Ductile electroless copper

An electroless copper deposit having an elongation capability of at least 10 percent as determined by a mechanical bulge test on a foil having a thickness of between 1.5 and 2.0 mils.

This invention relates to an electroless copper deposit having an 
elongation capability of ten percent or greater, as measured by a 
standardized procedure described in detail below, and to a plating 
solution capable of plating such a deposit. 
DESCRIPTION OF THE PRIOR ART 
Electroless copper deposition refers to the chemical plating of copper over 
a clean, catalytically active surface by chemical reduction in the absence 
of an external electric current. Processes and compositions for 
electroless deposition of copper are known in the art and are in 
substantial commercial use. They are disclosed in a number of prior art 
patents, for example, U.S. Pat. Nos. 3,615,732; 3,615,733; 3,615,735; 
3,728,137; 3,846,138; 4,229,218; and 4,539,044, all incorporated herein by 
reference. 
Known electroless copper deposition solutions generally comprise four 
ingredients dissolved in water. They are (1) a source of cupric ions, 
usually a copper salt such as copper sulfate, (2) a reducing agent such as 
formaldehyde, or preferably a formaldehyde precursor such as 
paraformaldehyde, (3) hydroxide, generally an alkali metal hydroxide and 
usually sodium hydroxide, sufficient to provide the required alkalinity 
necessary for said composition to be effective and (4) a complexing agent 
for copper sufficient to prevent its precipitation in alkaline solution. A 
large number of complexing agents are known and described in the aforesaid 
cited patents and elsewhere. 
Electroless copper plating solutions have many industrial uses. One use is 
for the manufacture of printed circuit boards where an electroless copper 
deposit plated over the walls of through-holes, vias, interconnects, etc. 
provides conductivity between surfaces of a board and/or between circuit 
layers. In additive circuit manufacture, in addition to providing 
conductivity between surfaces and/or circuit layers, the deposit also 
serves as the conductor lines. 
With increased circuit density, and with more rigorous specifications for 
circuit boards, the mechanical properties of a deposit become increasingly 
important, especially deposit ductility. For example, in the manufacture 
of electronic devices, it is necessary to solder components to a circuit 
board. The solder increases the temperature of the electroless deposit 
which causes it to expand and then contract with cooling. The coefficient 
of expansion of the copper differs from the coefficient of expansion of 
the surface over which the copper is plated. Therefore, stress is created 
in the copper which can cause cracking of the deposit. A crack in the 
deposit can result in failure of the circuit board. To determine the 
ability of an electroless copper deposit to withstand soldering, a test 
method has been developed known as the solder shock test. This test will 
be described in greater detail below. 
Electroless copper is significantly less ductile than other forms of copper 
such as electrolytically deposited copper. For example, electroless copper 
typically possesses elongations of about 0.5 to 3.5 percent while 
electrolytic copper, as used in the manufacture of through-hole printed 
circuit boards, typically possesses elongations in the range of from about 
6 to 15 percent. As reported by Nakahara et al, Acta Metall, Volume 3, No. 
5. pp. 713-724, 1983, United Kingdom, the art has attributed poor 
ductility in part to hydrogen embrittlement. The need for a ductile 
electroless copper for the manufacture of printed circuit boards is 
recognized throughout the industry and has been discussed in technical 
papers including Nakaso et al, Mechanical Properties of Electroless Copper 
Deposits, (Technical Paper No. WCIII-10, Kamiyama et al, High Density 
Printed Wiring Boards by Additive Processes, (Technical Paper No. 
WCIII-11) and Debrita, High Reliability Electroless Copper For Fully 
Additive Printed Wiring Board Manufacture, (Technical Paper WCIII-69), all 
presented at the Printed Circuit World Convention III held in Washington, 
D.C. on May 22 through May 25, 1984, incorporated herein by reference. 
