U.S. patent application number 09/822502 was filed with the patent office on 2002-07-04 for method and apparatus for improving interfacial chemical reactions in electroless depositions of metals.
Invention is credited to Grunwald, John.
Application Number | 20020086102 09/822502 |
Document ID | / |
Family ID | 26324001 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020086102 |
Kind Code |
A1 |
Grunwald, John |
July 4, 2002 |
Method and apparatus for improving interfacial chemical reactions
in electroless depositions of metals
Abstract
A method and apparatus for improving interfacial chemical
reactions in electroless depositions of metals, in which the
substrate to be plated is pre-heated prior to its immersion in the
various processing solutions that require elevated temperatures,
and especially before immersion in the electroless plating
solution. The pre-heating is carried out to a temperature that is
needed to bring about the desired chemical reaction at the
substrate-solution interface, allowing the bath of that process
step to operate significantly below the temperature that would have
been needed if the panel had not been pre-heated, and below the
solution temperature of current practice. According to another
aspect of the present invention, the electroless plating apparatus
for plating a workpiece operates in a vertical mode and it
comprises a heating station, with the panel to be plated returning
to the heating station as dictated by the temperature required for
a given process step. Alternatively, the heating station is
incorporated into the hoist system of such apparatus. In still
another aspect of the present invention, the electroless plating
apparatus operates in a horizontal mode and comprises at least one
heating station.
Inventors: |
Grunwald, John; (Ramat-Gan,
IL) |
Correspondence
Address: |
Edward Langer
c/o Landon & Stark Associates
One Crystal Park Suite 210
2011 Crystal Drive
Arlington
VA
22202
US
|
Family ID: |
26324001 |
Appl. No.: |
09/822502 |
Filed: |
April 2, 2001 |
Current U.S.
Class: |
174/256 ;
118/400; 174/266; 427/314; 427/437; 427/443.1; 427/97.2; 427/97.9;
427/98.1; 427/98.9 |
Current CPC
Class: |
C23C 18/1865 20130101;
C23C 18/1678 20130101; C23C 18/32 20130101; H05K 3/187 20130101;
C23C 18/1653 20130101; C23C 18/1889 20130101; C23C 18/1893
20130101; C23C 18/38 20130101 |
Class at
Publication: |
427/98 ; 427/314;
427/437; 427/443.1; 118/400 |
International
Class: |
B05D 003/02; B05D
001/18; B05D 005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2001 |
IL |
141179 |
Jan 2, 2001 |
IL |
140680 |
Claims
I claim:
1. A method for improving interfacial chemical reactions in
electroless depostion of metals on the surface of a workpiece using
an electroless plating solution in an electroless plating bath,
said method comprising pre-heating the workpiece, prior to
immersion thereof in the electroless plating solution, to a
temperature approximately equal to or above the operating
temperature of the electroless plating bath.
2. The method of claim 1, said method further comprising the step
of heating the surface of the workpiece prior to immersion in a
processing solution, said heating being such that the workpiece is
heated to a temperature approximately equal to or above the
temperature of the processing solution.
3. The method of claim 1 wherein part of the surface of a workpiece
is non-metallic.
4. The method of claim 1 wherein the workpiece is flat.
5. The method of claim 1 wherein the workpiece is a copper-clad
polymer.
6. The method of claim 5, wherein said copper-clad polymer
comprises holes.
7. The method of claim 5 wherein said polymer is epoxy.
8. An article produced by the method of claim 1.
9. A PCB produced by the method of claim 1.
10. A hoist-operated vertically operated plating apparatus, wherein
a workpiece to be plated is mounted on vertical jigs and is carried
by a hoist through a sequence of processing solutions and rinses,
said apparatus comprising a heating station wherein the workpiece
to be plated is preheated prior to immersion in the processing
solutions to a temperature approximately equal to or above the
operating temperature of the processing solutions.
11. The apparatus of claim 10, wherein said heating station is
incorporated in the hoist.
12. The apparatus of claim 10 wherein the preheating temperature
for each process solution is controlled by software.
13. The apparatus of claim 10 comprising an electroless plating
station.
14. The apparatus of claim 10 comprising a palladium-bearing
activator station.
