Printed circuit board structure and manufacturing method, and RF generator

Abstract

In particular, a printed circuit board structure (10) for an RF generator includes a heat spreader (14), a base printed circuit board (22) having a top surface (18) and a bottom surface (28), and a contact layer (26), the base PCB (22) having a metal layer (30) on the bottom surface having at least one conductor line (24) and / or at least one contact pad (38), the contact layer (26) having a top surface that is directly connected to the metal layer (30) of the base PCB (22), and a bottom surface (20), the bottom surface (20) of the contact layer (26) being bonded to the heat spreader (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, to a method for manufacturing such a printed circuit board structure, and to an RF generator having such a printed circuit board structure. [Background technology]

[0002] The RF power stage of an RF generator usually consists of a power amplifier and a power combiner stage (when multiple power amplifiers are combined to obtain higher power). These circuits contain planar inductors and power transmission lines that can become hot during operation. As an example, RF generators rated to deliver high-power signals of more than 1 kW typically contain power modules that can consume more than 20% of their rated power under certain conditions, e.g. in applications with unmatched impedances. Such RF generators for frequencies from 30 kHz to 300 MHz are frequently used in plasma generation systems for semiconductor manufacturing, e.g. in deposition and etching processes.

[0003] State-of-the-art power amplifier designs employ planar structures embedded in the metal layers of a printed circuit board (PCB). Excess heat generated by such planar inductors and power transmission line structures during RF power operation must be removed by adequately cooling the inductors and transmission lines. Typically, this is done by bonding the PCB to a heat spreader.

[0004] Due to the different coefficients of thermal expansion (CTE) of the PCB and heat spreader, changes in temperature of the PCB-heat spreader composite create stresses in the bond between these materials.

[0005] In the manufacturing process, the highest temperatures are applied during soldering of electronic components. In industrial oven soldering processes, PCBs are heated to 200-250 °C. In normal operation in an RF generator, the described PCB heat spreader composite can experience temperature cycles from room temperature to well over 100 °C. In either case, soldering and thermal cycling during operation can create stresses in the bond between the different materials of the PCB and the heat spreader, leading to delamination, i.e., separation of the PCB and the heat spreader, and / or cracking of the bond layer. Such structural defects affect the heat removal efficiency. As a result, overheating can occur during operation, further damaging the circuitry.

[0006] In particular, the soldering problem can be solved by avoiding industrial processes that require heating the entire PCB heat spreader complex.

[0007] As an alternative to oven soldering, components can be hand soldered one at a time with a soldering iron. This method only localizes heat to the solder joint, minimizing thermal stress. However, hand soldering lacks the repeatability of an automated process and can result in cold solder joints, damaged components, or short circuits if done improperly.

[0008] On the other hand, automated piece-by-piece soldering processes, such as automated iron soldering or laser soldering, that can be applied to selectively solder individual components are more expensive and time consuming than standard industrial processes.

[0009] EP2725880A1 describes the integration of heat generating components with planar circuit boards.

[0010] US6653741B2 describes the use of special materials, in particular CTE-modified components, to reduce or prevent damage due to thermal cycling. The CTE-modified components are special solder materials and additional materials with a thermal expansion coefficient intermediate between two extremes.

[0011] WO2020040725A1 describes a multi-stack cooling structure using various elements and arrangements, such as frames, to achieve cooling of individual components. In particular, a heat spreader made of adjacent plates of copper and ceramic for cooling different parts of the circuit is described. In contrast to the present invention, in the referenced prior art, the RF structures to be cooled are formed in the bottom metal of the PCB, which is directly bonded to the heat spreader. Such structures can be further improved, especially with regard to cooling and reliability requirements related to demanding applications, such as RF generators. This will be made clearer in the following description. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] European Patent Application Publication No. 2725880 [Patent Document 2] U.S. Patent No. 6,653,741 [Patent Document 3] International Publication No. 2020 / 040725 Summary of the Invention [Problem to be solved by the invention]

[0013] SUMMARY OF THE PRESENT EMBODIMENT It is therefore an object of the present invention to provide a PCB structure with improved heat resistance that improves the life and reliability of the PCB heat spreader composite.