Though the literature deals extensively with the ductility of copper, the 
measurement of electroless copper ductility is complex because the 
literature reports ductility as determined by a variety of test methods 
which do not correlate with each other. A method frequently used to 
determine ductility is the bend test where a copper foil is repeatedly 
folded and opened until it breaks. The number of bends achieved is used as 
a measurement of ductility. Though the test is easy to perform, it is not 
a true measure of ductility in metallurgical terms and it is unreliable 
because the foil becomes work hardened by bending which makes it difficult 
to refold the foil exactly at the point of the initial crease. In summary, 
the bend test is a measure of bendability, not a true measure of ductility 
or elongation. Foils that have passed the bend test in accordance with 
U.S. Pat. No. 3,257,215 possess elongations as determined by the 
mechanical bulge test of less than 5% and are not ductile deposits in 
accordance with the invention described herein. 
Another test used by the art is one where the copper is stretched in one 
dimension until it yields. Percent elongation is then determined. This 
test, though superior to the bend test, is nonetheless unreliable because 
the ductility of an electroless copper deposit can vary from one axis to 
another. 
A further complication encountered in the literature dealing with the 
ductility of an electroless deposit is that specimen thickness is not 
standardized. It is known in the art that ductility is related to the 
cross-sectional thickness of a copper foil. However, foil thickness is 
often not standardized during ductility testing, and it is difficult to 
repeat and confirm ductility measurements reported in the literature. The 
importance of foil thickness on ductility determination is described by 
Nakahara et al, A Simple Ductility Tester for Metal Films, Journal of 
Testing and Evaluation, JTEVA, Volume 5, No. 3, May 1977, pp. 178-182, 
incorporated herein by reference. 
Regardless of the method used to determine ductility of an electroless 
copper deposit and regardless of values reported in the literature, it is 
believed that the art of manufacturing printed circuit boards has been 
impeded by the unavailability of a stable electroless copper plating 
solution capable of yielding an electroless copper deposit of satisfactory 
ductility within a reasonably acceptable plating time. Consequently, the 
art is continuously searching for electroless copper deposition solutions 
capable of depositing ductile deposits at plating rates in excess of 0.05 
mils per hour. 
SUMMARY OF THE INVENTION 
The invention described herein is a deposit of electroless copper having an 
elongation capability, expressed as percent elongation, of at least ten 
percent or greater. As will be discussed in detail below, elongation 
capability is determined using standardized equipment and procedures. 
The ductile copper deposit, in accordance with the invention, is obtained 
from a stable electroless copper plating solution capable of plating 
copper at a rate in excess of 0.05 mils per hour and preferably in excess 
of 0.10 mils per hour. Each component of the plating solution is believed 
to be known in the art, but the specific admixture of solution components 
in accordance with the invention provides a stable plating solution 
capable of plating a deposit having ductility substantially in excess of 
the ductility of copper deposited from prior art plating solutions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To clearly define elongation capability, determinations were made by the 
procedure established by Nakahara et al reported in the Journal of Testing 
and Evaluation, referenced above. All tests were performed on a copper 
foil having a thickness within the range of 1.5 and 2.0 mils, this 
thickness having been selected because it has been found that though 
elongation readings vary appreciably with thickness, the variation becomes 
minimal as the thickness exceeds 1.5 mils. 
The procedure used to determine elongation capability utilized a mechanical 
bulge tester illustrated in FIG. 1 of the drawings. A foil specimen 1 
placed over an O ring 2 within groove 3 is clamped between two 
doughnut-shaped platens 4 and 5. The platens are joined with screws 6 and 
7. Specimen 1 is then slowly deformed into a nearly spherical shape by 
steel ball 8 driven by micrometer 9. By observing deformation under a low 
magnification eye piece (not shown), the bulge height at the onset of 
fracture can be accurately determined. The sample tested is large relative 
to the size of the ball and multiple drformations are used per test to 
determine elongation capability. On a sample measuring 4 inches by 4 
inches, 25 or more deformations can be used to determine the elongation 
capability. Using the calculations given by Nakahara et al, a graph 
relating bulge height with percent elongation can be constructed. The 
graph used for determining elongation values reported herein is depicted 
in FIG. 2 of the drawings. The above procedure will be referred to as the 
mechanical bulge test in the discussion which follows. 
