Process for production of printed circuit boards and use thereby

The invention relates to a process for production of a copper clad, electrically insulated base for printed circuit boards, wherein a foil, of copper, copper alloy, aluminum or aluminum alloy by electroplating on preferably both sides furnished with a 1-35 .mu.m thick unpatterned layer of copper or copper alloy, is under heat and pressure laminated with the copper surfaces facing an electrically insulating, resin containing base. The copper layers are electroplated onto the foil in such a way that the layers after lamination strongly adhere to the insulating base and at the same time exhibit a very poor adhesion to the foil, which easily can be stripped from the copper layers without splitting these. The foil works during the lamination as a mold plate, whereby conventional mold plates can be excluded. The invention also comprises the use of such a copper clad foil as a combined mold plate and base for the copper layers during the lamination.

The present invention relates to a process for the production of printed 
circuit boards and a use thereby. 
Normally a printed wiring pattern is used for the production of printed 
circuit boards in the electronics industry. This technique means the use 
of so called printed circuits, which usually are manufactured using a 
copper clad plastic laminate as starting material. A copy of the wiring 
pattern is transferred to the copper layer by e.g. printing or by a 
photochemical process. The applied pattern, the so called etch resist, 
acts as a protection during the subsequent removal by etching of 
superfluous copper. The electronic components are thereafter mounted on 
the laminate holding the finished wiring pattern and a complete printed 
circuit board of the most plain kind is thus obtained. The copper 
conductors provide the necessary electrical connections and the laminate 
provides mechanical support. The possibilities of a space and weight 
saving build up of the electronic unit are good. The method gives a high 
reliability and a rational production. 
The most common bases are paper reinforced phenolic laminates which are 
used for rather plain circuits and glass cloth reinforced epoxy resin 
laminates which are used when the technical requirements are high. Other 
types of fibre reinforced laminates are also used. Base materials of the 
type plastic films and plastic coated metal plates are, furthermore, used 
to a certain extent. 
It is common when preparing the copper coating to use copper foils, which 
are placed on a base forming fibre impregnated material impregnated with a 
partially cured resin (so called prepreg), followed by moulding at high 
pressure and elevated temperature. The final curing of the resin is hereby 
effected and the fibre material is compressed to a sheet to which also the 
copper foil is bonded. The copper foil usually has a thickness of 35 
.mu.m, but thicker or thinner foils also exist. A further common process 
is the gluing of a copper foil onto a plastic film using heat and 
pressure. 
Due to the rapid development in the field of electronics, the need for 
printed circuits with good dimensional precision especially in circuitry 
with a small line width and a space between the conductors, is increasing. 
Presently printed circuits with a line width of 0.1-0.2 mm and with an 
equally small conductor space are already required. Dimensions that are 
yet smaller will be required in the future. This development has led to a 
demand for laminates with thinner copper layers. During the last years 
laminates with a copper foil thickness of 6-12 .mu.m have increasingly 
come into use. By using a thinner copper foil advantages, such as a 
reduced so called "undercut", are obtained. Undercut is understood as the 
removal of copper underneath the etch resist caused by the etch solution 
which while dissolving the unprotected parts of the copper layer also eats 
into the copper covered by the etch resist. Undercut is a difficult 
problem causing an unacceptably low dimensional precision especially at 
small line widths. The effect of undercut on small conductor widths will 
below be further explained. It is by means of an accurate technique 
possible to obtain a high precision in the application of the etch resist 
itself. 
However, owing to the undercut, difficulties arise in maintaining the good 
dimensional precision of e.g. the line widths and conductor spaces, which 
the technique of masking with an etch resist per se makes possible. 
It is not solely with small line widths and small conductor spaces that a 
thinner copper layer is of advantage. Thinner copper layers result in an 
improved dimensional precision also with larger line widths and conductor 
spaces, which is an advantage, e.g. in those cases in which the electrical 
influence between the conductors must be considered when building up the 
electronic construction. These demands are expected to increase in the 
future i.a. in electronic systems operating at high frequences. 
Further advantages by using thinner copper layers are that the etching time 
is considerably reduced and that the amount of etching solution consumed 
is lesser. The consumption of copper is likewise reduced when using 
thinner copper layers. From the latter reasons thin copper layers may 
prove to be an advantage also at less high demands for dimensional 
precision. 
