Patent Application
20220361341
This disclosure relates to a method of producing printed circuit boards, and printed circuit boards produced in accordance with the method.
A printed circuit board (PCB) serves as a carrier for electronic components and ensures the electrical contacting thereof. Virtually every electronic device contains one or more printed circuit boards.
Printed circuit boards always comprise a base substrate, which is embodied in an electrically non-conductive fashion and has on at least one substrate side a structure composed of conductor tracks (conductor structure) for electrically contacting the electronic components. In general, base substrates for printed circuit boards consist of fiber-reinforced plastic, of plastic films or hard paper. The conductor tracks usually consist of a metal such as copper.
In the simplest form, only one side of the base substrate has a conductor structure. However, more complex circuits often require more than one conductor track plane; a multilayer printed circuit board (multilayer board (MLB)) is then required. In these instances, for example, both sides of a carrier layer can be provided with a conductor structure, or a plurality of base substrates each having a conductor track plane are combined to form an MLB. In particular, base substrates provided with a conductor structure on both sides can also form a basis for multilayer constructions. The conductor tracks of the different conductor track planes can be electrically connected to one another by way of vias. For this purpose, by way of example, holes can be drilled into the base substrates and the walls of the drilled holes can be metallized.
Formation of the conductor structures on a base substrate is usually carried out subtractively in a multi-stage photolithographic process using a photoresist (resist), the solubility of which in a developer solution can be influenced by radiation, in particular by UV radiation. In a routine procedure, a metal layer is formed on the base substrate and is covered with a layer of the photoresist. The layer of the photoresist can be laminated onto the metal layer, for example. Afterwards, the layer of the photoresist is subjected to the aforementioned radiation in an exposure step, wherein partial regions of the layer are protected against exposure to radiation by an exposure mask. Depending on the photoresist used and the developer solution used, after the exposure step either the exposed or the unexposed partial regions of the layer of the photoresist are soluble in the developer solution and can be removed in a subsequent step. In this subsequent step, the developer step, partial regions of the metal layer on the base substrate are uncovered, and they can be removed wet-chemically in a further subsequent step, an etching step. The remnants of the metal layer that remain after the resist has subsequently been completely removed form the desired conductor structure. Optionally, the latter can be reinforced in a deposition step, for example, by electrodeposition of a suitable metal.
Owing to the dictates of production, the conductor tracks are thus situated on the surface of a base substrate. This may be disadvantageous in the production of MLBs. If a surface of a base substrate provided with conductor tracks is pressed together with a further base substrate, afterwards there is often a need for monitoring and correction owing to deviations that are caused by the pressures and temperatures occurring during pressing. Conductor tracks on the surface of base substrates are subjected to such loading to a particular extent. The smaller the distances and dimensions of the conductor tracks on the substrate, the greater generally the corresponding need for monitoring and correction, for example, with regard to existing impedance and signal speed requirements.
A general disadvantage of the traditional subtractive processes, moreover, is that resolution of the conductor structures to be produced is limited. Conductor tracks having widths in the low two-digit or even one-digit μm range are thus virtually impossible to produce.
In recent years, additive and semi-additive approaches have increasingly also been employed to form conductor tracks (SAP: Semi-Additive Process and mSAP: Modified Semi-Additive Process). These approaches make it possible to produce conductor track structures with a higher resolution than is possible with subtractive processes. However, such progress is made at the expense of enormous technical complexity, which is reflected in losses of yield and high production costs.
It could therefore be helpful to develop a procedure for producing printed circuit boards which makes it possible to avoid or at least reduce the problems described.
I provide a method of producing a multilayer printed circuit board including a metallic conductor structure, including: a. providing a base substrate including a film or plate and having a first substrate side and a second substrate side, which base substrate at least partly consists of an electrically non-conductive organic polymer material and wherein the first substrate side is covered with a cover metal layer, b. partially removing the cover metal layer while subdividing the first substrate side into at least one first partial area, in which the first substrate side is free of the cover metal layer, and into at least one second partial area, in which the first substrate side is covered with the cover metal layer, c. causing a plasma to act on the first substrate side with the aid of which plasma the polymer material is removed in the at least one first partial area while forming at least one trench, d. filling the at least one trench with a filling metal, and e. completely removing the cover metal layer in the at least one second partial area while forming a first conductor structure or a part of a first conductor structure, f. optionally planarizing the first substrate side with the at least one trench that has been filled, g. covering the first conductor structure with an insulation layer, which in a composite assembly with the base substrate has an underside directly in contact with the first conductor structure and a top side facing away from the first conductor structure and which at least partly consists of an electrically non-conductive organic polymer material, h. if not yet present, forming a cover metal layer on the top side of the insulation layer, i. partially removing the cover metal layer while subdividing the top side into at least one first partial area, in which the top side is free of the cover metal layer, and into at least one second partial area, in which the top side is covered with the cover metal layer, j. causing a plasma to act on the top side, with the aid of which plasma the polymer material is removed in the at least one first partial area while forming at least one trench, k. filling the at least one trench with a filling metal, and l. completely removing the cover metal layer in the at least one second partial area while forming a second conductor structure or a part of a second conductor structure.
