Method for manufacturing printed circuit boards

Abstract

The invention relates to a method for manufacturing a printed circuit board, comprising the operations that consist in: - continuously supplying a sheet of electrically conductive material (1); - continuously coating at least one layer of electrically insulating material (2) over at least one area of the surface of one of the faces of the sheet of electrically conductive material (1), this electrically insulating material (2) comprising at least one epoxy resin; - continuously photopolymerising at least some regions of the epoxy resin under UV radiation; and - producing at least one pattern by photolithographic etching in the sheet of electrically conductive material (1), this pattern being supported by the photopolymerised epoxy resin.

Description

Description Title of the invention: Method for manufacturing printed circuit boards technical field

[0001] The invention relates to the field of printed circuits and printed circuit board manufacturing processes. For example, it relates to the field of flexible printed circuits for medical biology testing devices. State of the art

[0002] Some printed circuit boards are made from multilayer structures comprising at least one sheet of electrically conductive material laminated onto a layer of electrically insulating material. The sheet of electrically conductive material is used to create conductive features (conductive traces, contacts, antennas, electrodes, etc.). The layer of electrically insulating material serves as a support and dielectric substrate.

[0003] For example, the electrically conductive sheet is made of a metal or a metal alloy of one or more of the following metals: copper, aluminum, copper alloy, aluminum alloy, steel, etc. For example, the electrically insulating layer is a dielectric substrate of glass-epoxy, polyimide, polyethylene terephthalate, etc. An adhesive layer may optionally be used to bond the electrically conductive sheet to the electrically insulating layer. For example, the adhesive layer is made of epoxy resin.

[0004] Multilayer structures thus created by laminating together a sheet of electrically conductive material and a layer of electrically insulating material (possibly with a layer of adhesive material between the two) exhibit defects at the interface between the layers. An illustration of this type of defect is given in Figure 1. In Figure 1, a pattern 1 can be seen, etched into a sheet of copper alloy laminated onto a dielectric substrate 2 (in this example, the dielectric substrate 2 is formed from a layer of polyethylene terephthalate). Holes and / or channels 3 are present on the surface of the dielectric substrate 2. After lamination of the sheet Electrically conductive material and layer of electrically insulating material, and etching patterns 1 into the sheet of electrically conductive material, the holes and / or channels 3 form openings 4 at the edges of the patterns 1. More specifically, these openings 4 can form entry points for the etching solutions used to create patterns 1 in the sheet of electrically conductive material, by chemical or electrochemical etching. This can then lead to uncontrolled etching of the sheet of electrically conductive material, around the periphery of the patterns 1 etched into the sheet of electrically conductive material.This is illustrated by figure 2 in which we can see that an opening 4, on the edge of the pattern 1, at the interface between the sheet of electrically conductive material and the layer of dielectric material 2, is the origin of the engraving of a cavity 5 in the sheet of electrically conductive material, and this even if a layer of adhesive 6 is present, between the sheet of electrically conductive material and the layer of dielectric material.

[0005] This uncontrolled engraving can lead to many problems.

[0006] This is particularly relevant when these multilayer structures are used to create electrodes for medical biology testing devices, especially point-of-care (POC) devices. These electrodes are used to detect one or more molecules in human fluids and, potentially, to measure the concentration(s) of these molecules. Detection, and the possible concentration measurement, are performed electrochemically by immersing one or more electrodes made from these multilayer structures in the fluids. These fluids may contain one or more reagents designed to react with the electrodes to provide an electrical measurement that represents information about their nature and / or composition.

[0007] However, it has been observed that the measurements taken at the electrodes mentioned above are not always stable and / or reproducible. The applicant investigated the cause and determined that the presence of four openings at the interfaces between the layers, such as those mentioned above, and highlighted in figures 1 and 2, can modify the response of these electrodes. Summary of the invention

[0008] The invention aims to at least partially overcome this drawback.

[0009] To this end, a method for manufacturing a printed circuit board is proposed. This printed circuit board is advantageously flexible enough to allow continuous implementation ("Reel-to-reel" or "Roll-to-roll" in English) of the method according to the invention.

[0010] The process according to the invention comprises the following operations: - to continuously supply a sheet of electrically conductive material, this sheet comprising two main faces, - to continuously coat at least one layer of electrically insulating material on at least one area of ​​the surface of one of the main faces of the sheet of electrically conductive material, this electrically insulating material comprising at least one epoxy resin, - to continuously photopolymerize at least certain regions (i.e., at least those areas where patterns will be created) of the epoxy resin; and - to produce at least one pattern by photolithography in the sheet of electrically conductive material, this pattern being supported only by said photopolymerized epoxy resin.