To further standardize the results obtained, the plating procedure used to 
obtain electroless copper test foils was standardized and elongation 
values were derived from foils plated in accordance with this procedure. 
The procedure comprised: 
a. provide a flat molded ABS plaque of suitable size sufficient to provide 
several 1" by 1" squares; 
b. immerse ABS plaque in a solution of PM 940 chromic acid conditioner for 
from 5 to 10 minutes at 155.degree. F. and water rinse thoroughly; 
c. immerse ABS plaque in a 5% aqueous solution of Neutralizer Cleaner 
Conditioner 1175 for 5 minutes at 180.degree. F. and water rinse 
thoroughly; 
d. immerse ABS plaque in a solution of Cataprep 404 conditioner for 1 
minute at 110.degree. F.; 
e. immerse ABS plaque in a 3% solution of Cataposit 44 tin palladium 
catalyst for 5 minutes at a temperature of 110.degree. F. and water rinse 
thoroughly; 
f. immerse ABS plaque in a solution of Accelerator 19 for 5 minutes at room 
temperature and water rinse thoroughly; 
g. immerse ABS plaque in a solution of the test electroless copper solution 
at a temperature suitable for the test solution and for a time sufficient 
to build a deposit of from 1.5 to 2.0 mils; and 
h. cut a sample of the copper deposit in a square measuring about 1" by 1", 
peel from the ABS plaque and submit to mechanical bulge testing as 
described above. In the aforesaid procedure, Cataprep and Cataposit are 
registered trademarks of Shipley Company Inc. Those solutions identified 
as PM 940, Neutralizer Cleaner Conditioner 1175, Cataprep 404, Cataposit 
44 and Accelerator 19 are all commercially available materials from 
Shipley Company Inc. of Newton, Mass. 02162. 
The mechanical bulge test procedure permits the operator to microscopically 
view the stretching and expansion of the deposit sample as the test 
proceeds and the bulge forms. This permits the operator to determine if 
the fracture emanates from the apex of the bulge or at some other point in 
the bulge. 
To accurately determine elongation capability using the mechanical bulge 
test, it is necessary to measure defect-free electroless copper. 
Imperfections in the sample may cause premature fracture of the sample. 
These imperfections may be caused by sample preparation rather than an 
inherent defect in the sample. For example, preparation of defect free 
samples by the plating process described above is difficult, primarily 
because of the necessity to etch roughen the surface of the ABS plaque to 
improve adhesion of the copper deposit to the plaque. The bond between the 
substrate and the deposit is apt to cause defects during the step of 
peeling the copper from the plaque which are not in the deposit as plated, 
but rather are formed as the sample is removed from the substrate for 
purposes of testing. 
When a sample tested using the mechanical bulge tester fractures at a point 
beneath the apex of the bulge, it is likely that the sample has an 
imperfection that results in premature fracture. If the tested sample 
fractures at the apex, the sample is typically free of obvious defects 
through the sample may still contain a defect at the apex. For this 
reason, when reporting elongation capability herein, a sample is subjected 
to multiple testing using the mechanical bulge tester and those tests 
where fracture occurs at the apex of the bulge and where maximum 
elongation values are obtained are used to determine elongation 
capability. 
Elongation capability values given herein have been determined using the 
standardized procedure described above with the test specimen having a 
thickness of from 1.5 to 2.0 mils. This is not to be interpreted to mean 
that the invention is limited to foils having a thickness within the range 
of 1.5 to 2.0 mils. The invention is directed to a copper foil of any 
desired thickness, but if plated to a thickness of from 1.5 to 2.0 mils, 
would exhibit an elongation capability of 10 percent or greater as 
determined by the mechanical bulge test. 