15. A horizontally operated plating apparatus wherein a workpiece
to be plated is placed on a conveyor that travels and carries the
workpiece through a sequence of process modules in which liquid is
applied to said workpiece by at least one of spraying and
submersion comprising a heating station wherein the workpiece to be
plated is heated prior to being conveyed into said process modules
to a temperature approximately equal to or above the operating
temperature of the process solutions.
16. The apparatus of claim 15 wherein the preheating temperature
for each process solution is controlled by software.
17. The apparatus of claim 15 comprising an electroless plating
station.
18. The apparatus of claim 12 comprising a palladium-bearing
activator station.
19. The method of claim 1 wherein the workpiece to be plated is
pre-heated in a suitable chemical solution to a predetermined
temperature, prior to immersion thereof in the electroless plating
bath.
20. The method of claim 19 wherein said chemical solution is an
alkaline aqueous accelerator.
21. The method of claim 19 wherein said predetermined temperature
is at least 5.degree. C. above ambient.
22. The method of claim 19, wherein said workpiece is immersed in
the electroless plating bath without being exposed to a water
rinse.
23. A method for improving solid/solution interfacial reactions in
a surface chemical process, said method comprising pre-heating a
workpiece, prior to immersion thereof in a solution, to a
temperature approximately equal to or above the operating
temperature of that solution.
Description
FIELD OF INVENTION
[0001] The invention describes a method and apparatus for improving
interfacial chemical reactions in electroless depositions of
metals, and more particularly for producing a through-hole-plated
Printed Circuit Board (PCB).
BACKGROUND OF THE INVENTION
[0002] U.S. Pat. Nos. 4,279,948, 4,265,943, and 4,209,331, which
are hereby referenced in their entirety, summarize the prior art of
using electroless copper to prepare non-conductive through-hole
walls for electrolytic copper plating, said through-hole walls
predominantly consisting of electrically insulating glass-epoxy
composites.
[0003] In this application the terms process tanks, process
solutions, process steps and process stations are interchangeable.
The term "substrate" is meant to denote workpiece, panel, board,
PCB, with or without through-holes, these terms being
interchangeable.
[0004] Indeed, the substrate, with its trough holes, has to undergo
a long, intricate sequence of process steps before the hole walls
are electrically conductive and electroplatable. Many of these
process steps require bath temperatures well above ambient for the
desired chemical reactions to take place. In current practice, the
through-hole panels enter the process solutions mostly following
cold water rinses, resulting in significant heat transfer related
time delays before the panel/solution interface reaches the
"threshold" temperature, below which, the desired reaction will not
take place satisfactorily.
[0005] U.S. Pat. Nos. 3,532,518 and 3,011,920 are referenced
herewith to show the crucial importance of adequate catalytic
preparation of the polymer hole walls for electroless copper to
take place on non-conductive substrates.
[0006] Perhaps the most critical step in the PCB production
process, is the electroless copper bath, known also as chemical
copper. In order to achieve complete, void-free coverage of the
through-hole walls, the electroless copper bath must operate at a
sufficiently elevated temperature to provide the necessary
"activation energy" for the reduction of cupric and/or cuprous ions
in solution to metallic copper. Indeed, electroless copper
deposition, whether obtained via formaldehyde, or via the
environmentally friendlier hypophosphite as the main reducer, is
understood to be the result of a complex sequence of intermediate
reactions that take place at the interface of the Pd-catalyzed
non-conducting polymer hole walls and the electroless Cu solution.
Excessively low interfacial temperatures will retard copper
initiation to a point where coverage will be incomplete, resulting
in rejects.
[0007] More importantly, significant delays in chemical copper
initiation are especially harmful in hyophosphite-reduced baths,
because unlike formaldehyde baths, they are not autocatalytic, and
Cu deposition is understood to essentially cease after the Pd layer
over the hole wall surface is coated with a continuous coating of
Cu. Contamination, or poisoning of the Pd catalytic layer on the
hole wall occasioned by slow initiation of copper deposition due to
low interfacial temperature in the electroless copper bath may
cause deposition of poorly conductive copper oxides, areas with no
deposit at all, or a combination of both. Also, while formaldehyde
type electroless copper baths operate at temperatures in the range
of 25.degree. C., the recommended operating temperature of
hypophosphite-based electroless copper systems is 70.degree. C.,
perceived by both equipment manufacturers and PCB producers as
restrictive and not user-friendly.
