Note: Descriptions are shown in the official language in which they were submitted.
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WO 99/42402 ' PCT/US98/03406
COMPOSITION AND METHOD FOR REDUCING
COPPER OXIDE TO METALLIC COPPER
Related Application
This application is a continuation-in-part of U.S. Patent No 5,721,014, issued
on
February 24, 1998.
Field of the Invention
The present invention relates to a composition and method for reducing copper
oxide to metallic copper in the fabrication of multilayer printed circuit
boards.
Background of the Invention
Successful fabrication of multilayer printed circuit boards requires bonding
together of copper and resin layers. However, direct bonding of copper and
resin
layers does not provide sufficient bonding strength. Therefore, it is common
to
improve copper resin bonding strength by depositing on the copper surface an
oxide
layer, such; ~e cuprous oxide, cupric oxide, or the like. Formation of the
oxide layer,
which turns the pink copper surface a black-brown color, creates minute
unevennesses on the copper surface which provide an interlocking effect
between the
copper surface and resin, thus improving bonding strength.
However, copper oxides are readily hydrolyzed and dissolved upon contact with
acid. Because various acid treatments are used in later stages of fabrication
of
~0 multiiayer circuit boards, oxide layer deposition has been problematic at
best. Acid
attack on the oxide layer is commonly referred to in the industry as "pink
ring",
because as acid strips the black-brown oxide layer from the surface, a ring of
bare
pink copper becomes evident.
The problem of vulnerability of the oxide layer to acid was solved by the
method
described in U.S. Patent No. 4,642,161 to Akahoshi et al. The Akahoshi patent
has been
assigned ,
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' WO 99142402 PCT/US98/03406
to Hitachi, Ltd. The Akahoshi et al. method is also described in Akahoshi et
al.,
Circuit World 14(1) (1987), and in the Hitachi, Ltd. technical publication
"The
Chemical Reduction Treatment of Copper Oxide, DMAB Method (Technoloqv for the
Elimination of Pink Ring"
In the Akahoshi et al. method, the copper oxide layer i,s reduced to metallic
copper by means of a reducing solution containing an amine b~rane compound as
the
active reducing agent. The minute unevennesses created on the copper surface
from
oxidation'remain following reduction, so that the metallic copper surface
produced as
a result of the reduction process will form a sufficiently strong bond with a
resin. In
contrast to cupric oxide and cuprous oxide, which are both soluble in acid,
the
metallic copper surface resulting from the reduction process, which is the
same black
brown color as the oxide layer, has good acid resistance. Therefore by
reducing the
copper oxide to metallic copper, the acid resistance of the surface or panel
is
increased, and there is a reduced likelihood of the appearance of "pink ring".
The presently known reducing agents which are capable of reducing cupric
oxide to metallic copper are amine boranes represented by the general formula:
BH3NHRR' (wherein R and R' are each a member selected from the group
consisting
of H, CH3, and CH2CH3?, such as dimethylamine borane (DMAB1 and ammonia
borane.
Because amine boranes are costly to manufacture, they are quite expensive,
which
results in high operating costs for the reduction process.
Thus, it is clearly desirable to develop an alternative reducing agent which
is
less expensive than the amine boranes; while ensuring that the metallic copper
layer
resulting from such an alternative reducing agent has good bonding properties
and
acid resistance.
Summary of the Invention
In one embodiment, this invention provides a composition for reducing a copper
oxide layer to metallic copper so as to facilitate bonding a resin to the
metallic copper.
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The composition includes an aqueous reducing solution containing a cyclic
borane
compound. Examples of such cyclic borane compounds include morpholine borane,
piperidine borane, pyridine borane, piperazine borane, 2,6-lutidine borane,
N,N-
diethylaniline borane, 4-methylmorpholine borane, and 1,4-oxathiane borane.
In another embodiment, the present invention provides an improvement to a
method of bonding copper and a resin together wherein a copper oxide layer is
reduced to metallic copper and the metallic copper is bonded to a resin. The
improvement includes reducing the copper oxide layer to metallic copper with
an
aqueous reducing solution containing a cyclic borane compound.
It is an object of the present invention to provide an alternative reducing
agent
to the amine boranes used for reducing a copper oxide layer to metallic copper
so as
to facilitate bonding a resin to the metallic copper.
It is another object of the present invention to decrease operating costs
associated with copper oxide reduction processes.
Other objects, advantages, and features of the present invention will become
apparent from the following specification.
Detailed Description of the Invention
The present specification describes a composition and method for reducing
copper oxide to metallic copper in the fabrication of multilayer printed
circuit boards.
As previously described, copper oxide reduction is particularly useful in the
manufacturing of multi-layer printed circuit boards because formation of the
oxide
layer, which turns the pink copper surface a black-brown color, creates minute
unevennesses on the copper surface which provide an interlocking effect
between the
copper surface and resin, thus improving bonding strength of the layers.
However,
because copper oxides are soluble in acid, the oxide layer is vulnerable to
acid attack.
