METHOD OF NODULARIZING A METAL SURFACE

FORMATION DE NODULES SUR UNE SURFACE METALLIQUE

English Abstract

Abstract of the Disclosure
A method of nodularizing a metal surface, e.g., a
copper, copper alloy or nickel surface, by electroplating the
surface with copper from a copper sulfate plating solution
containing nitrate ions. The nodules formed are generally
spherical and are firmly attached to the underlying metal
surface and assist in mechanically locking it to another mem-
ber, for example, the resinous substrate used in forming a
printed circuit board.

Claims

The embodiments of the invention in which an exclu-
sive property or privilege is claimed are defined as follows:
1. A method of nodularizing a metal surface
which comprises rendering the surface electronegative with
respect to a relatively electropositive electrode in an
electrolytic plating cell, and contacting said surface under
electroplating conditions with a plating solution comprising
an acid solution containing copper ions in a concentration
of between about 5 to about 70 g/l and nitrate ions in a con-
centration of between about 3 to about 50 g/l, maintaining
the temperature of the plating solution between about 22°C
to about 50°C, maintaining the cathode current density within
a preselected range between about 50 to about 300 amps/ft2,
and continuing said contacting of said surface for a time
period of between about 10 seconds to about 3 minutes.
2. The method of Claim 1 wherein said metal surface
is formed of copper.
3. me method defined in Claim 2, wherein said ni-
trate ion concentration is provided by a compound selected from
the group consisting of ammonium nitrate, copper nitrate,
sodium nitrate and potassium nitrate.
4. The method defined in Claim 3 wherein said metal
surface is the surface of a copper foil.
5. The method defined in Claim 4 wherein the
contacting of the surface of said foil is conducted at a
cathode current density within the range of about 75 to about
300 amps/ft2.
16

6. The method defined in Claim 5 wherein at least
the nodularized surface of said copper foil is further treated
by depositing thereon a layer of a material selected from the
group consisting of zinc, indium, brass, nickel and cobalt.
7. The method defined in Claim 6 wherein said layer
has a thickness within the range of about 10 to about 20
microinches.
8. A method of nodularizing the exposed surface of
a thin copper foil deposited on a temporary strippable carrier
comprising the steps of contacting at least the exposed surface
of said foil as a cathode with an aqueous composition comprising
from about 5 to about 70 g/l copper ions from about 25 to about
100 g/l sulfuric acid, from about 3 to about 50 g/l nitrate
ions in the presence of a copper anode, and at a cathode current
denisty within the range of about 75 to about 300 amps/ft2 for
a time within the range of from about 10 seconds to about 3
minutes.
9. The method defined in Claim 8 wherein said copper
ions are provided by copper sulfate pentahydrate and said
nitrate ions are provided by a compound selected from the group
consisting of ammonium nitrate, copper nitrate, sodium nitrate
and potassium nitrate.
10. The method defined in claim 8 wherein said
temporary strippable carrier is an aluminum alloy.
11. The method defined in claim 8 wherein at least
the exposed, nodularized surface of said thin copper foil is
17

further treated by depositing thereon a layer of a material
selected from the group consisting of zinc, indium, brass,
nickel and cobalt.
12. The method defined in Claim 10 wherein at least
the exposed, nodularized surface of said thin copper foil is
further treated by depositing thereon a layer of zinc, having
a thickness within the range of from about 10 to about 20
microinches.
13. The method defined in Claim 11 wherein said
layer is electrodeposited to a thickness within the range of
from about 10 to about 20 microinches.
14. The method defined in Claim 4 wherein said
composition comprises from about 28 to about 50 g/l copper,
from about 50 to about 100 g/l sulfuric acid, and from about
15 to about 30 g/l nitrate ions.
15. The method defined in Claim 8 wherein said
composition comprises from about 28 to about 50 g/l copper ions
from about 50 to about 100 g/l sulfuric acid, and from about
15 to about 30 g/l nitrate ions, and the current density is
within the range of from about 75 to about 250 amps/ft2 and
the contact time is within the range of about 10 to about 120
seconds.
16. The method of Claim 1 wherein said solution
further includes sulfuric acid in a concentration of between
about 25 to about 100 g/l.
18

