Electromagnetic shielding film and circuit board

Abstract

The invention discloses an electromagnetic shielding film and a circuit board. The electromagnetic shielding film comprises a protective layer, a metal layer and an adhesive film layer which are sequentially stacked, the electromagnetic shielding film comprises a protective layer and a metal layer, the protective layer and the metal layer are stacked to form a plating sheet, the first strain strength sigma 1 of the plating sheet and the first strain strength sigma 2 of the whole electromagnetic shielding film meet the following condition: the first strain strength refers to the stress when a stress-strain curve deviates from a linear section to reach a preset multiple of plastic strain a, and the range of a is 0.00018-0.00035. According to the invention, the electromagnetic shielding film can be effectively prevented from being damaged during lamination, and the overall quality of the electromagnetic shielding film is ensured.

Description

Technical Field

[0001] The present invention relates to the field of electronic technology, and in particular to an electromagnetic shielding film and a circuit board. Background Art

[0002] In the modern electronics manufacturing industry, especially in the application scenarios of multi-layer boards and high-frequency signal transmission, the complexity and functional requirements of printed circuit boards (FPCs) are constantly increasing.

[0003] When laminating circuit boards, the height difference in the step area causes the shielding film to deform in both the height and width directions during lamination. This can easily damage the shielding film, affecting signal transmission and product reliability. Therefore, to ensure signal integrity, reduce electromagnetic interference (EMI), and improve product reliability, preventing the electromagnetic shielding film from breaking during lamination is a major challenge. Summary of the Invention

[0004] The purpose of the embodiments of the present invention is to provide an electromagnetic shielding film and a circuit board, which can effectively prevent the electromagnetic shielding film from being damaged during pressing and ensure the overall quality of the electromagnetic shielding film.

[0005] To achieve the above-mentioned object, an embodiment of the present invention provides an electromagnetic shielding film, the electromagnetic shielding film comprising a protective layer, a metal layer, and an adhesive film layer stacked in sequence; the protective layer and the metal layer are stacked to form a plated sheet, and the first strain strength σ1 of the plated sheet and the first strain strength σ2 of the electromagnetic shielding film as a whole satisfy:

[0006]

[0007] The first strain intensity refers to the stress when the stress-strain curve deviates from the linear segment and reaches a preset multiple a of plastic strain, and the range of a is 0.00018 to 0.00035.

[0008] As an improvement of the above solution, the first strain strength σ3 of the protective layer and the first strain strength σ2 of the entire electromagnetic shielding film satisfy: |σ3-σ2|≤1MPa.

[0009] As an improvement of the above solution, the first strain strength σ4 of the adhesive film layer is 5 to 15 MPa.

[0010] As an improvement to the above solution, the difference between the thermal expansion coefficient CTE1 of the adhesive film layer and the thermal expansion coefficient CTE2 of the metal layer is less than 100 ppm / °C.

[0011] As an improvement to the above solution, the elastic modulus T1 of the adhesive film layer and the thermal expansion coefficient CTE1 of the adhesive film layer satisfy: 0.0025≤T1 / CTE1≤0.0417, where the unit of T1 is MPa, the unit of CTE1 is ppm / °C, and the calculation is dimensionless.

[0012] As an improvement to the above solution, the elastic modulus T2 of the plated sheet and the first strain strength σ1 of the plated sheet satisfy: 0.8≤T2*σ1≤3, where the unit of T2 is MPa.

[0013] As an improvement of the above solution, the glass transition temperature Tg of the adhesive film layer is ≤ 120°C.

[0014] As an improvement to the above solution, the metal layer is undulating, and the height difference of the undulations of the metal layer ranges from 3 to 7 μm.

[0015] As an improvement to the above solution, the adhesive film layer is provided with conductive particles, and the ratio of the maximum width of the conductive particles to the minimum thickness of the adhesive film layer is less than 0.8.

[0016] As an improvement of the above solution, the protective layer includes a support layer and a filling layer, and the support layer is arranged on a surface of the filling layer that is away from the metal layer.

[0017] An embodiment of the present invention provides a circuit board, comprising a circuit board body and an electromagnetic shielding film as described above; the electromagnetic shielding film is pressed onto the circuit board body via an adhesive film layer.

[0018] Compared with the prior art, the electromagnetic shielding film and circuit board disclosed in the present invention can effectively avoid the stress concentration phenomenon caused by sudden change in interlayer strength by limiting the change in the first strain strength of the electromagnetic shielding film before and after coating to no more than 40%, reduce problems such as interlayer peeling and wrinkling caused by stress concentration, and avoid the electromagnetic shielding film from being damaged and torn at the break during the pressing process. The present invention can make the electromagnetic shielding film and substrates such as circuit boards fit more tightly and smoothly, and improve the tensile strength and elongation of the electromagnetic shielding film, reduce the scrap rate, and improve the production yield of the electromagnetic shielding film. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic structural diagram of a first electromagnetic shielding film provided by an embodiment of the present invention;

[0020] Figure 2 is a structural schematic diagram of a second electromagnetic shielding film provided by an embodiment of the present invention;

[0021] Figure 3 is a schematic structural diagram of a third electromagnetic shielding film provided by an embodiment of the present invention;

[0022] Figure 4 is a schematic structural diagram of a fourth electromagnetic shielding film provided by an embodiment of the present invention;

[0023] Figure 5 This is a schematic structural diagram of a circuit board provided by an embodiment of the present invention;

[0024] In the figure, 10, electromagnetic shielding film; 11, protective layer; 111, support layer; 112, filling layer; 12, metal layer; 13, film layer; 131, conductive particles; 20, circuit board; 21, circuit board body. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0026] In the description of this application, it should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

[0027] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature specified as "first" or "second" may explicitly or implicitly include one or more of such features. Throughout this application, unless otherwise specified, "plurality" means two or more.

