base of the several thousand wave solder machines that
are
running around the world at the moment, some for more than five years.
The
need to manage copper content means that users are taking regular
samples for analysis, sometimes weekly, or even daily and plotting the
results to reveal any
trends.   Most of that data is considered proprietary but the trends are
to
some extent predictable.

The copper level in a working solder bath is the result of a balance
between the copper coming in to the solder bath and the copper being
carried out (see
attached animated PowerPoint slide).    If conditions are stable the
copper
content eventually reaches an equilibrium level and the rate at which it
achieves that equilibrium and the level it reaches depends on factors
such as bath
capacity, rate of dross removal etc.   I attach a live Excel spreadsheet
that
provides a simulation which, although simplistic, does generate trends
that are consistent with real life experience qualitatively if not
quantitatively.
You can see the effect of the various parameters by changing the numbers
in the boxes and noting the resulting change in the %Cu time curve.

If the equilibrium copper level is higher than the maximum specified for
that
alloy then corrective action has to be taken.   If the solder is not so
aggressive towards copper the level can usually be maintained by using a
low-copper
top up alloy.   Silver seems to increase the rate at which the alloy
dissolves copper so that sometimes it is not possible to keep the copper
level in specification simply by topping up with a copper-free alloy and
from time to time some of the bath has to be removed and replaced with
fresh copper-free solder.

The upper and lower limits of copper in the solder bath are determined
largely by the slope of the liquidus line.   If the copper content gets
too high for
a particular operating temperature excess copper starts to precipitate
out as intermetallic crystals that interfere with the flow of the solder
with a
consequent increase in the incidence of shorts.   Typically the copper
content
should be kept below 1.0% and for some solders at least, preferably
under 0.85%.

The experience in the Japanese industry, which started lead-free
soldering long before lead-free components were widely available, was
that lead levels crept up typically to about 0.2- 0.25% which before
RoHS was tolerable without
joint degradation in some true eutectic alloys.   In non-eutectic alloys
the
lead tends to segregate and compromise reliability so that the level
that can be
tolerated is lower.     For alloys that do not contain silver there is
also a
gradual build up to a level around the 0.02% as a result of leaching
from components with silver finishes or metallisation.

Keith Sweatman
Nihon Superior Co., Ltd

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