17. The method of claim 1, wherein said copper ion con-
centration is maintained between about 28 to about 50 g/l, said
nitrate ion concentration is maintained between about 15 to
about 30 g/l, said cathode current density is maintained
between about 75 to about 250 amps/ft2 and said contacting of
said surface is continued for a time period of between about 10
seconds to about 120 seconds, and the temperature of said
plating solution is maintained between about 24°C and about
28°C.
18. The method of claim 17, wherein said solution
further includes sulfuric acid in a concentration of between
about 50 to about 100 g/l.
19. The method of claim 1, wherein said concentration of
nitrate ions is 20 g/l, and the cathode current density
generally is increased for increased concentrations of copper
ion within the ranges set forth hereinbelow:
<IMG>
20. The method of claim 1, wherein said concentration of
copper ions is 35 g/l, and the range of cathode current density
is increased with increasing nitrate concentration calculated
as ammonium nitrate, as follows:
19

<IMG>

Description

D-5656
,:
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The present invention relates to the metal treating
art, and more particularly, to a method of nodularizing a
metal surface to enhance its bonding characteristics. The
invention is particularly applicable to the treatment of printed
circuit grade copper foil, wherein a surface of a copper foil is ~
adhesively bonded to a substrate used in forming a printed -
circuit board. The invention, however, can also be used to
treat copper alloy and nickel surfaces as well as the surfaces
of other metals.
When nodularized in accordance with the present in- -
vention, with the nodularized metal surface laminated, for
example to a resinous substrate, the metal surface becomes very
strongly anchored to the substrate. This is reflected in high
"peel strengths" which are a measure of the force required to
delaminate the substrate and the metal surface.
Although the invention ~ill be described with refer-
ence to the treatment of a surface of copper foil preparatory
to its utilization in the manufacture of printed circuit boards,
it will be understood that this is for illustrative purposes
only and is not to be ~onstrued as~a limitation on the scope
of applicability of the invention.
BACKGROUND OF TH~ INVENTION
One of the commercial methods of forming printed
circuit boards is to laminate a thin foil of copper to a
resinous substrate, e.g., a fiberElass reinforced epoxy resin.
Th~s can be accompllshed by laminating the foil to a surface
of an uncured or partially cured resinous substrate, and then
applying heat and pressure to effect simultaneous curing and
bonding. Alternatively, the copper foil can be laminated to a
fully cured resinous substrate through the use of an adhesive
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D-5656
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system which may be formed with the same resin the substrate ;
ls composed of, or alternatively, a resin which is compatible
with the resin the substrate i8i composed of. Heat and pressure
are generally applied to cure the adhesive.
It is important that the bond between the copper foil
and the resinous substrate be sufficiently strong so that de-
lamination does not occur during handling, subsequent treatment
(e~g., etchinE), or during the service life of the printed
circuit board. The strength of the bond between the foil and
the reæinous substrate is conventionally referred to as the
; ~ "peel strength". Also, by convention, peel strengtb is a ~ -
measurement of the force in pounds required to separate a one
inch wide strip of one ounce copper foil from the substrate
when pulled at an angle of 90 to the surface of the substrate.
One ounce foil is foil which weighs one ounce per square foot.
~; In the case of copper foil, one ounce foil has a thickness of
about 1.4 ml. Peel strengths in excess of about 7 lbs.~in.
are generally deemed necessary to satisfy printed circuit re- -
i: :
quirements.
~ Although the prior art teaches various methods for
nodularizing copper foil, as exemplified by U.S. Patent
3,220,897, it has been found that under some circumstances the
nodules formed by such prior~art methods are too fragile to ; ;~
wlthstand the heat and presisure applied during lamination of .
the foll to a resinous substrate, and break off. The broken
pieces tend to migrate into and become encapsulated by the
resin forming the substrate. In extreme cases, the density of ~ -
.
broken pieces of nodules can become sufficiently high to
~; render the substrate electroconductive, thereby rendering it
lneffective as in insulative element in a printed circuit board.
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D-5656
