Printed circuit board structure and manufacturing method

Abstract

A printed circuit board structure (10), in particular for an RF generator, comprises: a heat diffuser (14); a base printed circuit board, namely a base PCB (22), having an upper surface (18) and a lower surface (28), where the base PCB (22) comprises a metal layer (30) having at least one conductor line (24) and / or at least one contact pad (38) at the lower surface; a contact layer (26), wherein the contact layer (26) comprises an upper surface and a lower surface (20) in direct connection with the metal layer (30) of the base PCB (22); wherein the lower surface (20) of the contact layer (26) is bonded to the heat diffusion member (14) by a bonding layer (16).

Description

Technical Field

[0001] The present invention relates to a printed circuit board structure, in particular for a radio frequency (RF) generator, a method for manufacturing such a printed circuit board structure and an RF generator comprising such a printed circuit board structure. Background Art

[0002] The RF power stage of an RF generator typically consists of a power amplifier and a power combiner stage (if several power amplifiers are to be combined to obtain higher output power). These circuits contain planar inductors and power transmission lines that may become hot in operation. For example, RF generators rated to provide high power signals in excess of 1 kW typically contain power modules that may dissipate 20% or more of the rated power under certain conditions (e.g., in impedance mismatch applications). Such RF generators for frequencies in the range of 30 kHz to 300 MHz are often used in plasma power generation systems for semiconductor manufacturing (e.g., deposition and etching processes).

[0003] State-of-the-art power amplifier designs employ planar structures integrated into the metal layers of a printed circuit board (PCB). Excessive heat generated by such planar inductor or power transmission line structures during RF power operation must be removed by properly cooling the inductor or transmission line. Typically, this is accomplished by bonding the PCB to a heat spreader.

[0004] Due to the different coefficients of thermal expansion (CTE) of the PCB and the heat spreader, the joint between these materials is stressed when the temperature of the PCB heat spreader composite changes.

[0005] During manufacturing, the highest temperatures are applied during the soldering of electronic components. For industrial furnace soldering processes, the PCB is heated to temperatures between 200°C and 250°C. In conventional operation of an RF generator, the described PCB heat diffusion composite may experience temperature cycles between room temperature and temperatures much higher than 100°C. In both cases, soldering and thermal cycling in operation cause stresses in the joints between the different materials of the PCB and the heat diffusion, which may cause delamination (i.e., separation of the PCB from the heat diffusion) and / or cracks in the joint layer. Such structural defects affect the heat dissipation efficiency. Therefore, during operation, overheating and further damage to the circuit may occur.

[0006] In particular, soldering issues may be addressed by avoiding industrial processes that require heating of the entire PCB heat spreader compound.

[0007] Instead of using furnace soldering, components can be manually soldered one by one using a soldering iron. In this way, heat is only applied locally to the solder joint, and thermal stress is minimized. However, manual soldering lacks the reproducibility of automated processes and, if not performed correctly, can result in cold solder joints, component damage, or short circuits.

[0008] On the other hand, automated one-by-one welding processes (eg, automated iron welding or laser welding that may be applied to selectively weld individual components) are expensive and take longer than standard industrial processes.

[0009] Prior art

[0010] EP 2 725 880 A1 describes the integration of heat generating components with a planar circuit board.

[0011] US6653741B2 describes the use of special materials, in particular CTE-modifying components, to reduce or prevent damage due to thermal cycling. CTE-modifying components are special solder materials and are additional materials with an intermediate coefficient of thermal expansion between two extreme values.

[0012] WO2020040725A1 describes a multi-stack cooling structure using various elements and arrangements (such as frames) to achieve cooling of various components. In particular, a heat spreader made of adjacent copper plates and ceramic plates for cooling different parts of a circuit is described. Compared with the present invention, in the cited prior art, the RF structure to be cooled is formed in the bottom metal of the PCB, which is then directly bonded to the heat spreader. This structure can be further improved, especially with respect to the cooling requirements and reliability requirements associated with demanding applications (e.g., applications of RF generators). This will be further clarified in the following description. Summary of the invention

[0013] It is therefore an object of the present invention to provide a PCB structure having improved heat resistance, thereby increasing the life and reliability of the PCB heat spreader composite.

[0014] The abovementioned problem is solved by a printed circuit board structure according to claim 1 , a method for producing such a printed circuit board structure according to claim 13 and an RF generator according to claim 16 .

