Large-current transmission method for flexible printed circuit board in weak magnetic environment

Abstract

The invention discloses a large-current transmission method for a flexible printed circuit board in a weak magnetic environment. The method comprises the following steps: designing a wiring structure, decomposing a current loop into an array form, and performing high-symmetry arrangement; a flexible PCB lamination structure is planned, the line width, the wiring spacing, the geometric dimension and the like are designed according to the determined wiring structure, and the current wires of different layers are connected through via holes; electrical connection is completed according to the structure and the function of a transmission object, the current with the wiring design completed is connected to a bonding pad, a connector and other structures, electrical connection is formed, and current transmission is completed. According to the invention, a large current can be applied in a weak magnetic environment, a magnetic field of a hundred milliampere current is reduced to a Nattesla magnitude, and a complex structure design and an electrical connection function are realized at the same time.

Description

Technical Field

[0001] The present invention relates to the field of precision magnetic field measurement, and more specifically to a method for reducing the magnetic field generated during the transmission of a 100-milliampere current to the nanotesla level by designing the wiring method and laminate structure of flexible printed circuit board (PCB) cables, thereby enabling the use of large currents in weak magnetic environments. Background Art

[0002] According to Biot-Savart's law, electric current and magnetic field are a pair of deeply coupled physical quantities. Therefore, in many precision magnetic field measurement environments, the current in the environment must be kept at a low level. For example, magnetoencephalography (MEG) studies brain function by detecting the weak picotesla-level magnetic fields (approximately one billionth of the Earth's magnetic field) generated by brain neuronal activity; magnetocardiography (MCG) captures the sub-nanotesla-level magnetic fields generated by cardiac electrical activity. In these types of measurements, active or passive shielding techniques are first required to shield against external strong magnetic field interference sources such as the Earth's magnetic field, reducing the background magnetic field to the nanotesla level.

[0003] In addition, new interference sources, such as electronic components and power supply lines, should be avoided within the magnetic shielding device to avoid problems such as static magnetic fields or magnetic noise caused by strong power supply currents. Therefore, apart from magnetic field detectors, it is difficult for other auxiliary equipment to enter the measurement environment. However, some weakly magnetic electronic components and optical chips can provide some auxiliary functions for magnetic field measurement in a weak magnetic detection environment, such as communications, electrophysiological acquisition, and wearable medical monitoring sensors. The magnetic compatibility requirements for such devices, in addition to the material of the chip itself, usually require a strong current for the chip's power supply. Therefore, the power supply and signal current also need to be designed specifically for magnetic compatibility.

[0004] Current transmission occurs in a loop. Because the current's return and return directions differ, the magnetic field generated also differs in direction. Therefore, when the current's return paths are the same, the smaller the distance between the two paths, the more opposite the magnetic field directions, and the greater the cancellation effect. Everyday twisted-pair and coaxial cables have the ability to cancel each other out and suppress magnetic field leakage. However, traditional coaxial and twisted-pair cables only provide simple connection and transmission functions, rely on connectors, and are relatively small and inflexible. Flexible PCBs, on the other hand, can adapt to a wide range of deformation scenarios, and their routing schemes can be precisely designed, allowing for targeted magnetic field design. They also offer greater flexibility in mounting with various chips, allowing for balanced magnetic field considerations while also meeting process requirements for wire bonding and on-chip packaging.

[0005] In summary, the flexible design and application of flexible PCBs to transmit current in weak magnetic environments while reducing magnetic field interference on measurements can open up new opportunities for the application of more electronic devices in precision magnetic field measurements. However, a complete and practical method for current transmission on flexible PCBs that takes magnetic fields into account has not yet been proposed. Summary of the Invention

[0006] In view of the above, an object of the present invention is to provide a method for transmitting a current of hundreds of milliamperes in a weak magnetic environment while reducing the magnetic field generated at close range to the nanotesla level (nT).

[0007] The technical solution adopted in the present invention is as follows:

[0008] A method for transmitting high current on a flexible printed circuit board in a weak magnetic environment comprises the following steps:

[0009] Divide the conductors running back and forth in the multi-layer flexible PCB into an even number of equal parts, into conductors in a first current direction and conductors in a second current direction, wherein the first current direction is the opposite direction of the second current direction;

[0010] Arranging the conductors in the first current direction and the conductors in the second current direction into an N×N distribution pattern for canceling magnetic fields, wherein the conductors in the first current direction and the conductors in the second current direction are alternately arranged in both the layer width direction and the layer thickness direction of the multilayer flexible PCB;

[0011] The N×N distributed conductors in the multi-layer flexible PCB are used to achieve high current transmission and reduce the magnetic field.