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1()44~;36
Nodularization of a surface of one ounce copper
foll in accordance with the present invention increases
the peel strength of the bond between the copper foll and
-~ the substrate to which it is lamlnated. Also, the nodules
5 formed are sufficlently strong to reslst breaklng of~ when -~
exposed to the heat and pressure employed during lamination.
The present invention is also applicable to
the manufacture of printed circuit Erade copper foil, where-
in the copper foil has very thin dimensions, e.g., on the
~, ~ ;.: . .
order of five mlcrons. The foil is deposited on a tempor-
ary carrler, for example, aluminum foil, and ultimately -
removed from the carrier after the copper foil (patterned .
or~unpatterned) has been adhesively secured to a permanent
ubstrate in the manufacture of a printed circuit board.
15Extra-thin copper foll produced ln accordance
wlth this technology~generally has an extremely smooth
urface~due~to the facts that the temporary carrier has
an extremely æmooth surface, and that the lay-down of
copper~conforms itself faithfully to the surface of the
20~ temporary carrier.
When a smooth surface of an ~xtra-thin copper `~
oil~is~1aminated~to a reslnous substrate ln the pro-
duct~lon~of~prlnted clrcuit boards, the resultlng peel
erengths~ra~nge from border-line to unsatlsfactorily low.
25~ Thè~nodular-zatlon method of the present lnventlon sub-
stantially improves the peel strength of lamlnates ~ormed
extra-thin copper foil and resinous substrates.
SUMMARY
OF THE INVENTION
30The present lnvention i8 directed to a method of
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1(~44636 ;-
nodularizing a metal surface to provide the surface with firmly
attached nodules which assist in mechanically locking the metal
surface to another member. Copper, copper alloy, nickel, as
well as other metal surfaces may be nodularized in accordance
with the method of the present invention.
The nodules formed in accordance with the present -
invention are sufficiently strongly attached to the metal sur-
face to minimize or eliminate breakage when the forces of heat
and pressure sufficient to laminate the metal surface to a -
resinous substrate are applied.
The invention is applicable to the treatment of a ~ -
copper surface, e.g., the surface of a conventional one ounce
and thicker copper foil, or a very thin copper foil which is
seventeen or eighteen microns, or significantly less, in r
thickness. ~!
It is therefore an object of the present invention
to provide an improved method of nodularizing a metal surface,
such as a copper, copper alloy or nickel surface~
A further object of the invention is to prov~de a ,'r
~20 nodularized copper foil which, when laminated to a resinous
substrate, ha~ a peel strength in excess of seven pounds per
inch. ~`
Still another object of the invention is to provide
;; a method of nodularizing extra-thin copper foil while the
foil is secured to a temporary carrier. ~
In accordance with these and other objects, one aspect ~ -
of the invention provides a method of nodularizing a metal ,
surface, suitably a copper foil, which comprises rendering the
surface electronegative with respect to a relatively electro-
positive electrode in an electrolytic plating ~cell, and con-
tacting the surface under electroplating conditions `
.
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~V44~;3~;
with a composition comprising an acid solution containing copper
ions and a nitrate ion concentration within the range of
about 3 to about 50 g/l.
In the latter aspect of the invention the acid solution
is suitably a plating solution in which the copper ions are in
a concentration of between about 5 and about 70 g/l, preferably
about 28 to about 50 g/l. The nitrate ion concentration is
preferably about 15 to about 30 g/l and it is especially
preferred to maintain the temperature of the plating solution
at about 22C to about 50C, to maintain the current density
within a preselected range between about 50 and about 300
amps/ft2, and to continue the contacting for a time period of
about 10 seconds to about 3 minutes. ;~
,
In accordance with a further aspect of the invention,
there is provided a method of nodularizing the exposed surface
of a thin copper foil deposited on a temporary strippable carrier
comprising the step of contacting at least the exposed surface
of the foil, under electroplating conditions, with a composition
comprising an acid solution of copper ions and a nitrate ion ~-
concentration within the range of from about 3 to about 50 g~l.
In the latter aspect of the invention the -acid solution
is suitably an aqueous composition having a copper ion content
of from about 5 to about 70 g/l, preferably about 28 to about
