The present invention describes a method for selectively metallizing dielectric materials such as those used in the field of electronics. The products manufactured using that method include, e.g., printed circuit boards, wiring elements, chip carriers, interposers, lead frames, or even entire components. The prior art primarily for the production of printed circuit boards will be described below.
Printed circuit boards are wiring carriers featuring a wiring structure (usually printed circuits made of thin copper layers generated by way of printing) on an insulating carrier/supporting boards, and serve to accommodate components. There are various types of such printed circuit boards, including, e.g., rigid, flexible or rigid-flex, drilled or non-drilled, through hole plated or non-through hole plated supporting boards. Depending on the layers and the number of wiring levels, they are called one-sided, double-sided or multi-layer printed circuit boards. Three-dimensional circuits wherein the circuit structures go across more than two levels are also known in the art.
For many years various methods for the production of printed circuit boards have been known (e.g., Günther Hermann, Handbuch der Leiterplattentechnik, Eugen Leuze Verlag, 1982, D-88348 Saulgau).
The simplest, original version is based on copper laminated dielectric materials, to which a positive print in the shape of the desired conductive pattern is applied by way of silk screen-printing or through phototechnical means. During the subsequent etching, this positive print serves as a protective layer (etching resist). The exposed copper-plated areas, which are not protected by such an etching resist, are removed using suitable etching solutions. The etched copper is a waste product. The etching resist is afterwards stripped using inorganic or organic solvents.
In other types of this so-called subtraction method, a negative print of the conductive pattern is generated on the dielectric material to be metallized as a galvanic resist by way of silkscreen printing or through phototechnical means. Then the strip conductors are galvanized by metallizing the strip conductors up to the desired layer thickness (mostly copper). The areas that are free of strip conductors are protected by the galvanic resist. Afterwards a metal resist in the form of, e.g., a tin or tin/lead layer is applied to the metallized strip conductors galvanic-technically using a common technique, which layer protects the strip conductors during the subsequent subtractive removal (etching) of the original copper laminate. Afterwards the metal resist is removed. Other types are known as well. It should be noted, however, that this method is not suitable for the production of fine and very fine (<50 μm) strip conductors, because etching occurs not only downward but also sideward on the flanks of the future strip conductors, the so-called sub-etching phenomenon, with the resulting technical disadvantages of breaking of the edges as well as a potential for future short circuits. The thicker the copper layer to be etched off, the greater the degree of sub-etching.
In order to avoid the problem of partial sub-etching, one frequently resorts to the semi-additive method, which uses non-laminated supporting boards onto which a thin galvanic conductive layer is applied, generally copper precipitated in the absence of external power. Additional processing is essentially similar to the methods described above, except that no so-called positive etching resist is applied. That means that only the desired areas of the printed circuit board are galvanically reinforced with copper. Afterwards the copper areas of varying thickness are etched down by the degree of the initial copperplating in the absence of external power. This method, therefore, produces copper as a waste product as well. In addition, special care is needed with regard to a very consistent layer thickness distribution of the copper to be applied electrolytically.
The full-additive technique does away with copper etching altogether, because copper is applied only where it is needed, i.e., to strip conductors, lands for soldering, etc. Therefore, this method is widely used. Due to a lack of contacting options, the copper-plating takes place in the absence of external power. Most of the time a bonding and activating layer is applied onto the non-laminated dielectric material, which layer contains catalyst germs for setting off the copper-plating in the absence of external power. Once any areas not to be copper-plated are covered by means of silk screen-printing or with a photo mask, the bonding and activating agent is solubilized so that copper-plating in the absence of external power can take place directly thereafter.
The masks, which generate the discrete strip conductors, can be generated by way of silk screen-printing or through phototechnical means. What they have in common; is that an individual mask for every one of the strip conductor patterns to be generated needs to be produced respectively. When structuring is performed using a photo process, so-called “photosensitive resists” are applied as photosensitive materials onto the dielectric material to be metallized. Afterward only specific areas are exposed to light, which represent either the positive or the negative of the desired strip conductors, depending on which substance is used.
Aside from structuring the strip conductors using phototechnical processes, use of an immersion tin layer with subsequent structuring by laser is another option, particularly for three-dimensional parts such as the so-called molded interconnect devices. Here, use of the laser represents an additional subtraction technology step. The biggest disadvantage is the large amount of time required for structuring. On the one hand the thickness of the tin layer must provide sufficient etching protection, on the other hand the layer must be as thin as possible to ensure rapid laser ablation.