The Akahoshi et al. method solved the problem of the acid vulnerability of the
oxide layer. According to that method, the copper oxide layer is reduced to
metallic
copper by means of a reducing solution containing an amine borane compound as
the
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active reducing agent. The minute unevennesses created on the copper surface
from
oxidation remain following reduction, so that the metallic copper surface
produced as
a result of the reduction process will form a sufficiently strong bond with a
resin, and
the metallic copper surface resulting from the reduction process, which is the
same
black-brown color as the oxide layer, has good acid resistance. Because good
acid
resistance indicates that the copper oxide layer has been successfully reduced
to
metallic copper (despite no change in color of the surface), and because such
a
metallic copper surface retains the minute unevennesses created by the earlier
oxidation process, good acid resistance also indicates that the metallic
copper surface
resulting from the reduction process will have an excellent ability to bond
with resins
such as those described in the Akahoshi et al. patent. Such resins include
epoxy
resins, polyir~ide resins, polyamide resins, polyester resins, phenolic
resins, and
thermoplastic resins such as polyethylene, polyphenylene sulfide, polyether-
imide
resins, and fluororesins.
The amine boranes disclosed in the Akahoshi et al method are represented by
the general formula: BH3NHRR' wherein R and R' are each a member selected from
the group consisting of H, CH3, and CH2CH3, as represented below:
H3
BH3N BH3NH3
H3
BH3NHZCH3 BH3NHZCH2CH3
H3 HzCH3
BH3N BH3N
HZCH3 HZCH3
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These amine boranes include dimethylamine borane and ammonia borane. However,
such amine boranes are expensive; furthermore, after completion of the desired
reduction of copper oxide surfaces, the amine boranes are consumed in the
reduction
solution in amounts greatly in excess of the amount stoichiometrically
required for
such reduction. Thus, reduction of copper oxide by amine borane compounds
typically results in high operating costs.
The present inventor discovered that an aqueous solution containing a cyclic
borane, in particular, cyclic borane compounds having nitrogen or sulfur as a
ring-
forming member, such as the cyclic compound morpholine borane, OC4HeNH:BH3, is
very effective in reducing copper oxide to metallic copper so as to facilitate
bonding
of a resin to the metallic copper in the course of fabrication of multilayer
printed
circuit boards. The compound morpholine borane contains nitrogen as a ring-
forming
member, as shown below.
BH3
Morpholine borane is less expensive to manufacture than the amine boranes,
such as DMAB, presently used in the reduction of copper oxide to metallic
copper.
The melting point of morpholine borane is 98 ° (208 ° F),
compared to DMAB, which
has a melting point of 36 ° (97 ° F). Because of its higher
melting point, purified
morpholine borane can be obtained by distillation at ambient pressure. On the
other
hand, to purify the lower-melting point compound DMAB, vacuum distillation
must
be used, resulting in higher manufacturing costs for morpholine borane as
compared
to DMAB. This relative ease in manufacturing morpholine borane as compared to
amine boranes such as DMAB, results in up to a 50% cost savings, thereby
significantly decreasing operating costs for the copper oxide reduction
process.
The effectiveness of morpholine borane as a reducing agent was determined
by processing a copper oxide coated copper panel through a reducing solution
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containing morpholine borane. Because the metallic copper surface produced
from
reduction is the same brown-black color as the oxide layer, there are several
parameters other than appearance which are measured to test the effectiveness
of
the reducing process. Such parameters include initiation time, acid
resistance, and
weight loss of the copper oxide coating after reduction.
The initiation time is the time required for the reduction of the copper oxide
to
initiate. When the reduction reaction initiates, hydrogen bubbles rapidly form
from
the copper oxide and continue until the reaction is complete; preferably,
initiation
occurs within 4 minutes. The acid resistance is determined by immersing the
reduced
copper in an acid bath. Metallic copper survives longer in acid than do copper
oxides;
one suggested measure of acid resistance is whether the metallic copper layer
can
withstand acid resistance for at least about 30 minutes. Thus, the
effectiveness. of
the reduction process can be determined by resistance to acid attack. The
weight
loss of the panel also determines the effectiveness of the reduction process.
A
copper oxide panel loses weight when the copper oxide is reduced. By measuring
the
weight loss of the copper oxide panel, the completeness of reduction can be
determined. A low weight loss indicates that the copper oxide is not fully
being
reduced; preferably, weight loss is greater than 15%. Copper oxide panels were
made by growing an oxide layer on a metallic copper panel.
Successful reduction by morpholine borane, as measured by the above
parameters, ensures that the minute unevennesses created on the copper surface
from oxidation remain following reduction. Those minute unevennesses enable
the
metallic copper surface produced from morpholine borane reduction to form a
sufficiently strong bond with a resin. Such resins include epoxy resins,
polyamide
resins, phenoloic resins, and thermoplastic resins such as polyethylene,
polyphenylene
sulfide, polyether-imide resins, and fluororesins. By bonding together copper
and
resins layers, multilayer printed circuit boards can be successfully
fabricated.