50 g/l, which preferably contalns from about 25 to about 100 g/l
sulfuric acid. The nitrate ion concentration is preferably
about 15 to about 30 g/l. In carrying out the method the ~`
exposed surface of the foil forms a cathode and there is suitably
employed a copper anode, the cathode current density is suitably
within the range of about 75 to about 300 amp9~ft2, preferably
about 75 to about 250 amps/ft : and the electroplating is suit-
ably carried out for a time in the range of from about 10 seconds
to about 3 minutes, preferably about 10 to about 120 seconds.
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`` 1l)~4t~3~;
In accordance with a further aspect of the invention,
there is provided a method by which a surface of copper foil,
nodularized in accordance with the method of the present ;
invention, is further treated by depositing thereon a layer of
material selected from a group consisting of zinc, indium, ~
brass, nickel, cobalt, and possibly chromium and stainless ~ -
steel to further enhance the adhesion between the copper foil
and a resinous substrate.
Other objects, features, aspects and advantages of
the present invention will become apparent from the following
detailed description thereof which includes the best mode
presently contemplated for practicing the invention.
The improved nodularizing method of the present
invention relies for its efficacy on the incorporation of
nitrate ions in an acid copper sulfate plating solution, and
then cathodically electroplating on the copper foil with the
nitrate ion-modified plating solution to provide nodules on
the surface of the copper foil.
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In general, the plating solution will contain copper -
ions introduced, for example, as copper sulfate pentah~drate,
sulfuric acid, and nitrate ions, introduced, for example, as
copper nitrate, ammonium nitrate, potassium nitrate or sodium
nitrate.
The method of the invention may be conducted with com~ -
positions falling within a broad range of proportions and under
widely varying, although interdependent, operating conditions, `~
as set forth in Tables I and II below:
TABLE I
Proportions
Ingredient Operative Ranqe q/l Preferred Ran~e q/l
Copper ion (as
CuSo4.5H2O) 5_70 28- 50
Sulfuric acid 25-100 50-100 ~
Nitrate ion (as copper ~ -
nitrate, ammonium nitrate,
potassium nitrate, or
sodium nitrate) 3-50 15- 30
TABLE II
. .~ .
Condition Broad Ranqe Preferred Ran~e
Cathode current 2 ;-
density (amps/ft ) 50 - 300 75 - 250
Time 10 sec.-3 min. 10 sec. - 120 sec.
Temperature (C) 22 - 50 24 - 28
Anodes Copper Copper
It has been found, experimentally, that for any given ~`~
plating solution, treatment time and operating temperature,
there is a critical minimum current density and a critical max-
imum current density between which satisfactory nodularization
will take place.
i
Below the critical minimum current density, nodul-
arization takes place so slowly that the increase in thickness
of the copper foil is unacceptably high. In general, a buildup
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of not more than about 0.2 mils is regarded as satisfactory.
The critical maximum current density is established
by that current density above which weakly adherent powdery
nodules are formed.
The concentration of copper and nitrate ions in the
solution, the degree of agitation of the solution, the tem- -
perature of the solution and th~ treatment time all contribute
to establishing critical minimum and critical maximum current
densities. Thus, for example, raising the temperature of a
given solution, or agitating the solution tend to raise the
critical minimum current density. Lowering the copper or
nitrate ion concentration in a plating solution lowers the
maximum critical current density.
The following experiments were conducted to confirm
some of the foregoing relationships and to establish critical
minimum and critical maximum current densities by varying a
single parameter: ~
EXAMPLE I ~- ;
, ~ .
The effect of varying the copper ion concentration
in a plating solution on the critical minimum and critical
maximum current densities were established in the following
manner. An aqueous solution containing 100 g/l sulfuric acid
and 20 g/l of nitrate ion, introduced as ammonium nitrate was 7 ',', ~ ~
prepared and varying amounts of copper ion, introduced as copper - -
sulfate pentahydrate were added, as set forth in the left-hand ~;
column of Table III below. The temperature of the bath was i~
maintained within the range of from about 26 to about 28C.
The cathode was the matte side of a 1.3 mil (one ounce) copper
foii. The anode was copper. A treatment time of 90 seconds `~
was held constant throughout.
For each concentration of copper sulfate pentahydrate,

