Thin circuit board accessible from both sides

Abstract

Printed circuit board (100) of a double-sided type, comprising a top side (F1) and a bottom side (F2) intended for receiving electronic components or connections to external circuits; wherein the circuit board (100) comprises a single layer of conductive material (3) into which conductive traces (32) are etched and which is arranged between a first layer (2) of insulating material and a second layer (3) of insulating material; wherein the first layer (2) of insulating material has openings (25) to allow access to the layer (3) of conductive material corresponding to areas which must form uncovered areas in the printed circuit board (100) intended for soldering electronic components or connections to external circuits on the top side (F1) of the printed circuit board (100); and wherein the second layer (4) of insulating material has openings (45) to allow access to the layer (3) of conductive material corresponding to areas which must form uncovered areas in the printed circuit board (100) intended for soldering electronic components or connections to external circuits on the underside (F2) of the printed circuit board (100); characterized by the fact that the conductive layer (3) is a thin, flexible layer, and the first insulating layer (2) and the second insulating layer (4) are both thin, flexible layers to obtain a thin, flexible printed circuit board; and the conductive traces (32) of the conductive layer are produced by screen printing or photographic masking, by chemical etching of a pattern printed on the conductive layer or on a photosensitive film connected to the conductive layer.

Description

[0001] The present invention relates to the field of double-sided printed circuit boards, i.e., a printed circuit board with two sides on which electronic components can be soldered or connections to external circuits can be made. The present invention relates in particular to a printed circuit board with a single conductive layer that is accessible from both sides of the printed circuit board.

[0002] Printed circuit boards require at least one electronic component to be connected to at least one external connection. Generally, hundreds or even thousands of components need to be connected to each other and to external connections according to a defined circuit diagram.

[0003] The prior art in this field typically involves the production of a multilayer printed circuit board consisting of alternating, pressed-together insulating and conductive layers, with connections between the different conductive layers made by through holes or blind holes that are internally metallized using a method known to those skilled in the art. This method has the advantage of enabling the connection of a large number of electronic components to form even a very complex network of connections that constitute a functional circuit or a module of a device.

[0004] A second case concerns circuits designed for relatively high current flows, where the need to have access to the conductive layers from both sides of the printed circuit board is usually solved by using at least two conductive copper layers arranged on the outer sides, connected to each other by a large number of metallized interconnect holes.

[0005] The current method also has a number of disadvantages, including the complexity of the process, which involves dozens of steps such as laminating, etching, pressing, drilling, metallizing, screen printing, contouring, etc. Furthermore, the resulting circuit is rigid, and the internal metallization of the connection holes is delicate and difficult to control. According to the prior art, the circuit is fabricated starting with a coating of glass fiber-filled epoxy material, onto which two copper layers are deposited on opposite sides. After etching to create conductive traces on these copper layers, further insulating and conductive layers can be added, ultimately resulting in two outer conductive layers on which the electronic components are mounted and the connections are made.

[0006] With reference to Fig. 1 The printed circuit board (1) is manufactured starting from a layer (10) of insulating material as a substrate, onto which two layers (11, 12) of conductive material are laminated.

[0007] The layers (11, 12) of conductive material typically consist of annealed copper, but in some cases may consist of a different type of metal.

[0008] With reference to Fig. 2 at least one through hole (13) is made in the circuit board (1).

[0009] With reference to Fig. 3. The two layers (11, 12) of conductive material are electrically connected by metallizing (14) the inner walls of the through-hole (13). Subsequently, the circuit board (1) is subjected to a screen printing or photographic masking process, and conductive traces are etched onto the two layers (11, 12) of conductive material according to a defined circuit diagram.

[0010] With reference to Fig. Figure 4 finally shows that after etching the conductor tracks on the two layers (11, 12) of conductive material, two further layers (15, 16) of insulating material with protective functions can optionally be applied, leaving areas (17, 18) of conductive material uncovered on which electronic components can be soldered or connections to other circuits can be made.

[0011] This well-known process involves a sequence of chemical and electroplating processing steps with significant environmental impacts and therefore requires the purification and recovery or disposal of residues, the costs of which considerably affect the overall process. The connections between electronic components and external connections are typically made by soldering with a tin-based alloy, using various manual or automated methods, typically wave soldering, selective wave soldering, or the melting of a screen-printed solder paste, or by bonding with conductive resins.