[0015] According to the present invention, a printed circuit board structure, in particular for an RF generator, is provided. The printed circuit board structure comprises a heat diffusion member having an upper surface and a lower surface. In addition, the printed circuit board structure comprises a base printed circuit board (PCB) having an upper surface and a lower surface. Wherein, according to the present invention, the PCB comprises a metal layer at least at its lower surface, the metal layer having at least one conductor line and / or at least one contact pad formed by the metal of the metal layer. Preferably, a plurality of conductor lines and / or contact pads are formed by the metal layer. In addition, the PCB structure comprises a contact layer, wherein the contact layer comprises an upper surface directly connected to the metal layer, and a lower surface. Wherein, the lower surface of the contact layer is bonded to the heat diffusion member through a bonding layer, and the bonding layer can be a liquid glue, a prepreg or a bonding film. Thus, the lower surface of the contact layer is directly attached to the upper surface of the heat diffusion member through the bonding layer to form the printed circuit board structure. Therefore, by the contact layer, the lower surface of the base PCB is covered, and the conductor lines and / or contact pads of the metal layer are encapsulated.

[0016] By adding an additional contact layer to the metal layer of the base PCB, the metal layer becomes an inner layer. Due to the nature of the material used to form the contact layer becoming soft during the lamination process, the height difference in the structured metal layer is eliminated by completely filling the gaps between the individual metal traces of at least one conductor line and / or at least one contact pad. Thus, a flat lower surface of the contact layer is provided. Therefore, when the base PCB including the contact layer is bonded to the heat diffusion member through the bonding layer, uniform bonding strength is ensured over the entire surface, thereby avoiding delamination and thermal cracks under the heat of welding or operation. Therefore, the life of the circuit can be improved. In addition, in the process of laminating the contact layer onto the base PCB, which is usually achieved by applying pressure and / or high temperature, uniform pressure can be applied to the contact layer so that a flat surface is produced. In this way, a stable and reliable connection between the contact layer and the metal layer is produced and bubbles / voids in the contact layer are avoided, otherwise the bubbles / voids will make the heat dissipation towards the heat diffusion member worse. Due to the separation of the steps of attaching the base PCB to the heat spreader by first adding an additional contact layer and then bonding the base PCB with the flat surface contact layer to the heat spreader, a stronger bond is achieved compared to conventional techniques, and delamination and cracks due to thermal effects can be reduced compared to the conventional case where the heat spreader is directly glued to the metal layer.

[0017] Preferably, the contact layer can be made of prepreg or non-reinforced adhesive. Prepreg describes a composite material made of reinforcing fibers that have been pre-impregnated with a resin system. Among them, the fibers of the prepreg can be unidirectionally arranged, can be used as a woven fabric or as layered unidirectionally arranged fibers. In addition, non-reinforced adhesives as sheets based on various resin systems can be used to form the contact layer. In a lamination process where pressure and heat are applied to the contact layer material, the contact layer is attached to the lower surface of the base PCB. In the case of prepreg as the contact layer material, the contact layer is cured and hardened in the process.

[0018] Preferably, the bonding layer is a prepreg or a non-reinforced adhesive sheet. Alternatively, a liquid glue system may be used, such as epoxy, acryl or polyurethane.

[0019] Preferably, the thickness of the resulting contact layer may be in the range from 30 μm to 500 μm, preferably between 50 μm and 300 μm. On the one hand, by these measures, a uniform and flat surface is provided at the lower surface of the contact layer by filling the gaps between the corresponding conductor lines and / or contact pads of the base PCB. On the other hand, sufficient heat transfer is still possible.

[0020] Preferably, the thickness of the contact layer is adapted to provide a planar or at least substantially planar lower surface with limited corrugations. As mentioned above, the thickness of the contact layer may be a compromise between the requirement to provide a flat and uniform surface at the lower surface of the contact layer and maintaining sufficient heat transfer from the base PCB to the heat spreader. In particular, the thickness is adjusted relative to the thickness of the (Cu) metal traces (e.g. at least one conductor line and / or at least one contact pad) of the metal layer of the base PCB such that the gaps between the traces are completely filled and the corrugations of the surface of the cured contact layer are less than 30 μm, preferably less than 15 μm.

[0021] Preferably, the contact layer may be chosen to be approximately 30%-200% thicker than the metal layer of the base PCB, preferably 40%-150% thicker, more preferably 50%-100% thicker.

[0022] For example, for a Cu thickness of 35 μm for the metal layer, the thickness of the contact layer may be chosen to be 60 μm, for example 1 layer of 106 type FR4 prepreg.

[0023] For a 70 μm thick Cu layer of the metal layer, the thickness of the contact layer may be 120 μm, for example two layers of 106 type FR4 prepreg.

[0024] For an even thicker 105 μm Cu layer of the metal layer, the thickness of the contact layer may be 180 μm, for example three layers of 106 type FR4 prepreg or one layer of 7628 type FR4 prepreg.

[0025] If the operating temperature of the PCB structure in the application is higher than 100° C., high temperature materials and / or prepregs may be selected for the PCB structure and for the contact layer or the base PCB.

[0026] For high frequencies in the GHz range, the material systems PTFE or polyimide with ceramic fillers can be used instead of the standard FR4 epoxy-based PCB material filled with glass fibers.