[0014] This 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. [Means for solving the problem]

[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 spreader having an upper surface and a lower surface. Furthermore, the printed circuit board structure comprises a basic printed circuit board (PCB) having an upper surface and a lower surface. Here, according to the present invention, the PCB comprises a metal layer at least on 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. Furthermore, the PCB structure comprises a contact layer, the contact layer having an upper surface directly connected to the metal layer, and a lower surface. Here, the lower surface of the contact layer is bonded to the heat spreader by a bonding layer, which may be a liquid adhesive, a prepreg, or a bonding film. Thereby, the lower surface of the contact layer is directly attached to the upper surface of the heat spreader by the bonding layer to form the printed circuit board structure. In this way, the contact layer covers the lower surface of the basic PCB and encapsulates the conductor lines and / or contact pads of the metal layer.

[0016] By adding an additional contact layer on the metal layer of the basic PCB, the metal layer becomes an internal layer. Due to the nature of the material used to form the contact layer, which becomes soft during the lamination process, the gaps between the individual metal traces of the at least one conductor line and / or at least one contact pad are completely filled and the height differences of the structured metal layer are leveled. A flat underside of the contact layer is thereby provided. Thus, when the basic PCB including the contact layer is bonded to the heat spreader by the bonding layer, a homogenous bonding strength is ensured over the entire surface, and thermal delamination and thermal cracking during soldering and operation are avoided. The life of the circuit can therefore be extended. Furthermore, during the lamination process of the contact layer to the basic PCB, which is usually achieved by applying pressure and / or high temperature, a homogenous pressure can be applied to the contact layer to form a flat surface. In this way, a stable and reliable connection between the contact layer and the metal layer is obtained and the occurrence of bubbles / cavities in the contact layer, which would worsen the heat dissipation to the heat spreader, is avoided. By separating the steps of attaching the basic PCB to the heat spreader by first adding an additional contact layer to the heat spreader and then bonding the basic PCB with a flat contact layer to the heat spreader, a much stronger bond is achieved compared to the prior art, reducing thermally-induced delamination and cracking compared to the traditional situation where the heat spreader is bonded directly to a metal layer.

[0017] Preferably, the contact layer is formed of prepreg or non-reinforced adhesive. Prepreg refers to a composite material made from reinforced fibers pre-impregnated with a resin system. Here, the fibers of the prepreg can be unidirectionally arranged as a woven fabric or as fibers unidirectionally arranged in layers. Also, non-reinforced adhesive as a sheet based on various resin systems can be used to form the contact layer. The contact layer is attached to the bottom surface of the basic PCB in a lamination process, in which pressure and heat are applied to the contact layer material. If a prepreg is used as the contact layer material, the contact layer is cured and solidified in this process.

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

[0019] Preferably, the contact layer obtained can have a thickness in the range of 30-500 μm, preferably 50-300 μm, whereby on the one hand, by these means, a homogeneous and flat surface is provided on the underside of the contact layer by filling the gaps between the respective conductor lines and / or contact pads of the basic printed PCB, and on the other hand, sufficient heat conduction is also possible.

[0020] Preferably, the thickness of the contact layer is adapted to provide a flat or at least substantially flat underside with limited corrugation. As mentioned above, the thickness of the contact layer may be a trade-off between the requirement to provide a flat and homogenous surface on the underside of the contact layer and the requirement to maintain sufficient heat transfer from the basic PCB to the heat spreader. In particular, the thickness is adjusted in relation to the thickness of the (Cu) metal traces of the metal layer of the basic PCB, i.e. the at least one conductor line and / or the at least one contact pad, such that the gaps between the traces are completely filled and the corrugation of the surface of the hardened contact layer is less than 30 μm, preferably less than 15 μm.

[0021] Preferably, the contact layers are selected to be approximately 30% to 200%, preferably 40% to 150%, more preferably 50% to 100% thicker than the metal layers of the base PCB.

[0022] For example, if the metal layer has a Cu thickness of 35 μm, the contact layer may have a thickness of 60 μm, for example one layer of type 106 FR4 prepreg.

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

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

[0025] If the operating temperature of the PCB structure in the application exceeds 100°C, high temperature materials and / or prepregs can be selected for the PCB structure as well as the contact layers or basic PCB.

[0026] At higher frequencies in the GHz range, ceramic filled PTFE or polyimide material systems can be used instead of the standard fiberglass filled FR4 epoxy resin based PCB material.