[0012] In most practical applications, a high level of complexity in printed circuit boards (PCBs) is not always necessary. There are some applications where a PCB only needs to accommodate a few components, even though it is relatively large due to geometric constraints. In these cases, one is forced to use a relatively expensive technology in a large-format product, even though high complexity is not actually required. Examples of such cases include PCBs used to connect the terminals of photovoltaic solar cells or PCBs placed behind the dashboards of devices. In these cases, the PCB has geometric dimensions defined by mechanical or aesthetic requirements, but only needs to accommodate a small number of components and connections per unit area.In this type of application, in order to create relatively few connections over a large area, most of the copper surface is often removed during the etching phase, wasting a very expensive material.

[0013] US3072734A (Fox et al., 1963) and US2613252 (Heibel, 1952) disclose a double-sided printed circuit board with a layer of conductive material sandwiched between two layers of insulating material. The conductive layer is a thick, rigid layer in which holes and traces are produced by punching. The insulating layers are also rigid, and holes are produced in them by mechanical punching.

[0014] This results in a rigid, i.e., inflexible, circuit board with a large thickness.

[0015] Furthermore, the manufacturing process of this circuit, based on the mechanical stamping of the conductive layer and the insulating layers, can only be industrialized at high cost and for large-scale production, and is not suitable for miniaturized circuits or those with a complex design of the conductor tracks.

[0016] AT273542B (Stillit, 1963) and NL7601969 (Publisher, 1975) reveal circuit boards for games. These circuits are very simple, however, and are produced by shearing or punching to merely create a connection between two contact pads on the circuit, thus defining whether the answer is correct or incorrect in the game. Certainly, these circuit boards are not intended for the production of complex electronic circuits or for mounting electronic components.

[0017] One object of the present invention is to eliminate the disadvantages of the prior art by providing a double-sided printed circuit board that saves conductive metal material and can be industrialized for the production of any quantity, even for prototypes and small series, and is suitable for creating complex designs, miniaturizing the printed circuit board and attaching electronic components.

[0018] Another task is to provide a printed circuit board manufactured using a process that is simple, fast, efficient and reliable.

[0019] These problems are solved according to the invention with the features of the independent claims.

[0020] Further advantageous embodiments of the invention are evident from the dependent claims.

[0021] The proposed embodiment essentially reverses the order in which the printed circuit board is mounted and processed by placing the etched conductive layer in the center of the circuit board and laminating the insulating layers to both sides of the conductive layer.

[0022] The manufacturing process includes the following steps: - Preparing a first layer of insulating material by creating the necessary holes and slots to expose the conductive material to which the first layer of insulating material will be bonded; - Laminating a layer of conductive material with the first layer of insulating material to obtain a connected arrangement; - Etching of the conductive material to create conductor tracks according to a desired electrical scheme; - Drilling the laminate to create through holes for the installation of electronic components; - Preparing a second layer of insulating material by creating the necessary holes and slots to expose the conductive material; - Applying the second layer of insulating material onto the layer of conductive material, so that the layer of conductive material is positioned between the first and second layers of insulating material.

[0023] The special feature of the invention is that the layers of insulating material are flexible films of low thickness and the layer of conductive material is a flexible film of low thickness in order to obtain a flexible printed circuit board.

[0024] In addition, the conductive traces of the conductive layer are produced by chemically etching a pattern printed onto the conductive layer using screen printing or by photographic masking of a photosensitive film connected to the conductive layer.

[0025] As an alternative to the previous drilling and slotting, it is possible to drill or cut one or both of the two insulating layers by laser ablation, even after the coupling phase, to expose the conductive material if it is necessary to expose only small areas of the conductive layer for soldering electronic components or for areas intended for making external connections or to make changes to the already connected circuit board.

[0026] Further chemical or electroplating treatments are possible to achieve the desired level of surface protection of the conductive material, as well as further finishing processes according to known methods.

[0027] Further features of the invention will become apparent from the following detailed description, which relates to a purely exemplary and therefore non-limiting embodiment thereof, illustrated in the accompanying drawings, of which: Fig. 1, Fig. 2, Fig. 3 and Fig. Figure 4 shows sectional views illustrating the four steps involved in manufacturing a double-sided printed circuit board according to the state of the art; Fig. Figure 5 is a perspective view showing a first layer of insulating material used to manufacture a printed circuit board according to the invention; Fig. 6 is a cross-sectional view of the first layer of insulating material made of Fig. 5; Fig. Figure 7 is a sectional view showing the first layer of insulating material. Fig. Figure 6 shows which is connected to a layer of conductive material; Fig. Figure 8 is a perspective view of the connection from Fig. 7 in reverse orientation, showing the etching of conductor tracks on the conductive layer; Fig. Image 9 is a perspective view showing the laminate made of Fig. 8 and shows an exploded view of a second layer of insulating material; Fig. Figure 10 is a sectional view showing a printed circuit board according to the invention, which consists of the in Fig. The connection shown in 9 will be obtained; and Fig. 11 is a sectional view showing a printed circuit board similar to the one in Fig. Figure 10 shows the area on which some electronic components have been attached.