The above criteria were used to identify that morpholine borane is effective
in
reducing copper oxide to metallic copper at concentrations ranging from about
1 g/1
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to saturation, in order to facilitate bonding a resin to the metallic copper
in the
fabrication of multilayer printed circuit boards. The preferred morpholine
borane
concentration is in the range of about 2.7 g/1 to 16.8 g/1, in particular, 2.7
g/1. The
invention is further described and illustrated with reference to the following
examples:
Example 1
An aqueous reducing solution was prepared using 1.6 g/L dimethylamine
borane and 1 5.2 g/1 sodium hydroxide. The solution, at room temperature, was
used
as a control to reduce copper oxide formed on the above mentioned copper
panels by
immersing the panels in the reducing solution for a 4 minute dwell time. The
initiation
time was recorded. The percent weight loss of the copper oxide coating and the
time
that the resultant coating survived a 10 % by volume hydrochloric acid bath
were
recorded. -
An aqueous reducing solution was prepared using 2.7 g/L morpholine borane
and 15.2 g/1 sodium hydroxide. This was the experimental formula used to
determine
the effectiveness of morpholine borane as a reducing agent, and contains the
stoichiometric equivalent composition of -BH3 to that contained in a 1.6 g/L
DMAB
reducing solution. The panels were immersed in the reducing solution for a 4
minute
dwell time. The initiation time was recorded. The percent weight loss of the
oxide
coating and the time that the resultant coating survived a 10 % by volume
hydrochloric acid bath were recorded.
Both DMAB and morpholine borane were obtained from Aldrich Chemical
Company, Milwaukee, Wisconsin. Sodium hydroxide was obtained from Hill
Brothers
Chemical Company, Orange, California.
The results, presented in Table 1, below, indicate that morpholine borane
effectively reduces copper oxide to metallic copper.
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TABLE 1
Reducing Agent Initiation % Weight Acid
Time Loss Resistance
Dimethylamine 26.4 sec 19.72 % 51 .3 min
Borane
Morpholine Borane14.3 sec 19.77 % 54.5 min
Example 2
The present inventor tested the effectiveness of morpholine borane at other
concentrations. The saturation concentration of morpholine borane in an
aqueous
solution of 15.2 g/1 sodium hydroxide in the range of approximately 50 g/1 -
60 g/1.
The results, presented in Table 2, below, indicate that morpholine borane is
an
effective reducing agent at concentrations ranging from about 1 g/1 to
saturation_
TABLE 2
Concentration Initiation % Weight Acid
of Time Loss Resistance
Morpholine Borane
0 g/1 > 4 min < 5 % < 10 sec
1 g/1 69 sec 17 % 45 min
g/1 9 sec 19 % 50 min
. 50 g/1 7 sec 20 % 50 min
With respect to the amine borane reducing agents, it is known that the amine
20 boranes continue to be consumed even after all of the cupric oxide on the
panels has
been reduced to copper metal, and no additional cupric oxide is introduced
into the
solution. Consumption of the amine boranes continues after reduction of the
copper
oxide panels because, it is theorized, the reduced copper oxide from the
panels is still
present in the reducing solution, and may be reoxidized or may catalyze the
hydrolysis
25 of the amine boranes. Therefore, the amine boranes are consumed in greater
than the
stoichiometric amount necessary to reduce the copper oxide on the panels. The
excessive consumption of the reducing agent shortens the usable lifetime of
the
reducing solution, and ultimately results in higher operating costs for the
process.
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In U.S. Patent No 5,721,014 , the present inventor discloses that addition of
reduction stabilizers to amine borane reducing solutions decreased amine
borane
consumption to between about 11 % to 92% of amine borane consumption observed
in the absence of such stabilizers. Suitable reduction stabilizers disclosed
in the co-
y pending application include thio-containing (-C( =S)NHZ) compounds such as
thiourea,
triazole-containing (CZH3N3) compounds such as tolytriazole and benzotriazole,
isoxazole-containing (-C3HN0) compounds such as 3-amino-5-methylisoxazole,
thiazole-containing (-NCS-) compounds such as mercaptobenzothiazole, imidazole
containing (-NCN-) compounds such as benzimidazole, and sulfone-containing (-
S03H)
compounds such as sulfamic acid.
As described in U.S. Patent No 5,721,014 , the selection of a particular
stabilizer and stabilizer concentratio,~ is based .on several factors,
including the
following: whether the reduction process stabilized by a given concentration
of
selected stabilizer is initiated within a reasonable time (preferably in less
than about
4 minutes), whether the metallic copper layer resulting from the thus
stabilized
reduction process is resistant to acid attack (one suggested measure of such
resistance is whether the metallic copper layer can withstand acid attack for
at least
about 30 minutes), and finally, whether the given concentration of selected
stabilizer
actually results in a decreased consumption of the amine borane reducing
agent.
In U.S. Patent No 5,721,014 , with respect to an amine borane solution
consisting of 1.6 g/1 dimethylamine borane and 15.2 g/ sodium hydroxide at
80°F,
the above criteria were used to identify the following preferred stabilizers
and
effective concentrations: thiourea (about 1 ppm - 13 ppml; tolytriazole (about
0.50
ppm); benzotriazole (about 1.0 ppm); 3-amino-5-methylisoxazole (about 100
ppm);
mercaptobenzo-thiazole (about 10 ppm); benzimidazole (about 10 ppm); and
sulfamic
acid (about 10 g/1). The co-pending application also discloses that increases
in
reducing agent concentration and temperature can increase the upper limits of
effective stabilizer concentrations.
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The present inventor discovered that copper oxide reduction by morpholine
borane can be similarly stabilized so as to reduce consumption of morpholine
borane.