[0011] Thus, thanks to the process according to the invention, a layer of electrically insulating material is directly applied by coating to one face of the sheet of electrically conductive material. The layer of electrically insulating material is therefore applied in liquid or paste form so as to create a continuous interface between this layer and the face of the sheet of electrically conductive material to which the layer of electrically insulating material is applied. In other words, the layer of electrically insulating material conforms to and fills any surface irregularities of the sheet of electrically conductive material. Therefore, when engraving designs into the sheet of electrically conductive material, the engraving liquid(s) do not penetrate between the sheet of electrically conductive material and the layer of material. electrically insulating. Similarly, when used in a medical biology testing device, the fluids to be analyzed do not penetrate between the sheet of electrically conductive material and the layer of electrically insulating material. The measurement taken from the electrode(s) etched into the sheet of electrically conductive material is therefore more reliable and more precise.

[0012] In this document, the term "continuous interface" means that the layer of electrically insulating material completely wets the surface of the sheet of electrically conductive material to which it is applied. In other words, at the micrometer or a few micrometer scale, or even at a smaller (nanometer) scale, the interface is homogeneous and continuous between the layer of insulating material and the sheet of electrically conductive material. Put another way, at these scales, there are no holes, channels, openings, cavities, or other such defects between the layer of insulating material and the sheet of electrically conductive material.

[0013] Without this type of defect, the substances (reagents and other liquids) in which the electrode is immersed do not seep between the insulating layer and the electrically conductive sheet. Only the interactions between these substances and the electrode on one or more well-defined surfaces of the electrode are considered. This results in more stable, reliable, and reproducible measurements.

[0014] Furthermore, the implementation of coating and photopolymerization operations on the resin allows the process according to the invention to be carried out continuously and rapidly. The process therefore enables relatively fast and efficient manufacturing speeds.

[0015] The method according to the invention further comprises one of the following characteristics, considered in isolation and independently of each other, or in combination with one or more others:

[0016] - the pattern is an electrode configured for use in a medical biology examination device;

[0017] - the epoxy resin(s) represent at least 30% by dry weight of the insulating material layer coated on one side of the sheet of electrically conductive material; with such a concentration, a continuous layer of insulating material is obtained more easily and with better repeatability;

[0018] - the insulating material layer coated on one side of the electrically conductive material sheet includes a mineral filler representing between 10% and 70% of its dry weight; for example, the mineral filler is essentially made up of glass microspheres; a mineral filler of glass microspheres may possibly allow the viscosity of the resin(s) forming the insulating material layer to be adjusted, in order to improve the quality of the coating formed by this layer; a mineral filler of glass microspheres may also possibly allow the aggregation of other additives such as fumed silicas (or their derivatives);

[0019] - the epoxy resin(s) are chosen from among the following monomers: 3,4-epoxycyclohexylmethyl 3,4 epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate, and other cyclo-aliphatic epoxy resins; the monomer resins can advantageously provide some flexibility to the insulating material layer so that it adapts better to the surface of the conductive material and thus avoid, or at least limit, the formation of holes;

[0020] - the layer of electrically insulating material includes at least one photoinitiator (in other words, an element, molecule or compound that promotes polymerization under UV radiation), for example, the photoinitiator(s) are chosen from the following molecules: bis(4,4'-(C10-C13)-alkylphenyl)-iodonium hexafluoroantimonate, propylene carbonate, and other cationic photoinitiator; a cationic photoinitiator is advantageously used to obtain a faster activation of the curing process; for example, to allow a curing process with a duration of between 5 seconds and 5 minutes;

[0021] - the electrically insulating material layer includes a photopolymerization kinetics regulator or photosensitizer; for example, the photopolymerization kinetics regulator is phenothiazine; a photosensitizer can allow the photoinitiator to absorb a wider range of UV wavelengths, which can also accelerate the UV curing process;

[0022] - the layer of electrically insulating material comprises at least one rheological agent; for example, the rheological agent(s) are silicas pyrogenated (or their derivatives); such rheological agents can also help to adjust the viscosity of the resin(s) forming the insulating material layer, in order to improve the quality of the coating formed by this layer;

[0023] - the layer of electrically insulating material has a thickness between 50 and 350 micrometers; with this thickness range, a wide range of products can be made;

[0024] - It further includes an incubation operation consisting of maintaining, at room temperature for a period of 1 hour or more, the sheet of electrically conductive material supporting photopolymerized epoxy resin under UV radiation in at least certain regions; for example, the incubation operation is carried out for a period of time between 1 and 12 hours, immediately after the operation of continuously photopolymerizing at least certain regions of the epoxy resin; and

[0025] - the operation of continuously photopolymerizing at least certain regions of the epoxy resin under UV radiation is carried out under the following conditions: exposure to UV radiation emitted by light-emitting diodes, for a period of time between 5 seconds and 5 minutes, and with an energy between 60 and 140 mW per square centimeter. Brief description of the figures