[0028] The printed circuit board according to the invention is manufactured using the Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9 to Fig. 10 described, the circuit board in Fig. 10 is shown and is designated overall with the reference number 100.

[0029] The terms "below" and "above" refer in the following to the arrangement of the items in Fig. The printed circuit board (100) shown in Figure 10 is double-sided and has a top side (F1) and a bottom side (F2). Electronic components or connections to external circuits are to be placed on each side (F1, F2) of the printed circuit board (100).

[0030] Referring to the Fig. 5 and Fig. 6. The printed circuit board (100) is manufactured starting from a first layer (2) of insulating material. The first layer (2) of insulating material has a top side (20) which is to face outwards and a bottom side (21) opposite the top side (20).

[0031] The first layer (2) of insulating material is a flexible layer with a thinness between 12 and 250 µm. The first layer (2) is a film made of polymer material, such as polyimide (PI), polyester (PL), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyvinyl fluoride (PVDF) or polytetrafluoroethylene (PTFE).

[0032] In the first layer (2) of insulating material, openings (25) are created that correspond to the areas on the printed circuit board (100) that are to serve as uncovered areas for soldering electronic components or connections to external circuits on the top surface (F1) of the printed circuit board (100). The openings (25) can include holes (22) and slots (23). The openings (25) can be produced by machining, chemical etching, or laser cutting. Advantageously, the openings (25) of the first insulating layer can be produced by chemical etching to follow a pattern corresponding to the conductive traces of the conductive layer (3).

[0033] Referring to Fig. 7. The first layer (2) of insulating material is connected to a layer (3) of conductive material to form a connected arrangement (5). The openings (25) in the first layer of insulating material make the layer (3) of conductive material accessible from the top (F1) of the printed circuit board.

[0034] The conductive material layer (3) has a top (30) and a bottom (31). Subsequently, the bottom (21) of the first insulating layer (2) is connected to the top (30) of the conductive material layer (3).

[0035] The conductive material layer (3) is a flexible layer with a small thickness between 17 and 105 µm. The conductive material layer (3) can be a copper layer.

[0036] After joining, the layer of conductive material (3) is etched to create conductive tracks (32). Fig. Figure 8 shows the conductive traces (32) on the underside (21) of the first layer of insulating material, which are obtained by etching the layer of conductive material (3). The conductive traces can be produced using methods such as screen printing or photographic masking, which enable the creation of complex patterns and an industrial mass production process of the printed circuit.

[0037] For example, a screen print of the pattern of the conductor tracks (32) can be applied to the layer of conductive material (3), followed by chemical etching of the layer of conductive material (3) using strong acids or alkalis.

[0038] As an alternative to screen printing, a light-sensitive protective film (photoresist) can be laminated onto the conductive layer (3), whereupon the light-sensitive film is exposed with an image of the desired circuit layout, subsequently the light-sensitive film is developed and the layer of conductive material (3) is chemically etched using strong acids or alkalis, leaving the conductive traces (32) behind.

[0039] Referring to Fig. 9 A second layer (4) of insulating material is prepared to be connected to the conductive layer (3) of the connected arrangement (5).

[0040] The second layer (4) of insulating material is a flexible layer with a thinness between 12 and 250 µm. The second layer (4) of insulating material is a film made of polymer material, such as polyimide (PI), polyester (PL), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyvinyl fluoride (PVDF) or polytetrafluoroethylene (PTFE).

[0041] The second layer (4) of insulating material has a bottom side (41) which is to face outwards and a top side (40) opposite the bottom side (41).

[0042] Openings (45) are created in the second layer (4) of insulating material to allow access to the layer (3) of conductive material corresponding to areas on the printed circuit board (100) for soldering electronic components or making connections to external circuits on the underside (F2) of the printed circuit board (100). The openings (45) of the second layer (4) of insulating material can include holes (42) and slots (43). The openings (45) of the second layer (4) of insulating material can be produced by machining, chemical etching, or laser cutting. Advantageously, the openings (45) of the second insulating layer can be produced by chemical etching to follow a pattern corresponding to the conductive traces of the conductive layer (3).

