WO2014095007A1

WO2014095007A1 - Circuit board with real wood ply composite material

Info

Publication number: WO2014095007A1
Application number: PCT/EP2013/003756
Authority: WO
Inventors: Thomas Hofmann
Original assignee: Thomas Hofmann
Priority date: 2012-12-20
Filing date: 2013-12-12
Publication date: 2014-06-26
Also published as: DE102012025409A1; TW201438902A

Abstract

The invention relates to a method for the manufacture of a printed circuit board (20) that includes a real wood ply (11), with the method comprising the steps for producing a real wood ply composite material (10) that includes a real wood ply (11) and a synthetic resin (12), providing one or more pre-pregs (21), applying a copper foil to one or both sides of the real wood ply composite material and/or applying a copper foil to one or both sides of at least one pre-preg, patterning the copper foil or copper foils to form one or more conductive structures (22) and forming electrically conductive connections between conductive structures formed from different copper foils as appropriate, pressing the real wood ply composite material (10) and the one or more pre-pregs together for forming a composite printed circuit board, and at least partially curing the synthetic resin portions of the real wood ply composite material and the pre-preg (s). The step for producing a real wood ply composite material (10) hereby comprises substeps for providing a real wood ply (S1, S1*), impregnating the real wood ply (11) with a fluid synthetic resin (S2, S4*), and placing the real wood ply (11) in a receptacle (S3, S2*) and creating a negative pressure in the receptacle (S4, S3*).

Description

CIRCUIT BOARD WITH REAL WOOD PLY COMPOSITE MATERIAL

The present invention relates to decorative printed circuit boards and in particular to printed circuit boards provided with a real wood ply that is resistant to temperatures occurring when soldering components located on or within the printed circuit board.

It is known to use real wood plies on circuitry manufactured using printed circuit board technology for decorative purposes. German patent application DE 10 2010 010 750 discloses for example an active multi layer substrate comprising an outer ply formed by a covering layer with high light absorbing properties that inter alia may be implemented using a real wood ply.

As used in this document, the term "real wood ply" is to be understood as a thin, sheet-like wood. The thickness of a real wood ply is preferably less than 1 mm and in particular less than 0.7 mm, for instance 0.5 mm. The lower limit of the thickness is governed by the respective type of wood used, and is in particular defined by the desired or otherwise tolerated degree of translucency of the real wood ply. Preferred are real wood plies with thicknesses of 0.2 mm to about 0.3 mm and thinner, if applicable. The term "multi layer substrate" refers to a composite multilayered printed circuit board with circuit components disposed inside the printed circuit board. Just as multi layer circuit boards (substrates having no internally disposed components) , multi layer substrates are also manufactured by laminating pre-pregs. The term pre-preg is used in printed circuit technology for indicating resin impregnated mats of glass fibers with the resin being already pre- polymerized to an extent that the pre-pregs are not tacky when touched. At higher temperatures the resin starts to flow again and cures completely upon application of pressure and temperature .

In printed circuit technology, real wood plies are used for decorative purposes and are therefore preferably applied to visible faces of a circuit manufactured using printed circuit technology. In printed circuit technology, real wood plies are, however, dissimilar materials that cannot be integrated into the general manufacturing processes.

Accordingly, real wood plies are currently applied to printed circuit boards very rarely and only after their production, which requires an additional process step and increases the overall thickness of the circuitry considerably, because a real wood ply requires a minimum thickness for creating the impression of a wood surface, whereby the minimum thickness depends on the respective type of wood used. When components are placed on the surface of a circuit board, a real wood ply to be applied to that surface would further have to be patterned such that the areas occupied by the components are exposed. This, however, greatly limits the scope of application of printed circuit boards that serve, in addition to their circuit functionality, as decorative elements on and in surfaces of equipment, for instance vehicle instrumentation, floor installations or the like. To resolve this, a real wood ply may be impregnated with a synthetic resin suited for processes in the production and manufacturing of circuit boards. The resin penetrates the real wood ply upon impregnation, and thus enables use of the real wood ply as an integral part of a circuit board. All resins that are produced synthetically by polymerization, polyaddition or polycondensation reactions are referred to as synthetic resins.

When producing circuits using printed circuit boards, electric and electronic components disposed on and/or inside the circuit board have to be electrically interconnected using conductive tracks located on and/or within the circuit board. However, when applying the soldering processes necessary for this purpose, a delamination of the synthetic resin impregnated real wood plies is frequently observed.