The morpholine borane reduction process, stabilized by thiourea, is initiated
in a
reasonable time and the metallic copper layer resulting from the stabilized
reduction
process is resistant to acid attack. For example, the present inventor
discovered that
a reducing solution consisting of 2.7 g/1 morpholine borane and 15.2 g/1
sodium
hydroxide at 80°F could be stabilized by the addition of thiourea in a
concentration
in the range of about 2.5 ppm to about 1 5 ppm.
The present inventor also discovered that, as was the case in his co-pending
application, reducing agent concentration and temperature have an effect on
the
effective concentration range of the stabilizer. For example, in a reducing
solution
consisting of about 2.7 g/L morpholine borane and 15 g/L sodium hydroxide,
raising
the temperature from about 80 ° F to 120 ° F increases the upper
limit of the effective
concentration range of thiourea stabilizer from about 15 ppm to about 20 ppm.
1 5 Increasing the morpholine borane concentration to 16.8 g/L, at about
80°F, increases
the upper limit of the effective concentration range of thiourea stabilizer to
about 160
ppm; at a morpholine borane concentration of 16.8 g/L, if the temperature is
increased to 120°F, the upper limit of the effective concentration
range of thiourea
stabilizer increases to about 200 ppm.
General Procedures for Testing Stabilizers
In order to test the potential stabilizer, the present inventor created
reducing
solutions similar to those used in an actual copper oxide reduction process.
The
reducing solutions were then poisoned with copper oxide, because during an
actual
copper oxide reduction process, copper oxide remains in the reducing solution
after
the reduced copper panels are removed. The remaining copper oxide consumes
further amounts of the reducing agent, resulting in overall consumption of
reducing
agent in greater than the stoichiometric amount necessary for reduction of
copper
oxide on the panels. Thus, the consumption of reducing agent per gram of
copper
oxide and the consumption of reducing agent per time could be determined. The
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concentration of the reducing agent was analytically determined initially and
after 24
hours, via iodometric titration.
The possible negative effects the potential stabilizer could have on the
copper
oxide reduction process were determined by processing a copper oxide panel
through
the reducing solution containing a given potential stabilizer. As described
above,
there were several parameters examined to determine the effectiveness of such
stabilized reducing processes; reduction process initiation time, acid
resistance of the
metallized copper surfaces resulting from the stabilized reduction, and weight
loss of
the copper oxide coating after reduction.
Initiation time is the time required for copper oxide reduction to begin. When
the reduction reaction begins, hydrogen bubbles rapidly form and continue
until the
reaction is complete. Acid resistance was determined by immersing the
resulting.
metallized copper surface in an acid bath. Metallic copper survives longer
than
copper oxides in an acid bath. Thus, the effectiveness of a given stabilized
reduction
process in producing the desired metallic copper surface, was determined by
resistance to acid attack.
Weight loss of the tested panel also determines the effectiveness of the
reduction process. Copper oxide loses weight when it is chemically reduced to
copper metal. By measuring the weight loss of the panels, the completeness of
reduction was determined. A low weight loss indicates that the copper oxide is
not
fully being reduced. Copper oxide panels were made by growing an oxide layer
on
a metallic copper panel.
Example 3
To test the effect of the potential reducing stabilizer thiourea, an aqueous
reducing
solution was prepared using 2.7 grams per liter morpholine borane and 15.2 g/L
sodium hydroxide. DMAB was obtained from Aldrich Chemical Company, Milwaukee,
Wisconsin. Sodium hydroxide was obtained from Hill Brothers Chemical Company,
Orange, California. That solution, at room temperature, was used as the
control
solution. An experimental solution was prepared by adding 2.5 ppm of thiourea
to
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a solution identical in composition to the control solution. Both the control
and the
experimental solutions were poisoned with 0.075 g/L of copper oxide. The
concentration of morpholine borane in both solutions was analyzed before the
copper
oxide was added, and 24 hours after the copper oxide was added. The results
are
provided in Table 3.
TABLE 3
Morpholine Initial Stoichio. Consumption
Borane Concent. Consume. of of
Solution Morpholine Morpholine Morpholine
Borane Borane to Borane Over
Reduce All 24 Hours
Copper Oxide
Control 2.70 g/L 0.0317 g/L 1 .1745 g/L
Ex erimental 2.70 /L 0.0317 /L 0.2784 /L
Example 4
The effective stabilization range of thiourea for morpholine borane was tested
to a solution of 2.7 g/L morpholine borane and 15.2 g/L sodium hydroxide,
thiourea
was added at 5 ppm intervals, and the solution's ability to reduce copper
oxide and
stability were determined. The measured parameters were initiation time,
percent
weight loss; acid resistance, and consumption over 24 hours. The results are
presented in Table 4.
TABLE 4
Concent. Initiation % Acid Consume.
of Time Weight Resistance of
Thiourea Loss Morpholine
Borane Over
24 Hours
0 ppm 25 sec 20 % 40 min 1 .78 g/L
5 ppm 30 sec 20 % 40 min 0.25 g/L
10 ppm 30 sec 18 % 45 min 0.19 g/L
1 5 ppm 40 sec 1 6 % 35 min 0.14 g/L
20 m 60 sec 9 % < 1 min 0.1 1 /L
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The above results indicate that in a 2.7 g/L morpholine borane solution,
thiourea can be added to a concentration up to about 15 ppm, and good copper
reduction can be achieved. Also evident from the above data is the fact that
the
more thiourea added to the reducing solution, the less morpholine borane is
consumed.