The thickness can, if an increased thickness of the copper of the 
conductors is desired, be increased by chemical deposition or by 
electro-deposition of copper according to known processes. This means that 
copper is employed only on that part of the finished printed circuit which 
is covered by metal, which in most cases is a minor portion of the total 
surface of the circuit. The increase of the copper thickness of the 
conductors can when using a suitable technique be performed with a good 
dimensional precision and can in the case of insulation bases, which on 
both sides are furnished with a wiring pattern of copper, suitably be 
carried out in connection with the so called through holes plating, which 
provides electrical connections between the printed circuits of the two 
sides of the base as well as holes for mounting of electrical components. 
Any additional processing stage involving plating of the through holes is 
in this case not required. The main portion of the conductors and the 
plating of the through holes will constitute a homogeneous and 
simultaneously deposited metal, which is an advantage with regard to 
reliability. 
Besides above described comparatively simple printed circuit boards with a 
wiring pattern on one or two sides, so-called multilayer printed circuit 
boards are available on the market. 
These multilayer printed circuit boards are composed of several so called 
inner layers, each consisting of an insulating base of a thin laminate of 
a thermosetting resin impregnated reinforcement material with a layer of 
metal or metal alloy on preferably both sides, in which layers a wiring 
pattern has been formed, and so called prepreg sheets of reinforcement 
material impregnated with a thermosetting resin, whereby the resin is not 
fully cured, which prepreg sheets are placed between the inner layers. 
Multilayer printed circuit boards are used for advanced electronic purposes 
and are often composed of two or more inner layers of the above kind with 
intermediate prepreg sheets. Moreover an outer layer consisting of a 
laminate of the above kind having a thin layer of metal or metal alloy on 
one side is placed on each side of the stack of inner layers. The metal 
layer is directed outwards. In certain cases the outer layers are composed 
of a thin foil of metal or metal alloy instead of said laminate. Prepreg 
sheets are also placed between the utmost of the inner layers and the 
outer layers. 
A process for production of printed circuit boards having thin copper 
layers is disclosed in the Swedish patent 7110929-2. In this case a 
temporary base of e.g. aluminium is used onto which a thin continuous 
copper layer is electroplated. The copper layer of the temporary base 
faces an insulating base and is laminated to this under heat and pressure. 
The temporary base is, subsequent to the lamination removed by means of 
etching or by mechanical stripping. Several copper clad laminates or 
multilayer boards are during this process, as well as during other 
processes disclosed above, moulded simultaneously. Mould plates are, in 
order to render the produced laminates a sufficient quality with regard to 
surface smoothness and cleanliness, placed between these various so-called 
lamination stacks. Above mould plates are most often made of steel with a 
thickness of at least 1.5 mm. Prior to each mould the mould plates must be 
carefully polished and cleaned in order to be able to keep the amount of 
discarded products at a low level. 
The unavoidable dust present in the factory premises also contribute to big 
discard problems. 
The handling of mould plates is expensive and difficult but has for decades 
been regarded as necessary. 
It has according to the present invention been possible to avoid above 
difficult and expensive use of conventional mould plates. The invention 
relates to a process for production of a copper or copper alloy clad, 
electrically insulated base for printed circuit boards. A foil of copper, 
copper alloy, aluminium or aluminium alloy, by electro-plating on one or 
preferably both sides furnished with a 1-35 .mu.m, preferably 1-18 .mu.m, 
thick unpatterned layer of copper or copper alloy, is under heat and 
pressure laminated with the copper surfaces facing an electrically 
insulating, resin containing base. The copper layers are electroplated 
onto the foil in such a way that the layers after lamination strongly 
adhere to the insulating base and at the same time exhibit a very poor 
adhesion or to an intermediate adhesion reducing layer to the foil, which 
foil with possible adhesion reducing layer easily can be separated from 
the copper layers without splitting these. The invention is characterised 
in, that the foil during the lamination works as a mould plate, whereby 
conventional mould plates can be excluded. 
A new foil of a disposable type is throughout the invention used, onto 
which a layer of copper is electroplated. All cleansing and dusting 
problems as above when using conventional steel plates are thus avoided. 
Aluminium and copper are better heat conductors than steel and this 
results in a more uniform and improved quality of the produced copper 
laminate, i.a. due to a more uniform degree of curing of the thermosetting 
resin. According to the invention a laminate with a very even copper 
surface is obtained. This depends on the fact that the electroplated 
copper layer at the laminating step is attached to a rather thick and soft 
foil of aluminium or copper. Therefore, the structure of the glass fibre 
cloth situated underneath will not be transferred to the copper surface. 