[0028] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0029] See also Figure 1, is a structural schematic diagram of the first electromagnetic shielding film provided by an embodiment of the present invention. An embodiment of the present invention provides an electromagnetic shielding film 10, which includes a protective layer 11, a metal layer 12 and a film layer 13 stacked in sequence, and the protective layer 11 and the metal layer 12 are stacked to form a plated sheet.

[0030] It should be noted that the manufacturing process of the electromagnetic shielding film is as follows: a protective layer 11 is formed by coating on a carrier film, a metal seed layer is vacuum-sputtered on the protective layer 11, thereby forming a nano-scale metal seed layer on one side of the protective layer, and the protective layer with the metal seed layer is electroplated to thicken the metal layer to 1-3 μm to form a metal layer 12. The structure of the protective layer 11 and the metal layer 12 is called a plated sheet, and an adhesive film layer 13 is coated on the metal layer to form a finished electromagnetic shielding film. That is, before adhesive coating, the structure is a two-layer structure of a protective layer and a metal shielding layer, which is simply called a plated sheet; after adhesive coating, the structure is a three-layer structure of a protective layer, a metal shielding layer, and an adhesive film layer, which is the finished electromagnetic shielding film. Optionally, the protective layer is an insulating layer.

[0031] In existing technology, when some electromagnetic shielding films are laminated onto circuit boards with typical step heights of 20 to 100 μm (including cover film and etched pad steps), they can easily break, impacting signal transmission and product reliability. Step heights refer to height differences, such as uneven areas formed by pads, vias, or device bumps on a circuit board.

[0032] The reason for the damage of the shielding film is usually that when the shielding film is pressed onto the printed circuit board, when it is pressed onto the circuit board with a step on the surface, the film layer at the step will be subjected to complex stresses such as vertical pressure, shear stress and bending stress.

[0033] Due to the presence of vertical pressure, the uniform pressure applied during lamination causes the adhesive layer to compress and deform perpendicular to the PCB surface. Furthermore, the height difference on either side of the break causes the adhesive layer to shift parallel to the PCB surface, generating shear forces. Furthermore, when the metal shielding layer is deformed by pressure, it bends at the break edge, generating tensile / compressive stresses. Microscopically, this manifests as tension on the outside and compression on the inside.

[0034] For electromagnetic shielding films with low tensile strength and low elongation, when the product is pressed and filled, the film layer begins to flow when it encounters heat. At the bonding point between the film layer and the step difference of the circuit board, the film layer will drive the metal layer to slide downward, and eventually tear the upper metal layer and the insulation layer together, resulting in post-pressing damage problems.

[0035] Therefore, to address the issue of electromagnetic shielding films being susceptible to breakage at the gap during lamination, embodiments of the present invention optimize the first strain strength σ of the plating sheet and the electromagnetic shielding film as a whole. The first strain strength σ refers to the stress at which the stress-strain curve deviates from the linear segment and reaches a predetermined multiple a of plastic strain, with a ranging from 0.00018 to 0.00035. It is a key indicator of the critical value at which a material enters the plastic deformation stage. For example, in a tensile test, if the strain-stress curve exhibits an inelastic strain of 0.2% at σ, then σ is the first strain strength.

[0036] Then, the first strain strength σ1 of the plated sheet and the first strain strength σ2 of the entire electromagnetic shielding film satisfy:

[0037]

[0038] In the embodiment of the present invention, the intensity change Δσ is the ratio of the difference between the first strain intensity σ2 of the electromagnetic shielding film and the first strain intensity σ1 of the plating sheet to the first strain intensity σ1 of the plating sheet, that is, It is used to characterize the change in the first strain strength of the electromagnetic shielding film before coating the adhesive film layer (ie, the plated structure) and after coating the adhesive film layer (ie, the finished electromagnetic shielding film).

[0039] If the strength change Δσ>0.4, it means that the strain strength mutation between the plating sheet and the overall film material is too large. During the lamination process of the electromagnetic shielding film, the external pressure will cause the layers of material to deform, and at this time, stress concentration points will be formed at the places where the strength mutation between the layers occurs. This is because when materials of different strengths are combined together to withstand external forces, the areas with lower strength cannot deform synchronously with the areas with higher strength, resulting in uneven strain distribution and a sharp increase in stress at the junction. Stress concentration will make the material more susceptible to crack initiation and expansion, which will cause the shielding film to break and tear at the fault.

[0040] Therefore, an embodiment of the present invention limits the change in the first strain strength of the electromagnetic shielding film before coating the adhesive film layer (i.e., the plated structure) and after coating the adhesive film layer (i.e., the finished electromagnetic shielding film) to no more than 40%. For example, the strength change can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 13%, 25%, 27%, 30%, 31%, 34%, 35%, 38%, 39% or 40%, etc. Of course, it can also be set to other qualified values ​​according to actual conditions, and no specific limitation is made here.

[0041] By limiting the strength change to less than or equal to 40%, it is ensured that the overall mechanical properties after the film layer is coated are gradually controllable, and the strain strength transition between the plated sheet and the overall film material can be relatively smooth. Each layer can deform more coordinated when subjected to force, reducing the stress difference at the interface between layers, avoiding the occurrence of stress concentration due to sudden changes in interlayer strength, and forming a "gradient buffer structure" for the electromagnetic shielding film during pressing. The metal layer is not easily torn, thereby improving the resistance to damage and ensuring the signal integrity and reliability of the circuit board after pressing.

[0042] By adopting the technical means of the embodiments of the present invention, by limiting the change in the first strain strength of the electromagnetic shielding film before and after coating to no more than 40%, it is possible to effectively avoid the occurrence of stress concentration caused by sudden changes in interlayer strength, reduce problems such as interlayer peeling and wrinkling caused by stress concentration, and avoid the electromagnetic shielding film from being damaged and torn at the break during the pressing process. The present invention can make the electromagnetic shielding film fit more tightly and smoothly with substrates such as circuit boards, and improve the tensile strength and elongation of the electromagnetic shielding film, reduce the scrap rate, and improve the production yield of the electromagnetic shielding film.