[0027] Preferably, the bonding layer between the contact layer and the heat diffusion member is two different layers from the contact layer. In particular, the contact layer is formed together in the process of manufacturing the base PCB. Subsequently, the adhesive of the bonding layer is applied to the upper surface of the contact layer and / or the heat diffusion member to connect the contact layer to the heat diffusion member. Therefore, the bonding layer and the contact layer are implemented in different steps of the process and are intended to achieve different functions.

[0028] The base PCB can be a one-layer structure, a two-layer structure, or a multi-layer structure. Depending on the complexity of the electronic circuit to be implemented, the wiring of the interconnections between the various electronic components may require a PCB with 3 or more metal layers, while simpler circuits can be implemented with 2 or even 1 metal layer.

[0029] The PCB structure with contact layer and base PCB according to the present invention can be produced in a standard PCB manufacturing process using standard steps like drilling, plating, applying photoresist, etching, removing photoresist, oxidizing copper, laminating prepreg and / or copper foil. For power electronic circuits according to the present invention, a ceramic or metal plate acting as a heat spreader can be added.

[0030] Preferably, the contact layer is added together with a metal (eg Cu) foil. In particular, the metal foil pressed onto the contact layer has a high roughness, preferably Rz between 5 μm and 20 μm.

[0031] For circuits with only ceramic heat spreaders, such as power combiners or hybrid combiners with only inductors or power lines to be cooled, the metal layer is completely etched away so that only the contact layer remains. In this way, the lower surface of the contact layer has acquired an embossed roughness corresponding to the roughness of the metal foil, which increases the surface area for bonding and promotes adhesion of the adhesive bonding layer to the heat spreader.

[0032] For circuits that require a ground connection, such as a power amplifier that requires a combined ceramic and copper heat spreader, the metal layer is etched away in most PCB areas, but in some areas where a conductive connection to the metal heat spreader is required, the contact pads are protected by the photoresist during etching and remain at the lower surface of the contact layer. The copper foil acts as an additional contact metal layer at the lower surface of the contact layer in these areas. In addition, in this variation, the roughness of the copper foil is retained on the surface of the contact layer, and the adhesion of the bonding layer to the contact layer is improved.

[0033] In an embodiment where a ground connection from a metal layer to a heat spreader is required, an additional process step in the manufacturing sequence is required. In such an embodiment, the contact layer and the additional metal foil are laminated to the base PCB in one step. The additional metal foil, which is now the bottom metal layer, is then patterned using standard photolithography and etching processes so that metal pads are left only in those areas where electrical contact with the heat spreader is required. Through holes are used to make electrical connections from other layers to these pads. As a result, most of the lower surface of the base PCB is covered by an exposed insulating contact layer, while in some limited areas, the metal pads provide electrical connections to the heat spreader. When the base PCB is later laminated to the heat spreader, the insulating bonding layer (e.g., epoxy prepreg or acrylic adhesive sheet) has cutouts at the locations of the contact pads. Appropriately sized pads of conductive bonding film are placed in these cutouts. When this combination of insulating layer and conductive adhesive layer is used to laminate the base PCB to the heat spreader, the contact pads are electrically connected to the heat spreader via the conductive bonding film, while all other parts of the circuit (particularly the planar inductor and / or power transmission lines encapsulated in the contact layer) are bonded to the heat spreader through the electrically insulating bonding layer.

[0034] Preferably, the heat diffuser is made of a ceramic material and / or a metal material. Metal heat diffusers are preferably used to cool discrete components or planar components, especially on the upper surface of a PCB, where the thickness of the base PCB is sufficient to achieve the required inductance of the planar inductor or the impedance of the transmission line. For other planar components that require higher inductance or impedance, especially for planar structures in the metal layer at the lower surface of the base PCB, non-conductive heat diffusers are used because their volume can store alternating magnetic fields. Therefore, preferably, heat diffusers made of ceramics with high thermal conductivity, such as aluminum oxide (Al2O3) or aluminum nitrate (AlN), are used. However, especially when a ground connection is required, a conductive heat diffuser can be used in at least part of the PCB structure, which is preferably made of copper or any other good thermal and electrical conductor. Therefore, the heat diffuser can be a ceramic substrate, a metal substrate or a combination thereof.

[0035] The heat spreader may be mounted to a heat sink. The heat sink may have integrated water cooling and / or may include fins exposed to airflow for cooling. In some embodiments, the heat spreader and the heat sink may be integrally formed as a heat spreader composite. In other embodiments, the heat spreader and the heat sink are two parts connected to each other.

[0036] Preferably, the at least one conductor line is a power transmission line or a power planar inductor. In particular, these types of conductor lines generate a lot of heat, which has to be transferred to the heat spreader. At the same time, local overheating may occur for these types of conductor lines. Therein, by using an additional contact layer, the adhesion of the base PCB to the heat spreader - in particular in areas where heat is added by the power line and / or the inductor - is improved and a reliable connection can be achieved, thereby also improving the heat transfer to the heat spreader.

