Method of making a laminated structure with shear force delamination resistance

The surface of a metal foil to be used as an intermediate layer in printed circuit boards is chemically roughened. In one embodiment, fine depressions are etched out of the metal coating. In another embodiment, protuberances are plated thereon. The use of the method permits the use of the foil in the production of multilayer printed circuit boards with coatings having different thermal expansion characteristics and improves the adhesion with the next conductor layer applied thereto to a significant extent and prevents delamination.

FIELD OF THE INVENTION 
The invention relates to the field of connection carriers for electronic 
components and specifically relates to the structuring of the surface of a 
metal foil in conjunction with a method for the production of a 
multilayer, laminated printed circuit board, as well as to a printed 
circuit board produced by this method. 
BACKGROUND OF THE INVENTION 
Printed circuit boards for producing electronic circuits carry and connect 
electronic components, compactness and high interconnection density being 
sought. It therefore forms part of the prior art to directly solder onto 
the surface connection layer of printed circuit boards integrated circuits 
in the form of surface mounted devices (abbreviated SMD) or leadless chip 
carriers (abbreviated LCC). The printed circuit boards generally have a 
multilayer construction, i.e. they have several conductor layers. 
The reliability of SMDs soldered onto a printed circuit board is greatly 
reduced if the thermal expansion coefficient of the components differs 
significantly from that of the printed circuit board. Different thermal 
expansions of the board and components leads to high shear stresses at the 
soldered joints, which can then lead to cracks. The problem does not occur 
to the same extent when using components having metal leads, because the 
latter serve as flexible intermediate members between the board and the 
component and in this way the stresses can be compensated. 
As a function of the materials used, conventional printed circuit boards 
have a thermal expansion coefficient of 15 to 20 ppm/.degree. C., but the 
coefficient approximately 6 ppm/.degree. C. for components in ceramic 
packages without leads, i.e. leadless ceramic chip carriers (LCCC). The 
use of LCCC's consequently requires printed circuit boards whose thermal 
expansion is reduced by suitable measures, so that the stresses are 
limited to an acceptable amount. 
Standard measures for reducing the thermal expansion of printed circuit 
boards are the fitting of cores or intermediate layers of thermally stable 
materials. For example metal sheets or foils formed from a combination of 
copper and an alloy sold under the trademark INVAR.TM. (an alloy of nickel 
and iron with a very low expansion coefficient), of molybdenum (with a 
thermal expansion coefficient of approximately 5 ppm/.degree. C.) or cores 
with a carbon fiber reinforcement are known. Further information on this 
problem are provided by EP-A-393312 which is assigned to the assignee of 
this application. 
As a result of these measures the composite of thermally stable 
reinforcement and conductor layers, i.e. the printed circuit board, 
acquires an adapted, low expansion factor and in this way the thermal 
stresses at the soldered joints are reduced. However, it leads to a new 
problem. Because of temperature changes, shear stresses act at the 
interface between the cores or intermediate layers of the thermally more 
stable materials and the remaining layers of the printed circuit board. 
This occurs because the cores or intermediate layers, due to their high 
mechanical strength, force the board to remain dimensionally stable, 
whereas the remaining layers of the board, having the tendency to expand 
or contract more with the temperature change, are subject to a restriction 
of their freedom of movement because of their adhesive attachment to the 
cores or intermediate layers. If untreated intermediate layers are 
laminated together with conductor layers to form a printed circuit board, 
due to the normal temperature cycles, delamination must be expected, i.e. 
the union between the conductor layer and the intermediate layer is 
broken. This separation can in particular be observed if glass 
fibre-reinforced polyimide resin is used as the laminate base material. 
However, poor results are also obtained with base materials from cyanate 
ester resins. 
Solutions for overcoming this problem are also known. The surfaces of the 
metal foils (copper/INVAR.TM., copper/molybdenum) are pretreated by black 
oxidation, sand blasting or the application of a treatment coating. The 
normally used black oxidation is admittedly sufficient to prevent the 
delamination of glass fibre-reinforced epoxy materials, but is not 
sufficient in the case of polyimide or cyanate ester resin base materials. 
The necessary adhesion coefficients are also not provided by sand 
blasting. There also is a process in which an additional coating, normally 
of copper, is so electrodeposited on the stabilizing intermediate layer, 
that a rough surface is obtained. However, this process is complicated and 
expensive. It is always used over the entire surface, so that no desired 
or even necessary recesses are possible. The product also suffers from the 
disadvantage of being sensitive to pressure and scratching and cannot 
subsequently undergo photochemical processing. 
SUMMARY OF THE INVENTION 
An object of the invention is to find a measure for preventing the 
delamination of coatings having different thermal expansion 
characteristics in a laminated, multilayer printed circuit board, without 
having to accept the aforementioned inadequacies and disadvantages. This 
object is achieved by a special structuring of the surface of metal foils 
and the assembly of the foils in the printed circuit boards. 