[0043] As a preferred embodiment, the embodiment of the present invention is further implemented on the basis of the above embodiment, and the first strain strength σ3 of the protective layer 11 and the first strain strength σ2 of the entire electromagnetic shielding film satisfy: |σ3-σ2|≤1MPa.

[0044] It should be noted that the protective layer 11, as the outer layer of the electromagnetic shielding film, primarily supports and protects the electromagnetic shielding film. If the first strain strength of the protective layer 11 differs significantly from the first strain strength of the entire film, delamination may occur during lamination or use due to uneven stress.

[0045] In an embodiment of the present invention, the strain strength difference between the protective layer 11 and the overall shielding film is limited to 1 MPa or less. For example, it can be 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, or 1 MPa. Of course, it can also be set to other suitable values ​​according to actual conditions, and no specific limitation is given here. By limiting the strain strength difference between the protective layer 11 and the overall shielding film to 1 MPa or less, the mechanical properties between the layers can be matched. When the strain strengths of the two layers are close, the load between the layers is evenly transferred, which can effectively reduce the risk of surface delamination.

[0046] The technical means adopted in the embodiments of the present invention can effectively enhance the collaborative deformation ability between the layers of the shielding film, avoid excessive pulling of the metal layer due to excessive stiffness of the protective layer, and at the same time improve the bending resistance of the overall structure, which is suitable for complex bending scenarios of flexible circuit boards.

[0047] As a preferred embodiment, the embodiment of the present invention is further implemented on the basis of any of the above embodiments, and the first strain strength σ4 of the adhesive film layer 13 is 5 to 15 MPa.

[0048] In an embodiment of the present invention, the first strain strength σ4 of the film layer 13 is limited to between 5 and 15 MPa, for example, it can be 5 MPa, 5.5 MPa, 6 MPa, 6.5 MPa, 7 MPa, 7.5 MPa, 8 MPa, 8.5 MPa, 9 MPa, 9.5 MPa, 10 MPa, 11 MPa, 12 MPa, 13 MPa, 13.5 MPa, 14 MPa, 14.5 MPa or 15 MPa, etc. Of course, it can also be set to other qualified values ​​according to actual conditions, and no specific limitation is made here.

[0049] It should be noted that when the first strain strength σ4 of the adhesive layer 13 is greater than the aforementioned range, the adhesive layer becomes too rigid, making it difficult to fill the gap at the bottom of the step during lamination. This causes the metal layer to hang loose at the edge of the step, forming a cantilever-like structure. Stress is concentrated at the edge of the metal layer, which can easily cause the metal layer to tear. When the first strain strength σ4 of the adhesive layer 13 is lower than the aforementioned range, the adhesive layer suffers from insufficient cohesion after lamination, making creep or interlayer slip more likely to occur. This can affect the reliability of the electromagnetic shielding film. For example, at high temperatures, the adhesive layer can cause the shielding film to shift due to fluid flow.

[0050] In an embodiment of the present invention, by limiting the first strain strength σ4 of the film layer 13 to be within the low strain strength range of 5 to 15 MPa, it is ensured that the film layer has good fluidity and that the film layer preferentially undergoes plastic flow during lamination. The discontinuity can be filled by plastic flow and the shear stress at the discontinuity can be dispersed. At the same time, the change in the first strain strength of the electromagnetic shielding membrane before and after coating the film layer is limited to no more than 40%, thereby avoiding stress concentration due to sudden changes in interlayer strength and effectively preventing the electromagnetic shielding membrane from being easily broken and damaged at the discontinuity during lamination.

[0051] Optionally, the material system of the adhesive film layer is preferably selected to use a flexible polymer material as a substrate, more preferably an elastomer substrate.

[0052] Preferably, the adhesive film layer 13 is made of at least one of an acrylic adhesive, a silicone rubber adhesive, and a polyurethane adhesive.

[0053] Specifically, the molecular characteristics of the acrylic adhesive are: a flexible ester group (-COOR) in the main chain and a long alkyl group in the side chain, such as C8-C12 acrylate, which can reduce intermolecular forces and enhance segment mobility.

[0054] Optionally, the adhesive layer is composed of 70% to 85% isooctyl acrylate, 10% to 20% methyl methacrylate, and 0.5% to 1.5% N-hydroxymethyl acrylamide. The use of an acrylic adhesive as the adhesive layer has the advantage of being in a rubbery state at room temperature, which further reduces the first strain strength of the electromagnetic shielding film and improves its elongation.

[0055] The molecular characteristics of the silicone rubber adhesive are: the main chain is a -Si-O- bond, with low bond energy and extremely high flexibility, and the side chain methyl (-CH3) further reduces the glass transition temperature of the film layer, has better high and low temperature resistance, and is beneficial to increasing the overall elongation of the electromagnetic shielding film and reducing the first strain strength.

[0056] Optionally, the adhesive film layer is composed of polytetramethylene glycol (PTF) accounting for at least 80%, with the hard segment adopting a low crosslink density structure. The performance can be controlled by adjusting the ratio of the soft segment to the hard segment, thereby reducing the first strain strength and increasing the elongation. The hard segment can be a polyether / polyester diol, and the soft segment can be a combination of an isocyanate and a chain extender.

[0057] In another embodiment, the adhesive film layer 13 contains a crosslinking agent, which accounts for 0.3% to 1.0% of the total mass of the adhesive liquid in the adhesive film layer; and / or the adhesive film layer 13 contains a plasticizer, which accounts for 10% to 25% of the total mass of the adhesive liquid in the adhesive film layer. This helps to reduce the crosslinking density and enhance the molecular chain slippage ability.

