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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