This makes it possible to use a glass fibre cloth which is coarser than 
usually utilised in this connection. The coarser cloth gives a laminate 
with better dimensional stability than a thinner cloth. In addition the 
coarser cloth is cheaper. 
The foil of copper, copper alloy, aluminium or aluminium alloy is suitably 
of a thickness of 50-600 .mu.m, preferably 100-400 .mu.m and most 
preferably 150-400 .mu.m. As disclosed above ordinary mould plates of 
steel normally exhibit a thickness of about 1.5 mm. According to the 
invention the capacity of the press hence increases, due to a reduction of 
space occupied by mould plates. The time consumed by the mould cycle can, 
furthermore, be reduced due to the higher heat conductivity of aluminium 
and copper compared to steel. 
The resin containing, insulating base can consist of many different 
materials, i.a. of glass fibre cloth impregnated with epoxy resin or 
polyimide or of paper impregnated with phenol-formaldehyde resin. 
Several different processes can be used in order to obtain the, according 
to the invention, necessary poor adhesion between the copper or aluminium 
foil and the copper layers after the lamination step. The foil can i.a. be 
furnished with an adhesion reducing coating of oxide such as aluminium 
oxide. Such a process is disclosed in the Swedish patent 7405449-5. 
According to another method a chromium layer is electroplated onto the 
foil. If an aluminium foil is used, it is suitable to electroplate a 
0.1-10 .mu.m, preferably 0.1-5 .mu.m thick layer of copper or copper alloy 
on the foil followed by a 0.01-1.5 .mu.m, preferably 0.05-1.0 .mu.m thick 
electroplated chomium layer and then the above-mentioned 1-35 .mu.m, 
preferably 1-18 .mu.m thick layer of copper or copper alloy. 
If instead a foil of copper or copper alloy is used it is suitable to 
electroplate a 0.01-1.5 .mu.m, preferably 0.05-1.0 .mu.m thick chromium 
layer on the foil followed by the above-mentioned 1-35 .mu.m, preferably 
1-18 .mu.m thick layer of copper or copper alloy. 
Regardless of whether a copper or aluminium foil is used it is suitable to 
provide the foil with a surface magnifying layer by electroplating before 
the application of the chromium layer. Preferably the surface magnifying 
layer consists of copper or copper alloy. 
Another method to obtain a poor adhesion is described in the U.S. Pat. No. 
3,969,199. The foil is according to this method treated with an alkaline 
aqueous solution containing alkaline metal zincate, a water soluble salt 
of iron, cobalt or nickel and a complexing agent. The foil is then treated 
with an acid in order to remove the applied coating. Copper is thereafter 
electroplated onto the foil. 
A further method to obtain a poor adhesion is a currentless treatment of 
the foil at 30.degree.-95.degree. C., preferably 50.degree.-75.degree. C., 
in an aqueous solution of copper pyrophosphate. Electro-plating of copper 
is then performed, e.g. in the same solution, at the same temperature and 
at a current density of 3-8 A/dm.sup.2. 
In example 1 of the U.S. Pat. No. Re. 29,820 a process is also disclosed by 
which said poor adhesion is obtained. 
Regardless of which process according to above that is used to obtain the 
poor adhesion, the subsequent electro-plating of copper can be performed 
in one or more steps using the same or a different current density. The 
various copper plating baths can be of several different compositions. The 
electro-plating is suitably performed in such a way that a surface 
magnifying, dendrite resembling surface layer is obtained, which after the 
lamination provides a good adhesion to the insulating base. The surface 
magnifying layer is then normally furnished with a barrier layer. This can 
be applied by e.g. electro-plating or by chemical processes. The barrier 
layer can i.a. consist of zinc and is intended to passivate the copper 
surface. 