It is therefore desirable to provide a method for manufacturing a printed circuit board that includes a real wood ply and is adapted for use in conventional processes for manufacturing circuits in printed circuit board technology without risk of a delamination of the real wood portion from the circuit board. Embodiments with respect to a method for the manufacture of a printed circuit board that includes a real wood ply comprise the steps for producing a real wood ply composite material that includes a real wood ply and a synthetic resin, providing one or more pre-pregs, applying a copper foil to one or both sides of the real wood ply composite material and/or applying a copper foil to one or both sides of at least one pre-preg, patterning the copper foil or copper foils to form one or more conductive structures and optionally to form electrically conductive connections between conductive structures formed from different copper foils, pressing the real wood ply composite material and the one or more pre-pregs together to form a composite printed circuit board, and at least partially curing the synthetic resin portions of the real wood ply composite material and the pre- preg (s) . The step for producing a real wood ply composite material hereby comprises substeps for providing a real wood ply, impregnating the real wood ply with a fluid synthetic resin, and placing the real wood ply in a receptacle and creating a negative pressure in the receptacle.

Placing the real wood ply in a receptacle and generating a negative pressure results in an efficient removal of air entrapped in the real wood ply and of air adhering to a surface of the real wood ply. A circuit board that includes a composite material manufactured in this manner from real wood and synthetic resin shows no delamination of the real wood ply composite material at temperatures used for soldering components to circuit boards.

It should be noted in this context that the terms "include", "comprise", "having", "contain", and "with", as well as grammatical modifications thereof used for listing features in this specification or the claims, are generally to be regarded as indicating a non-exhaustive listing of features such as method steps, elements, ranges, quantities or the like, and do by no means preclude the presence of other or additional features or groups of other or additional features. According to one type of embodiment of the method, the substep for placing the real wood ply in a receptacle and for creating a negative pressure in the receptacle is performed after the substep for impregnating the real wood ply with a fluid synthetic resin. Respective embodiments enable an application of the synthetic resin on and/or into the real wood ply without much engineering effort. By placing the synthetic resin impregnated real wood ply in a receptacle and generating a negative pressure in the receptacle thereafter, the external pressure to which the synthetic resin and the real wood ply are exposed is lowered so that the pressure of the air entrapped in the real wood and the synthetic resin is higher than that of the material surrounding it, resulting in a movement of the air pockets to the surfaces of the synthetic resin impregnated real wood ply where they vanish.

According to an alternative type of embodiment of the method, the substep for impregnating the real wood ply with a fluid synthetic resin is performed after the substep for placing the real wood ply in a receptacle and creating a negative pressure in the receptacle. With this embodiment, the gas pressure present in the pores of the real wood ply is already low enough for the synthetic resin to penetrate into the cavities of the real wood ply and fill them without hindrance.

In further types of embodiments, the substep for impregnating the real wood ply with a fluid synthetic resin includes an immersion of the real wood ply in fluid synthetic resin and/or a coating of at least one face of the real wood ply with the fluid resin and/or a forcing of the fluid synthetic resin into the real wood ply using pressure. All substeps can be implemented without much engineering effort and outside or within a receptacle that is under negative pressure. An impregnation of real wood plies by immersion into a fluid synthetic resin allows the synthetic resin to penetrate into the real wood ply from all sides, thus enabling an improved permeation of the real wood ply with the resin. The penetration of the resin may be enhanced by pressurizing the fluid resin with the real wood ply placed therein for forcing it into the wood structure. When coating the real wood plies on only one side, the amount of synthetic resin required is minimized, which is particularly beneficial for low volume productions. Also in this case a penetration of the synthetic resin into the real wood ply may be speeded up or improved by a pressurized coating or by pressurizing the coating subsequently. In configurations of such embodiments, the pressurized injection of fluid synthetic resin into the real wood ply comprises a rolling of the synthetic resin impregnated real wood ply, for example by rolling a roller pressed on the impregnated real wood ply across its surface or by urging the impregnated real wood ply between two rollers of e.g. a mangle pressing on it.