The materials now used for cores or intermediate layers normally have an 
outermost coating of electrically and thermally conductive, 
electroplatable metal, particularly copper. However, it is also possible 
to obtain foils made solely from INVAR.TM. or solely from molybdenum. The 
invention therefore aims at improving the adhesion between the outer metal 
surface and the conductor layer to be applied thereto. This can be 
efficiently and inexpensive achieved in that the surface of the metal 
coating is given a fine structure by photochemical treatment. The 
photochemical process makes it possible using per se known means in a few, 
relatively uncomplicated operations to obtain a fine and almost randomly 
selectable pattern of etched-out depressions and/or electrodeposited on 
protuberances. Structured in this way the contact surface between the 
metal and the connecting material of the next, laminated-on layer, 
normally a resin, is much larger than with the untreated metal coating, 
which increases the overall adhesiveness. The largely freely selectable 
shape of the depressions and protuberances also makes it possible to 
effectively counteract on an overall basis the shear forces directed 
parallel to the surface. The production of patterns for photomasks can 
take place in a very simple manner by providing a basic element of the 
pattern and then repeating it in a computer assisted manner. It is also 
easy to make different patterns in the same mask and in particular larger 
parts can be excluded from the patterning with random boundaries. 
The invention has the advantage of being based on standard technology. It 
can be used in virtually any printed circuit board fabrication without 
special means having to be used. The process is also much less expensive 
than "treating" copper surfaces. The product is robust with respect to 
scratches and compressive loads and can be treated and further processed 
in the same way as a normal copper surface. Where it is possible to work 
with conventional-metal and in particular copper-coated foils, the etching 
out of depressions also has the positive effect of weakening the lateral 
cohesion of the outer metal coating, so that the inner, thermally more 
stable coating can better develop its dimensional stabilization action.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an example of a multilayer, laminated printed circuit board, 
whose thermal expansion is kept within limits by the incorporation of two 
metal foils 3 each having a low thermal expansion coefficient which is, as 
a function of the structure, approximately 1 to 6 ppm/.degree. C. Such 
foils can e.g. comprise a molybdenum or INVAR.TM. coating with a copper or 
copper-nickel coatings of varying thickness rolled onto one or both sides. 
Between the two foils 3 and on the two outer faces is in each case 
provided a generally multilayer conductor 2, whose thermal expansion 
coefficient is between approximately 12 and 20 ppm/.degree. C. The average 
thermal expansion coefficient for the entire printed circuit board is then 
7 to 10 ppm/.degree. C. There is a delamination risk in the case of 
temperature changes at the contact surface where the metal foil 3 and the 
conductor layer 2 are laminated together. To prevent this specific metal 
foil surface treatment measures are taken. These measures are shown at the 
larger scale detail A of FIG. 1. 
FIG. 2 shows an enlarge view of detail A of FIG. 1 with a known surface 
treatment of the metal foil for improving the adhesion of the laminate. 
Thus, FIG. 2 shows the prior art. The upper part of the foil 3 is shown 
and comprises the Invar layer 4, the copper coating 5 and an 
electrodeposited "treatment" coating 6 with its rough surface. Above this 
can be seen the lower part of the laminated-on conductor layer 2 
comprising the adhesive 7, the plastic carrier 8 and the electrically 
conductive connections 9 and this sequence can be repeated in the 
conductor layer 2. 
Foils with a treatment coating are able to prevent delamination, but not in 
all cases. Moreover, although "treating" can be used on copper coatings, 
molybdenum or INVAR.TM. are too hard. 
FIGS. 3a-3d show four successive steps in a method in accordance with the 
invention for producing a structured surface in the metal coating of one 
layer of a multilayer printed circuit board. FIG. 3a shows the starting 
material, in this case a metal foil 3 having a core part 4 covered with 
the metal coating 5. Such foils are commercially available and are 
typically formed from a composite of INVAR.TM. or molybdenum in the core 
part 4 and rolled on copper as the metal coating 5. A copper/INVAR.TM. 
foil is used in the example described manner hereinafter. The method steps 
are shown for one of the two copper coatings 5. However, the method can be 
readily used in the same operation on both sides. Considered individually, 
all the partial steps are known per se from other sequences for the 
production of printed circuit boards. 
First a photoresist coating 10 is applied to the copper coating 5. The 
photoresist, usually a polymer, under the action of light locally changes 
its resistance to certain chemicals used for development, so that recesses 
can be produced in the photoresist coating. The photoresist is resistant 
to etchants for the metal which it covers. FIG. 3b shows the copper 
coating covered by the already exposed and developed photoresist coating 
10. The latter has recesses 11 at the points where depressions are to be 
formed in the metal surface. The shape of the recesses can be freely 
selected. The dimensions are preferably approximately 50 to 100 
micrometers. 
In the next step the copper is partly etched, i.e. it is not entirely 
etched away. At the locations of the recesses 11 in the photoresist 
coating 10, depressions 12 are formed in the underlying copper coating 5. 
FIG. 3c shows the situation following this partial step. 
With the conventional etching process, preference being given to spray 
etching, the successive material removal commences at the surface 
uniformly over the entire opening in the photoresist. With progressive 
etching out depth, there is a slight expansion of the depression. This 
known undercutting leads in the case of the small depressions as produced 
here to a bulging cavity. The etching depth is decisively determined by 
the running speed or residence time of the workpiece in the etching 
process. These are known, readily controllable processes. Preferred depths 
are between 10 and 50 micrometers. 