[0058] It should be noted that increasing the amount of crosslinker improves the first strain strength but decreases the elongation. Therefore, in the embodiments of the present invention, the crosslinker content of the adhesive film layer is controlled to 0.3% to 1.0% of the total adhesive mass to achieve a lower first strain strength and higher elongation. Specifically, the crosslinker can be N,N'-methylenebisacrylamide, a type of acrylic ester.

[0059] Preferably, the crosslinking agent is a monofunctional or low-functional crosslinking agent, such as monoacrylates.

[0060] It should be noted that small molecule plasticizers inserted between polymer chains can weaken van der Waals forces, lower the glass transition temperature (Tg), and enhance chain segment mobility. For example, small molecule plasticizers can be dioctyl phthalate (DOP) or paraffin oil.

[0061] Therefore, in the embodiment of the present invention, the plasticizer content of the adhesive film layer is controlled to be 10% to 25% of the adhesive solution mass. As the plasticizer content increases, the first strain strength decreases and the elongation increases, but excessive plasticizer content can lead to migration failure.

[0062] Preferably, the plasticizer is a high molecular weight plasticizer, such as a polyester plasticizer. Furthermore, the plasticizer is migration-resistant to avoid the loss of plasticizer during long-term use, which may lead to hardening and brittleness of the film.

[0063] Preferably, the surface of the adhesive film layer 13 is filled with elastic fillers, which are silicone rubber microspheres or acrylic elastomers. Furthermore, the molecular chains of the constituent materials of the adhesive film layer 13 are connected with flexible segments, which are polyethylene glycol or polydimethylsiloxane.

[0064] In an embodiment of the present invention, flexible segments such as polyethylene glycol (PEG) and polydimethylsiloxane (PDMS) are inserted into the molecular chains of the film layer, which can effectively improve the flexibility of the molecular chain, thereby reducing the first strain strength and improving the elongation. For example, in an acrylate-PEG block copolymer film, the elongation is greatly improved and the first strain strength is reduced. Alternatively, by adding surface-modified elastomer particles to the film layer, the stress dispersion effect can prevent crack propagation while avoiding a significant increase in strength. For example, silicone rubber microspheres and acrylic elastomers are added to the film layer with a particle size of 50 to 200 nm and an amount of 5% to 10%.

[0065] Alternatively, during the coating and curing process, a slow, low-temperature curing method can be used to avoid the internal stress accumulation caused by rapid, high-temperature curing, allowing the molecular chains to fully stretch and align, thereby improving elongation. For example, during UV curing of the film layer, the light intensity can be reduced to 50-100mW / cm2 and the irradiation time can be extended to 30-60 seconds.

[0066] Alternatively, a gradient crosslinking process can be used, where a partial network structure is formed through low-temperature pre-crosslinking before the temperature is raised to complete the crosslinking, thus reducing microcracks caused by shrinkage. Specifically, the low temperature range is 40-60°C, and the rising temperature range is 80-100°C.

[0067] Preferably, the thickness of the adhesive film layer 13 is 5-15 μm.

[0068] It should be noted that in the context of electromagnetic shielding films, an overly thin film layer is prone to tearing due to stress concentration; an overly thick film layer may result in insufficient cohesive strength, resulting in viscous flow rather than elastic extension. The embodiments of the present invention control the thickness of the film layer of the electromagnetic shielding film to be between 5 and 15 μm, for example, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 8 μm, 9 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, etc., so that the thickness of the film layer is within a reasonable range, effectively balancing stress strength and fluidity.

[0069] In addition, when coating the film layer on the plated sheet, a micro-concave roller or comma scraper is used to control the thickness tolerance of ±1μm to avoid uneven performance caused by local excessive thickness or thinness, thereby improving the uniformity of the film layer.

[0070] Preferably, during the coating process of the film layer, the film is stretched unidirectionally or bidirectionally with a stretching ratio of 1.5 to 2.0 times, so that the molecular chains are oriented along the force direction, thereby increasing the elongation in the main direction. At the same time, since the molecular chains are easier to slip after pre-stretching, it is beneficial to reduce the first strain strength.

[0071] By adopting the technical means of the embodiments of the present invention, through limiting the first strain strength, thickness and mechanical properties of the film layer, a film layer with both low first strain strength and high elongation can be prepared. The film layer with low strain strength is convenient for deformation and energy consumption, and the film layer with high elongation can effectively adapt to step deformation, thereby effectively improving the tear resistance of the electromagnetic shielding film and reducing the occurrence of tearing after the electromagnetic shielding film is pressed onto the circuit board.

[0072] As a preferred embodiment, this embodiment of the present invention is further implemented based on any of the above embodiments, wherein the difference between the thermal expansion coefficient CTE1 of the adhesive film layer 13 and the thermal expansion coefficient CTE2 of the metal layer 12 is less than 100 ppm / °C. Furthermore, the difference between the thermal expansion coefficient CTE1 of the adhesive film layer 13 and the thermal expansion coefficient CTE3 of the plated sheet is less than 60 ppm / °C.

[0073] It should be noted that the coefficient of thermal expansion (CTE) of a material is a core parameter that characterizes the material's ability to expand and contract in size when the temperature changes. The larger the value, the more significant the expansion and contraction of the material under the same temperature fluctuation.

[0074] When the difference between the thermal expansion coefficient CTE1 of the film layer and the thermal expansion coefficient CTE2 of the metal layer is greater than or equal to 100ppm / °C, changes in ambient temperature will cause thermal expansion or thermal contraction stress between the layers. In scenarios such as reflow soldering, ambient temperature fluctuations, and heat generation during equipment operation, the expansion and contraction of the two layers of material will differ significantly. For example, if the thermal expansion coefficient of the film layer is much greater than that of the metal layer, the elongation of the film layer in a high-temperature environment will far exceed that of the metal layer. At this time, the metal layer will be subjected to tensile stress imposed by the film layer; in a low-temperature environment, the contraction of the film layer will be more obvious, which will produce compressive stress on the metal layer. This repeated alternating thermal stress will be concentrated at the interface between the metal layer and the film layer. As the number of temperature cycles increases, fatigue damage will gradually accumulate inside the metal layer, which may eventually cause cracks, resulting in a decrease in local shielding effectiveness, or even complete rupture of the metal layer, losing its shielding function.