Preferred embodiments have the negative pressure corresponding to a gas pressure of less than 300 mbar, which results in practically all entrapped air being removed from the resin impregnated real wood ply that might cause a delamination of the real wood ply composite material at a later stage of a manufacturing process. For some embodiments, the step for producing a real wood ply composite material comprises a further advantageous substep for pre-polymerizing the synthetic resin that is carried out after the substeps for impregnating the real wood ply with fluid synthetic resin and for placing the real wood ply in a receptacle and creating a negative pressure inside the receptacle. A respective pre-polymerization enables a formation of a real wood ply composite material that is not tacky when touched, and that can be used for the manufacture of printed circuit boards because it is like a pre-preg adapted to the typical circuit board technology processes. In other embodiments, the step for producing a real wood ply composite material comprises a substep for curing the synthetic resin comprised in the real wood ply after impregnation, i.e. before the real wood ply composite material is laminated onto other circuit board components.

In some embodiments, the fluid synthetic resin comprises a blend of a thermosetting resin and a curing agent that is configured for being stored in a cool place or where required at room temperature for some time without setting, and that sets at raised temperatures. In some of these embodiments, the thermosetting resin is an epoxy based resin or a vinyl ester based resin. Other embodiments use resins of a different basis. The substep for providing a real wood ply comprises in some embodiments preferably one or more actions for pretreating the real wood ply for a subsequent resin impregnation. In particular, the substep for providing a real wood ply may in some embodiments comprise a fine sanding of the real wood ply's surface reducing length and density of wood fiber ends protruding from the surface of the real wood ply. The fine sanding procedure may be repeated several times for an improved result, whereby the surface of the real wood ply is suitably moistened between each of two fine sandings so as to have the remaining fiber ends stand up before the next sanding procedure. The term "fine sanding" as used here is to be understood as a surface treatment using abrasives affixed on substrates like e.g. sanding paper or abrasive cloth, fine steel wools and non- woven abrasives. For achieving real wood ply composite material with smooth surfaces, the substep for providing a real wood ply further comprises in some embodiments a dust removal from the real wood ply. In order to prevent a delamination of a real wood ply composite material owing to an evaporation of moisture inclusions in the real wood ply upon lamination, the substep for providing a real wood ply comprises in some embodiments also a drying of the real wood ply.

Further features of the invention will be apparent from the following description of exemplary embodiments together with the claims and the figures. In the figures, alike or similar elements are assigned alike or similar reference numerals. It is appreciated that the invention is defined by the scope of the attached claims, and not limited to the configurations of the exemplary embodiments described. Embodiments of the present invention may in particular implement individual features in different numbers and combinations than the exemplary embodiments described below. When explaining the present invention in the following with respect to a special embodiment, reference is made to the enclosed figures, of which Figure 1 is a schematic cross-sectional view of a resin impregnated real wood ply having air entrapped therein,

Figure 2 is a flowchart showing the essential steps of a first type of embodiment according to the present invention,

Figure 3 is a flowchart showing the steps for pretreating a real wood ply,

Figure 4 is a flowchart showing the essential steps of a second type of embodiment according to the present invention, and

Figure 5 is a perspective view showing in a schematic representation a printed circuit board having one ply formed by a synthetic resin impregnated real wood ply.

The schematic cross-sectional view of Figure 1 illustrates a microstructure of a real wood ply 11 impregnated with a fluid resin 12 in a conventional manner. The assignment of the hatches to the individual components of the synthetic resin impregnated real wood ply 10 is shown in the lower portion of the Figure. Although the microstructure illustrated in Figure 1 corresponds to that of a flat sawn larch wood, the representation is representative for all types of real wood plies, since wood of all sorts has pores or cavities.

The synthetic resin impregnated real wood ply 10 shown in Figure 1 comprises several air pockets 13 that are at some locations of the real wood ply's 11 porous structure not displaced by the penetrated synthetic resin 12. Real wood plies further have wood fiber ends (not shown in the Figures) standing up from their surfaces trapping air bubbles formed when impregnating a real wood ply with a synthetic resin.

It has been recognized that the pressure inside air pockets and air bubbles increases at the temperatures usually involved whilst soldering components on circuit boards (about 220°C to 260°C) to an extent that causes a deformation of the surrounding synthetic resin impregnated real wood ply- When using a synthetic resin impregnated real wood ply as part of a printed circuit board or as part of an active multi layer substrate, the deformation results in a separation of the synthetic resin impregnated real wood ply from the rest of the circuit board. This is called delamination and renders the circuit board unusable .