The undercutting in the etched-out depression is desired for the sought 
structuring purpose. After laminating on a further coating, the resin in 
the cavities hardens. This leads to a positive connection, which makes a 
separation of the coatings virtually impossible. Only the elasticity of 
the materials allows separation when very high forces are exerted by 
overcoming the pushbutton effect. 
A further improvement regarding the shape of the depressions can be 
obtained by prior hardening of the surface of the copper coating with 
respect to the etchant. For this purpose, prior to the application of the 
photoresist, nickel is applied to the copper and diffused in by a heat 
treatment. The etching process through the top coating then initially 
takes place slowly until the hardened coating has been etched through and 
then there is a pronounced undercutting of the harder top coating. The 
overhanging edges of the depressions are thicker and stronger than in the 
previously described variant. 
Following the etching, the photoresist is stripped, i.e. removed again in 
known manner. What is left behind is the original metal foil 2 with the 
recesses 12 in the copper coating 5 as shown in FIG. 3d which represents 
the desired structuring of the surface as regards pattern and shape. 
In the subtractive method the copper coating is weakened, which must be 
taken into account for the electrical function if this is significant. 
This weakening has an advantageous effect with respect to the 
mechanical-thermal characteristics. As there is less copper with a 
comparatively high thermal expansion coefficient, the INVAR.TM. can even 
better fulfil its dimension-stabilizing action. Moreover, in its 
broken-away form the copper coating is less stiff than in the form of a 
uniform thick foil. The method is not restricted to copper coatings. 
Uncovered INVAR.TM. or molybdenum foils can also be appropriately 
structured in accordance with the above-described steps. 
For the choice of the pattern and shape of the depression, decisive 
importance is attached to the characteristics of the materials used. In 
order to effectively counteract the shear forces, it may e.g. be 
advantageous to incorporate appropriate barriers. FIGS. 4a-4c are plan 
view of structured surfaces with possible patterns of the recesses 12 in 
three variants in an exemplified, but non-limitative manner. 
According to another embodiment which is referred to herein as the additive 
method, protuberances of the material are electrodeposited on the metal 
coating 5, the associated steps being shown in FIG. 5. Starting takes 
place from the same method stage as described hereinbefore in conjunction 
with FIG. 3a. FIG. 5a again shows the copper/INVAR.TM. foil and FIG. 5b 
the state following the application, exposure and development of the 
photoresist 10 on the copper coating 5. The recesses 11 are here patterned 
and shaped in accordance with the protuberances applied at these 
locations. Otherwise the partial steps do not differ up to now from those 
of the subtractive method. 
In a next step the copper is electroplated on. Copper protuberances 15 are 
formed in the recesses 11, in the manner shown in FIG. 5c. The thickness 
of the electroplated-on coating or islands, i.e. the protuberances 15, is 
determined by the duration and intensity of the electroplating process. 
Here again the pattern and shape of the protuberances can be freely 
selected. It has the major advantage that overhanging structures can be 
produced, as shown in FIG. 5d. The electroplating process is continued 
beyond the state of FIG. 5c. Copper is not only deposited in the recesses 
11, but also, starting from the protuberances 15, over the photoresist 
coating 10. As a result, collars 16 are formed on the protuberances 15, 
which therefore acquire a mushroom-shaped configuration. After stripping 
the photoresist the mushroom-shaped protuberances 15 with their collars 16 
are left behind, as shown in FIG. 5e. Following a subsequent method step, 
in which the next conductor is laminated on, the collars 16 bring about an 
extremely favourable anchoring of the resin or adhesive on the metal 
surface. However, electroplating can be stopped at the state of FIG. 5c 
and the photoresist stripped (not shown). This leads to similar structures 
and shapes to those of the subtractive method. 
Generally additional requirements will decide the choice of method. Metal 
foils of the described type can fulfil thermal, electrical and mechanical 
functions. If exclusively a depression and a limited thermal expansion is 
sought, it is sufficient to have foils of Invar, molybdenum, etc. Both 
methods can be used and plating-on can e.g. take place with nickel or 
copper. If additionally a good electrical conductivity is sought, in order 
to obtain a screening effect, a copper coating, structured according to 
the subtractive method, may well be the correct choice. However, if the 
main significance is attached to the heat conduction function, then a 
relatively thick copper coating is appropriate and the additive method 
should be used. 
With these photochemical methods it is readily possible to only structure 
parts of the foil and to recess other parts, which are subsequently e.g. 
not to be covered, but which are to provide a screening connection. 
The method and the resulting product have been explained using the example 
of a copper/INVAR.TM. foil, as used as an intermediate layer in printed 
circuit boards. The delamination problem occurs most acutely on the said 
foil surface which, for producing a multilayer printed circuit board, is 
laminated together with another conductor layer. However, the method and 
resulting product are in no way limited to such intermediate layers. With 
the knowledge of the invention the expert is in a position to use the same 
for substrates formed in different ways.