[0043] Referring to Fig. 10 The top (40) of the second layer (2) made of insulating material is connected to the bottom (31) of the layer (3) made of conductive material.

[0044] In this way, a flexible printed circuit board (100) with a small thickness between 41 and 605 µm is obtained.

[0045] The conductive layer (3) has through-holes (33) arranged to connect the openings (25) of the first insulating layer to corresponding openings (45) of the second insulating layer. Through-holes (33) may be required for mounting electronic or electrical components with pins or terminals, such as connectors. The through-holes (33) of the conductive layer can be produced by machining or laser cutting on the laminate (5) or on the printed circuit board (100).

[0046] Fig. Figure 10 shows an example where some holes (22) of the first insulating layer correspond to respective holes (42) of the second insulating layer. In this case, in addition to the conductive traces (32), through holes (33) were created in the conductive layer (3) that correspond to the holes (22) of the first insulating layer and the corresponding holes (42) of the second insulating layer.

[0047] In some cases it may be advantageous to produce the openings (25, 45) by laser ablation of the first layer (2) or the second layer (4) of insulating material in a subsequent step of the production of the already connected printed circuit board (100).

[0048] Fig. Figure 11 shows the circuit board (100) which includes: - a first electronic component (C1) mounted on the first side (F1) of the circuit board (100) with connecting terminals (61) arranged according to two slots (23) of the first layer of insulating material and connected to the layer of conductive material (3) via solder joints (61'); - a second electronic component (C2) mounted on the second side (F2) of the circuit board (100), with connecting terminals (62) arranged according to two slots (43) of the second layer of insulating material, and connected to the layer of conductive material (3) via solder joints (62'); - a third electronic component (C3) mounted on the first side (F1) of the printed circuit board, with two conductive pins (63) inserted into two holes (22) of the first layer of insulating material, and connected to the layer of conductive material (3) by means of solder joints (64), the solder joints being arranged on the second side (F2) of the printed circuit board (100) corresponding to two holes (42) of the second layer of insulating material.

[0049] The differences and advantages of the printed circuit board (100) according to the invention compared to the printed circuit board (1) of the prior art are obvious.

[0050] The prior art printed circuit board (1) has a layer (10) of insulating material arranged between two layers (11, 12) of conductive material. Furthermore, the insulating layer (10) has holes (13) with metallization (14) to connect the two conductive layers (11, 12). Consequently, such a printed circuit board involves a considerable waste of very expensive conductive metal material.

[0051] The printed circuit board (100) according to the invention has a single layer of conductive material (3) arranged between two layers (2, 4) of insulating material. Furthermore, this printed circuit board does not require metallization in the openings (25, 45) of the insulating layers. Consequently, such a printed circuit board results in a significant saving of very expensive conductive metal material.

[0052] Appropriate variations and modifications of the present embodiment of the invention can be made by the person skilled in the art within his field, which in any case fall within the scope of the invention as expressed by the attached claims. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 3072734A

[0013] US 2613252

[0013] AT 273542B

[0016] NL 7601969

[0016]