I also provide a printed circuit board including: a composite composed of a base substrate as first printed circuit board layer and an insulation layer as second printed circuit board layer, wherein the base substrate includes a first substrate side and a second substrate, the base substrate has at least one trench on the first substrate side, a first conductor structure being embedded into the at least one trench, the base substrate has a planarized surface on the first substrate side, the insulation layer includes a top side and an underside, the first conductor structure is covered by the insulation layer, wherein the underside of the insulation layer is directly in contact with the first conductor structure, and the insulation layer has at least one trench on the top side, a second conductor structure being embedded into the at least one trench.
My method of producing a printed circuit board comprising a metallic conductor structure always comprises steps a. to e.:
The method is suitable both for producing monolayer printed circuit boards that comprise only one base substrate with one conductor structure in one plane, and producing multilayer printed circuit boards that comprise conductor structures in at least two planes and usually also a plurality of base substrates. In the latter configuration, removing the cover metal layer in step e. results in formation of only part of the entire conductor structure of the printed circuit board. In the first configuration, preferably the entire conductor structure of the printed circuit board is formed in step e.
Particularly preferably, the method additionally comprises step f.:
In this respect, even more detailed explanations follow.
Choice of the base substrate
Preferably, the method comprises at least one of a. to c.:
Preferably, a. and b. directly above are realized in combination with one another.
Particularly preferably, the base substrate is a film composed of a polymer material, in particular one of the polymer materials mentioned. This holds true particularly if the printed circuit board to be produced is embodied in a multilayer fashion. A comparatively thicker base substrate embodied as a plate is preferably chosen in a monolayer printed circuit board.
Optionally, the base substrate can comprise fillers, in particular dielectric fillers. By way of example, the base substrate can be a film composed of one of the polymer materials mentioned, into which silicon dioxide particles are embedded.
Appropriate dielectric fillers include, in particular, metal or semimetal oxides (besides silicon dioxide in particular also aluminium oxide, zirconium oxide or titanium oxide) and other ceramic fillers (in particular silicon carbide or boron nitride or boron carbide). Silicon, too, can optionally be used.
The fillers are preferably present in a particulate form, in particular having an average particle size (d50) in the nanorange (<1 μm).
Particularly preferably, the method is thus distinguished by at least one of a. to c.:
Preferably, a. and b., in particular also a. to c., directly above are realized in combination with one another.
To facilitate handling, the base substrate can be applied to a carrier or an auxiliary substrate, for example, composed of glass or aluminium, for processing purposes.
Formation and properties of the cover metal layer
Further preferably, the method comprises at least one of a. and b.:
Preferably, a. and b. directly above are realized in combination with one another.
Ideally, the cover layer should be closed and the preferred minimum thickness mentioned should not be undershot such that the cover metal layer can fulfil its technical function as an etching barrier as explained below.
As an alternative to the copper or the copper alloy, titanium and nickel-chromium alloys are also appropriate as cover metal.
To form the cover metal layer, it is possible, in principle, as the cover metal layer, for a thin metal film, in particular a thin copper film, to be laminated or applied as cladding onto the first substrate side. It is preferred, however, if the method comprises at least one of steps a. to c.:
Preferably, a. and b. directly above are realized in combination with one another.
Metallizations by physical and chemical vapor depositions and also the production of metal layers by wet-chemical coating processes are known and need no further explanation.
Particularly preferably, as the cover metal layer, a copper layer is sputtered onto the base substrate.
It may be preferred for an adhesion-promoting adhesion layer to be applied to the first substrate side before formation of the cover metal layer or during the application of the cover metal layer.