In order to prevent a respective delamination, the air pockets or air bubbles are in some embodiments removed from the synthetic resin after the impregnation of the real wood ply by application of a negative pressure. In other embodiments, the air present in the real wood ply is removed by applying a negative pressure before impregnating the real wood ply inside the negative pressure atmosphere. For reducing the density and size of air bubbles forming on the surfaces of the synthetic resin impregnated real wood ply, the surface of the real wood ply is in configurations of the embodiments specified above pretreated in order to achieve a wood surface that is as smooth as possible, characterized by a low density of wood fibers protruding from the surface and a short length of these wood fibers .

Figure 2 shows a flowchart F0 illustrating the essential method steps of a first type of embodiment. The method starts with step SI providing a real wood ply for the manufacture of a real wood ply composite material. In the following step S2, the real wood ply is impregnated with a fluid synthetic resin, and in step S3 finally placed in a closed receptacle, wherein a negative pressure is created in the following step S4. The negative pressure is preferably created by evacuating the air from the receptacle, which air may possibly also contain some volatile components outgassed from the synthetic resin.

The air is preferably evacuated until the negative pressure in the receptacle stabilizes and thus indicates that no further gaseous components leave the synthetic resin impregnated real wood ply. In preferred embodiments, the evacuating of the receptacle is favorably further continued until the probability of air pockets still being left in the synthetic resin impregnated real wood ply that might affect its adhesion on the other circuit board materials is virtually zero. The exact period of time depends on the negative pressure chosen and on each real wood ply's type, and is expediently determined experimentally. Air pockets and air bubbles can be removed reliably using negative pressures corresponding to a gas pressure of less than 300 mbar und thus a vacuum condition in the technical sense. After having the air pockets removed, which likewise includes the air bubbles at the surface of the real wood ply, the real wood ply is removed from the receptacle.

In order to enhance the storage stability of a thus impregnated real wood ply, the synthetic resin contained in there is in some embodiments pre-polymerized and in other embodiments cured. A respective pre-polymerization or curing can be performed before or after the impregnated real wood ply is taken out of the negative pressure atmosphere.

Providing the real wood ply comprises in preferred embodiments a pretreatment of the real wood ply for achieving a surface of the real wood ply that is as smooth as possible with a low density of so-called free wood fiber ends, i.e. wood fiber ends that protrude from the real wood ply surface like hair. The length of the remaining free wood fiber ends is thereby kept as short as possible. Which steps are carried out for a pretreatment of real wood plies depends on the type and surface quality of a respective real wood ply.

Real wood plies with a rough surface are preferably first finely sanded. The fine sanding may be carried out in several steps and includes for instance a smoothening of the surface using sanding paper, abrasive cloth or non-woven abrasive, and possibly a stripping of the surfaces with a steel or plastics wool or possibly a non-woven abrasive. To have the free fiber ends stand up so they can be removed more easily in a following sanding or stripping action, the surface of the real wood play may be moisturized between consecutive sanding or stripping actions. When high quality real wood plies are used, fine sanding of the real wood ply surfaces may not be necessary.

For achieving a real wood ply with a surface smooth enough for adhering well to other circuit board components, preferably any dust is removed from the real wood plies before any subsequent treatment. In order to prevent any adverse effect to the adhesion caused by residual moisture left in the real wood ply, the latter is dried before being impregnated with synthetic resin or placed within a negative pressure receptacle.

The flowchart Fl of Figure 3 illustrates the steps specified above for the pretreatment of a real wood ply. After starting the pretreatment in step S10, the pretreatment comprises an optional step Sll for smoothing the surface of the real wood ply by fine sanding, whereby this step may be subdivided into several individual steps as explained above. The pretreatment further includes a step S12 for removing dust from the real wood ply's surfaces as well as a step S13 for drying the real wood ply. Step S13 is advisable but with real wood plies of low moisture content not always necessary. The termination of the pretreatment is indicated in flowchart Fl by step S14.