[0026] Other aspects, purposes and advantages of the invention will become apparent from the detailed description that follows, as well as from the accompanying figures, given by way of non-limiting examples:

[0027] - Figure 1 is a scanning electron microscope image of the interface between the insulating material layer and the electrically charged material sheet, in a prior art multilayer structure,

[0028] - Figure 2 is also a scanning electron microscope image of a section at the interface between the insulating material layer and the electrically conductive material sheet, in a prior art multilayer structure;

[0029] - Figure 3 is a scanning electron microscope image of the interface between the insulating material layer and the electrically charged material sheet, in a multilayer structure according to the invention; and

[0030] - Figure 4 is also a scanning electron microscope image of a section at the interface between the insulating material layer and the electrically conductive material sheet, in a multilayer structure according to the invention. Detailed description

[0031] The invention is exemplified below in the context of a method for manufacturing an electrode for a medical biology examination device. However, the method described below can be adapted to other applications.

[0032] According to this process, an electrically insulating material is prepared in a sufficiently liquid form to be coated onto a sheet of conductive material.

[0033] Advantageously, the electrically insulating material has a viscosity of the order of 1 Pa.s.

[0034] According to the invention, this electrically insulating material is a photopolymerizable epoxy resin under UV radiation. To this end, a homogeneous mixture of liquid monomers and / or prepolymers is prepared in the absence of UV radiation.

[0035] These liquid monomers and / or prepolymers are epoxy resins. They are chosen from the following prepolymers or monomers: 3,4-epoxycyclohexylmethyl 3,4 epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate, or other UV-curable cycloaliphatic epoxy resins.

[0036] For example, a mixture of 35% by weight of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate and 65% by weight of bis(3,4-epoxycyclohexylmethyl) adipate is prepared. The weight concentration of the resins is adjusted to be between 30 and 90% in the final mixture (including the epoxy resins, fillers, and other additives).

[0037] When a homogeneous mixture is obtained from epoxy resins, fillers or reinforcing additives are added to this mixture. For example, these fillers are added gradually to the epoxy resin mixture, in a shear mixer, until a homogeneous mixture is obtained again.

[0038] The weight concentration of the filler(s) is adjusted to be between 30 and 90% in the final mixture. For example, these fillers are mineral fillers. For instance, a mineral filler consisting primarily of glass microspheres 7 is used (microspheres can be seen in Figure 4). The weight concentration of these glass microspheres in the final mixture is then, for example, close to 50%.

[0039] Once a homogeneous mixture is obtained, at least one photopolymerization kinetics regulator and at least one photoinitiator are added and mixed with the resins and the filler(s). For example, the weight concentration of the photopolymerization kinetics regulator(s) is less than or equal to 1% in the final mixture. For example, the photopolymerization kinetics regulator is phenothiazine. For example, the weight concentration of the photoinitiator(s) is less than or equal to 5% in the final mixture. For example, the photoinitiator(s) is bis(4,4'-(C10-C13)-alkylphenyl)-iodonium hexafluoroantimonate, or another cationic photoinitiator.

[0040] At least one rheological agent is optionally added at a level of approximately 1% by weight in the final mixture. For example, this rheological agent is fumed silica.

[0041] Furthermore, a sheet of electrically conductive material in a coil is unrolled. For example, the sheet of electrically conductive material is a sheet of a flexible copper alloy with a thickness of 35 micrometers.

[0042] The sheet of electrically conductive material is continuously coated, on at least a portion of the surface of one of its main faces, with the previously prepared mixture. The sheet of electrically conductive material is thus at least partially covered with a layer of electrically insulating material. For example, the thickness of the layer of electrically insulating material is between 50 and 350 micrometers. For instance, this thickness is approximately 100 micrometers to form a suitable support and dielectric substrate for supporting patterns subsequently created in the sheet of electrically conductive material.

[0043] For example, the coating of the sheet with electrically conductive material is carried out using a slot die type technology.

[0044] Continuously, the layer of electrically insulating material that at least partially coats the sheet of electrically conductive material is exposed to ultraviolet light. For example, this ultraviolet light is emitted by light-emitting diodes (LEDs) or a mercury vapor lamp. For instance, the layer of electrically insulating material is continuously exposed for 10 seconds to light with a wavelength of approximately 385 nm, emitted by LEDs, at an energy of 100 mW per square centimeter. As another example, the layer of electrically insulating material is continuously exposed for 60 seconds to light emitted by a mercury vapor lamp, at a power of 100 milliwatts per square centimeter.