[0075] Therefore, in an embodiment of the present invention, the difference between the thermal expansion coefficient CTE1 of the film layer 13 and the thermal expansion coefficient CTE2 of the metal layer is controlled to be less than 100 ppm / °C, for example, it can be 5ppm / °C, 10ppm / °C, 15ppm / °C, 20ppm / °C, 25ppm / °C, 30ppm / °C, 40ppm / °C, 50ppm / °C, 60ppm / °C, 65ppm / °C, 70ppm / °C, 75ppm / °C, 80ppm / °C, 85ppm / °C, 90ppm / °C, 95ppm / °C, etc. Of course, it can also be set to other qualified values ​​according to actual conditions, and no specific limitation is made here.

[0076] The embodiments of the present invention limit the CTE difference between the film layer and the metal layer to less than 100ppm / °C. This significantly reduces the difference in expansion and contraction between the layers, effectively reducing thermal stress during temperature changes. When the ambient temperature changes, the deformation trends of the film layer and the metal layer become closer, and the thermal stress at the interface is controlled within the fatigue threshold that the metal layer can withstand, thereby preventing cracking of the metal layer due to stress overload. The smaller the CTE difference between the film layer and the metal layer, the lower the thermal stress, which can better prevent the metal layer from tearing due to concentrated thermal stress.

[0077] Similarly, the embodiment of the present invention controls the difference between the thermal expansion coefficient CTE1 of the adhesive film layer and the thermal expansion coefficient CTE3 of the plating sheet to be less than 60ppm / °C, which can also effectively reduce the thermal stress between layers during temperature changes and avoid cracking between electromagnetic shielding film layers due to stress overload.

[0078] By adopting the technical means of the embodiments of the present invention and controlling the difference in thermal expansion coefficients, the high and low temperature resistance reliability of the shielding film can be effectively improved, the adhesive film layer and the metal layer can be prevented from separating or cracking after long-term thermal cycling, the stable adhesion of the adhesive film layer to the metal layer can be maintained, and the overall failure of the shielding film caused by delamination can be avoided. The continuity of the shielding path can be ensured, and stable shielding performance can be maintained. It is particularly suitable for circuit boards in harsh environments such as automotive electronics, aerospace, etc.

[0079] As a preferred embodiment, this embodiment of the present invention is further implemented based on any of the above embodiments, wherein the elastic modulus T1 of the adhesive layer 13 and the thermal expansion coefficient CTE1 of the adhesive layer 13 satisfy the following: 0.0025≤T1 / CTE1≤0.0417. Wherein, T1 is in MPa, and CTE1 is in ppm / °C, and the calculation is dimensionless.

[0080] It should be noted that the elastic modulus reflects the material's ability to resist deformation. The lower the value, the easier it is for the material to undergo plastic flow; the thermal expansion coefficient reflects the expansion and contraction characteristics when the temperature changes.

[0081] The embodiment of the present invention balances temperature sensitivity and deformation resistance by using the ratio of the elastic modulus of the film layer to the thermal expansion coefficient (T1 / CTE1), thereby achieving dynamic performance adaptation of the film layer throughout the entire lamination process. In the embodiment of the present invention, the ratio T1 / CTE1 of the elastic modulus T1 of the film layer to the thermal expansion coefficient CTE1 of the film layer is controlled to be between 0.0025 and 0.0417, for example, 0.0025, 0.0052, 0.0063, 0.0084, 0.0117, 0.0155, 0.0218, 0.0369, 0.0417, etc. Of course, it can also be set to other suitable values ​​according to actual conditions, and is not specifically limited here.

[0082] During the heating phase of lamination, the rising ambient temperature intensifies the molecular chain motion of the film layer. At this time, if T1 / CTE1 is between 0.0025 and 0.0417, the film layer exhibits low elastic modulus and moderate thermal expansion. The low elastic modulus facilitates plastic flow under pressure, while the matching thermal expansion coefficient with the metal layer prevents interlaminar stress caused by excessive expansion. In this state, the film can quickly fill gaps on the PCB, such as those in coverlay film windows and etched pad height differences. By flowing to fill tiny gaps, it ensures a tight fit with the metal layer and the PCB surface, minimizing the risk of bubbles or voids.

[0083] During the cooling phase, the temperature drops, causing the film's molecular chains to gradually shrink and solidify, with the elastic modulus increasing as the temperature decreases. The upper limit of T1 / CTE1, 0.0417, ensures that the modulus increase remains within a reasonable range. This prevents excessive flow during cooling and shrinkage due to a low modulus, which could prevent overall displacement or edge overflow of the shielding film, and also prevents a sudden increase in modulus from losing its stress-buffering capacity. This moderate hardening property allows the film to maintain a stable filling shape while absorbing thermal stress differences between the metal layer and the substrate through a certain degree of elastic deformation, preventing tensile forces generated by cooling and shrinkage from damaging the interlayer bond.

[0084] If the T1 / CTE1 ratio is lower than 0.0025, the film layer will be too soft and prone to creep. After cooling, the film will have insufficient modulus, which can easily lead to shielding film offset or uneven thickness due to continued flow. If the T1 / CTE1 ratio is higher than 0.0417, the film layer will be too rigid but have poor cushioning properties. During heating, the film will be too rigid to fully fill the gap, and after cooling, it may become brittle due to the high modulus, losing its stress-buffering function. Therefore, a T1 / CTE1 ratio within the range of 0.0025 to 0.0417 allows the film to release thermal stress through elastic deformation during temperature changes while maintaining sufficient cohesion.