[0008] A technical document entitled "Process Operating Guide, M
system" by MacDermid Inc., is referenced herewith, as indicative of
the elaborate process steps required for satisfactory PCB
production, with some process steps using solutions that require
elevated temperatures. With some few exceptions, PCB production
facilities currently use automatic plating machines, wherein a
computer-programmed hoist carries panels to be plated through the
various process steps, with the panels mounted on vertical jigs,
also called racks. The software that directs the movement of the
hoist is called timeway. The jigs, with the mounted panels, enter
and exit the process solutions in a vertical position.
[0009] Relatively recently, some PCB producers have changed to
horizontal, conveyorized equipment, wherein PCB panels move through
the process solutions in a horizontal, as opposed to vertical mode.
Horizontal machines are gaining attention, principally because of
reduction in labor expenses One such horizontal PCB machine is
called Uniplate LB and can be obtained from Atotech.
SUMMARY OF THE INVENTION
[0010] A central consideration behind the present invention hinges
on the fact that one deals mainly with interfacial chemical
reactions, as opposed to predominantly bulk reactions, when
practicing Pd activation and electroless depositions on a
substrate. The prior art use of large bath volumes is in most cases
superfluous, such that the reduction in bath volume in the present
invention results in considerable savings due to reduction in waste
disposal volumes and inherent environmental problems.
[0011] Thus, the present invention provides according to a first of
its aspects, an improved method for manufacturing a PCB, wherein
the board to be plated is pre-heated prior to its immersion in an
electroless plating solution. The pre-heating is carried out at a
temperature that is needed to bring about the desired chemical
reaction at the panel-solution interface, allowing the bath of that
process step to operate significantly below the temperature that
would have been needed if the panel had not been pre-heated, and
below the solution temperature of current practice.
[0012] According to another aspect of the present invention, the
electroless plating apparatus for plating a workpiece, operates in
a vertical mode and it comprises a heating station, with the panel
to be plated returning to the heating station as dictated by the
temperature required for a given process step. Alternatively, the
heating station is incorporated into the hoist system of such
apparatus.
[0013] In still another aspect of the present invention, the
electroless plating apparatus operates in a horizontal mode and
comprises at least one heating station. The station element where
the preheating is to take place is stationary and the panel to be
plated can only move in a forward direction, as dictated by
conveyorized, moving belt type equipment. Preheating therefore
needs to be limited to the most critical process steps such as for
example activation or electroless plating.
[0014] In a preferred embodiment, a workpiece to be plated is
preheated in an appropriate hot chemical solution, thus entering
the subsequent process tank without the usual water rinses, as
accepted in the industry. By enveloping the workpiece in a
desirable, compatible hot liquid layer that will preferably
participate in the reaction, one can greatly enhance the desired
effect of a given process. This embodiment is referred to
hereinbelow as Participant Liquid Layer (PLL) technology.
[0015] Thus, the method of the present invention provides an
economical approach for interfacial chemical reactions, enabling a
reduction in bath volume, and therefore a reduction in waste
disposal volumes, which is environmentally desirable. The method
can be used with both horizontal and vertical type plating
appartuses, so that it is easily and conveniently adaptable to
current manufacturing practices. Use of the PLL technology, as
stated, enhances the results of the process.
[0016] Additional features and advantages of the present invention
will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The preheating of the through-hole panel to be plated can be
accomplished by numerous techniques, such as for example IR,
thermal laser, hot air, high pressure steam, pressurized hot water
spray, microwave, etc. With time, new technologies will be
available to impart the desired temperature to the copper-clad
panel with the through-holes in it. Desirably, the heating should
be focused or directed into the holes, in relation to the method
chosen, reducing the heat sink effect of the copper cladding.
Indeed, the elevated temperature is most needed at the
hole-wall/solution interface, and not at the interface of the
copper cladding/solution. A recently developed new technology by
IBM, known as FLUID BEAD DRYER MANIFOLD, is a potential preferred
heating mode of the through-holes in the laminate. The technique is
described in U.S. Pat. No. 5,289,639, referenced herewith.