[0045] After the photopolymerization step, the layer of electrically insulating material is maintained at room temperature (e.g., between 18 and 24°C; more specifically, 20°C or approximately 20°C) during an incubation period of at least 1 to 12 hours. An incubation period following the photopolymerization step is advantageous. If the photopolymerization time is too short, polymerization may be insufficient. However, if the photopolymerization time is too long, the insulating material may overheat. The photopolymerization time and the incubation time are interdependent. For example, a relatively short photopolymerization time combined with a relatively long incubation time can lead to sufficient polymerization.In addition, the waiting or incubation time can be shortened to varying degrees, for example by increasing the temperature during insolation / exposure to UV radiation.

[0046] It is important that the layer of electrically insulating material obtained at the end of the steps described above has properties suitable for its implementation as a dielectric substrate in a continuous printed circuit board manufacturing process.

[0047] For example, this layer of electrically insulating material must have the following properties: - elongation: 1.9% - Tensile strength: 55 MPa, - good flexibility and not being brittle - good resistance to chemical baths, quantified by a mass difference of less than 1% of the material after immersion for 5 minutes in the bath followed by drying for 4 hours, - good punching quality (limiting the amount of dust generated during cutting and leaving perfectly defined holes).

[0048] The values ​​mentioned above correspond to those measured in the example described here. They provide an order of magnitude from which variations can be observed and tolerated.

[0049] According to an example of implementation of the process according to the invention, the sheet of electrically conductive material coated with the electrically insulating layer obtained as indicated above, can undergo suitable steps to create, in the sheet of electrically conductive material, patterns such as conductive tracks, electrical contacts, electrodes, etc.

[0050] For example, drive holes are punched along the edges of a strip formed from a sheet of electrically conductive material, and photolithography operations are performed to define the patterns in the sheet. Optionally, operations to deposit one or more metals onto at least some of these patterns may also be performed. Optionally, operations to deposit one or more protective layers onto at least some of these patterns may also be performed.

Claims

Claims

1. A method of manufacturing a printed circuit, comprising the operations of: - continuously supplying a sheet of electrically conductive material (1), to - continuously coating at least one layer of electrically insulating material (2) on at least one area of ​​the surface of one of the faces of the sheet of electrically conductive material (1), this electrically insulating material (2) comprising at least one epoxy resin, - continuously photopolymerize at least certain regions of the epoxy resin under UV radiation, and - producing at least one pattern by photolithography in the sheet of electrically conductive material (1), this pattern being supported by said photopolymerized epoxy resin.

2. The method of claim 1, wherein the pattern is an electrode configured for use in a medical biology testing device.

3. Method according to claim 1 or 2, in which the epoxy resin(s) represent by dry weight of the layer of insulating material between 30% and 70% coated on one of the faces of the sheet of electrically conductive material (1).

4. Method according to one of the preceding claims, in which the layer of insulating material (2) coated on one of the faces of the sheet of electrically conductive material (1) comprises between 10% and 70% by dry weight of a mineral filler.

5. A method according to claim 4, wherein the mineral filler consists essentially of glass microspheres (7).

6. Method according to one of the preceding claims, in which the epoxy resin(s) are chosen from UV-polymerizable cycloaliphatic epoxy resins.

7. Method according to the preceding claim, in which the epoxy resin(s) are chosen from the following molecules: 3,4-epoxycyclohexylmethyl 3,4 epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate.

8. Method according to one of the preceding claims, in which the layer of electrically insulating material comprises at least one photoinitiator.

9. The method of claim 8, wherein the photoinitiator is a cationic photoinitiator.

10. The method of claim 9, wherein the photoinitiator is bis(4,4'-(C10-C13)-alkylphenyl)-iodonium hexafluoroantimonate, or propylene carbonate.

11. Method according to one of the preceding claims, in which the layer of electrically insulating material (2) comprises a regulator of the kinetics of photopolymerization.

12. The method of claim 11, wherein the photopolymerization kinetics regulator is phenothiazine.

13. Method according to one of the preceding claims, in which the layer of electrically insulating material (2) comprises at least one rheological agent.

14. Method according to claim 13, in which the rheological agent(s) are chosen from the following molecules: silicones, siloxanes, silica fume.

15. Method according to one of the preceding claims, in which the layer of electrically insulating material (2) has a thickness of between 50 and 350 micrometers.

16. Method according to one of the preceding claims, further comprising an incubation operation consisting of maintaining, at room temperature for a period greater than or equal to 1 hour, the sheet of electrically conductive material supporting photopolymerized epoxy resin under UV radiation in at least certain regions.

17. A method according to the preceding claim, wherein the incubation operation is carried out for a period of time between 1 hour and 12 hours, immediately after the operation of continuously photopolymerizing at least certain regions of the epoxy resin.

18. A method according to any preceding claim, wherein the step of continuously photopolymerizing at least some regions of the epoxy resin under UV radiation is carried out in the following conditions: exposure to UV radiation emitted by light-emitting diodes, for a period of time between 5 seconds and 5 minutes, and with an energy between 60 and 140 mW per square centimeter.

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