Possible procedures when partially removing the cover metal layer
Further preferably, the method comprises steps a. or b.:
Removing the first cover metal layer using the masking and the wet-chemical etching step is a traditional procedure that does not require detailed explanation. For masking purposes, the cover metal layer can be coated with a photoresist in a first step, for example, the photoresist being partially exposed and removed in the exposed regions with the aid of a developer solution, as was described in the introduction. The uncovered cover metal layer is then etched away in the wet-chemical etching step. This can be done, for example, with the aid of etching solutions based on copper chloride or ammonium persulfate.
Alternatively, the cover metal layer can be removed by the laser.
Preferred examples when removing the polymer material by the plasma
Further preferably, the method comprises one of steps a. and b.:
Preferably, a. and b. directly above are realized in combination with one another.
Particularly preferably, the process gas used for providing the plasma in the context comprises at least one of the reactive gases from the group comprising CF4, C3F8 and CHF3.
Etching by a plasma is also known. Process gasses that can convert the material to be etched into the gas phase are used during plasma etching. The gas enriched with the etched-away material is pumped away and fresh process gas is fed in. A continuous removal is thus achieved.
Particularly preferably, an inductively coupled plasma (ICP plasma) is used, for example, generated by an ICP generator with a DC bias.
The process gasses mentioned directly above are particularly well suited to the etching of the preferred polymer materials mentioned above.
It is of importance that in the at least one first partial area of the first substrate side, the base substrate consisting of the polymer material can make contact directly with the plasma, while the at least one second partial area of the first substrate side is covered with the cover metal layer. Generally, metals are etched more slowly than polymer materials by a plasma, particularly when the process gasses mentioned are used. Consequently, when the plasma is allowed to act, trenches form exclusively in the region of the at least one first partial area of the first substrate side, while the cover metal layer forms a barrier that shields the at least one second partial area from the plasma. The surface of the base substrate can thus be structured with trenches in a targeted manner.
During the plasma treatment, moreover, if appropriate, in the second partial area residual photoresist is also removed from the first substrate side. Consequently, separate resist stripping is not absolutely necessary.
Particularly preferably, the plasma is used in the context of an anisotropic etching process. Ideally, in this configuration, ions of the plasma are accelerated perpendicularly to the surface of the substrate to be etched. The accelerated ions provide for physical sputtering removal.
Examples of reactive ion etching (RIE) and of reactive ion beam etching (RIBE) are particularly suitable as an anisotropic etching process.
Accordingly, preferably, the method is distinguished by at least one of a. to c.:
Preferably, a. and b. directly above, particularly preferably a. to c. directly above, are realized in combination with one another.
Surprisingly, we found that the presence of the particulate fillers mentioned above has a particularly advantageous effect on the result of the material removed by the plasma.
By the plasma it is not only possible to introduce trenches into the first substrate side, rather it is also possible to deepen further trenches that already exist in the surface of the first substrate side, for example, to produce blind holes or even through holes in the base substrate. For this purpose, the method can comprise the additional steps a. and b.:
If a base substrate that has been pretreated in this way is subjected to the plasma treatment described, then the latter results in the described formation of the at least one trench in the at least one first partial area of the first substrate side; and where the trenches introduced into the base substrate during the pretreatment overlap the first partial region, they are deepened further by the plasma treatment.
Introducing the at least one trench in the context of the pretreatment is preferably likewise carried out by allowing a plasma to act, analogously to step c. To form a mask required for this purpose, the pretreatment can also comprise photolithographic steps.
Selection and application of the filling metal
Further preferably, the method comprises one of a. to c.:
Preferably, a. and b., in particular a. to c., directly above are realized in combination with one another.
Preferably, a thin layer composed of copper or a copper alloy is formed in the context of the metallization.
In a wet-chemical metallization, the metallization is effected by depositing copper from a solution, for example.
Filling with the filling metal is preferably carried out by electrochemical deposition. Particularly preferably, filling is carried out by a so-called via fill method that enables the deposition to be carried out primarily in the at least one trench and optionally in drilled holes or blind holes while simultaneously minimizing an undesired deposition on the first substrate side, while reinforcing the cover metal layer in the at least one second partial region.
A metallization layer applied over the whole area enables an electrical contacting of the first substrate side to be able to position there a cathodic contact for a subsequent electrochemical deposition and to ensure that the entire substrate side can be coated.