Example 5
In this example, the concentrations of both morpholine borane and thiourea
were varied. Reducing solutions of (1 ) 8.4 g/L MB, 1 5.2 g/L sodium
hydroxide, and
(2) 16.8 g/L MB, 1.2 g/L sodium hydroxide were made. Thiourea was added over
the
side of the reducing baths, and the reduction of copper oxide panels was
determined.
The measured parameters were initiation time, percent weight loss, and acid
resistance. The results are presented in Table 5. - .
TABLE 5
Concept. Thiourea Initiation % Weight Acid
of MB Concept. Time Loss Resistance
8.4 g/L 0 ppm 10 sec 21.3% 50 min
8.4 g/L 10 ppm 10 sec 18.7% 50 min
8.4 g/L 20 ppm 15 sec 20.9% 50 min
8.4 g/L 30 ppm 15 sec 16.5 % 50 min
8.4 g/L 40 ppm 15 sec 18.3% 45 min
8.4 g/L 50 ppm 20 sec 16.1 % 50 min
8.4 g/L 60 ppm 25 sec 15.8% 45 min
8.4 g/L 70 ppm 25 sec 15.7% < 1 min
16.8 g/L 0 ppm 5 sec 22.8% 50 min
16.8 g/L 25 ppm 5 sec 17.6% 50 min
16.8 g/L 50 ppm 10 sec 16.1 % 50 min
16.8 g/L 75 ppm 10 sec 19.7% 50 min
16.8 g/L 100 ppm 10 sec 18.3% 45 min
16.8 g/L 120 ppm 15 sec 17.5% 45 min
16.8 g/L 140 ppm 1 5 sec 17.1 % 50 min
16.8 g/L 160 ppm 15 sec 16.8% 50 min
16.8 /L 180 m 15 sec 16.4% < 1 min
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These results indicate that thiourea may be used as a stabilizer in a
morpholine
borane reducing solution at concentrations ranging from greater than O ppm
thiourea
to 160 ppm. Based on the trends evident from these results, it is probable
that
reducing solutions of morpholine borane having concentrations greater than
16.8 g/L
MB may still use thiourea as a stabilizer at concentrations greater than 160
ppm
thiourea.
Example 6
As seen in Example 5, Table 5, the effective concentration range of stabilizer
in the reducing solution is dependent on the concentration of reducing agent.
The
effective concentration range of stabilizer in the reducing solution is also
dependent
on the temperature of the reducing solution.
To illustrate the effects of temperature on a morpholine borane reducing
solution with thiourea as a stabilizer, reducing solutions of similar
morpholine borane
concentrations and thiourea concentrations were made and the ability to reduce
1 5 copper oxide panels at various temperatures were tested.
An aqueous solution of 2.7 grams per liter dimethylamine borane, 15.2 g/L
sodium hydroxide, and 20 ppm thiourea was made. The solution's ability to
reduce
copper oxide panels was determined for several temperatures. The results are
listed
in Table 6 below.
TABLE 6
Morpholine Thiourea Temperature % InitiationAcid
Borane Concentration Weight Time Resistance
Concentration Loss
2.7 g/L 20 ppm 80F 9.0 60 sec < 1 min
2.7 g/L 20 ppm 100F 15.6 45 sec 35 min
2.7 /L 20 m 120F 19.8 40 sec 40 min
An aqueous solution of 8.4 grams per liter morpholine borane, 15.2 g/L sodium
hydroxide and 70 ppm thiourea was made. The solution's ability to reduce
copper
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oxide panels was determined at several temperatures. The results are listed in
Table
7 below.
TABLE 7
Morpholine Thiourea Temperature % InitiationAcid
Borane Concentration Weight Time Resistance
Concentration Loss
8.4 g/L 70 ppm 80 F 15.7 25 sec < 1 min
8.4 g/L 70 ppm 100F 18.1 25 sec 35 min
8.4 /L 70 m 120F 19.3 20 sec 40 min
An aqueous solution of 16.8 grams per liter morpholine borane, 15.2 g/L
sodium hydroxide and 200 ppm thiourea was made. The solutions ability to
reduce
copper oxide panels was determined at several temperatures. The results are
listed
_ .
in Table 8 below.
TABLE 8
Morpholine Thiourea Temperature % InitiationAcid
Borane Concentration Weight Time Resistance
Concentration Loss
16.8 g/L 200 ppm 80 F 15.1 20 sec < 1 min
16.8 g/L 200 ppm 100F 17.0 15 sec 35 min
16.8 /L 200 m 120 F 18.8 15 sec 40 min
The present inventor also discovered an additional improvement to the
reduction process which can be realized by using morpholine borane instead of
alkane
boranes. Morpholine borane stabilized with thiourea shows more stability after
the
reducing bath has been continuously processed and replenished than a
comparable
DMAB reducing solution stabilized with thiourea which has been continuously
processed and replenished.
This improvement can be explained by examining and comparing the DMAB and
morpholine borane reduction processes more closely. In DMAB reduction, a
reducing
bath containing 1 .5 g/L DMAB and 1 5.2 g/L NaOH is initially made. Copper
oxide
panels are then processed through the reducing bath. The copper oxide is
reduced
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by the dimethylamine borane. In the reaction, the boron atom in the DMAB is
oxidized and the copper oxide is reduced. It is believed that the
dimethylamine
functional group is left as a side product.