Copper plating solutions capable of depositing copper having an elongation 
capability of 10 percent or greater in accordance with the invention use 
ingredients that have been used in prior art copper plating solution, but 
not in the specific combinations disclosed herein. In addition, the 
plating solutions of the invention contain nickel and cyanide ions in 
concentrations that do not deleteriously affect elongation. Further, the 
plating solutions of the invention are free or ingredients, additives, 
impurities, and other substances in concentrations that deleteriously 
affect ductility. Finally, to plate copper to a desired thickness, the 
plating solutions should contain one or more stabilizers capable of 
adequately stabilizing the solution in concentrations that do not 
deleteriously affect elongation capability. 
The copper plating solutions of the invention necessarily include a source 
of cupric ions, formaldehyde and hydroxide. These constituents are 
conventional in electroless plating solutions and are used in known 
concentrations. Hydroxide is used in an amount sufficient to adjust 
solution pH to between about 11.0 and 14.0 and preferably between about 
12.0 and 13.0, the preferred operating pH of the bath. 
The copper plating solutions of the invention also require a complexing 
agent. The solutions can be formulated with known complexing agents that 
do not interfere with ductility in required concentrations. Since one 
object of the invention is a ductile deposit from a plating solution 
capable of plating copper at a rapid rate, the most preferred complexing 
agent is N,N,N',N'-tetrakis (2-hydroxpropyl) - ethylene diamine 
commercially available under the tradename "Quadrol". This complexing 
agent and similar complexing complexing agents are disclosed in U.S. Pat. 
No. 3,329,512 incorporated herein by referenee. Another preferred 
complexing agent is the alkali metal salt of ethylene diamine tetraacetic 
acid which can be used alone or in combination with Quadrol. The 
complexing agent is used in an amount preferably in excess of the amount 
required to complex the copper ions in solution, but not in an amount that 
would deleteriously affect the ductility of the deposit. 
As stated above, to obtain a solution capable of plating a copper deposit 
containing codeposited nickel and having an elongation capability of 10 
percent or greater as determined using the mechanical bulge tester, it is 
necessary to include a minor amount of a source of nickel ions and cyanide 
and/or ferrocyanide ions in the plating solution. With an adequate 
concentration of cyanide ions, an increase in elongation capability is 
realized when the nickel ion content is as low as about 5 parts per 
million parts of solution (ppm). For the test solutions described below, 
it was necessary that nickel content reach 40 ppm to obtain an elongation 
of 10 percent. As nickel ion concentration increases, elongation 
capability increases, then levels off, and then begins to gradually 
decrease. Preferably, nickel ion concentration varies between about 40 and 
1,000 ppm and more preferably, between about 40 and 500 ppm. 
Cyanide as a stabilizer and an agent that increases the bendability of a 
copper deposit is shown in U.S. Pat. No. 3,257,215, incorporated herein by 
reference. It can be introduced into the electroless copper solution in 
the form of its salts such as sodium cyanide, potassium cyanide, copper 
cyanide, nickel cyanide, etc. The cyanide ion in solution is a solution 
poison i.e., it will first retard and then prevent deposition of copper 
from solution. Therefore, it can be added to solution in limited 
concentration. Cyanide ions are used in an amount of from about 1 ppm to 
that amount that retards or prevents deposition of copper from solution. 
The plating solutions described above plate copper having an elongation 
capability of 10% or greater but it is desirable to add other ingredients 
to the solution such as stabilizers, exaltants, brighteners, etc. Such 
additives can be used in the solutions of this invention if used in 
concentrations that do not deleteriously affect elongation. 
Stabilizers prevent copper plating solutions from undergoing spontaneous 
decomposition. Many stabilizers have been disclosed in the prior art as 
suitable for stabilizing electroless copper plating solution. A 
conventional stabilizer comprises a divalent sulfur compound such as 
mercapto benzo thiazole. Divalent sulfur compounds embrittle the copper 
deposit when used in interfering concentrations. 
Alkyne alcohols disclosed in U.S. Pat. No. 3,661,597, in stabilizing 
concentrations, stabilize the plating solutions of the invention without 
noticeable affect on deposit ductility. Methyl butynol is the preferred 
alkyne alcohol. Another stabilizer that can be used alone or in 
combination with the alkyne alcohol is 2,2'-dipyridyl disclosed in U.S. 