[0085] By controlling the ratio of the elastic modulus and thermal expansion coefficient of the adhesive film layer within an appropriate range, the low modulus flowability during the heating phase ensures that the adhesive film can deeply fill micron-scale discontinuities, reducing gaps between the shielding film and the substrate and preventing shielding effectiveness degradation caused by gaps. A moderate increase in the modulus after cooling locks the adhesive film's shape, preventing displacement of the shielding film due to subsequent processing or temperature fluctuations. This ensures precise alignment of the shielding area with the design target and avoids covering critical pads or exposing the circuitry to be shielded. The dynamically balanced modulus properties of the embodiments of the present invention allow the adhesive film layer to release local stress through flow during lamination and, after cooling, to buffer differential thermal expansion and contraction between the metal layer and the substrate through elastic deformation, reducing warping or cracking of the shielding film caused by stress accumulation. This makes it particularly suitable for applications with complex discontinuities, such as thick copper circuit boards. Furthermore, this ratio range reduces the stringent requirements for lamination temperature and pressure. Even with slight fluctuations in process parameters, the adhesive film can achieve stable filling and curing through its own performance adjustment, reducing batch defects caused by process fluctuations and improving production yield.

[0086] As a preferred embodiment, the present invention is further implemented on the basis of any of the above embodiments, wherein the elastic modulus T2 of the plated sheet and the first strain strength σ1 of the plated sheet satisfy the following relationship: 0.8≤T2*σ1≤3. Wherein, the unit of T2 is MPa, and the unit of σ1 is MPa.

[0087] The plate is composed of a composite protective layer and a metal layer. Its elastic modulus (T2) reflects the material's ability to resist elastic deformation, while the first strain intensity (σ1) represents the critical stress at which the material enters the plastic deformation stage. The present invention uses the product of these two factors (T2 × σ1) to quantitatively evaluate the plate's rigidity-toughness balance. This product balances the material's deformation resistance with the plastic buffer space, ensuring that the electromagnetic shielding film, when subjected to stress, neither fails due to excessive deformation nor suffers damage due to brittle fracture.

[0088] When T2×σ1<0.8, the rigidity and plastic critical stress of the plated sheet are both at low levels. During the electromagnetic shielding film lamination process, if it encounters local pressure generated by the circuit board's step difference or mechanical impact during subsequent processing, the electromagnetic shielding film will be unable to resist external forces due to insufficient rigidity and is prone to buckling. For example, at the height difference between thick and thin copper lines, the local pressure during lamination can cause the low-product plated sheet to bend toward the step difference, forming wrinkles or collapse, resulting in gaps between the metal layer and the film layer, destroying the shielding continuity and affecting the shielding effectiveness and grounding resistance.

[0089] When T2×σ1>3, the rigidity and plastic critical stress of the plated sheet are too high, and the electromagnetic shielding film material exhibits high rigidity and low ductility. At this time, although the plated sheet can resist large elastic deformation, once the force exceeds the first strain strength σ1, due to the lack of sufficient plastic deformation space and poor ductility, stress will quickly concentrate locally, causing brittle fracture of the electromagnetic shielding film. For example, in scenarios where the shielding film is bent or vibrated, the high-product plated sheet cannot release stress through moderate plastic deformation, and cracks are easily generated at the bending point or the edge of the fault, eventually causing the metal layer to break, causing the electromagnetic shielding film to lose its shielding function.

[0090] Therefore, the embodiment of the present invention controls the product T2×σ1 of the elastic modulus T2 of the plating of the electromagnetic shielding membrane and the first strain strength σ1 of the plating of the electromagnetic shielding membrane to be in the range of 0.8 to 3, for example, it can be 0.8, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0, 2.2, 2.3, 2.6, 2.8, 2.9, 3.0, etc. Of course, it can also be set to other qualified values ​​according to actual conditions, and no specific limitation is made here.

[0091] When T2×σ1 is between 0.8 and 3, the plate's rigidity and ductility are balanced. It possesses sufficient elastic modulus to resist localized pressure during lamination, preventing buckling instability. Furthermore, when stress approaches the first strain intensity σ1, it can buffer external forces through moderate plastic deformation, preventing brittle fracture caused by stress concentration and thus maintaining structural integrity. This range allows the plate to withstand certain bending stresses during lamination while deforming with the film without tearing.

[0092] By adopting the technical means of the embodiments of the present invention, by controlling the product of the elastic modulus and the first strain strength of the plating sheet to be within a reasonable range, it is ensured that the electromagnetic shielding film will not produce buckling and wrinkling due to insufficient stiffness, nor will it suffer brittle fracture due to poor ductility under working conditions such as pressing, bending, and vibration. The bending resistance of the plating sheet in the step area is effectively improved, and the metal layer is not prone to forming a "cantilever beam" structure at the edge of the step, thereby reducing damage caused by stress concentration, ensuring the continuous shielding path of the metal layer, and improving the quality of the electromagnetic shielding film.

[0093] As a preferred embodiment, the embodiment of the present invention is further implemented on the basis of any of the above embodiments, and the glass transition temperature Tg of the adhesive film layer 13 is ≤ 120°C.

[0094] It should be noted that the glass transition temperature (Tg) is the critical temperature at which a polymer material changes from a glassy state to a highly elastic state, that is, from a hard, brittle, and low-elastic state to a soft, highly elastic state.

[0095] When the Tg of the adhesive film layer exceeds 120°C, the film layer remains in a glassy state during thermal lamination. This restricts the movement of the film's molecular chains, resulting in high modulus and low fluidity, making it difficult to adapt to the complex shape of the circuit board's steps. For example, when using epoxy resin adhesive with a high glass transition temperature (Tg) as the adhesive film material, if the step height is greater than 50μm, the film layer cannot effectively flow and fill the step, and the metal layer is easily ruptured.