[0018] The optimal surface temperature of the panel can be varied,
and computer-controlled to suit the bath in a given process step.
It should take into consideration the fact that the heated panel
will undergo some cooling before and during its immersion in a
given process bath, and therefore the surface temperature of the
panel is likely to be raised considerably above the recommended
bath temperature of a given process step. The upper limit of the
surface temperature will be dependent on the dielectric material,
to avoid degradation.
[0019] When the invention is practiced in a vertical, automatic
hoist apparatus, one can envision a single heating station, with
the panel to be plated returning to the heating station as dictated
by the temperature required for a given process step. The circuit
board to be plated, will preferably be sandwiched between two rigid
sliding/movable insulating polymer sheets mounted on the jig, that
will minimize cooling of the heated surface to be plated, as it
travels to the designated process station. As the panel to be
plated enters the solution, the two insulating polymer sheets
either slide/retract upwards, or they can be immersed together with
the panel to be plated.
[0020] Another possibility to practice the invention involves
incorporation of the heating mechanism into the hoist with the
heating being done while the hoist travels between process tanks
and additionally, while the racks are moving upwards vertically
when exiting a given process tank, and also while moving vertically
downwards to enter the next process tank, as dictated by design.
Such arrangement results in greater through-put, thanks to optimal
use of hoist time.
[0021] When the invention is practiced in a horizontal mode, the
station/heating element where the preheating is to take place, is
stationary and the panel to be plated can only move in a forward
direction, as dictated by conveyorized, moving belt type equipment.
Preheating therefore needs to be limited to the most critical
process steps, i.e. prior to electroless plating, and perhaps
activation.
[0022] A further embodiment of the invention contemplates
pre-heating of the workpiece to be plated in an appropriate hot
chemical solution, and entering the subsequent process tank without
the usual water rinses, as accepted in the prior art. By enveloping
the workpiece in a desirable, compatible hot liquid layer that will
preferably participate in the interfacial chemical reaction, one
can greatly enhance the desired effect of a given process.
Electroless deposition, whether copper or nickel, is illustrative
of one such potential advantage of this invention. While the
mechanism of electroless deposition is still not properly
understood, its initial stages are known to be critical and will
essentially determine the success or failure of the desired
electroless coating. Hence, entering, by way of example, the
electroless tank directly following immersion in a hot alkaline
aqueous accelerator, and without the recommended water rinse, can
often be beneficially practiced. Alkaline accelerators are taught
by patents referenced in the background section of the application
and are standard practice with hypophosphite-reduced electroless
coppers. Indeed, electroless deposition reaction consumes hydroxyl
ions, and supplying them in the form of a heated layer at the
workpiece/electroless bath interface, will greatly increase the
flexibility and parameters of the initial deposition process, also
known as "initiation".
[0023] There are numerous other industrial applications where the
method of the present invention will be beneficial. Specifically,
the method of introducing a substrate into a process tank with a
chemical layer of a solution at the appropriate temperature,
thereby promoting its participation in the desired reaction,
enhances the final, desired reaction result. Thus, a preferred
embodiment of this invention envisions applying to a workpiece a
Participant Liquid Layer (PLL). PLL is defined in this invention as
a heated liquid layer that coats the workpiece as it enters a given
process tank such as electroless copper, nickel and enhances the
desired reaction. The temperature of that liquid layer should be at
least 5.degree. C. above ambient, but in most cases will be
significantly higher.
[0024] The practice of entering into a process tank with a
preheated panel, with or without a heated PLL on the surface of
said workpiece, offers many benefits, inter alia the following:
[0025] 1. Maximization of initiation speed of chemical reactions at
the PCB/solution interface, by eliminating or at least greatly
reducing time delays due to heat transfer from the hot solution to
the panel to be plated. Indeed, the invention allows the panel to
enter the solution at the required temperature, which is especially
beneficial for electroless deposition and also, though perhaps to a
lesser degree, for adequate and speedy activation of the hole walls
in the colloidal tin/palladium suspension.