~4~;36
the cell was operated at a series of different cathode current
densities, as set forth in Table III below. The treated
copper foil was then laminated to a substrate consisting of
an epoxy/glass resinous substrate. The peel strengths -
developed are reported in the Table below. As a control, the -
untreated one ounce copper was laminated to epoxy/glass
resinous substrate and produced a peel strength of 6 lbs./inch.
TABLE III
Cathode Current Density - amps/ft
~ 1 Copper Ion 50 75 100 130 170 200 230
6 9.0 9.8 10.0 *
8.0 8.5 10.010.5 * :
8.0 9.0 10.510.8 11.5 * -
8.5 9.5 11.011.5 11.8 *
6.0 6.0 10.012.5 13.0 13.5 14.0
* Weakly adherent powder starts to form.
These data show that increasing the copper ion con- -
centration from 6 to 35 g/l has virtually no impact on the
critical minimum current density, but does increase the critical
maximum current density. Copper ion concentrations in excess
of 35 g/l tend to increase the critical minimum current density.
EXA~.PLE II
The impact of varying the nitrate ion concentration
on critical minimum and critical maximum current densities were
established in a manner similar to that described under Example
I. Here, however, there was prepared an aqueous solution con-
taining 35 g/l of copper introduced as copper sulfate penta-
hydrate, and 100 g/l of sulfuric acid. The nitrate ion concent-
ration introduced, as ammonium nitrate, was varied as described
below in Table IV. The temperature, treatment time, and the nature
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of the electrodes were as described above under Example I.
The data generated by this experimental program are reported
below in Table IV:
TABLE IV
g/l ~litrate Cathode Current Density - amps/ft
Ion 45 50 75 100 130 170 200 230300 ;
0 9.5 *
9.2 10 *
8.0 9.5 10 *
8.0 8.5 11 12 *
8.5 9.5 11 11.5 11.8 * ;
8.0 8.8 10 11.5 12 13 13.5 *
* Weakly adherent powder begins to form.
It will be seen from the foregoing data that increasing '~
the nitrate ion concentration slightly increases the critical
minimum current density while at the same time substantially
increasing the critical maximum current density. ~ ~;
EXAMPLE III
The procedure described in Example II was repeated
except that the copper ion concentration introduced as copper
sulfate pentahydrate was established at 25 g/l, and the contact
time was reduced from 90 seconds to 15 seconds. The data for two
concentrations of nitrate ions introduced as ammonium nitrate
are reported below in Table V. -
TABLE V `
g/l ~itrate Cathode Current Density - amps/ft -
Ion 130 200 265_ 300 330 .-
6 - 7 9.5 10 * *
5 - 7 9.0 1010.8 *
* Loose powder.
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1~ 636
Ignoring the change in copper ion concentration from
35 to 25 g/l (this is essentially insignificant as established
by the data in Table III), the reduction in contact time from -
90 seconds to 15 seconds shifted very substantially the
critical maximum current density by about threefold.
EXAMPLE IV
One ounce copper foil (about 0.0014" thick) was
treated cathodically in the following electrolyte: "!'
35.7 g/l copper ion, introduced as copper sulfate
pentahydrate
100 g/l sulfuric acid
10 g/l nitrate ion, introduced as ammonium
nitrate.
The cell was operated at a temperature within the
range of from about 26 to about 28C, and at a current density
of about 230 amps/ft . The contact time was 15 seconds. -
The treated copper foil was laminated to an
epoxy/glass resinous substrate. The resulting peel strength
was measured at 10.5 lbs./inch. The peel strength of the -~
untreated copper was about 6 lbs./inch. Omission of the ammon~
ium nitrate ingredient from the above electrolyte produced a
treated surface coated with loosely adherent copper powder
instead of the firmly attached nodular growth, which developed ~x
on copper treated with electrolyte containing a nitrate ion
ingredient. `~
EXAMPLE V
A temporary, strippable aluminum foil carrier was
etched by dipping it in a two percent sodium hydroxide solu-
tion, followed by a water rinse. The aluminum foil was then
desmutted by dipping it in a 20-30 percent by volume nitric `-
.
acid solution, followed by a water rinse.
-- 11 -- ,,
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1~4~3~
The etched and desmutted aluminum foil was then
plated with 0.1 mil copper using a standard pyrophosphate
copper bath operated at a current density of 50 amps/ft2 for
two minutes at a pH within the range of about 8 to about 8.5,
and a bath temperature of about 60C. This was followed by a
water rinse.
Thereafter, the copper plated aluminum foil was
further electroplated with a 0.2 mil copper layer using - ~
electrolytes having compositions falling within the following ; -
ranges:
20-48 g/l copper ion,-introduced as copper sulfate
pentahydrate
55-60 g/l sulfuric acid
18-22 g/l nitrate ion, introduced as ammonium nitrate
In each case, the cells were operated at a current -~
density of 100 amps/ft2 with a contact time of two minutes
and a bath temperature within the range of from about 21 to
about 24C. The aluminum foil was then rinsed and dried.
Examination of the plated aluminum foil under a micro-
. .
scope revealed the nature of the nodular growths to be generally ~ ;
spherical in shape with firm attachment to the underlying ,~ -
copper layer. The shape provided a re-entrant angle which
could be expected to promote mechanical locking of the nodules
to a mating surface, such as a resinous substrate.
The aluminum-backed copper was laminated to an
epoxy/glass resinous substrate. The aluminum foil carrier was
then stripped from the copper. Thereafter, the then nodularized ;
copper that adhered to the resin was overplated with additional
copper until a total copper thickness was built up to 1.4 mil
(one ounce copper) and the peel strength of the copper was `~
measured. Values in the range of 8.8 to 9.8 lbs./in. were '!,, ' ,'
achieved.