[0027] Preferably, the bonding layer between the contact layer and the heat spreader and the contact layer are two different layers. In particular, the contact layer is formed simultaneously in the process of manufacturing the basic PCB. Then, the bonding layer adhesive is applied on the top surface of the contact layer and / or the heat spreader to connect the contact layer and the heat spreader. In this way, the bonding layer and the contact layer are applied in different steps of the process and serve different functions.

[0028] A basic PCB can be one, two or even multiple layers. Depending on the complexity of the electronic circuit to be realized, a PCB with three or more metal layers may be required to set up the interconnections between the various electronic components, but simpler circuits can be realized with two or even a single metal layer.

[0029] The PCB structure with the contact layers and the basic PCB according to the invention can be provided in a standard PCB manufacturing process by using standard processes such as drilling, plating, applying photoresist, etching, removing photoresist, copper oxidation, lamination of prepreg and / or copper foil, etc. The power electronic circuit according to the invention may be supplemented with a ceramic or metal plate acting as a heat spreader.

[0030] Preferably, the contact layer is applied with a metal (e.g. Cu) foil. In particular, the metal foil pressed onto the contact layer has a high roughness, preferably Rz of 5 to 20 μm.

[0031] In circuits with only a ceramic heat spreader, such as a power combiner or a hybrid combiner with only inductors or power lines to be cooled, the metal layer is completely etched away, leaving only the contact layer, which transfers a roughness to the underside of the contact layer that corresponds to the roughness of the metal foil, increasing the surface area for bonding and facilitating the bonding of an adhesive bonding layer to the heat spreader.

[0032] In circuits requiring a ground connection, for example a power amplifier requiring a combined ceramic-copper heat spreader, the metal layer is etched away in most of the PCB area, but in some areas where a conductive connection to the metal heat spreader is required, contact pads are protected by photoresist during etching and remain on the underside of the contact layer. In these areas, the copper foil acts as an additional contact metal layer on the underside of the contact layer. Also, in this variant, the contact layer surface reflects the roughness of the copper foil, improving the adhesion of the bonding layer to the contact layer.

[0033] In embodiments where a ground connection from the metal layer to the heat spreader is required, an additional step is required in the manufacturing sequence. In these embodiments, the contact layer and the additional metal foil are laminated to the basic PCB in one step. The additional metal foil, which is now the bottom metal layer, is then patterned by standard lithography and etching steps to leave metal pads only where an electrical connection to the heat spreader is required. Electrical connections to these pads from other layers are made using vias. As a result, most of the bottom surface of the basic PCB is covered with an exposed insulating contact layer, and in some defined areas the metal pads provide electrical connection to the heat spreader. When the basic PCB is later laminated to the heat spreader, the insulating bonding layer, e.g. an epoxy prepreg or an acrylic adhesive sheet, has cuts at the locations of the contact pads. Pads of a conductive bonding film of appropriate size are placed in the cuts. When this combination of insulating and conductive adhesive layers is used to laminate the basic PCB to a heat spreader, the contact pads are electrically connected to the heat spreader via the conductive adhesive film, and all other parts of the circuitry encapsulated in the contact layer, in particular the planar inductors and / or power transmission lines, are bonded to the heat spreader by the electrically insulating adhesive layer.

[0034] Preferably, the heat spreader is made of ceramic and / or metallic materials. Metallic heat spreaders are preferred for cooling discrete or planar components, especially the top surface of a PCB where the thickness of the basic PCB is sufficient to achieve the required inductance or transmission line impedance for the planar inductor. For other planar components where a higher inductance or impedance is required, especially planar structures in the metal layer on the bottom surface of the basic PCB, electrically non-conductive heat spreaders are used, since their volume can store alternating magnetic fields. Thus, preferably, heat spreaders made of ceramics with high thermal conductivity, such as alumina (Al2O3) or aluminum nitrate (AlN), are used. However, especially when a connection to ground is required in at least a part of the PCB structure, a heat spreader preferably made of copper or other good thermal and electrical conductors can be used. Thus, the heat spreader may be a ceramic substrate, a metal substrate, or a combination of these.

[0035] The heat spreader may be attached to a heat sink, where the heat sink may have integrated water cooling and / or may have a radiator exposed to airflow for cooling. In some embodiments, the heat spreader and heat sink may be integrally formed as a heat spreader composite. In other embodiments, the heat spreader and heat sink are two pieces connected together.