Pat. No. 4,099,974. Mercury compounds disclosed in U.S. Pat. Nos. 
3,663,242 and 3,649,308 stabilize the solutions of the invention, but 
excessive amounts embrittle the deposit. Phenyl mercuric acetate is 
preferred. 
Aeration, as known in the art, can assist in stabilizing the solutions of 
the invention without noticeable affect on deposit ductility. 
The electroless copper solutions of the invention are operated in a manner 
similar to prior art solutions. A part is immersed in the solution for a 
time sufficient to plate the part to desired thickness. The plating 
solution is preferably operated at an elevated temperature, preferably 
within a range of from 100.degree. to 175.degree. F. and more preferably 
within a range of from 135.degree. to 150.degree. F. Solution agitation is 
desirable. 
The following examples will better illustrate the invention. 
Example 1 
______________________________________ 
Copper nitrate dihydrate 
6.8 gms 
nickel sulfate hexahydrate 
0.2 gms 
Quadrol 12.8 gms 
paraformaldehyde 1.5 gms 
methyl butynol 35 mgs 
phenyl mercuric acetate 
3 mgs 
potassium ferrocyanide 
60 mgs 
trihydrate 
2,2'-dipyridyl 30 mgs 
Pluronic F68 25 mgs 
sodium hydroxide (liquid form) 
16.0 gms 
water to 1 liter 
Operating temperature 
135-140.degree. F. 
______________________________________ 
The above solution constitutes the most preferred embodiment of the 
invention. 
The above solution was used to plate an ABS part to a thickness of 1.5 
mils. The plated deposit was subjected to the mechanical bulge test 
described above and found to have an elongation capability of greater than 
12%. For purposes of comparison, the plating procedure described above was 
used to plate a 1.5 mil thick copper deposit from a commercially available 
electroless copper plating solution identified as Cuposit.RTM. CP 70 
available from Shipley Company Inc. of Newton, Mass. The elongation 
capability was found to be less than 3%. 
The above solution used cupric nitrate as the source of cupric ions rather 
than cupric sulfate as is conventional in the formulation of electroless 
copper plating solutions. While cupric sulfafe may successively be 
utilized, it has disadvantages during replenishment of cupric ions because 
of accumulation of excessive quantities of sulfate ion in solution. 
Excessive sulfate ion interferes with ductility. Conversely, it is a 
discovery of this invention that monovalent anions, most preferably 
nitrate and formate ions, can be tolerated in higher concentration without 
deleteriously effecting ductility. 
When used for the fabrication of plated through hole printed circuit 
boards, it is essential that the electroless copper plated onto the hole 
walls pass a solder shock test. The solder shock test used herein involved 
plating of a test solution onto the walls of through holes in a double 
copper clad FR4 glass epoxy substrate having a thickness of either 62 or 
92 mils and 30 to 40 mil holes. The test comprises floatation of the 
plated part on molten solder at a temperature of about 500.degree. F. for 
10 seconds. The copper deposits of this invention pass a solder shock test 
without evidence of fracture, whereas parts plated with Cuposit.RTM. CP-70 
repeatedly fail the solder shock test. Using a bend test, the Cuposit.RTM. 
CP-70 deposits exhibit excellent bendability, but do not possess the 
elongation capability of the deposits of this invention. 
To obtain consistent results, electroless copper solutions need to be 
maintained near full strength with respect to the concentrations of 
copper, nickel, formaldehyde and hydroxide. On a laboratory scale, this 
can be accomplished by preparing at least 12 liters of plating solution 
without addition of sodium hydroxide until the solutions are ready for 
use. When ready to test the plating solution, the liquid sodium hydroxide 
is added to 1 liter of solution and a part is immersed in the solution to 
commence plating. After 1 hour of plating, the plated part is transferred 
to a second "fresh solution" to which sodium hydroxide has been added and 
the part is again plated for 1 hour. This sequence is continued until full 
desired thickness is obtained. If more than 10% of the original copper is 
consumed during plating from a 1 liter solution, it is necessary to change 
solutions. If the operating temperature of the solution is higher than 
about 140.degree. F., it might be necessary to change solutions more often 
than every hour as formaldehyde consumption might become excessive. Gentle 
agitation of the solution and the part is desirable during plating. 