[0096] When the Tg of the adhesive layer is ≤120°C, at the lamination process temperature of the electromagnetic shielding film, typically 120-180°C, the film layer will cross the glass transition point and enter a highly elastic or even viscous state, exhibiting low modulus and high fluidity. In this state, the film layer undergoes plastic flow under the lamination pressure, filling tiny gaps along the contours of the discontinuity on the circuit board surface. When the temperature cools, the film layer returns to the glassy state and solidifies, providing sufficient cohesion. For example, when a circuit board has a discontinuity of 5-50μm, the film layer with a low glass transition temperature can further conform to the uneven structure of the discontinuity through molecular chain slippage and rearrangement, eliminating air gaps.

[0097] By adopting the technical means of the embodiments of the present invention, the adaptability of the lamination process is optimized by controlling the characteristics of the film layer with a low glass transition temperature, thereby avoiding insufficient fluidity during lamination due to the glass transition temperature of the film layer being too high, and the metal layer being torn in the air at the edge of the step. The high fluidity of the film layer with a low glass transition temperature at the lamination temperature can quickly fill the gaps of different sizes. At the same time, the film layer with a low glass transition temperature can also enable the film to be quickly cured and shaped at a lower temperature.

[0098] As a preferred embodiment, the present invention is further implemented on the basis of any of the above embodiments. Figure 2 , is a schematic structural diagram of a second electromagnetic shielding film provided by an embodiment of the present invention, wherein the metal layer 12 is undulating, and the height difference of the undulation of the metal layer 12 ranges from 3 to 7 μm.

[0099] In an embodiment of the present invention, the metal layer 12 includes a first surface and a second surface facing each other. The first surface and / or the second surface are uneven, with an undulating degree of 3 to 7 μm. The undulation is defined as the distance between the highest and lowest points of the surface. Furthermore, the undulation of the metal layer extends in two dimensions, i.e., it has undulations in both the X and Y directions.

[0100] In this embodiment, the metal shielding layer is designed with an undulating pattern, with a height difference of 3 to 7 μm. This microstructure increases the surface area and alters the stress distribution pattern. The undulating structure distributes tensile stress over a wider area, avoiding localized stress concentration. Furthermore, the peaks and valleys of the undulating layers can be embedded in the adhesive layer, enhancing mechanical interlayer engagement.

[0101] The technical means adopted in the embodiments of the present invention can effectively improve the flexibility and tear resistance of the metal layer, especially when laminating to fill the gap. The undulating structure disperses the tensile stress to more areas, avoiding local stress concentration. At the same time, it can anchor the film layer, reduce the relative slippage between the metal layer and the film, and reduce the risk of tearing. In addition, the undulating metal layer can pierce the film layer and ground during the lamination process, and has a larger grounding area, thereby ensuring the electromagnetic shielding effectiveness.

[0102] As a preferred embodiment, the present invention is further implemented on the basis of any of the above embodiments. Figure 3 , is a schematic structural diagram of the third electromagnetic shielding film provided by an embodiment of the present invention, wherein the adhesive film layer 13 is provided with conductive particles 131, and the ratio of the maximum width of the conductive particles 131 to the minimum thickness of the adhesive film layer 13 is less than 0.8.

[0103] In this embodiment of the present invention, by controlling the ratio of the maximum width W of the conductive particles in the adhesive film to the thickness H of the adhesive film to less than 0.8, this prevents excessive particle size from causing localized strength loss in the adhesive film. If the particle size approaches or exceeds the film thickness, it could puncture the metal layer or create stress concentration points during lamination. Controlling this ratio ensures uniform particle dispersion without disrupting the continuity of the adhesive film.

[0104] The technical means of the embodiments of the present invention are used to ensure the balance between the mechanical integrity and conductivity of the film layer, avoid shielding failure or the formation of stress concentration points due to conductive particles piercing the metal layer, and at the same time ensure the deformation ability of the film at the break, thereby improving the overall tear resistance.

[0105] As a preferred embodiment, the present invention is further implemented on the basis of any of the above embodiments. Figure 4 , is a structural schematic diagram of the fourth electromagnetic shielding film provided by an embodiment of the present invention, the protective layer 11 includes a support layer 111 and a filling layer 112, and the support layer 111 is arranged on the side surface of the filling layer 112 facing away from the metal layer 12.

[0106] In this embodiment of the present invention, protective layer 11 is divided into a support layer 111 and a filling layer 112. Support layer 111 provides structural rigidity, while filling layer 112 adapts to the contour of the step. During lamination, filling layer 112 flows into the bottom of the step to fill the gap, while support layer 111 prevents excessive deformation of the protective layer. The two layers work together to ensure a more uniform stress distribution across the metal layer.

[0107] Optionally, the filling layer 112 is elastic resin.

[0108] The technical means adopted in the embodiments of the present invention can effectively improve the protective layer's ability to wrap and protect the metal layer, especially in high-break scenarios. The supporting layer plays a better supporting role, and the filling layer can serve as a pre-buffer layer to reduce the shear stress directly borne by the metal layer and reduce the probability of press-fit breakage. At the same time, the filling layer can also fill the breakage to avoid breakage and damage to the metal layer.

[0109] The various properties of the conventional electromagnetic shielding film and the electromagnetic shielding film according to the embodiment of the present invention were tested respectively using specific examples.