1~44~3~
EXAMPLE VI
. . .
The procedure described in Example V was repeated
except that the arnmonium nitrate ingredient was omitted, The
treated surface was powdery, and when the copper layer was
built up to 1.4 mil, a peel strength of less than 8 lbs./inch
developed. It was also found that upon lamination, the powdery
surface transferred to the resin, leaving a residue of copper
particles er~edded in the resin underlying portions of the
copper which was etched away to form a printed circuit.
EXAMPLE VII
A procedure of Example V was repeated except that
copper nitrate was substituted for ammonium nitrate in the ~-
following plating solution: ;
35.7 g/l copper ion, introduced as copper sulfate
pentahydrate
g/l sulfuric acid
6.6 g/l nitrate ion, introduced as cupric nitrate
trihydrate
As another departure from Example V, the above solu- ~`
tion was plated at a current density of 150 arnps/ft for two
minutes.
The peel strength of the nodularized copper surface
when built up to a thickness of 1.4 mil (one ounce copper)
was 9.0 lbs./inch. -
In the procedure described in Examples ~-VII, the
temporary aluminum foil carrier can be stripped from the
copper foil plated thereon. Under circumstances where it is
desirable to render the temporary aluminum foil carrier remov-
able by dissolving in acid or alkali that does not attack the
copper, rather than strippable from the copper, this can be
accomplished by treating the aluminum surface after etching ~ -
and desmutting, but prior to the pyrophosphate with a zincate
solution. A quitable composition for this purpose is as follows:
- 13 _
, ~