[0037] Preferably, one or more electronic components are placed onto the upper surface of the base PCB and can be connected to at least one conductor line and / or at least one contact pad on the lower surface of the base PCB via through-holes. Such connections to the metal layer at the lower surface of the base PCB and to the heat spreader allow efficient removal of heat from the power components located on the upper surface. By using an array of many through-holes, the thermal resistance of such connections from the upper surface of the PCB to the heat spreader and thus to the heat sink can be reduced.

[0038] Before applying the contact layer to the metal layer using standard PCB manufacturing processes, the metal traces are also preferably treated with an inner layer bonding coating, which can be a conversion coating such as MultiBond TM or oxides, for achieving a high reliability bond between the metal and the prepreg. This compares favorably to previous approaches without a contact layer. In state of the art approaches, the uncovered metal on both sides of the PCB is treated with a PCB surface finish (e.g., ENIG, etc.) that is specifically targeted for good solderability, not particularly for good bonding characteristics.

[0039] Preferably, the base PCB may include at least one cutout to mount the electronic components directly onto the heat diffusion member. The cutout may extend from the upper surface of the PCB to the lower surface and the contact layer, exposing the heat diffusion member directly. Power components that need to be effectively cooled in a direct manner may be directly attached to the heat diffusion member. Such power components may be power transistors (e.g., LDMOS, VDMOS, bipolar transistors) or power diodes that generate more heat. For example, diodes for RF power amplifiers may generate about several watts of heat, LDMOS transistors may generate about 100 watts or more of heat, and typical power amplifier modules used in RF generators may generate hundreds of watts or more of heat. Components that generate less than 20W of heat may be assembled to the surface of the PCB, while components that generate more than 50W of heat may preferably be directly attached to the heat diffusion member. However, the present invention is not limited to these components as power components that are directly attached to the heat diffusion member. Other components (e.g., power resistors) may also be directly mounted to the heat diffusion member inside the cutout. Such a direct connection of components to the heat diffusion member may be achieved, for example, by welding, mechanical clamps, or screws. Depending on the type of direct connection, these components can be attached to the heat spreader before or after the base PCB is bonded to the heat spreader.

[0040] Preferably, the base PCB comprises at least one expansion slot. For large PCBs, e.g. more than 10 cm in length or more than 100 cm 2 The area of ​​the base PCB and the heat diffusion piece, the thermal stress caused by the different thermal expansion coefficients of the base PCB and the heat diffusion piece may even exceed the improved bonding strength between the base PCB and the heat diffusion piece provided by the additional contact layer. In order to prevent delamination, the thermomechanical stress can be reduced by using expansion slits in the base PCB. Such expansion slits can be realized by milling small strips of the base PCB at suitable positions, and the suitable positions are preferably distributed at several positions on the entire PCB area. Therein, the expansion slits can partially or completely form a gap between the upper surface and the lower surface of the base PCB and can extend through the contact layer. Therefore, by means of the expansion slits, the overall thermomechanical forces are reduced by dividing the overall thermomechanical forces into multiple smaller areas, thereby preventing cracks in the bonding layer and delamination between the base PCB and the heat diffusion piece.

[0041] Preferably, more than one expansion slot is provided in the base PCB and the contact layer, wherein in particular at least two expansion slots are arranged perpendicular to each other.Thus, thermal expansion in two directions in the plane of the base PCB can be reduced by the perpendicular expansion slots.

[0042] Preferably, the expansion slits segment the base PCB into more than one segment. Thus, the total thermomechanical forces can be reduced to an acceptable value by dividing it into individual segments, thereby avoiding cracks or delamination.

[0043] Preferably, the segment size is less than 50 cm 2 , more preferably less than 30 cm 2 , and most preferably less than 15 cm 2 However, the exact number of segments of allowable size depends on the difference in coefficient of thermal expansion between the base PCB and the heat spreader material and the expected amount of heat to be dissipated by each segment.

[0044] In another aspect of the present invention, there is provided a method for manufacturing a printed circuit board structure, i.e., a PCB structure, comprising:

[0045] - providing a base PCB having an upper surface and a lower surface, wherein the metal layer at the lower surface comprises at least one conductor line and / or at least one contact pad;

[0046] - laminating a contact layer onto the metal layer, the contact layer having an upper surface directly connected to the metal layer of the base PCB, and a lower surface;

[0047] - Bonding the lower surface of the contact layer to the heat diffusion member through the bonding layer to form a PCB structure.

[0048] In a first step, a base PCB having an upper surface and a lower surface is provided. A metal layer (e.g. made of copper) is provided at the lower surface. The metal layer is structured to provide at least one conductor line and / or at least one contact pad in the metal layer. Preferably, a plurality of conductor lines and / or contact pads are formed by the metal layer. The construction of the metal layer can be accomplished by any conventional process known in the art.