[0110] The electromagnetic shielding film S of the embodiment of the present invention includes samples S1 to S5, each of which includes a film layer, a metal layer, and a protective layer stacked in sequence, wherein the metal layer and the protective layer form a plated sheet; the control electromagnetic shielding film R includes controls R1 and R2, each of which includes a film layer, a metal layer, and a protective layer stacked in sequence. Compared with the control electromagnetic shielding film R, the electromagnetic shielding film S of the embodiment of the present invention has test data of the first strain strength σ1 of the plated sheet, the first strain strength σ3 of the protective layer, the first strain strength σ4 of the film layer, and the first strain strength σ2 of the entire electromagnetic shielding film as shown in Table 1:

[0111] Table 1

[0112]

[0113] The test data of the thermal expansion coefficient CTE1 of the adhesive layer, the thermal expansion coefficient CTE2 of the metal layer, the elastic modulus T1 of the adhesive layer, and the elastic modulus T2 of the plated sheet in the electromagnetic shielding film S of the embodiment of the present invention and the control electromagnetic shielding film R are shown in Table 2:

[0114] Table 2

[0115]

[0116]

[0117] The comparison results of the tensile strength, elongation and damage and tearing at the break of the electromagnetic shielding film S of the embodiment structure of the present invention and the control electromagnetic shielding film R are shown in Table 3:

[0118] Table 3

[0119] electromagnetic shielding film Tensile strength / MPa Elongation / % Damage and tearing at the fault Sample S1 20.4 7.1 No damage or tearing issues Sample S2 18.1 9.3 No damage or tearing issues Sample S3 19.5 11.9 No damage or tearing issues Sample S4 17.1 8.7 No damage or tearing issues Sample S5 16.6 11.6 No damage or tearing issues Control R1 19.4 2.1 There is damage or tearing Control R2 11.8 4.0 Presence of wrinkles and fine lines

[0120] It can be seen from the experimental data in Tables 1 to 3 that, compared with the electromagnetic shielding film in the comparative example, the electromagnetic shielding film of the present invention has higher tensile strength and elongation, the integrity of the electromagnetic shielding film is better, and there are fewer cases of damage and tearing at the break during the lamination process. It can be seen that the covering film of the present invention has more superior performance.

[0121] See also Figure 5 , is a structural schematic diagram of a circuit board provided by an embodiment of the present invention. The embodiment of the present invention also provides a circuit board 20, wherein the circuit board 20 includes a circuit board body 21 and an electromagnetic shielding film 10 as described in any one of the above embodiments; the electromagnetic shielding film includes a protective layer 11, a metal layer 12 and a film layer 13 stacked in sequence, and the electromagnetic shielding film 10 is pressed together with the circuit board body 21 through the film layer 13.

[0122] It should be noted that the structure of the electromagnetic shielding film may refer to the structure of the electromagnetic shielding film described in any of the above embodiments, and will not be described in detail here.

[0123] In an embodiment of the present invention, the electromagnetic shielding film of the above-mentioned embodiment is applied to a circuit board. By limiting the change in the first strain strength of the electromagnetic shielding film before and after coating to no more than 40%, the stress concentration phenomenon caused by the sudden change in interlayer strength can be effectively avoided. By utilizing the low first strain strength characteristic of the adhesive film layer, the plastic flow of the adhesive film layer is used to fill the gap during lamination, thereby dispersing the stress of the metal layer and avoiding the electromagnetic shielding film from being damaged or torn at the gap during the lamination process.

[0124] The technical means adopted in the embodiments of the present invention can effectively improve the reliability of circuit boards in multi-layer lamination and high-frequency signal transmission scenarios, effectively solve the problem of shielding film damage in the discontinuity area, ensure signal integrity, reduce EMI interference, and are suitable for the manufacture of precision circuit boards for wireless communications, high-end electronic equipment, etc.

[0125] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. An electromagnetic shielding film, characterized in that The electromagnetic shielding film comprises a protective layer, a metal layer and an adhesive film layer stacked in sequence; the protective layer and the metal layer are stacked to form a plated sheet, and the first strain strength σ1 of the plated sheet and the first strain strength σ2 of the entire electromagnetic shielding film satisfy: The first strain intensity refers to the stress when the stress-strain curve deviates from the linear segment and reaches a preset multiple a of the plastic strain, and the range of a is 0.00018 to 0.00035.

2. The electromagnetic shielding film according to claim 1, wherein The first strain strength σ3 of the protective layer and the first strain strength σ2 of the entire electromagnetic shielding film satisfy: |σ3-σ2|≤1MPa.

3. The electromagnetic shielding film according to claim 1, wherein The first strain strength σ4 of the adhesive film layer is 5 to 15 MPa.

4. The electromagnetic shielding film according to claim 1, wherein The difference between the thermal expansion coefficient CTE1 of the adhesive film layer and the thermal expansion coefficient CTE2 of the metal layer is less than 100 ppm / °C.

5. The electromagnetic shielding film according to claim 1, wherein The elastic modulus T1 of the adhesive film layer and the thermal expansion coefficient CTE1 of the adhesive film layer satisfy: 0.0025≤T1 / CTE1≤0.0417, wherein the unit of T1 is MPa, the unit of CTE1 is ppm / °C, and the calculation is dimensionless.

6. The electromagnetic shielding film according to claim 1, wherein The elastic modulus T2 of the plated sheet and the first strain strength σ1 of the plated sheet satisfy: 0.8≤T2*σ1≤3, where the unit of T2 is MPa; and / or, The glass transition temperature Tg of the adhesive film layer is ≤120°C.

7. The electromagnetic shielding film according to any one of claims 1 to 6, wherein The metal layer is undulating, and the height difference of the undulations of the metal layer ranges from 3 to 7 μm.

8. The electromagnetic shielding film according to any one of claims 1 to 6, wherein The adhesive film layer is provided with conductive particles, and the ratio of the maximum width of the conductive particles to the minimum thickness of the adhesive film layer is less than 0.

8.

9. The electromagnetic shielding film according to any one of claims 1 to 6, wherein The protective layer includes a supporting layer and a filling layer. The supporting layer is arranged on a surface of the filling layer facing away from the metal layer.

10. A circuit board, characterized in that: The circuit board comprises a circuit board body and the electromagnetic shielding film according to any one of claims 1 to 9; the electromagnetic shielding film is pressed together with the circuit board body via an adhesive film layer.

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