Example 2 
The procedure of Example 1 was repeated, but the nickel sulfate was omitted 
from solution. The copper deposit passed the bend test, but had an 
elongation capability approaching 3%. The deposits were not sufficiently 
ductile for purposes of this invention and the deposits failed the solder 
shock test. 
Example 3 
The procedure of Example 1 was repeated, but the potassium ferrocyanide was 
omitted from solution. As above, the copper deposit passed the bend test, 
but had an elongation capability approaching 3%. Again, the deposits were 
not sufficiently ductile for purposes of this invention and the deposits 
failed the solder shock test. 
Example 4 
The procedure of Example 3 was repeated, but the nickel sulfate 
concentration was increased to 700 ppm (156 ppm nickel ions). The deposit 
formed had an elongation capability of almost 4%, but failed the solder 
shock test. This example illustrates that ferrocyanide ions are needed in 
addition to nickel ions. 
Example 5 
The procedure of Example 1 was repeated, but the concentration of nickel 
ions in solution was varied from 0 to 450 ppm. Elongation capability is 
set forth in the following table: 
______________________________________ 
Nickel Sulfate 
Nickel ions Elongation Nickel in 
in solut. (ppm) 
in solut. (ppm) 
Capability (%) 
deposit (ppm) 
______________________________________ 
0 0 less than 3% 
0 
50 11 almost 6% 17 
200 45 more than 12% 
74 
400 90 more than 13% 
153 
700 156 more than 15% 
234 
1,000 223 more than 15% 
(1) 
1,500 335 more than 14% 
280 
2,000 446 more than 11% 
236 
______________________________________ 
(1) not measured. 
Deposits having an elongation capability of 10% or greater were obtained 
from solutions having a nickel ion concentration ranging from abut 40 ppm 
to substantially more than 450 ppm. 
Examples 6 to 10 
The procedure of Example 1 was repeated, but the concentration of nickel 
sulfate was increased to 0.7 grams and the concentrations of several other 
ingredients were modified as set forth in the following table: 
______________________________________ 
Elongation 
Solder 
Ex. No. 
Ingredient Amount Capability 
Test 
______________________________________ 
6 phenyl mercurio 
9 mgs less than 4% 
failed 
acetate 
7 potassium 250 mgs more than 15% 
passed 
ferrocyanide 
8 potassium 1,000 mgs more than 15% 
passed 
ferrocyanide 
9 2,2'-dipyridyl 
60 mgs more than 14% 
passed 
10 2,2'-dipyridyl 
90 mgs more than 14% 
passed 
______________________________________ 
The above examples illustrate that stabilizers vary in concentration 
tolerance. Thus, phenyl mercuric acetate may be used as a stabilizer in a 
limited concentration, but as its concentration increases, it 
deleteriously affects elongation capability. 
Example 11 
The procedure of Example 1 was repeated with 4 mgs of thiomalic acid (a 
known divalent sulfur stabilizer) added to the solution. The deposit 
obtained had an elongation capability of about 3%, passed the bend test, 
but failed the solder shock test. This example illustrates that a plating 
solution able to deposit copper having excellent elongation properties may 
be destroyed by extremely small amounts of an additive that embrittles the 
deposit. 
Example 12 
The procedure of Example 1 was repeated with variations in temperature of 
deposition. The temperature used and the affect on plating rate and 
elongation capability is set forth in the following table: 
______________________________________ 
Solution Plating Plating Elongation 
Temp. .degree.F. 
Time (hrs) Rate (mil/hr) 
Capability (%) 
______________________________________ 
120 16 0.09 more than 12 
140 11 0.14 more than 12 
165 7 0.21 more than 12 
______________________________________ 
Temperature affects rate, but not elongation capability, within the range 
set forth above.