The process according to the invention is especially suitable for 
production of so-called multilayer printed circuit boards. A foil of 
copper, copper alloy, aluminium or aluminium alloy, on both sides plated 
with copper or copper alloy, is then placed between as well as on top of a 
mould stack consisting of several inner layers of plastic laminate, on 
both sides furnished with a wiring pattern of copper or copper alloy, and 
so-called prepreg sheets placed between the inner layers as well as 
between the outside of the utmost inner layer of each stack and the copper 
clad aluminium foil. The prepreg sheets consist preferably of glass fibre 
cloth impregnated with a partially cured epoxy resin or the like. A stack 
of a number of such mould stacks is moulded under heat and pressure 
without ordinary mould plates. After the moulding the mould stack is 
separated along the foils of copper, copper alloy, aluminium or aluminium 
alloy or along an intermediate adhesion reducing layer applied to the 
foils. The uncovered copper surface on the two outsides of obtained 
multilayer board can then in the usual way be furnished with wiring 
patterns. 
The used foil is returned to the manufacturer where it is employed in the 
production of new foils. The process according to the invention is, hence, 
favourable also from an environmental and economical point of view. 
The invention also comprises a use of a foil of copper, copper alloy, 
aluminium or aluminium alloy, with a thickness of 50-600 .mu.m, preferably 
100-400 .mu.m and most preferably 150-400 .mu.m, which by electroplating 
on one or preferably both sides is furnished with a 1-35 .mu.m, preferably 
1-18 .mu.m thick layer of copper or copper layers during moulding of 
copper clad laminates for production of printed circuit boards. The copper 
layers are electro-plated onto the foil in such a way that these after 
moulding exhibit such a poor adhesion to the foil or to an adhesion 
reducing layer applied to the foil that the foil with a possible adhesion 
reducing layer easily can be separated from the copper layers.

The invention is further illustrated by the enclosed embodiment examples of 
which example 1 shows the production of copper clad laminates by means of 
copper clad aluminium foils according to one embodiment of the invention 
without using conventional mould plates. Example 2 illustrates the same 
process as example 1. However, here copper clad copper foils were used. 
EXAMPLE 1 
An aluminium foil with a thickness of 200 .mu.m was etched for 30 seconds 
at a temperature of 40.degree. C. in an aqueous solution containing 
______________________________________ 
NaOH 30 g/l 
Rochelle salt 46 g/l 
Sodium carbonate 46 g/l 
______________________________________ 
Then the foil was rinsed in deionized water. 
In a subsequent step the foil was zincated in the following aqueous 
solution 
______________________________________ 
NaOH 50 g/l 
Zinc oxide 5 g/l 
Rochelle salt 50 g/l 
Sodium nitrate 1 g/l 
Iron chloride 2 g/l 
______________________________________ 
at a temperature of 40.degree. C. 
The foil was then rinsed again in deionized water followed by an 
electroplating step where an aqueous solution containing 
______________________________________ 
potassium pyrophosphate 
290 g/l 
copper pyrophoshate 50 g/l 
______________________________________ 
was used. 
The pH of the solution was 8.5. The current density was 5 A/dm.sup.2 and 
the electroplating lasted 20 seconds. 
The foil was cleaned in deionized water. 
The foil was then provided with a rough surface structure (nodularized) by 
treatment for 20 seconds in an aqueous solution containing 
______________________________________ 
copper sulfate 7 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 80 g/l 
______________________________________ 
at a temperature of 28.degree. C. and a current density of 5 A/dm.sup.2. 
On top of this rough copper surface another sealing copper layer was 
applied by means of an aqueous solution containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 25 
A/dm.sup.2. The electroplating lasted 40 seconds. The total copper 
thickness obtained so far including the rough surface layer was about 2.5 
.mu.m. 
The foil was then provided with a chromate layer by electroplating for 20 
seconds in an aqueous solution containing 
______________________________________ 
chromic acid 30 g/l 
calculated as CrO.sub.3 
H.sub.2 SO.sub.4 0.3 g/l 
______________________________________ 
The temperature was 30.degree. C. and the current density was 20 
A/dm.sup.2. 
The foil was then electroplated by means of an aqueous solution containing 
______________________________________ 
potassium pyrophosphate 
290 g/l 
copper pyrophosphate 
50 g/l 
______________________________________ 
The pH was 8.5. The current density was 5 A/dm.sup.2 and the electroplating 
lasted 20 seconds. 
The foil was then electroplated for 60 seconds in an aqueous solution 
containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 35 
A/dm.sup.2. The thickness of the copper layer obtained was 7 .mu.m. 