[0036] Preferably, at least one conductor line is a power transmission line or a power plane inductor. In particular, these types of conductor lines generate a significant amount of heat that must be transferred to the heat spreader. At the same time, these types of conductor lines may experience local overheating. Here, the use of an additional contact layer improves the adhesion between the basic PCB and the heat spreader (especially in the areas where the heat is increased by the power lines and / or inductors), allowing for a reliable connection and thus also improving the heat transfer to the heat spreader.

[0037] Preferably, one or more electronic components may be disposed on the top surface of the basic PCB and connected by vias to at least one conductor line and / or at least one contact pad on the bottom surface of the basic PCB. Such connection to the metal layer and heat spreader on the bottom surface of the basic PCB allows efficient heat removal from the power components disposed on the top surface. The use of a large array of vias can reduce the thermal resistance of such connection from the top surface of the PCB to the heat spreader and thus to the heat sink.

[0038] Prior to applying a contact layer to the metal layers using standard PCB manufacturing processes, the metal traces are also preferably treated with an innerlayer bond coating, which may be a conversion coating such as Multibond™ or an oxide, to achieve a reliable bond between the metal and the prepreg. This is advantageous compared to traditional methods that do not use a contact layer. In state-of-the-art methods, the open metal on both sides of the PCB is treated with a PCB surface finish (e.g., ENIG or similar), which is specifically targeted for good solderability but not for particularly good bonding properties.

[0039] Preferably, the basic PCB may include at least one notch for mounting electronic components directly on the heat spreader, where the notch extends from the top to the bottom surface of the PCB and the contact layer directly exposes the heat spreader. Power components that require direct and effective cooling can be directly attached to the heat spreader. Such power components can be power transistors (e.g. LDMOS, VDMOS, bipolar transistors) or power diodes that generate larger amounts of heat. As examples, diodes used in RF power amplifiers can generate heat of the order of a few watts, LDMOS transistors can generate heat of 100 watts or more, and typical power amplifier modules used in RF generators can generate heat of several hundred watts or more. Components that generate less than 20 W of heat can be assembled on the surface of the PCB, while components that generate more than 50 W of heat can preferably be directly attached to the heat spreader. However, the invention is not limited to these components as power components that are directly attached to the heat spreader. Other components, such as power resistors, can also be directly attached to the heat spreader inside the notch. Such direct connection of components to the heat spreader can be achieved, for example, by soldering, mechanical clamps, or screws. Depending on the type of direct connection, the components can be attached to the heat spreader before or after gluing the base PCB to the heat spreader.

[0040] Preferably, the basic PCB includes at least one extended slit, e.g., a length of more than 10 cm or an area of ​​more than 100 cm. 2For larger PCBs, such as those exceeding 1000 Å, the thermal stresses due to the different thermal expansion coefficients of the base PCB and the heat spreader may exceed the improved bonding strength between the base PCB and the heat spreader provided by the additional contact layer. To prevent delamination, the thermomechanical stresses can be reduced by using extension slits in the base PCB. Such extension slits can be realized by milling small stripes in the base PCB at suitable locations, preferably distributed in several places over the entire PCB area. Here, the extension slits may form a gap between the top and bottom surfaces of the base PCB partially or completely and extend through the contact layer. Thus, with the extension slits, the overall thermomechanical forces are reduced by being divided into smaller areas, which prevents cracking of the bonding layer and delamination between the base PCB and the heat spreader.

[0041] Preferably, a plurality of expansion slits are provided in the basic PCB and in the contact layer, in particular at least two expansion slits arranged perpendicular to each other, in this way the perpendicular expansion slits make it possible to reduce the thermal expansion in two directions in the plane of the basic PCB.

[0042] Preferably, the basic PCB is divided into multiple segments by expansion slits, so that the overall thermo-mechanical force can be divided into individual segments, and cracks and delaminations can be avoided and reduced to acceptable values.

[0043] Preferably, the segment size is 50 cm 2 Less than or equal to 30cm, preferably 2 Less than 15cm, most preferably 2 However, the exact numerical allowable size of the segments depends on the difference in the thermal expansion coefficients of the base PCB and the heat spreader material, and on the expected heat dissipation by each segment.

[0044] In another aspect of the present invention, a method of manufacturing a printed circuit board (PCB) structure is provided, the method comprising: Providing a basic PCB having a top surface and a bottom surface, the metal layer of the bottom surface having at least one conductor line and / or at least one contact pad; Depositing a contact layer on the metal layer, the contact layer having a top surface and a bottom surface directly connected to the metal layer of the base PCB; The bottom surface of the contact layer is bonded to the heat spreader with a bonding layer to form a PCB structure. Includes.