[0026] 2. Minimization of energy consumption, by affording solution
operation at lower temperatures and making it unnecessary to keep
the baths hot during idle machine time. improvement of bath
stability, due to lower temperature of operation. This is
especially true for formaldehyde-type electroless coppers. Indeed,
because the solution can now be operated at temperatures of about
20.degree. C., bath decomposition and consumption due to the
well-known Canizzaro reaction can be significantly lower. This is
of special importance in horizontal equipment where even moderate
bath decomposition requires periodic stripping of copper metal
deposited on the walls and bottom of the copper tank, necessitating
a complete shutdown of the line.
[0027] 4. Minimization of cycle time, resulting in improved
through-put.
[0028] 5. Lower concentrations and reduced operating temperature of
Pd-based activators are possible, because of the higher temperature
at the panel/solution interface. Indeed, the tin/Pd catalyst
suspensions disclosed in U.S. Pat. No. 3,532,518 and 3,011,920
cannot sustain prolonged heating beyond 30-35.degree. C., or else
they will decompose. Also, it is known that Pd prices are, as of
late, cost-prohibitive. Lower Pd concentrations in working bath are
contemplated by this invention, with considerable cost savings.
[0029] In addition to above improvements, additional benefits and
possibilities will evolve with time, as this invention becomes
available to equipment designers, PCB manufacturers, electroless
platers in general, and others.
EXAMPLES
[0030] The benefits of the invention will be further illustrated by
the examples given below.
Example 1
[0031] Two 3".times.3" copper-clad glass epoxy FR-4 panels from
which the copper has been etched away, were conditioned, activated,
accelerated and prepared for electroless copper deposition in a
solution using hypophosphite as the sole reducer, in accordance
with supplier's instructions. The supplier, MacDermid Inc., offers
the electroless composition under the trade name of HM copper. Its
chemistry is disclosed in U.S. Pat. No. 4,265,943 referenced in the
prior art section of this application. The manufacturer's
instructions call for an operating temperature of 70.degree. C.,
and deposition time of about 20 min, prior to copper
electroplating.
[0032] One of the panels was preheated for 5 min. in an air
circulating oven set for 100.degree. C., ahead of immersion in the
HM copper solution at 55.degree. C., for approximately 10 minutes.
Within about one minute in the electroless solution, faint hydrogen
evolution commenced at the vicinity of the immersed sample,
indicating the onset of copper deposition. At the end of about 10
minutes in the HM solution, the panel was fully covered with Cu of
satisfactory appearance, suitable for copper electroplating.
[0033] The second panel was not exposed to preheating, and entered
the HM copper solution at 55.degree. C., following acceleration and
a water rinse. Inspecting the panel at the end of 20 minutes, it
showed a barely noticeable, dark coating, deemed unsuitable for
copper electroplating.
Example 2
[0034] Similar panels as in Example 1, were prepared for
electroless plating in Macudep 22, a formaldehyde type electroless
copper, offered by MacDermid Inc. Conditioning, activation and
acceleration were performed in accordance with manufacturer's
instructions. One panel was preheated in an air-circulating oven
set at 100.degree. C., as in Example 1, for about 5 min. It was
then immersed in a solution of Macudep 22 at 18.degree. C. Copious
hydrogen evolution was noticed at the vicinity of the immersed
panel almost immediately. At the end of 5 min. in the electroless
copper bath, the panel was fully covered with a pink Cu coating,
judged eminently suitable for Cu electroplating.
[0035] The other panel was immersed in the same electroless Cu
solution after acceleration, and no pre heating. Electroless Cu
initiation was slow and sluggish. Minimal hydrogen evolution was
noticeable in the vicinity of the immersed sample. After 10 minutes
in the electroless Cu bath the panel showed a dark deposit, deemed
of questionable quality for electroplating of Cu.
[0036] While the invention has been described in terms of
through-hole plated PCBs, it should be understood that it can
generally be applied to both metallic and non-metallic substrates
to be electrolessly plated. Also, electroless plating is not
limited to Cu, and the invention includes electroless nickel and
others. Furthermore, the invention is not limited to electroless
chemical reactions, in general.
[0037] Having described the invention with regard to certain
specific embodiments thereof, it is to be understood that the
description is not meant as a limitation, since further
modifications may now suggest themselves to those skilled in the
art, and it is intended to cover such modifications as fall within
the scope of the appended claims.
* * * * *