Processes for the ablation of very fine metal films are already being used in the production of printed circuit boards. Most of the time one is dealing with tin layers applied to copper, which are structured by laser and where the remaining copper is removed using an etching process (E. Tradic: “Haaresbreite Feinstrukturen für zukünftige Produktgestaltungen” SMT Edition 1-2/2000, 12).
Furthermore, there are already known methods for three-dimensional bodies through which very fine structures are generated through the use of laser technology. For example, methods for the laser-supported additive metallizing of thermoplastics for 3D MIDs have been described, which are based on doped plastics and can be laser-activated and are suitable for subsequent metallizing in the absence of power for the purpose of building strip conductors (SMT Edition 4/2000, 20).
Other methods where various metal layers are being ablated by laser are known as well. For example, D. Meier describes a method whereby thin gold layers are ablated by UV lasers in the course of a mask projection process and are subsequently reinforced in the absence of power (“Laser Structuring of Fine Lines”, 5th Annual Conference on Flexible Materials Denver/USA 1999, Proceedings). Laser ablation, particularly with palladium-doped organic layers, is known as well (J. Kickelhain: Promotionsarbeit an der Universität Rostock 1999). However, what all these methods have in common is the disadvantage that—following the laser structuring—an additive metallization in the absence of power for reinforcing the conductive pattern occurs.
Furthermore, conductive polymers have earned their place in the metallization of dielectric materials some time ago. They are used particularly for throughplating and manufacturing of two-layered and multi-layered printed circuit boards in the so-called direct plating technology. This is a subtractive technique where galvanic metallization occurs directly following activation of the dielectric material with the respective conductive polymer (PCT/EP 89/00204). The disadvantage—particularly in the case of two-dimensional metallization—is the relatively low electric conductivity of these conductive polymers.
There are also methods known, which suggest a selective direct galvanic metallization based on conductive polymers. U.S. Pat. No. 5,935,405, e.g., describes a method where the supporting boards are coated with a primer and conductive polymers. A photo-structurable galleon resist is used to generate the structure. Following the galvanic coating the resist is first stripped and then the conductive polymer, which was underneath the resist, is removed. Disadvantages are the use of a photo-structurable galvanic resist, because for realizing fine and very fine line structures high clean-room technology requirements are necessary, and the removal of conductive polymer, which will become necessary later on.
The subject of the invention was to find a simple, safe, inexpensive and environmentally safe method for selectively metallizing dielectric materials, which allows for precise structuring in the under μm range and furthermore does away completely with the use of resists.
The method according to the invention provides a technical solution, whereby the dielectric material to be metallized is coated with an adhesive conductive activating layer, and subsequently the desired conductive pattern is structured out of said layer by using a laser as a precision tool. The remaining structures of the activating layer continue to be conductive and can, without a problem, be galvanically metallized, preferably copper-plated, by applying electric power. This allows for structuring even within a range below 50 μm.
Through the method according to the invention limitations of the state of the art are therefore being overcome in an advantageous manner. The invention proposes operational steps that render the method technically advantageous, exceedingly economical, and effective. Hereinafter the method is described in an exemplary manner for the production of printed circuit boards, without limiting its scope to this electronics segment.
In a first approach to solving the problem, the electrolytical galvanization through the application of power is being provided by the method of the invention, through the application of a thin adhesive and conductive activating layer onto the dielectric material to be metallized. This conductive activating layer allows for the electrolytical galvanization.
The activating layer can vary widely, as long as it is sufficiently conductive. The use of polymerized or copolymerized pyrrole, furan, tiophene and/or their derivatives is preferable. Particularly preferable, however, is the use of a conductive polymer based on a polythiophene derivative (DMS-E). Furthermore, metal sulfide layers and/or metal polysulfide layers as well as Pd and/or copper catalysts can be used. There is also the option of applying thin metal layers onto the substrate. Copper layers can be applied in various ways, for example.