The typical concentration of DMAB used for reducing copper oxide panels is
1 .6 g/L DMAB. Therefore, an automatic control system is set up to replenish
the
DMAB and keep a constant concentration of 1.6 g/L DMAB. As replenishment
continues, the dimethylamine side product accumulates in the tank. Therefore,
a
constant concentration of DMAB is present in the reducing solution, 1 .6 g/L
DMAB,
but the concentration of the dimethylamine functional group is continuously
increasing.
The concentration of the dimethylamine functional group reaches an equilibrium
when the amount of the dimethylamine functional group being formed is
equivalent
to the amount of dimethylamine being dragged out of the reducing solution on
the
panels. (It is a rough estimate that 10 - 15 mL of reducing solution per
square foot
of panel is dragged out of the reducing solution tank.)
Therefore, after a period of replenishment, the process should finally be in
equilibrium with respect to the dimethylamine functional group. That is there
should
be 1.6 g/L DMAB, 15.2 g/L NaOH, and probably a constant concentration of
dimethylamine functional group side product, and probably a constant
concentration
of a boron containing side product resulting from the reduction process. This
more
accurately represents the reducing bath as it will be run in the process.
In summary, there are two main states of the bath, initial and at equilibrium.
The parameters of % weight loss, acid resistance, initiation time, consumption
over
1 /2 turnover, and consumption over 24 hours were tested. The bath is
initially made
up of 1.6 g/L DMAB and 15.2 g/L NaOH. To mimic equilibrium, which is reached
in
approximately 20 days, copper oxide is added to the initial bath to decrease
the
concentration of DMAB in half over 24 hours. This is continued until the DMAB
is
replenished 20 times. The sum total of the 20 replenishments of 1 /2 the
initial
concentration has been designated "10 turnovers," an initial bath is
designated "zero
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turnovers." After 10 turnovers, the reducing solution now has roughly an
equilibrium
amount of dimethylamine side products together with 1 .6 g/L DMAB and 1 5.2
g/L
NaOH. Note that this does not precisely mimic an actual process, because in an
actual process, DMAB is continuously replenished to maintain the DMAB level
constant at 1 .6 g/L DNAB, while the concentration of dimethylamine side
product
increases to the equilibrium amount, i.e. the controller does not let half of
the DMAB
present react before replenishment is made.
The above scenario is similar for morpholine borane, MB. The initial
concentration of morpholine borane is 2.7 g/L MB and 1 5.2 g/L NaOH. Side
product
formed during the reduction of copper oxide is probably the morpholine
complex. As
the morpholine borane is replenished during the reduction process, the
morpholine
borane remains at a constant concentration of 2.7 g/L MB, but the
concentration of
the suspected side product, the morpholine complex, continues to increase
until the
bath reaches equilibrium with respect to the morpholine complex. Testing for
the
same parameters as was done above with respect to DMAB was done initially and
after 20 replenishments of morpholine borane. As above, half of the morpholine
borane was consumed and added back, and again the sum total of the 20
replenishments of 1 /2 the initial concentration was designated 10 turnovers;
the
initial bath was designated zero turnovers.
The above discussion described the process without addition of the stabilizer
thiourea. When thiourea is used to stabilize the reduction, the stabilizer is
added back
with the DMAB or the morpholine borane, depending on which reducing agent is
used. For example, the concentration of thiourea is initially 2.5 ppm, then
the actual
concentration of thiourea starts at 2.5 ppm and increases with each
replenishment
of DMAB or morpholine borane, as the case may be, until equilibrium is
reached.
The results presented in Table 9 illustrate the excellent benefits of using MB
as
opposed to DMAB. When stabilizing a DMAB reducing solution with thiourea, the
consumption over 24 hours greater than doubles after 10 turnovers, compared to
the
initial consumption over 24 hours at zero turnovers lcompare row 8 to row 2,
and
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row 9 to row 3). In contrast, when using morpholine borane stabilized by
thiourea,
the consumption over 24 hours does not double, but only increases by 20 - 30%
of
the original consumption over 24 hours at zero turnovers (compare row 1 1 to
row 5,
and row 12 to row 6).
TABLE 9
Turnover (TO) Results: DMAB and Morpholine Borane (with and without Thiourea)
Initial DMAB concentration: 1 .6 g/L
Initial Morpholine Borane concentration: 2.7 g/L
At Zero Turnovers:
10 a uc ion o m . ci - onsum.
~
solution Loss Time Resist. over 24
comp. hrs.
(% consum.
24 hrs.) '
Row 1
DMAB 29.3 50.1 12.1
8 /
20
0.0 ppm ' sec min (42.3%)
Thiourea
Row 2
DMAB o 30.7 2.27
3 /o 5
21
2.5 ppm . sec mi ~ (8.06%)
Thiourea
Row 3
DMAB o 38.1 48.4 1.61
0 /o
20
5.0 ppm . sec min (5.76%)
Thiourea
Row 4
Morpholine
18 37 12
2 6 4
Borane 20.6% . . .
sec min (43.8%)
0.0 ppm
Thiourea
Row 5
Morpholine
0 48 2
20 1 92
Borane 23.2% . . .
sec min (10.6%)
2.5 ppm
Thiourea
Row 6
Morpholine 2.17
1 45
26 7
Borane 20.6% . . (7,96%)
5.0 ppm sec min
Thiourea
~ Consumption over 24 hours is expressed as mol. reducing agent (DMAB or
morpholine borane)/mol. copper oxide (add other notes).