[0049] Subsequently, a contact layer, preferably made of a reinforced adhesive (e.g., prepreg) or a non-reinforced adhesive, is laminated onto the metal layer. Thus, the upper surface of the contact layer is directly connected to the metal layer of the base PCB. In addition, the contact layer has an opposing lower surface. During the pressing step of the lamination process, uniform pressure is applied to the base PCB and the contact layer in order to ensure a reliable connection between those layers. Preferably, the pressing step is performed under vacuum to avoid bubbles / voids in the contact layer or any other layer of the base PCB.

[0050] In an additional step, the lower surface of the contact layer is bonded to the heat spreader through a bonding layer, which may also involve a pressing process at high temperature. Thus, the lower surface of the contact layer is bonded to the upper surface of the heat spreader, thereby connecting the base PCB and the contact layer to the heat spreader.

[0051] The described method provides a PCB structure comprising a base PCB, a contact layer, a bonding layer and a heat spreader, wherein the upper surface of the base PCB can be provided with electronic components for soldering advantageously in one step in an industrial furnace process.

[0052] Preferably, the method is also constructed together with the features described in conjunction with the printed circuit board configuration as described above.

[0053] Preferably, cutouts for the electronic components are cut or milled in the base PCB and preferably in the contact layer before bonding the contact layer to the heat spreader.

[0054] Preferably, at least one expansion slot is cut or milled in the PCB and preferably in the contact layer before bonding the contact layer to the heat spreader.Wherein, the milling of the expansion slot can be done in the same step as cutting the cutouts for the electronic components.

[0055] Preferably, after bonding the contact layer to the heat spreader, the electrical components are soldered to the upper surface of the base PCB and / or to the heat spreader within the cutout. During the soldering step, high temperatures, typically between 200° C. and 250° C., are applied to the PCB structure in a reflow oven. Thereby, delamination of the base PCB and the heat spreader or cracks in the structure of the base PCB are avoided, since the additional contact layer provides significantly higher peel strength.

[0056] In another aspect of the present invention, an RF generator for generating a high-power RF signal is provided, which comprises the PCB structure as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Hereinafter, the present invention is described in more detail with reference to the accompanying drawings.

[0058] The accompanying drawings show:

[0059] Figure 1 A first embodiment of a PCB structure according to the present invention is shown;

[0060] Figure 2A An example of a PCB structure according to the present invention is shown, which has electronic components and is in thermal contact with a heat spreader;

[0061] Figure 2B Show Figure 2A A detailed view of an example of

[0062] Figure 3 A flow chart showing a method of manufacturing a PCB structure according to the present invention; and

[0063] Figure 4 A top view of an exemplary PCB structure according to the present invention is shown. DETAILED DESCRIPTION

[0064] Referring to the PCB structure 10 shown Figure 1PCB structure 10 includes base PCB 22, contact layer 26 bonded to heat diffusion member 14 via bonding layer 16. Heat diffusion member 14 may be made of non-conductive material (such as ceramic material) or conductive material (such as copper). In addition, a combination of different materials may be used for one heat diffusion member 14. Figure 1 , the heat spreader 14 is shown as a simple substrate, while Figure 2A A multi-substrate heat spreader is shown. The base PCB 22 includes an upper surface 18 and a lower surface 28. Among them, the contact layer 26 is laminated to the lower surface 28 of the base PCB 22 and bonded to the heat spreader 14. Preferably, the upper surface of the heat spreader 14 is bonded to the lower surface 20 of the contact layer 26. At the lower surface 28 of the base PCB 22, a metal layer 30 is arranged. The metal layer 30 can be made of copper. The metal layer 30 can be structured as follows Figure 1 and Figure 2A One or more conductor lines 24 shown and / or as Figure 2A The contact pads 38 shown. The conductor lines 24 are covered by an additional contact layer 26, preferably made of a reinforced adhesive (e.g., prepreg) or a non-reinforced adhesive. By adding an additional prepreg or non-reinforced adhesive layer to the metal layer 30, the metal layer 30 becomes an inner layer. Due to the nature of the contact layer material becoming soft during the lamination process, the height differences in the structured metal layer 30 are eliminated by completely filling the gaps 25 between the individual copper traces or conductor lines 24. The amount of contact layer material must be sufficient to completely encapsulate the entire metal layer 30 and, through compression during lamination, produce a flat lower surface 20 of the resulting contact layer with limited grooves of less than 30 μm and preferably less than 15 μm as previously described. Since the conductor lines 24 typically have a height of about 20 μm to 100 μm and more preferably between 25 μm and 40 μm, the contact layer 26 can have a thickness of between 0.03 mm and 0.5 mm. More preferably, the contact layer 26 has a thickness of between 0.05 mm and 0.3 mm. By these measures, on the one hand, a uniform and flat surface is provided at the lower surface 20 of the contact layer 26 by filling the gaps 25 between the respective conductor lines 24 and / or contact pads 38. On the other hand, sufficient heat transfer is still possible.