In principle, all metals and alloys from which conductor track structures can be produced on printed circuit boards are suitable as filling metal. It is particularly preferred, however, for
Further preferably, the method comprises one of a. or b.:
The etching step is, for example, a traditional etching step using a strong acid such as hydrochloric acid.
If the cover metal layer is removed mechanically, then the cover metal layer can be removed by polishing and/or by grinding, for example. The aim is to completely remove the cover metal layer in the at least one second partial region. It is only by this means that the formation of the conductor structure or of the part of the conductor structure is concluded.
Completely removing the cover metal layer preferably also comprises removing filling metal in the at least one partial area and optionally also in the area of the at least one trench insofar as the filling metal projects beyond the edge or the edges of the at least one trench.
Particularly advantageously, during the mechanical processing of the first substrate side, it is possible not only to remove the cover metal layer but at the same time also to accomplish the planarization of the first substrate side. The aim of the planarization is to level the first substrate side such that it has no conductor tracks projecting from the surface. Instead, the conductor structure is preferably sunk completely in the at least one trench.
In the base substrates treated in accordance with the method, the conductor structure formed lies within the base substrate in one plane. However, the method also enables the production of MLBs, that is to say of printed circuit boards comprising conductor structures in different planes.
There are three particularly preferred examples of producing MLBs:
In this example, the method is distinguished by the two additional steps a. and b.:
In Example 1 , step b. thus necessarily comprises in detail
Preferably, the substrate sides are treated successively.
In this example, the method according is distinguished by a. to c.:
In Example 2, step c. thus necessarily comprises in detail
partially removing the cover metal layer on the second substrate side while subdividing the second substrate side into at least one first partial region, in which the second substrate side is free of the cover metal layer, and into at least one second partial region, in which the second substrate side is covered with the cover metal layer,
allowing a plasma to act on the second substrate side, with the aid of which plasma in the at least one first partial area the polymer material is removed while forming at least one trench,
filling the at least one trench with a filling metal, and
completely removing the cover metal layers in the at least one second partial area while forming a conductor structure.
Example 3
In this example, the method is distinguished by additional a. to d.:
In Example 3, d. thus comprises in detail
partially removing the cover metal layer on the top side while subdividing the top side into at least one first partial region, in which the top side is free of the cover metal layer, and into at least one second partial region, in which the top side is covered with the cover metal layer,
allowing a plasma to act on the top side, with the aid of which plasma the polymer material is removed in the at least one first partial area while forming at least one trench,
filling the at least one trench with a filling metal, and
completely removing the cover metal layer in the at least one second partial area while forming the second conductor structure.
While Examples 1 and 2 result in a base substrate with both substrate sides thereof provided respectively with a conductor structure sunk in at least one trench, Example 3 results in a multilayer printed circuit board comprising at least two printed circuit board layers each having a conductor structure. It goes without saying that base substrates treated in accordance with Examples 1 and 2, in which a conductor structure is already found respectively on the first substrate side and on the second substrate side, can also serve as first printed circuit board layer and be processed further in accordance with Example 3.
Example 3 enables sequential construction of printed circuit boards having, in principle, as many printed circuit board layers as desired. In this regard, by way of example, a further insulation layer can be applied to the second conductor structure and be subjected to the same treatment as the insulation layer applied to the first conductor structure. These steps can be repeated as often as desired.
In Examples 1 and 2, the preferred developments already described above are each applicable for carrying out b. to f., the selection and constitution of the base substrates and the selection and the application of the cover metal layers and of the filling metal.
For Example 3, the insulation layer can be embodied like the base substrate provided in a. Particularly preferably, the insulation layer is a film composed of one of the electrically non-conductive organic polymer materials mentioned. The preferred developments disclosed in connection with the treatment of the base substrate are likewise applicable for the application, carried out in accordance with Example 3, on and the partial removal of the cover metal layer from the insulation layer, the plasma treatment, the filling of the at least one trench and the complete removal of the cover metal layer.
Application of the insulation layer in Example 3 is preferably carried out by lamination, coating or by adhesive bonding.
Printed circuit boards produced in accordance with Example 3 have conductor structures in different layers. To electrically connect the conductor structures to one another, vias are required. Procedures A and B are particularly preferred for forming the vias:
The method with Example 3 is distinguished by the two directly following additional a. and b.:
The via can thus be integrated elegantly into the method without significant additional outlay arising.
The drilled hole is particularly preferably a laser drilled hole.