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At 10 Turnovers:
a uc ion o . m . ci onsum. over
solution Loss Time Resist. hrs.
comp. (% consum.
24 hrs.)
Row 7
m 18 36 45 g
6% 3 sec 6 min
0 0 pp . . . (5
4%)
Thiourea
Row 8
DMAB 1 g 41 44 4.85
1 % 1 sec 4 min
2.5 ppm , . . (19.6%)
Thiourea
Row 9
17 46 42
1 % 8 sec 0 min
5 0 ppm . . . (1 1.8%)
Thiourea
Row 10
Morpholine
1
17
Borane 19.5% 29.3 sec 40.4 min .
(66.1 %)
0.0 ppm
Thiourea
Row 11
Morpholine 3
31
Borane 18.2% 30.7 sec 38.2 min .
(12
7%)
2.5 ppm .
Thiourea
Row 12
Morpholine
51
2
Borane 16.2% 33.2 sec 39.3 min .
5.0 ppm (9.33%)
Thiourea
Consumption over 24 hours is expressed as mol. reducing agent (DMAB or
morpholine borane/mol. copper oxide f add other notes).
The present inventor discovered that there are cyclic borane compounds in
addition to morphine borane which are very effective in reducing copper oxide
to
metallic copper so as to facilitate bonding of a resin to the metallic copper
in the
course of fabrication of multilayer printed circuit boards. The cyclic borane
compounds include piperidine borane, pyridine borane, piperazine borane, 2,6-
lutidine
borane, 4-ethylmorpholine borane, N,N-diethylaniline borane, 4-
methylmorpholine
borane, and 1,4-oxathiane borane. As can be seen by referring to the
structures of
these compounds set forth below, as with morpholine borane, seven of the
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compounds (in other words, all except N,N-diethylaniline borane) contain
nitrogen or
sulfur as a ring-forming member.
~~8~3
t f \aN %~H3
piperidine borane pyridine borane
B tt~ G
N~~Nv ~ /~ ~ g~ 3
c N3
piperazine borane 2,6-Iutidine borane
~~\ 81-~~ ~ ~ ~- ~~2G~ ~
~N~c H3 ~ffzcH 3
4-ethylmorpholine borane N,N-diethylaniline borane
~./ ~G H 3
4-methylmorpholine borane 1,4-oxathiane borane
The effectiveness of the above cyclic borane compounds as reducing agents
was determined by processing a copper oxide coated copper panel through a
reducing
solution containing the cyclic borane compound. Because the metallic copper
surface
produced from reduction is the same brown-black color as the oxide layer,
there were
several parameters other than appearance which were measured to test the
effectiveness of the reducing process. Such parameters included initiation
time, acid
resistance, and weight loss of the copper oxide coating after reduction.
The initiation time is the time required for the reduction of the copper oxide
to
initiate. When the reduction reaction initiates, hydrogen bubbles rapidly form
from
the copper oxide and continue until the reaction is complete; preferably,
initiation
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occurs within 4 minutes. The acid resistance was determined by immersing the
reduced copper in an acid bath. Metallic copper survives longer in acid than
do
copper oxides; one suggested measure of acid resistance is whether the
metallic
copper layer can withstand acid resistance for at least about 30 minutes.
Thus, the
effectiveness of the reduction process can be determined by resistance to acid
attack.
The weight loss of the panel also determines the effectiveness of the
reduction
process. A copper oxide panel loses weight when the copper oxide is reduced.
By
measuring the weight loss indicates that the copper oxide is not fully being
reduced;
preferably, weight loss is greater than 15%. Copper oxide panels were made by
growing an oxide layer on a metallic copper panel.
Successful reduction by the cyclic borane compounds, as measured by the
above parameters, ensures that the minute unevenness created on the copper
surface
from oxidation remain following reduction. Those minute unevenness enable the
metallic copper surface produced from the cyclic borane compound reduction to
form
a sufficiently strong bond with a resin. Such resins include epoxy resins,
polyamide
resins, phenoloic resins, and thermoplastic resins such as polyethylene,
polyphenylene
sulfide, polyether-imide resins, and fluorocesins. By bonding together copper
and
resins layers, multilayer printed circuit boards can be successfully
fabricated.
The above criteria were used to identify which borane compounds were
effective in reducing copper oxide, in order to facilitate bonding a resin to
the metallic
copper in the fabrication of multilayer printed circuit boards. The invention
is further
described and illustrated with reference to the following examples:
Example 7
An aqueous reducing solution was prepared using 25 g/L dimethylamine
borane. The solution, at room temperature, was used as a control to reduce
copper
oxide formed on the above-mentioned copper panels by immersing the panels in
the
reducing solution for a 4-minute dwell time. The initiation time was recorded.
The
percent weight loss of the copper oxide coating and the time that the
resultant
coating survived a 10% by volume hydrochloric acid bath were recorded.