Claims

[1] Printed circuit board (100) of a double-sided type, comprising a top side (F1) and a bottom side (F2) designed to accommodate electronic components or connections to external circuits; wherein the circuit board (100) comprises a single layer of conductive material (3) into which conductive traces (32) are etched and which is arranged between a first layer (2) of insulating material and a second layer (3) of insulating material; wherein the first layer (2) of insulating material has openings (25) to allow access to the layer (3) of conductive material corresponding to areas which must form uncovered areas in the printed circuit board (100) intended for soldering electronic components or connections to external circuits on the top side (F1) of the printed circuit board (100); and wherein the second layer (4) of insulating material has openings (45) to allow access to the layer (3) of conductive material corresponding to areas which must form uncovered areas in the printed circuit board (100) intended for soldering electronic components or connections to external circuits on the underside (F2) of the printed circuit board (100); characterized by , that the conductive layer (3) is a thin, flexible layer, and the first insulating layer (2) and the second insulating layer (4) are both thin, flexible layers to obtain a thin, flexible printed circuit board; and the conductive traces (32) of the conductive layer are produced by screen printing or photographic masking, by chemical etching of a pattern printed on the conductive layer or on a photosensitive film connected to the conductive layer. [2] Printed circuit board (100) according to claim 1, wherein the layer of conductive material (3) is a flexible layer with a small thickness between 17 and 105 µm; the first layer (2) of insulating material is a flexible layer with a small thickness between 12 and 250 µm; and the second layer (4) of insulating material is a flexible layer with a small thickness between 12 and 250 µm. [3] Printed circuit board (100) according to claim 1 or 2, wherein the openings (25) of the first layer of insulating material and the openings (45) of the second layer of insulating material are produced by chemical etching. [4] Printed circuit board (100) according to one of the preceding claims, wherein the openings (25) of the first layer of insulating material comprise holes (22) and slots (23) and the openings (45) of the second layer of insulating material comprise holes (42) and slots (43). [5] Printed circuit board (100) according to one of the preceding claims, wherein the layer of conductive material (3) has through holes (33) arranged such that they connect openings (25) of the first layer of insulating material with respective openings (45) of the second layer of insulating material. [6] Printed circuit board (100) according to claim 5, comprising: - a first electronic component (C1) which is mounted on the first side (F1) of the circuit board (100), has connecting terminals (61) which are arranged according to two slots (23) of the first layer of insulating material, and is connected to the layer of conductive material (3) via solder joints (61'); - a second electronic component (C2) mounted on the second side (F2) of the circuit board (100), having connection terminals (62) arranged according to two slots (43) of the second layer of insulating material, and connected to the layer of conductive material (3) via solder joints (62'); - a third electronic component (C3) mounted on the first side (F1) of the circuit board, having two conductive pins (63) inserted into two holes (22) of the first layer of insulating material, and connected to the layer of conductive material (3) by means of solder joints (64), the solder joints being arranged on the second side (F2) of the circuit board (100) corresponding to two holes (42) of the second layer of insulating material. [7] A double-sided printed circuit board (100) comprising a top side (F1) and a bottom side (F2) designed to accommodate electronic components or connections to external circuits; wherein the printed circuit board is manufactured by a process comprising the following steps: - Cutting a first layer (2) of insulating material to create openings (25) corresponding to areas in the printed circuit board (100) that will have uncovered areas intended for soldering electronic components or connections to external circuits on the top side (F1) of the printed circuit board; - Connecting a layer (3) of conductive material to the layer (2) of insulating material to obtain a connected arrangement (5); - Etching the layer (3) of conductive material to obtain tracks (32) on the connected arrangement (5); - Cutting a second layer (4) of insulating material to create openings (45) in areas that will have uncovered areas in the printed circuit board (100) intended for soldering electronic components or connections to external circuits on the underside (F2) of the printed circuit board; and - Connecting the second layer (4) of insulating material with the layer of conductive material (3) of the connected arrangement (5) to obtain a single layer of conductive material (3), wherein tracks (32) are arranged between the first layer (2) of insulating material and the second layer (3) of insulating material, characterized by , that the conductive layer (3) is a thin, flexible layer and the first insulating layer (2) and the second insulating layer (4) are both thin, flexible layers to obtain a thin, flexible printed circuit board; and the conductive traces (32) of the conductive layer are produced by screen printing or photographic masking, by chemical etching of a pattern printed on the conductive layer or on a photosensitive film connected to the conductive layer. [8] Printed circuit board according to claim 7, wherein the layer of conductive material (3) is a flexible layer with a small thickness between 17 and 105 µm; the first layer (2) of insulating material is a flexible layer with a small thickness between 12 and 250 µm; and the second layer (4) of insulating material is a flexible layer with a small thickness between 12 and 250 µm. [9] Printed circuit board according to claim 7 or 8, wherein in the method the openings (25) of the first layer of insulating material and the openings (45) of the second layer of insulating material are produced by chemical etching. [10] Printed circuit board according to one of claims 7 to 9, wherein in the method through holes (33) are produced in the layer of conductive material (3) which are arranged such that they connect openings (25) of the first layer of insulating material with respective openings (45) of the second layer of insulating material. [11] Printed circuit board according to claim 10, wherein the method comprises the following steps: - Attaching a first electronic component (C1) to the first side (F1) of the printed circuit board (100), wherein connecting terminals (61) are arranged according to two slots (23) of the first layer of insulating material and are connected to the layer of conductive material (3) via solder joints (61'); - Attaching a second electronic component (C2) to the second side (F2) of the printed circuit board (100), wherein connecting terminals (62) are arranged according to two slots (43) of the second layer of insulating material and are connected to the conductive layer (3) via solder joints (62'); - Attaching a third electronic component (C3) to the first side (F1) of the printed circuit board (100), with two conductive pins (63) inserted into two holes (22) of the first layer of insulating material and connected to the layer of conductive material (3) by means of solder joints (64), wherein the solder joints on the second side (F2) of the printed circuit board (100) are arranged corresponding to two holes (42) of the second layer of insulating material.

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