[0045] In a first step, a basic PCB is provided having a top surface and a bottom surface. The bottom surface is provided with a metal layer, for example made of copper. The metal layer is configured to provide the metal layer with at least one conductor line and / or at least one conductor pad. Preferably, the metal layer forms a plurality of conductor lines and / or contact pads. The formation of the metal layer can be performed by any conventional process known in the art.

[0046] Thereafter, a contact layer, preferably formed of a reinforced adhesive, e.g. a prepreg or a non-reinforced adhesive, is laminated onto the metal layer. Thus, the top surface of the contact layer is directly connected to the metal layer of the basic PCB. Furthermore, the contact layer has an opposite bottom surface. In the pressing step of the lamination process, a homogenous pressure is applied to the basic PCB and the contact layer to ensure a reliable connection between these layers. Preferably, the pressing step is performed under vacuum to avoid the generation of bubbles / voids in the contact layer or other layers of the basic PCB.

[0047] An additional step is to bond the bottom surface of the contact layer to the heat spreader with a bonding layer, which may also include a pressing step at high temperature, which bonds the bottom surface of the contact layer to the top surface of the heat spreader, connecting the base PCB and the contact layer to the heat spreader.

[0048] The above-described method provides a PCB structure having a base PCB, contact layers, bonding layers, and a heat spreader on top of which electronic components can be placed for advantageous one-step soldering in an industrial oven process.

[0049] Preferably, the method further comprises the features described in relation to the printed circuit board configuration above.

[0050] Preferably, before gluing the contact layers to the heat spreader, recesses for the electronic components are cut or milled into the basic PCB and preferably into the contact layers.

[0051] Preferably, at least one extension slit is cut or milled into the PCB, and preferably into the contact layer, prior to bonding the contact layer to the heat spreader, whereby milling of the extension slit can be performed in the same process as cutting the notches for the electronic components.

[0052] Preferably, after bonding the contact layer to the heat spreader, the electrical components are soldered to the top surface of the basic PCB and / or to the heat spreader in the notches. The soldering process involves subjecting the PCB structure to high temperatures, typically between 200 and 250°C, in a reflow oven. This avoids delamination of the basic PCB and the heat spreader or cracks in the basic PCB structure, as the additional contact layer provides significantly higher peel strength.

[0053] In another aspect of the present invention, there is provided an RF generator for generating a high power RF signal, comprising a PCB structure as described above. [Brief description of the drawings]

[0054] In the following the invention will be explained in more detail with reference to the attached figures. [Figure 1] 1 is a first embodiment of a PCB structure according to the present invention; [Figure 2A]1 is an example of a PCB structure according to the present invention, with electronic components mounted thereon and in thermal contact with a heat sink. [Figure 2B] A detailed view of the example in FIG. 2A. [Diagram 3] 1 is a flow diagram of a method for manufacturing a PCB structure according to the present invention. [Figure 4] 1 is a top view of an exemplary PCB structure according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] Referring to FIG. 1 showing a PCB structure 10, the PCB structure 10 comprises a base PCB 22 and a contact layer 26 bonded onto a heat spreader 14 via a bonding layer 16. Here, the heat spreader 14 can be made of a non-conductive material such as a ceramic material, or may be made of a conductive material such as copper. It is also possible to combine different materials in one heat spreader 14. In FIG. 1, the heat spreader 14 is shown as a simple substrate, whereas FIG. 2A shows a multi-substrate heat spreader. The base PCB 22 has a top surface 18 and a bottom surface 28. Here, a contact layer 26 is laminated onto the bottom surface 28 of the base PCB 22 and bonded onto the heat spreader 14. Preferably, the top surface of the heat spreader 14 is bonded to the bottom surface 20 of the contact layer 26. A metal layer 30 is disposed on the bottom surface 28 of the base PCB 22. The metal layer 30 may be made of copper. The metal layer 30 may be configured as one or more conductor lines 24, as shown in Figures 1 and 2A, and / or as contact pads 38, as shown in Figure 2A. The conductor lines 24 are covered by an additional contact layer 26, preferably formed of a reinforced adhesive, e.g., a prepreg or non-reinforced adhesive. By adding an additional layer of prepreg or non-reinforced adhesive onto the metal layer 30, the metal layer 30 becomes an internal layer. Due to the nature of the contact layer material to soften during the lamination process, the height differences of the constructed metal layer 30 are leveled 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 due to compression during lamination, the resulting flat lower surface 20 of the contact layer has limited waviness of less than 30 μm, preferably less than 15 μm, as previously explained. Typically, the conductor lines 24 have a height of between about 20 μm and 100 μm, more preferably between 25 μm and 40 μm, so that 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.Here, by these measures, on the one hand, a homogeneous and flat surface is provided on the underside 20 of the contact layer 26 by filling the gaps 25 between the respective conductor lines 24 and / or between the conductor pads 38, while on the other hand sufficient heat transfer is still possible.