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An aqueous reducing solution was prepared using 42 g/L piperidine borane.
This was the experimental formula used to determine the effectiveness of
piperidine
borane as a reducing agent, and contains the stoichiometric equivalent
composition
of -BH3 to that contained in the control solution, a 25 g/L DMAB reducing
solution.
The panels were immersed in the reducing solution for a 4-minute dwell time.
The
initiation time was recorded. The percent weight loss of the oxide coating and
the
time that the resultant coating survived a 10% by volume hydrochloric acid
bath were
recorded.
Both DMAB and piperidine borane were obtained from Aldrich Chemical
Company, Milwaukee, Wisconsin.
The results, presented in Table 10, below, indicate that piperidine borane
effectively reduces copper oxide to metallic copper. - .
Table 10
Reducing Agent Initiation % Weight Acid
Time Loss Resistance
Dimethylamine 2C sec 19.7% 51 min
Borane
Pi eridine Borane4 sec 17.9% 35 min
Example 8
An aqueous reducing solution was prepared using 25 g/L dimethylamine
borane. The solution, at room temperature, was used as a control to reduce
copper
oxide formed on the above-mentioned copper panels by immersing the panels in
the
reducing solution for a 4-minute dwell time. The initiation time was recorded.
The
percent weight loss of the copper oxide coating and the time that the
resultant
coating survived a 10% by volume hydrochloric acid bath were recorded.
Aqueous solutions of each experimental borane compound were made,
containing the stoichiometric equivalent composition of -BH3 to that contained
in the
control, a 25 g/L DMAB reducing solution. The experimental borane compounds
included morpholine borane; piperidine borane, pyridine borane, piperazine
borane,
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2,6-iutidine borane, 4-ethylmorpholine borane, N,N-diethylanaline borane, 4-
methylmorpholine borane, and 1 ,4-oxathiane borane. The panels were immersed
in
the reducing solution for a 4-minute dwell time. The initiation time was
recorded.
The percent weight loss of the oxide coating and the time that the resultant
coating
survived a 10% by volume hydrochloric acid bath were recorded.
All of the above borane compounds were obtained from Aldrich Chemical
Company, Milwaukee, Wisconsin.
The results, presented in Table 1 1, below, indicate that the cyclic borane
compounds effectively reduce copper oxide to metallic copper.
Table 11
Bo~rane Compound, Initiation % Weight ~ Acid
Concentration Time Loss Resistance
Dimethylamine Borane,26 sec 19.18% 51 min
25 g/L
Morpholine Borane, 14 sec 19.7% 55 min
42
g/L
Piperidine Borane, 40 sec 17.9% 35 min
42
g/L
Pyridine Borane, 60 sec 16.7% 30 min
39 g/L
Piperazine Borane, 20 sec 18.9% 45 min
42
g/L
2,6-lutidine Borane,10 sec 15.5% 30 min
51
g/L
4-Ethylmorpholine 40 sec 17.7% 40 min
Borane, 55 g/L
N,N-Diethylaniline 5 sec 17.0% 50 min
Borane, 69 g/L
4-Methylmorpholine 10 sec 17.7% 35 min
Borane, 48 g/L
1,4-Oxathiane Borane,5 sec 17.3% 30 min
60 /L
Example 9 (Prophetic)
Based on observations made by the present inventor with respect to the
effectiveness ranges (about 1 .0 g/L to saturation) of morpholine borane for
reducing
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copper oxide to metallic copper on the observed data for the other cyclic
borane, and
on comparisons of the structure of morpholine borane with the structures of
the other
cyclic boranes, the present inventor expects that the ranges of concentrations
for
which the cyclic boranes will be effective in reducing copper oxide to
metallic copper
will be as follows:
piperidine borane: about 1 .0 g/L to saturation
pyridine borane: about 0.9 g/L to saturation
piperazine borane: about 1 .0 g/L to saturation
2,6-lutidine borane: about 1.2 g/L to saturation
4-ethylmorpholine borane: about 1 .3 g/L to saturation
N,N-diethylaniline borane: about 1 .3 g/L to saturation
4-methylmorphine borane: about 1 .1 g/L to saturation - .
1 ,4-oxathiane borane: about 1.2 g/L to saturation
Example 10 (Prophetic)
The procedures followed in the General Procedures for Testing Stabilizers and
in Examples 3-6 will be used, except that the following cyclic boranes will be
substituted for morpholine borane in respective testings of each cyclic
borane:
piperidine borane
pyridine borane
piperazine borane
2,6-lutidine borane
N,N-diethylaniline borane
4-methylmorpholine borane
1 ,4-oxathiane borane
Based on observations made by the present inventor as to the stabling effect
thiourea has on morpholine borane reduction, and on comparisons of the
structure of
morpholine borane with the structures of the other cyclic boranes, the present
inventor expects that the consumption of the other cyclic boranes will be
decreased
in a similar manner to the decreased morpholine borane consumption observed in
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Examples 3-6, when the reducing stabilizer thiourea is present in the reducing
solution
at a concentration in the range of about 2.5 ppm to 200 ppm.
It is to be understood that the present invention is not confined to the
particular
construction and arrangement herein illustrated and described, but embraces
such
modified forms thereof as comes within the scope of the following claims.
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