Impregnating the pretreated real wood ply with synthetic resin may be carried out in various ways. The real wood ply is preferably placed or submerged in a fluid synthetic resin. In other embodiments, the real wood ply is coated with synthetic resin, either on one side or on both sides, whereby the coating may include a cast coating, spread coating or spray coating with fluid synthetic resin on one or both of the real wood ply's surfaces, or a combination thereof. To enable the synthetic resin to penetrate deeply into the real wood ply and impregnate it completely, the synthetic resin is in preferred embodiments subsequently pressurized to force it into the real wood ply. In some embodiments, the resin is forced from the real wood ply's surface into the interior of the real wood ply using a roller. In some embodiments the rolling can be carried out simultaneous to the application of the synthetic resin, for instance by using a mangle with the synthetic resin being applied to one or both of its rollers and transferred in a pressurized manner to the real wood ply fed through the rollers. Instead of a mangle, a multistage roller mechanism may also be used.

Other embodiments have the real wood plies submerged in fluid synthetic resin that is located and pressurized in a receptacle. Pressurizing the synthetic resin usually takes place in a suitably closed receptacle.

Other types of embodiments have the pretreated or provided real wood plies placed into a receptacle before they are impregnated with synthetic resin, whereby a negative pressure is created in the receptacle after closing it for removing the air from within the real wood ply or plies. The real wood ply or plies is or are subsequently impregnated with fluid synthetic resin using one of the above ' .methods , whereby the impregnation takes place without any shutting down of the negative pressure. To accommodate devices required for impregnating real wood plies, the negative pressure receptacle has a bigger internal volume than a negative pressure receptacle required for carrying out the type of embodiments mentioned first. The creation of the negative pressure therefore requires more time. To keep the time required hereto short, the real wood plies may be transferred into the negative pressure receptacle using one or more evacuatable lock chambers. The steps essential for a method according to the type of embodiment described before are clearly illustrated in the flowchart F0* shown in Figure 4.

The method starts with providing the real wood ply in step SI*, possibly comprising a pretreatment of the real wood ply that includes at least one of the above substeps, i.e. a fine sanding of the real wood ply's surfaces and/or a removal of dust from the real wood ply's surfaces and/or a drying of the real wood ply. Different to the first type of embodiments, the real wood ply or plies provided in this second type of embodiment are in step S2* placed into a closed receptacle, for instance a vacuum chamber, first. The placement into as well as the removal from the receptacle can be accomplished using a evacuatable lock chamber, thereby avoiding a ventilation of the receptacles interior when transferring real wood plies into or out from the receptacle. When using a lock chamber, a negative pressure is created in its interior in step S3* before the real wood ply or plies is or are further transferred into the interior of the closed receptacle. Otherwise, the receptacle is closed and a negative pressure is created in its interior in step S3*. After the negative pressure has stabilized in the receptacle's interior and remained so for a time period sufficiently long enough to remove the air and as the case may be also any moisture from within the real wood ply structure, the real wood ply is impregnated with fluid synthetic resin in step S4*. The optional pressurizing of the impregnated real wood ply can be implemented by mechanical means as described above or by using a synthetic resin container, which interior is separated from the negative pressure receptacle ^'after having the real wood plies transferred from the negative pressure receptacle into the synthetic resin container under negative pressure conditions, and subsequently pressurized. The step for removing the synthetic resin impregnated real wood ply from the receptacle is referenced in Figure 4 with S5*, whereby the term receptacle refers according to the above explanation either to the negative pressure receptacle or the synthetic resin container.

Independent from the respective method employed for impregnating a real wood ply with synthetic resin, the impregnated real wood ply may further be treated as explained above for pre- polymerizing or curing the synthetic resin comprised in the real wood ply in order to enhance the storage stability of the final real wood ply composite material. Figure 5 illustrates a circuit board technology implemented circuitry 20 having an outer ply 10 formed by a synthetic resin impregnated real wood ply laminated onto the one or more pre- preg plies 21. A conductive track pattern 22 is formed on the surface of the pre-preg plies 21 interconnecting several active and/or passive components 23 and 24. Although in the figure several components are referenced with the same reference numerals, these normally represent different components such as resistors, inductors, capacitors, diodes, transistors, integrated circuits, displays and the like. The conductive track pattern and the components may also be further located within the circuit board formed by the real wood ply material board 10 and the pre-preg plies 21 as well as on the bottom side of the real wood ply material board 10. Conductive track patterns formed on different levels may be interconnected by plated through holes .