[0065] The advantage of this additional contact layer 26 is that it provides a consistent and uniform surface to which the typically ceramic substrate 34 or metal substrate 36 (e.g. copper substrate) of the heat spreader 14 can be bonded by using a bonding layer 16. This bonding layer 16 can be a liquid glue or preferably a reinforced or non-reinforced adhesive sheet.

[0066] In contrast, in the prior art, the base PCB is bonded to the heat spreader, i.e., a ceramic or copper substrate, with an exposed bottom metal layer. This can result in unacceptably poor adhesion strength of the bonding layer. In particular, incomplete filling of gaps between conductor lines and / or surface finish of the exposed bottom metal can result in poor adhesion of the bonding layer, which can lead to delamination and other reliability issues, especially due to temperature variations during soldering and / or during operation of high electrical power devices implemented on prior art PCB assemblies.

[0067] Still referring to the prior art, the surface of the outer metal layer of a PCB is usually polished in an electrochemical (e.g., electroplating) process, which adds an additional metal such as gold to prevent oxidation of the metal layer, which is usually made of Cu. The adhesion of the glue to the metal surface treated in this way depends on the surface polishing process. The polishing process is usually performed by the PCB manufacturer, who must ensure that the resulting metal surface is compatible with the soldering process. However, the applicable surface specifications for soldering compatibility are not specifically aimed at providing a surface with particularly good adhesion.

[0068] The above-mentioned disadvantages of the prior art process are mainly overcome by the additional contact layer 26 of the present invention. As with conventional PCB manufacturing of inner layers of multilayer boards, the same established processes for processing the metal layer 30 can be used to obtain optimal adhesion of the contact layer material to the metal of the metal layer and the exposed surface of the base PCB.

[0069] Therefore, the delamination problem caused by the bonding defects between the base PCB 22 and the heat diffusion member 14 (or the substrate 34, 36 of the heat diffusion member 14) is eliminated by adding the additional contact layer 26 to the metal layer 30 at the lower surface of the base PCB 22. In particular, due to the introduction of the additional contact layer 26, the risk of delamination of the base PCB 22 and the heat diffusion member 14 (or the substrate 34, 36 of the heat diffusion member 14) during the soldering of the electronic components to the top metal layer 31 is significantly reduced. The soldering of the electronic components 40, 46 to the PCB structure 10 can be performed in a conventional industrial furnace process, which heats the entire PCB structure and solders all components in one step.

[0070] The use of the present invention has the following benefits for the manufacture and use of RF power circuits:

[0071] ● Electronic components can be assembled onto the composite of the base PCB and heat spreader using standard industrial (furnace) soldering processes, significantly reducing the risk of delamination between the heat spreader and the base PCB.

[0072] • Since the bond is stronger, the bond lasts longer in thermal cycling or normal power cycling operations.

[0073] Reference Figure 2A, an example of a PCB structure 10 according to the present invention is provided, which has electronic components and is in thermal contact with a heat sink 37. In which the same or similar features are denoted by the same reference numerals.

[0074] Figure 2A The electronic circuit of FIG. 1 shows another variation of the PCB structure 10 according to the present invention, which comprises a base PCB 22. An additional contact layer 26 is connected to the lower surface 28 of the base PCB 22. Furthermore, the contact layer 26 is connected to the heat spreader 14 via the bonding layer 16.

[0075] The base PCB 22 comprises a metal layer 30, wherein the metal layer 30 is structured to provide a conductor line 24, which may be a transmission line or a planar inductor. Alternatively or additionally, the metal layer 30 may be structured to provide a contact pad 38. The structure of the metal layer 30 is encapsulated by a contact layer 26. By means of the contact layer 26, the height difference between the structure of the metal layer 30 and the surface of the base PCB without metal disappears, so as to provide a flat and uniform lower surface 20 of the contact layer 26. Thereby, a good surface match is provided between the lower surface 20 of the contact layer 26 and the upper surface of the heat spreader 14.

[0076] The heat diffusion member 14 may include a non-conductive ceramic substrate 34 (ceramic heat diffusion member) to provide a space for a magnetic field between the conductor line 24 of the metal layer 30 and the heat diffusion member 14. In addition, the heat diffusion member 14 may include a metal substrate 36 (metal heat diffusion member) which may be connected to a ground potential (not shown).

[0077] In areas of the planar inductor structure 24 (conductor lines 24) where additional space is needed for the magnetic field (in the non-conductive heat spreader 34), a non-conductive bonding layer 16 is used. Figure 2B In the area of ​​the metal substrate 36 that needs to be grounded and connected to the heat diffusion member 37 via the metal substrate 36, a conductive bonding film 60 is locally used in a limited area (for example, below the contact pad 38 of the metal layer 30). The contact pad 38 is connected to the contact metal layer 33 at the lower surface of the contact layer 26 through the through hole 49. In the area of ​​the metal substrate 36 that does not require electrical connection, a non-conductive bonding layer 16 can be used.