[0056] The advantage of this additional contact layer 26 is that it provides a uniform and homogenous surface to which a typical ceramic substrate 34 or metal substrate 36 (e.g., copper substrate) of the heat spreader 14 can be bonded using the bonding layer 16. This bonding layer 16 can be a liquid adhesive or, preferably, a reinforced or non-reinforced adhesive sheet.

[0057] In contrast, in the prior art, the basic PCB is bonded to the heat spreader, i.e., ceramic or copper substrate, with the bottom metal layer exposed. This can result in unacceptably low adhesive strength of the bonding layer. In particular, incomplete filling of the gaps between the conductor lines and / or surface finish of the exposed bottom metal can cause insufficient bonding of the bonding layer, resulting in delamination and other reliability issues, especially as a result of temperature changes during soldering and / or operation of high power devices mounted on the prior art PCB assemblies.

[0058] With reference to the prior art, the surface of the outer metal layer of a PCB is usually finished by an electrochemical (e.g. galvanic) process that adds an additional metal, such as gold, to prevent oxidation of the metal layer, which is typically formed of Cu. The adhesion of adhesives to such treated metal surfaces depends on the surface finishing process. The finishing process is usually performed by the PCB manufacturer, who must ensure that the resulting metal surface is compatible with the soldering process. However, the surface specifications that apply for soldering compatibility are not specifically aimed at providing a surface with particularly good adhesion.

[0059] The above-mentioned shortcomings of the prior art processes are primarily overcome by the additional contact layer 26 of the present invention. Established processes for processing the metal layer 30 can be used in the same manner as in normal PCB manufacturing of inner layers of multi-layer boards, such that optimal adhesion of the metal of the metal layer and the contact layer material to the exposed surfaces of the base PCB is obtained.

[0060] As a result, the problem of delamination due to imperfect bonding between the basic PCB 22 and the heat spreader 14 (or the substrates 34, 36 of the heat spreader 14) is eliminated by adding the additional contact layer 26 to the metal layer 30 on the lower surface of the basic PCB 22. In particular, the introduction of the additional contact layer 26 significantly reduces the risk of the basic PCB 22 delaminating from the heat spreader 14 (or the substrates 34, 36 of the heat spreader 14) during soldering of electronic components to the upper metal layer 31. Soldering of the electronic components 40, 46 to the PCB structure 10 can be performed in a normal industrial oven process, in which the entire PCB structure is heated and all components are soldered in one step.

[0061] The use of the present invention has the following advantages in the manufacture and use of RF power supply circuits: Standard industrial (oven) soldering processes can be used to assemble electronic components onto the base PCB and heat spreader composite, significantly reducing the risk of delamination between the heat spreader and the base PCB. Stronger bonds result in longer bond life under thermal or normal power cycling operation.

[0062] 2A, there is shown an example of a PCB structure 10 according to the present invention having electronic components disposed thereon and in thermal contact with a heat sink 37, where identical or similar elements are designated with the same reference numerals.

[0063] 2A shows another variant of the PCB structure 10 according to the invention, which comprises a basic PCB 22. An additional contact layer 26 is connected to the bottom surface 28 of the basic PCB 22. Furthermore, the contact layer 26 is connected to the heat spreader 14 via the bonding layer 16.

[0064] The base PCB 22 comprises a metal layer 30 configured to provide conductor lines 24 that can be transmission lines or planar inductors. Alternatively or additionally, the metal layer 30 may be configured to provide contact pads 38. The structures of the metal layer 30 are encapsulated by a contact layer 26. The contact layer 26 levels out the height difference between the structures of the metal layer 30 and the metal-free base PCB surface to provide a flat and uniform underside 20 of the contact layer 26. This provides a good surface match between the underside 20 of the contact layer 26 and the top surface of the heat spreader 14.