The invention described above enables an integration of real wood plies into the manufacturing processes for circuit board technology circuitry. Circuit boards manufactured with a real wood ply composite material manufactured as described above may be used for circuitry to be applied on visible faces of objects like for instance surfaces of furniture or equipment, instrumentation, dash boards, or the like. The visible face of a circuit board finished with a real wood ply composite material may thereby also be provided with a conductive trace pattern tha is electrically connected to components, if applicable.

The real wood plies described may be combined with any standard circuit board technology materials if desired enabling a manufacture of all possible types of circuit boards. Using a respective mix of materials enables the combining of the benefits of the different materials with the visual appeal of a real wood synthetic resin composite surface.

Claims

A method for the manufacture of a printed circuit board (20) having a real wood ply (11), comprising the steps for

producing a real wood ply composite material (10) that includes a real wood ply (11) and a synthetic resin (12), providing one or more pre-pregs (21),

applying a copper foil to one or both sides of the real wood ply composite material and/or applying a copper foil to one or both sides of at least one pre-preg,

patterning the copper foil or copper foils to form one or more conductive structures (22) and forming electrically conductive connections between conductive structures formed from different copper foils as appropriate, pressing the real wood ply composite material (10) and the one or more pre-pregs together for forming a composite printed circuit board, and

at least partially curing the synthetic resin portions of the real wood ply composite material and the pre-preg (s), whereby the step for producing a real wood ply composite material (10) comprises the following substeps:

providing a real wood ply (SI, SI*) ,

impregnating the real wood ply (11) with a fluid synthetic resin (S2, S4*), and

placing the real wood ply (11) in a receptacle (S3, S2*) and creating a negative pressure in the receptacle (S4,

The method according to claim 1, wherein the substep for placing the real wood ply (11) in a receptacle

(S3) and for generating a negative pressure in the receptacle (S4) is performed after the substep for impregnating the real wood ply (11) with the fluid synthetic resin (S2).

The method according to claim 1, wherein the substep for impregnating the real wood ply (11) with the fluid synthetic resin (S4*) is performed after the substep for placing the real wood ply in a receptacle (S2*) and for generating a negative pressure in the receptacle (S3*).

4. The method according to one of claims 1, 2, or 3, wherein the substep for impregnating the real wood ply with fluid synthetic resin (S2, S4*) comprises at least one of the following :

submersing the real wood ply (11) in the fluid synthetic resin ( 12 ) ,

coating at least one face of the real wood ply (11) with the fluid synthetic resin (12), and

forcing the fluid synthetic resin (12) into the real wood play (11) using pressure.

5. The method according to claim 4, wherein the forcing of the synthetic resin (12) into the real wood ply (11) under pressure comprises a rolling of the real wood ply having the fluid synthetic resin applied thereto.

6. The method according to one of the preceding claims, wherein the negative pressure corresponds to a gas pressure of less than 300 mbar.

7. The method according to one of the preceding claims, wherein the step for producing a real wood ply composite material (10) further comprises a substep for pre-polymerizing the synthetic resin (12), that is carried out after the substeps for impregnating the real wood ply with a fluid synthetic resin (S2, S4*), for placing the real wood ply in a receptacle (S3, S2*) and for creating a negative pressure in the receptacle (S4, S3*) .

8. The method according to one of the preceding claims, wherein the step for producing a real wood ply composite material (10) further comprises a substep for curing the synthetic resin (12), that is carried out after the substeps for impregnating the real wood ply with a fluid synthetic resin (S2, S4*), for placing the real wood ply in a receptacle (S3, S2*) and for creating a negative pressure in the receptacle (S4, S3*), or after the optional substep for pre- polymerizing the synthetic resin (12).

9. The method according to one of the preceding claims, wherein the fluid synthetic resin (12) comprises a blend of a thermosetting resin and a curing agent.

10. The method according to claim 9, wherein the thermosetting resin is an epoxy based resin or a vinyl ester based resin.

11. The method according to one of the preceding claims, wherein the substep for providing a real wood ply (SI, SI*) comprises a fine sanding of the real wood ply's surface.

12. The method according to one of the preceding claims, wherein the substep for providing a real wood ply (SI, SI*) comprises a removal of dust from the real wood ply.

13. The method according to one of the preceding claims, wherein the substep for providing a real wood ply (SI, SI*) comprises a drying of the real wood ply.

14. Printed circuit board (20) comprising one or more pre-pregs (21), one or more patterned copper foils, and one or more real wood ply composite materials (10) obtainable by the method according to one of claims 1 to 13.