[0078] On the upper surface 18 of the resulting PCB structure 10, electronic components 40, 46 can be attached to solder pads 42, 48 and electrically connected through conductor lines 44 arranged in the top metal layer 31 on the upper surface 18 of the PCB structure 10 or in the inner metal layer (not shown) of the base PCB 22. Among them, especially for power components 46 such as power transistors (e.g., LDMOS, VDMOS, bipolar transistors), power diodes or power resistors, the solder pads 48 can be connected to the contact pads 38 through a through hole or an array of through holes 49 to be grounded and / or cooled at the lower surface 28 of the base PCB 22, and the through hole or the array of through holes 49 is plated with metal or filled with metal to provide electrical and thermal connections to the contact pads 38.

[0079] The base PCB 22 and preferably the contact layer 26 may include cutouts 50, wherein in these cutouts 50, power components 52 such as power transistors (e.g., LDMOS, VDMOS, bipolar transistors) may be mounted directly onto the heat spreader 14. Therein, the power components 52 may be connected to the upper surface 18 of the base PCB 22 via connection elements 54 soldered 56 to the metal layer 31 on the upper surface 18 of the PCB structure 10.

[0080] If it is desired to directly connect the contact pads 38 at the lower surface of the base PCB 22 to the heat spreader 14 , particularly in the area of ​​the metal substrate 36 , the bonding layer 16 may include cutouts 58 filled with a conductive bonding film 60 .

[0081] Furthermore, the base PCB 22 and preferably the contact layer 26 may include expansion slots 62 to reduce the thermomechanical forces between the base PCB 22 and the bonding layer 16. Figure 4 , which shows a schematic top view of a PCB structure 10 having a heat-generating metal structure 68 and a plurality of expansion slits 62A and 62B. Therein, in order to reduce the thermomechanical forces in two directions in the plane of the PCB structure 10, the expansion slits 62A, 62B are arranged perpendicular to each other. By means of the thermal expansion slits 62A, 62B, the PCB structure 10 is segmented into different segments 66, thereby reducing the absolute value of the thermomechanical forces during the welding and operation of the PCB 64. The width of the expansion slits may be in the range of 0.5 mm to 5 mm, more preferably between 0.5 mm and 3 mm, and most preferably between 1 mm and 2 mm. By means of the expansion slits 62A, 62B, the PCB structure 10 may be segmented into segments having a width of less than 50 cm 2 , more preferably less than 30cm 2 , and most preferably less than 15 cm 2However, the exact number of segments of the allowed size depends on the PCB layout. The slits are preferably located in areas free of components and PCB tracks.

[0082] Reference Figure 3 , showing a method for manufacturing a PCB structure according to the present invention.

[0083] In step S01 , a base PCB 22 having an upper surface 18 and a lower surface is provided, wherein a metal layer 30 at the lower surface 28 has at least one conductor line ( 24 ) and / or at least one contact pad ( 38 ).

[0084] In step S02, a contact layer (26) is laminated onto the metal layer, the contact layer having an upper surface directly connected to the metal layer of the base PCB (22), and a lower surface (20). By means of the contact layer 26, a flat and uniform lower surface 20 is provided.

[0085] In step S03 , the lower surface 20 of the contact layer 26 is bonded to the heat diffusion member 14 through the bonding layer 16 to form the PCB structure 10 .

[0086] These process steps provide a PCB structure 10 to which electronic components 46, 40 can be soldered to solder pads 42, 48 on the upper surface 18 of the PCB structure 10. Soldering can be done in an industrial furnace process, and preferably in one step. Due to the contact layer 26, the bond strength between the base PCB 22 and the heat spreader 14 is improved, and the risk of delamination during the furnace process is reduced.

[0087] One or more cutouts 50 may be introduced in the base PCB 22 and preferably in the contact layer 26 before bonding the contact layer 26 to the heat spreader 14. In the same or additional steps, one or more expansion slots 62 may be provided in the base PCB 22 and preferably in the contact layer 26.

[0088] Thus, by the present invention, a PCB structure 10 is provided with an additional contact layer 26 that improves the bonding strength between the base PCB 22 and the heat spreader 14. Thereby, delamination or cracks caused by different thermal expansion coefficients of the base PCB 22 and the heat spreader 14 can be significantly reduced. Furthermore, by introducing expansion slots 62 into the base PCB 22 and preferably into the contact layer 26, stress caused by thermal expansion can be further reduced.