[0065] Here, the heat spreader 14 may include an electrically non-conductive ceramic substrate 34 (ceramic heat spreader) to provide a space for the magnetic field between the conductor wires 24 of the metal layer 30 and the heat spreader 14. Furthermore, the heat spreader 14 may include a metal substrate 36 (metal heat spreader) that can be connected to a ground potential (not shown).

[0066] In areas of the planar inductor structure 24 where additional space for the magnetic field (in the electrically non-conductive heat spreader 34) is required, an electrically non-conductive bonding layer 16 is used. Referring to Fig. 2B, in areas of the metal substrate 36 where a ground connection to a heat sink 37 via the metal substrate 36 is required, a conductive bonding layer 60 is used locally in defined areas, for example under a conductor pad 38 of the metal layer 30, where the conductor pad 38 is connected by a via 49 to the contact metal layer 33 on the underside of the contact layer 26. In areas of the metal substrate 36 where no electrical connection is required, an electrically non-conductive bonding layer 16 may be used.

[0067] At the top surface 18 of the resulting PCB structure 10, electronic components 40, 46 can be attached to solder pads 42, 48 and electrically connected by conductor lines 44 disposed on the top metal layer 31 of the top surface 18 of the PCB structure 10 or on an internal metal layer (not shown) of the basic PCB 22. Here, particularly for power components 46 such as power transistors (e.g., LDMOS, VDMOS, bipolar transistors), power diodes, or power resistors, the solder pads 48 may be connected to contact pads 38 for grounding and / or cooling at the bottom surface 28 of the basic PCB 22 by vias or an array of vias 49 that are metal plated or filled with metal to provide electrical and thermal connection to the contact pads 38.

[0068] The basic PCB 22 and preferably the contact layer 26 may have cutouts 50 in which power components 52, such as power transistors (e.g. LDMOS, VDMOS, bipolar transistors), may be mounted directly to the heat spreader 14. Here, the power components 52 may be connected to the top surface 18 of the basic PCB 22 via connection elements 54 to solder pads 56 of the metal layer 31 on the top surface 18 of the PCB structure 10.

[0069] If it is necessary to connect the contact pads 38 on the underside of the basic PCB 22 directly to the heat spreader 14, in particular in the area of ​​the metal substrate 36, the bonding layer 16 may have cuts 58 filled with a conductive bonding film 60.

[0070] Furthermore, the basic PCB 22 and preferably the contact layer 26 may include extension slits 62 to reduce the thermo-mechanical forces between the basic PCB 22 and the bonding layer 16. Reference is now made to FIG. 4, which shows a schematic top view of a PCB structure 10 having a heat-generating metal structure 68 and a number of extension slits 62A and 62B. Here, the extension slits 62A, 62B are arranged perpendicular to each other to reduce the thermo-mechanical forces in two directions in the plane of the PCB structure 10. The thermal extension slits 62A, 62B divide the PCB structure 10 into different segments 66, which reduces the absolute value of the thermo-mechanical forces during soldering and operation of the PCB 64. The width of the extension slits may range from 0.5 mm to 5 mm, more preferably from 0.5 mm to 3 mm, and most preferably from 1 mm to 2 mm. The extension slits 62A, 62B allow the PCB structure 10 to be divided ... 2 Less than 30cm, more preferably 2 Less than 15cm, most preferably 2 The slits can be divided into segments with an area of ​​less than 100 mm. However, the exact numerical value of the allowable size of the segments depends on the PCB layout. The slits are preferably placed in areas where there are no components or PCB tracks.

[0071] Referring to FIG. 3, a method for manufacturing a PCB structure according to the present invention is shown.

[0072] In step S01, a base PCB 22 is provided having a top surface 18 and a bottom surface 28, with a metal layer 30 on the bottom surface 28 having at least one conductor line (24) and / or at least one contact pad (38).

[0073] In step S02, a contact layer 26 is laminated onto a metal layer having a top surface and a bottom surface 20 that are directly connected to the metal layers of the base PCB 22. The contact layer 26 provides a flat, uniform bottom surface 20.

[0074] In step S03, the bottom surface 20 of the contact layer 26 is bonded to the heat spreader 14 by the bonding layer 16 to form the PCB structure 10.