Moreover, conductor structures formed in accordance with Examples 1 and 2 can also be electrically contacted analogously to this. If a conductor structure is formed on the second substrate side in accordance with Example 2 after allowing the plasma to act on the second substrate side, one or more of the trenches formed in the process can be connected to the conductor structure on the first substrate side by a drilled hole. The electrical contacting is then effected during the subsequent process of filling the drilled hole and the trenches with the filling metal.
The method in accordance with Example 3 comprises a. to f.:
As a result of allowing the plasma to act in f., the at least one trench introduced into the top side of the insulation layer in c. is deepened further, at least insofar as it lies in the at least one first partial region. Given a sufficiently long time of action, this can result in a perforation which, given corresponding metallization and/or filling with a filling metal, can serve to form a via contact, in particular to the first conductor structure.
The treatment of the top side of the insulation layer that is performed in c. is preferably effected by the above-described pretreatment of the base substrate.
Concluding method steps
Preferably, outer conductor structures formed in accordance with the method are coated with a solder resist for their protection. Free contacts can be coated with a noble metal, for example, with gold, silver or platinum.
Features and properties of printed circuit boards produced in accordance with the method
A printed circuit board produced in accordance with the above method is distinguished by:
Preferably, the printed circuit board is distinguished by a.:
Further printed circuit board layers can be provided. Preferably, the printed circuit board has between 2 and 20 printed circuit board layers.
Numerous preferred features relating to the printed circuit board produced and relating to the base substrate and the insulation layer have already been disclosed in the description of the method. It should be emphasized that the base substrate and the insulation layer are particularly preferably films.
It is thus possible to produce printed circuit boards with extremely high resolution in the μm range, specifically with less complexity and lower production costs in conjunction with a higher yield in comparison with what the prior art allows. The disadvantages of the subtractive and also additive and semi-additive methods are avoided.
The fact that the conductor structures are sunk in the base substrate has a positive effect during the production of MLBs, particularly in the sequential construction described. The pressures acting on the conductor structures when base substrates are joined together are comparatively low, which has a positive effect with regard to existing impedance and signal speed requirements. The fact that channels can be formed with extremely high precision by plasma etching also has a positive effect in this regard.
In principle, such channels would also be able to be formed with the aid of a laser. By comparison therewith, plasma etching affords the advantage that during plasma etching all channels and other trenches can be formed simultaneously and in one step, which is generally faster and more cost-effective by a multiple. Furthermore, higher resolutions can be obtained by plasma etching.
Further features, details and preferences are evident from the appended claims, the wording of which is incorporated by reference in the content of the description and to the drawing.
In accordance with
To form an MLB, in step J, an insulation layer 110 is laminated directly onto the substrate side 101a having the conductor structure 109. In step K, the top side 110a of the insulation layer is covered with a cover metal layer 111, which is partially removed again in steps L, M and N (analogously to steps C, D and E) by applying a resist 112 and partially removing the latter using an etching solution. The top side 110a of the insulation layer 110, originally completely covered with the cover metal layer 111, is now subdivided into the first partial regions 113, in which the top side is free of the cover metal layer 111, and into the second partial regions 114, in which the top side is still covered with the cover metal layer 111. In step O, a plasma is allowed to act on the top side 110a of the insulation layer 110. While the partial regions 114 are shielded from the plasma by the cover metal layer 111, in the partial regions 113 the plasma brings about material removal and consequently formation of the trenches 115. At the same time, the resist 112 is also completely removed in the process. In step P, one of the trenches 115 formed is connected by a drilled hole 116 to a trench 106 of the first conductor structure 109 that has already been filled with the filling metal 108. In step Q, the trenches 115 including the drilled hole 116 are metallized by sputtering, followed by filling the trenches 115 by electrochemical deposition of a filling metal 118 in step R. In step S, excess filling metal 118 is then mechanically removed together with the cover metal layer 111 in the partial regions 114. The conductor structure 119 sunk in the trenches 115 is formed in the process. In step T, a solder resist 120 is applied, followed by partial gold plating 121 of individual contacts of the conductor structure 119.
A reduction of the number of individual steps of the method described can be achieved by a structuring of the cover metal layers being performed by a laser instead of the photolithographic structuring of the cover metal layers 102 and 111 in steps C to E and L to N.
The base substrates illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 209 889.4 | Jul 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/067042 | 6/18/2020 | WO |