[0089] List of reference numerals:

[0090] 10: PCB structure

[0091] 14: Heat diffusion parts

[0092] 16: Bonding layer

[0093] 18: Upper surface of base PCB

[0094] 20: Lower surface of contact layer

[0095] 22: Basic PCB

[0096] 24: Conductor wire

[0097] 25: Clearance between metal traces

[0098] 26: Contact layer

[0099] 28: Bottom surface of base PCB

[0100] 30: Metal layer

[0101] 31: Top metal layer

[0102] 33: Contact metal layer

[0103] 34: Ceramic substrate

[0104] 36: Metal substrate

[0105] 37: Radiator

[0106] 38: Contact pad

[0107] 40: Electronic components

[0108] 42: Soldering pad

[0109] 44: Conductor wire

[0110] 46: Power Electronics

[0111] 48: Soldering pad

[0112] 49: Through hole

[0113] 50: Incision

[0114] 52: Power components

[0115] 54: Connecting elements

[0116] 56: Soldering pad

[0117] 58: Cutouts in the bonding layer

[0118] 60: Conductive bonding film

[0119] 62, 62A, 62B: Expansion Slits

[0120] 66: PCB segmentation

[0121] 68: Heating metal structure

Claims

1. A printed circuit board structure (10), in particular for a radio frequency generator, the printed circuit board structure (10) comprising: A heat diffuser (14); A base printed circuit board, i.e. a base PCB (22), having an upper surface (18) and a lower surface (28), wherein the base PCB (22) comprises a metal layer (30) having at least one conductor line (24) and / or at least one contact pad (38) at the lower surface; and a contact layer (26), wherein the contact layer (26) comprises an upper surface directly connected to the metal layer (30) of the base PCB (22) and comprises a lower surface (20); The lower surface (20) of the contact layer (26) is bonded to the heat diffusion member (14) via a bonding layer (16).

2. The printed circuit board structure (10) according to claim 1, wherein the contact layer (26) is made of prepreg or non-reinforced adhesive.

3. The printed circuit board structure (10) according to claim 1 or 2, wherein the bonding layer (16) comprises a resin-based adhesive.

4. The printed circuit board structure (10) according to any one of claims 1 to 3, wherein the bonding layer (16) comprises an adhesive sheet.

5. The printed circuit board structure (10) according to any one of claims 1 to 4, wherein the bonding layer (16) and the contact layer (26) of the PCB structure (10) are two different layers.

6. The printed circuit board structure (10) according to any one of claims 1 to 5, wherein the thickness of the contact layer (26) is between 0.03 mm and 0.5 mm, more preferably between 0.05 mm and 0.3 mm.

7. A printed circuit board structure (10) according to any one of claims 1 to 6, wherein the thickness of the contact layer (26) is adapted to provide a planar lower surface (20).

8. The printed circuit board structure (10) according to any one of claims 1 to 7, wherein the lower surface (20) of the contact layer (26) comprises a contact metal layer (33).

9. The printed circuit board structure (10) according to claim 8, wherein the bonding layer (16) has at least one cutout (58) for accommodating a piece of conductive bonding film (60) and providing electrical connection between the contact metal layer (33) and the heat diffusion member (14).

10. The printed circuit board structure (10) according to any one of claims 1 to 9, wherein the PCB comprises at least one expansion slot (62, 62A, 62B).

11. The printed circuit board structure (10) according to claim 10, comprising more than one expansion slot (62, 62A, 62B), wherein preferably at least two expansion slots (62A, 62B) are arranged perpendicular to each other.

12. The printed circuit board structure (10) according to claim 10 or 11, wherein the PCB structure (10) is segmented by the more than one expansion slots (62, 62A, 62B), and wherein preferably, the size of the segments is less than 50 cm 2 , more preferably less than 30 cm 2 , and most preferably less than 15 cm 2 .

13. A method for manufacturing a printed circuit board structure (10), the printed circuit board structure being a PCB structure, the method comprising: Providing (S01) a base PCB (22) having an upper surface (18) and a lower surface (28), wherein a metal layer (30) at the lower surface (28) has at least one conductor line (24) and / or at least one contact pad (38); laminating (S02) a contact layer (26) onto the metal layer (30), the contact layer (26) having an upper surface directly connected to the metal layer of the base PCB (22) and having a lower surface (20); The lower surface (20) of the contact layer (26) is bonded (S03) to the heat diffusion member (14) via a bonding layer (16) to form the PCB structure (10).

14. The method according to claim 13, wherein at least one expansion slot (62, 62A, 62B) is milled in the base PCB (22) and preferably in the contact layer (26) before joining the base PCB with the contact layer (26) to the heat spreader (14).

15. A method according to claim 13 or 14, wherein the PCB structure is constructed according to any one of claims 1-12.

16. A radio frequency generator for generating a high-power radio frequency signal, the radio frequency generator comprising a PCB structure according to any one of claims 1 to 12 or a PCB structure constructed by a method according to any one of claims 13 to 15.

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