Preferably, the thickness of the activating layer applied should be in the 0.1 μm range. The method of the invention discloses a possibility for structuring the conductive activating layer such that discrete conductive structures are generated. The invention proposes a new method for this. Pursuant to a particularly advantageous characteristic of the present invention, the precise structuring of the activating layer occurs by means of a laser. The structuring, i.e., the generation of the conductive pattern, can occur following application of the conductive polymers, following application of the metal sulfide and/or polysulfide layer, following application of the Pd or copper catalyst, or following application of a thin metal layer. The remaining structures of the activating layer continue to be conductive and can be metallized galvanically—as long as strip conductor connection is present. In doing so, insulated areas can be connected via so-called auxiliary conductors to form an artificial strip conductor bond. During the subsequent electrolytical galvanization, the remaining conductive structures of the activating layer are metallized by applying an external power source. Applying the method according to the invention, various metal electrolytes can be used for the electrolytical galvanization of the structured activating layer; preference is given to copper electrolytes however. Laser ablation thus allows for a precise and individual design of the strip conductors within a range of a few μm. In that context, the method according to the invention also provides the option of removing the so-called auxiliary conductors that served as a sort of power bridge between the insulated strip conductor areas during metallization, once the metallization has been completed. The destruction of the artificial strip conductor bond can, e.g., occur through laser treatment.
Hence, the method specified in the invention provides the option of doing away with metallizing baths without external power, such as those used in connection with the additive technology. Thanks to this, the pollution of the environment with chemical residue and wastewater is kept at low levels. Furthermore, metal layers precipitated in the absence of external power are clearly inferior with regard to their ductility to metal layers generated electrolytically, which are significantly more finely crystallized and dense. Due to the fact that, because of the activating layer, the method specified in the invention allows for direct electrolytic galvanization through the application of an external power source, it is being ensured in an advantageous manner, that the high ductility requirements necessary are being fulfilled, which will ensure that breakage of the bushes or edges can be avoided, e.g., by reason of the thermal stress of the soldering process during the subsequent assembly of the printed circuit board.
The method specified in the invention also provides for metallization in the absence of external power, should this be necessary and desirable. To that end the adhesive activating layer used must be adjusted for metallization in the absence of external power in such a way that the activating layer contains the catalyst germs required for copperplating in the absence of external power, and handling is adjusted accordingly.
The special characteristic of the method specified in the invention, i.e., structuring of the strip conductors with the help of a laser, provides another great advantage, namely rapid, individual design of the strip conductors. This results in more flexible production options, e.g., through quick reaction to any desired changes in the course of the strip conductors. This is achieved, e.g., due to the fact that contrary to any known methods, no silkscreen, photo or other masks are required that generate the outlines of the strip conductors, as said outlines are generated by the laser.
Due to the fact that traditional photo masks can be dispensed with, the method specified in the invention is also characterized advantageously in that it can dispense with resist substances of any kind, which, for example, form the masks or strip conductor courses in the subtractive technology. This makes it possible to save chemicals as well as operational steps, are neither applying them nor stripping them afterwards is necessary.
Another advantage is that due to the precise laser structuring there is no risk of wild growth of the metal layer. This ensures precise formation of the strip conductors, thus keeping the reject rate low.
The density of the conducting tracks, their structural density on the printed circuit boards, as well as their high quality and precision, which the method specified in the invention allows for, are characteristic of the advantages of the method specified in the invention, because to achieve the desired decrease in the mass and volume of electronic devices and to optimize their operating speed, very short conductive tracks are needed. Therefore, the method specified in the invention is particularly suitable for the production of products requiring large-scale integration. This applies to its use in fields such as computers, IT, connectivity or medical technology. Furthermore, the method specified in the invention allows for the problem-free production of 3D circuits where the circuit structures are no longer needed on two levels only.
The laser used for the method specified in the invention can, e.g., be a KrF, XeCl or Nd-YAG laser. The use of other lasers is possible as long as the precise laser ablation required can be achieved.
Furthermore, the method specified in the invention provides an option for specifically controlling the laser, e.g., in such a way that the device is connected to some sort of control module. This control module can be, for example, a computer.
Due to the exact structuring, the method specified in the invention offers the advantage that the amount of raw materials consumed during metallization is smaller than the amount consumed with traditional methods, because copper is applied additively only where it is needed. This fact also results in decreased consumption of chemicals because steps such as etching and stripping can largely be dispensed with. In the end, these characteristics of the method specified in the invention lead to advantageous cost saving and decreased environmental pollution because a smaller amount of chemical waste accumulates.
Furthermore, a great operational advantage with respect to any method based on photochemical processes is due to the fact that with the method specified in the invention, there is no need to undertake any light stabilizing measures. As a laser is being used, it is no longer necessary to provide special storage capacities and workshops for the photosensitive resists.
Additional advantages and characteristics result from the examples below, which serve to further elucidate the method specified in the invention: