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Minimizing Copper Plating Wastes While Enabling EDTA Recycling by Preventing Its Oxidation in the Deplater

IP.com Disclosure Number: IPCOM000120036D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 145K

Publishing Venue

IBM

Related People

Light, DN: AUTHOR [+4]

Abstract

Electroless copper plating baths have a finite lifetime which is determined by bath composition, usage, quality of maintenance, and functional requirements of plated films. Typically, these baths contain a chelating agent, such as ethylene diamine EDTA, which stabilizes the copper via complexion and prevents uncontrolled plating on non-catalytic substrates. EDTA for this application often must be extremely pure in order to ensure adequate plated film properties. Because of this, it is expensive to purchase. In addition, EDTA is difficult and costly to dispose of since it is classified as hazardous waste. For this reason, it is desirable to recover and reuse the EDTA from spent plating baths.

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Minimizing Copper Plating Wastes While Enabling EDTA Recycling by
Preventing Its Oxidation in the Deplater

      Electroless copper plating baths have a finite lifetime
which is determined by bath composition, usage, quality of
maintenance, and functional requirements of plated films. Typically,
these baths contain a chelating agent, such as ethylene diamine EDTA,
which stabilizes the copper via complexion and prevents uncontrolled
plating on non-catalytic substrates.  EDTA for this application often
must be extremely pure in order to ensure adequate plated film
properties.  Because of this, it is expensive to purchase.  In
addition, EDTA is difficult and costly to dispose of since it is
classified as hazardous waste.  For this reason, it is desirable to
recover and reuse the EDTA from spent plating baths.

      Typically, copper is removed from waste electroless plating
processes via plating at high current density in a single chamber
unit.  The process is referred to by the industry as electrowinning.
However, since the copper is being removed from the solution by
plating it out, the process is often referred to as 'deplating' and
the unit in which the process takes place is called a 'deplater'.

      After the copper is deplated from the solution, EDTA can be
recovered by lowering the pH of the remaining electrolyte.  This
causes most of the EDTA to precipitate out of solution.  Generally,
this EDTA is subsequently recystallized to purify it.  The EDTA
recovered in this manner is sometimes pure enough to reuse in
decorative or non- critical plating operations.  However, for
applications in the electronics industry, the purity is often
inadequate. It has been found that the EDTA undergoes oxidation at
the anode in a single-cell deplater.  If high-speed deplating is
carried out, a substantial portion of the EDTA is oxidized. Under
optimum conditions (i.e., low current density and controlled cell
voltage), at least 10% of the EDTA is still oxidized, and the
deplating rate is substantially reduced. Some of the products of EDTA
oxidation (which include secondary and tertiary amines) have a
deleterious impact on the properties of subsequent electroless copper
deposits when this EDTA is reused in plating baths.  These effects
include reduced ductility of the deposit and subsequent cracking of
the metal layer.  These effects are especially critical in large
board manufacturing.  These deleterious reaction products of EDTA are
not readily separable from the recovered EDTA.  While their
concentrations can sometimes be reduced by purification (e.g.,
recystallization), this involves extra processing steps and
substantially increases the recovery expense.  For some applications,
the EDTA is still unsatisfactory.

      It has been postulated previously that by careful control of
the cell voltages in a single chamber deplater, the oxidation of EDTA
and formation of deleterious byproducts could be avoided.  The
assumpti...