The foil was then electroplated for 30 seconds in an aqueous solution 
containing 
______________________________________ 
copper sulfate 7 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 80 g/l 
______________________________________ 
The temperature of the solution was 30.degree. C. and the current density 
was 10 A/dm.sup.2. At this step a rough copper surface was obtained. 
The foil was then electroplated in an aqueous solution containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 
The foil was then provided with a copper passivating layer by known 
methods. 
By the above method an aluminium foil on both sides provided with a 
releasable copper layer with a thickness of about 9 micron was obtained. 
In a conventional press the following sheets were stacked on top of each 
other; 
one copper coated aluminium foil obtained according to the above method, 
five sheets of prepreg consisting of glass fabric, style 7628 impregnated 
with epoxy resin, another copper coated aluminium foil, five prepreg 
sheets of the above kind and another copper coated aluminium foil. 
The above stack was pressed for 1.5 hours at 180.degree. C. and a pressure 
of 35 kp/cm.sup.2 without conventional press plates. Two laminates on both 
sides provided with a strongly adhering copper layer was obtained when the 
laminated product produced was easily separated. In spite of the rather 
thin press plate (the aluminium foil) and the rather coarse glass fabric 
used the copper surface of the laminates obtained had a roughness, R.sub.Z 
of 3,5 .mu.m and R.sub.A of 0.5 .mu.m. These are extremely good values. 
EXAMPLE 2 
A copper foil with a thickness of 200 .mu.m was carefully cleaned. 
The foil was then provided with a rough surface structure (nodularized) by 
treatment for 20 seconds in an aqueous solution containing 
______________________________________ 
copper sulfate 7 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 80 g/l 
______________________________________ 
at a temperature of 28.degree. C. and a current density of 5 A/dm.sup.2. 
On top of this rough copper surface another sealing copper layer was 
applied by means of an aqueous solution containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 25 
A/dm.sup.2. The electroplating lasted 40 seconds. The total copper 
thickness obtained so far including the rough surface layer was about 2.5 
.mu.m. 
The foil was than provided with a chromate layer by electroplating for 20 
seconds in an aqueous solution containing 
______________________________________ 
chromic acid 30 g/l 
calculated as CrO.sub.3 
H.sub.2 SO.sub.4 0.3 g/l 
______________________________________ 
The temperature was 30.degree. C. and the current density was 20 
A/dm.sup.2. 
The foil was then electroplated by means of an aqueous solution containing 
______________________________________ 
potassium pyrophosphate 
290 g/l 
copper pyrophosphate 
50 g/l 
______________________________________ 
The pH was 8.5. The current density was 5 A/dm.sup.2 and the electroplating 
lasted 20 seconds. 
The foil was then electroplated for 60 seconds in an aqueous solution 
containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 35 
A/dm.sup.2. The thickness of the copper layer obtained was 7 .mu.m. 
The foil was then electroplated for 30 seconds in an aqueous solution 
containing 
______________________________________ 
copper sulfate 7 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 80 g/l 
______________________________________ 
The temperature of the solution was 30.degree. C. and the current density 
was 10 A/dm.sup.2. At this step a rough copper surface was obtained. 
The foil was then electroplated in an aqueous solution containing 
______________________________________ 
copper sulfate 65 g/l 
calculated as metallic copper 
H.sub.2 SO.sub.4 70 g/l 
______________________________________ 
The temperature was 28.degree. C. and the current density was 35 
A/dm.sup.2. 
The foil was then provided with a copper passivating layer by known 
methods. 
By the above method an aluminium foil on both sides provided with a 
releasable copper layer with a thickness of about 9 .mu.m was obtained. 
In a conventional press the following sheets were stacked on top of each 
other; on copper coated aluminium foil obtained according to the above 
method, five sheets of prepreg consisting of glass fabric, style 7628 
impregnated with epoxy resin, another copper coated aluminium foil, five 
prepreg sheets of the above kind and another copper coated aluminium foil. 
The above stack was pressed for 1.5 hours at 180.degree. C. and a pressure 
of 35 kp/cm.sup.2. Without conventional press plates. Two laminates on 
both sides provided with a strongly adhering copper layer was obtained 
when the laminated product produced was easily separated. In spite of the 
rather thin press plate (the copper foil) and the rather coarse glass 
fabric used the copper surface of the laminates obtained had a roghness, 
R.sub.Z of 3,8 .mu.m and R.sub.A of 0.5 .mu.m. These are extremely good 
values.