[0075] These steps provide a PCB structure 10 in which electronic components 46, 40 are soldered to contact pads 42, 48 on the top surface 18 of the PCB structure 10. The soldering can be performed in an industrial oven process, and is preferably performed in one step. The contact layer 26 improves the bond strength between the base PCB 22 and the heat spreader 14 and reduces the risk of delamination during the oven process.

[0076] Prior to bonding the contact layer 26 to the heat spreader 14, one or more cuts 50 can be inserted into the base PCB 22 and preferably the contact layer 26. In the same step or an additional step, one or more expansion slits 62 can be provided in the base PCB 22 and preferably the contact layer 26.

[0077] Thus, the present invention provides a PCB structure 10 having an additional contact layer 26 that improves the bonding strength between the base PCB 22 and the heat spreader 14. This can significantly reduce delamination and cracks caused by different thermal expansion coefficients between the base PCB 22 and the heat spreader 14. Furthermore, by inserting the expansion slits 62 in the base PCB 22 and preferably in the contact layer 26, the thermal expansion induced stress can be further reduced. [Explanation of symbols]

[0078] 10:PCB structure 14: Heat spreader 16: Bonding layer 18: Top side of the basic PCB 20: Lower surface of contact layer 22: Basic PCB 24: Conductor wire 25: Gaps between metal traces 26: Contact layer 28: Bottom side of the basic PCB 30: Metal layer 31:Top metal layer 33: Contact metal layer 34: Ceramic substrate 36: Metal substrate 37: Heat sink 38: Contact pad 40: Electronic Components 42: Solder pad 44: Conductor wire 46: Power electronics parts 48: Solder pad 49: Beer 50: Cut 52: Power electronics parts 54: Connection element 56: Solder pad 58: Cutting of the bonding layer 60: Conductive adhesive film 62, 62A, 62B: Expanded slit 66: PCB segment 68: Heat-generating metal structure

Claims

1. Heat spreader, a base printed circuit board having a top surface and a bottom surface; a contact layer; the basic printed circuit board comprises on its lower surface a metal layer having at least one conductor line and / or at least one contact pad; the contact layer has an upper surface directly connected to the metal layer of the base printed circuit board, and a lower surface; The printed circuit board structure for an RF generator, wherein the bottom surface of the contact layer is adhered to the heat spreader by a bonding layer.

2. The printed circuit board structure of claim 1 , wherein the contact layer is formed of a prepreg or a non-reinforced adhesive.

3. The printed circuit board structure of claim 1 , wherein the bonding layer comprises a resin-based adhesive.

4. The printed circuit board structure of claim 1 , wherein the bonding layer comprises an adhesive sheet.

5. 10. The printed circuit board structure of claim 1, wherein the bonding layer and the contact layer are two different layers.

6. 10. The printed circuit board structure of claim 1, wherein the contact layer has a thickness between 0.03 mm and 0.5 mm or between 0.05 mm and 0.3 mm.

7. 7. The printed circuit board structure of claim 6, wherein the thickness of the contact layer is adapted to provide a planar lower surface.

8. The printed circuit board structure of claim 1 , wherein the lower surface of the contact layer comprises a contact metal layer.

9. 9. The printed circuit board structure of claim 8, wherein the bonding layer has at least one notch for receiving a conductive bonding film and providing an electrical connection between the contact metal layer and the heat spreader.

10. 10. The printed circuit board structure of claim 1, wherein the base printed circuit board has at least one expansion slit.

11. A printed circuit board structure as described in claim 1, wherein the basic printed circuit board has a plurality of expansion slits, at least two of the expansion slits being arranged perpendicular to each other.

12. The printed circuit board structure is segmented by the plurality of expansion slits, and the size of the segments is less than 50 cm. ２ , less than 30 cm ２ , or less than 15 cm ２ 12. The printed circuit board structure of claim 11, wherein:

13. Providing a basic printed circuit board having a top surface and a bottom surface having a metal layer with at least one conductive line and / or at least one contact pad; depositing a contact layer on the metal layer, the contact layer having an upper surface and a lower surface directly connected to the metal layer; A method of manufacturing a printed circuit board structure comprising bonding the underside of the contact layer to a heat spreader with a bonding layer.

14. 14. The method of claim 13, further comprising milling the base printed circuit board or the contact layer to form at least one expansion slit before bonding the bottom surface of the contact layer to the heat spreader.

15. 10. An RF generator for generating an RF signal, comprising the printed circuit board structure of claim 1.

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