`` 1~4~36
..^
` 70 oz./gal. sodium hydroxide
13 oz./gal. zinc oxide
0.13 oz./gal. ferric chloride, crystalline
1.3 oz./gal. Rochelle salt
~ If desired, a cyanide plate may be substituted for the pyro~
.~è,` phosphate plate. The following Rochelle salt-type cyanide`,
copper solution may be employed: ?~
5.5 oz./gal. copper cyanide ~-i
6.6 oz./gal. sodium cyanide ~-
~i~ 10 4.0 oz./gal. sodium carbonate
8.0 oz./gal. Rochelle salt ~;i--
'~ - Suitable plating conditions include a pH within the range of
about 10.2 to about 10.5, a temperature within the range of ~; -
about 38 to about 150C, and a current density of 24 amps/ft
for the first two minutes, and 12 amps/ft2 for subsequent
plating.
- ~ EXAMPLE VIII
Electroformed printed circuit grade copper foil was
electroplated on its matte surface for 90 seconds at a current
~20~ density of 130 amps/ft , with the following composition ;
maintained at a temperature within the range of from about 26
to about 28C~
35 g/l copper ion, introduced as copper sulfate
pentahydrate
lOO g/l ~ulfuric acid `?;~ -
16 ml./l nitric acid ~70~O reagent grade)
he untreated copper foil measured about 1.3 mil thick ~i
; (one ounce copper foil) and possessed a peel strength on an
epoxy/glass resinous substrate of about 4-5 lbs./inch. The
; same copper foil treated as described above to nodularize`- -
its surface, which added about 0.18 mil to thè thickness of
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~ D-5656
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~t~44636
the foil, had a peel strength of 12.5 lbs./inch when laminated
to an epoxy/glass resinous substrate.
i~ X~PL~ IX
~ The procedure described in ~xample V was repeated
- . :,?
except that the nodularizing plating solution of Example VIII
~9 was employed.
~3
The peel strength of the nodularized copper fbil
when built up to a thickness of 1.5 mil and an epoxy/glass
reslnous substrate was 7 lbs./inch. This comparatively lower
peel strength, compared to the 12.5 lbs./inch secured follow-
,s,~ ~
lng the procedure of ~xample V, is attrlbutable to the factthat the surface of the temporary aluminum foil carrier was
substantially smoother than the matte surface of the printed
clrcuit grade copper foil treated in ~xample VIII.
IP desirèd, to further enhance the bonding character-
,i8tlc8 of the copper ~oil at elevated temperatures, the sur-
ace ~o-dularized in accordance with the present invention may
be overplated wlth a layer of a material selected ~rom the
group con8isting of zinc, indlum, brass, nickel~ cobalt, and
20~;possibly~chromium and stainless steel. Layers ranEing in
thlckness ~rom about 10 to about 20 microinches are satisfact-
ory for this purpose. Reference may be had to U.S. Patent
3,585,010 and British Patent 1,211,494 for plating æolutions
and~operating conditlons which may be employed in effecting
the overplate.
WhiIe the invention has been described uith refer-
ence to certain specific embodiments, neither the embodiments
l}lustrated nor the terminology employed in describing them
i8 intended to be limiting; rather it ls intended that the
invention be limited only by the scope of the appended claims.
-15-
1,