[0012] Furthermore, the N×N distribution is a 2×2 distribution, indicating a 2-layer flexible PCB with 2 conductors laid on each layer.

[0013] Furthermore, the N×N distribution is a 4×4 distribution, indicating a 4-layer flexible PCB with 4 conductors laid on each layer.

[0014] Furthermore, the 4×4 distribution is one of the following three configurations, where “·” and “×” represent the first current direction and the second current direction, respectively:

[0015] Configuration A: The arrangement of the first layer of wires is “·×·×”, the arrangement of the second layer of wires is “×·×·”, the arrangement of the third layer of wires is “·×·×”, and the arrangement of the fourth layer of wires is “×·×·”;

[0016] Configuration B: The arrangement of the first layer of wires is “·××·”, the arrangement of the second layer of wires is “×··×”, the arrangement of the third layer of wires is “·××·”, and the arrangement of the fourth layer of wires is “×··×”;

[0017] Configuration C: The arrangement of the first layer of wires is “·××·”, the arrangement of the second layer of wires is “×··×”, the arrangement of the third layer of wires is “×··×”, and the arrangement of the fourth layer of wires is “·××·”.

[0018] Furthermore, based on the requirements of the test environment as well as cost and volume factors, the number of layers, line width, layer width spacing, and layer thickness spacing of the flexible PCB are selected, and the electrical connections are designed according to the transmission object, thereby forming a magnetically compatible current transmission solution.

[0019] Furthermore, by setting multiple vias, the wires of different layers of the flexible PCB are connected without interfering with each other, each layer is bonded with insulating material, and a protective layer is laid on the surface to form a laminated structure, thereby determining the total thickness of the flexible PCB.

[0020] Furthermore, after the wires are connected to each other through the vias, mounting pads are provided on the flexible PCB to form an electrical connection with the wires, and then connected to the chip through wire bonding to complete the electrical connection.

[0021] The present invention also provides a flexible printed circuit board in a weak magnetic environment for implementing the above method, which includes a multi-layer flexible PCB; the conductors running back and forth in the multi-layer flexible PCB are divided into an even number of equal parts, divided into conductors in a first current direction and conductors in a second current direction, wherein the first current direction is the opposite direction of the second current direction; the conductors in the first current direction and the conductors in the second current direction are arranged in an N×N distribution manner to offset the magnetic field, wherein the conductors in the first current direction and the conductors in the second current direction are arranged alternately in the layer width direction and the layer thickness direction of the multi-layer flexible PCB.

[0022] The flexible PCB high current transmission method in a weak magnetic environment of the present invention has the following advantages:

[0023] 1. This method can reduce the magnetic field generated by transmitting hundreds of milliamperes of current to the nanoT level within a short distance in precision magnetic field measurement scenarios. This provides the same shielding level as conventional magnetic shielding devices and is compatible with weak magnetic field detection requirements.

[0024] 2. The flexible PCB used is suitable for most deformation scenarios and can adapt to different structures and systems;

[0025] 3. Flexible PCB also supports the placement of chips and electronic components, and has the conditions to realize complex electronic functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the principle of the present invention;

[0027] Figure 2A schematic diagram of the laminate structure of a flexible PCB and an example of implementing chip current transmission. Coverlay represents the protective layer; Polyimide represents polyimide, the primary material for flexible printed circuit boards; Top Layer represents the surface layer; Middle Layer 1 represents middle layer 1; Middle Layer 2 represents middle layer 2; and Bottom Layer represents the bottom layer. DETAILED DESCRIPTION

[0028] The following description further illustrates the flexible PCB current transmission method of the present invention through specific embodiments, so that those skilled in the art can gain a more thorough understanding of the features and advantages of the present invention. It should be noted that the description below is merely a representative example of a typical application. Clearly, the present invention is not limited to any specific structure, function, device, or method described herein and may have other embodiments or combinations of other embodiments. The software / hardware modules described in the present invention or illustrated in the accompanying drawings may also be flexibly adjusted as needed.

[0029] The present invention provides a method for transmitting high current on a flexible printed circuit board in a weak magnetic environment, which can achieve magnetic field offset on a multi-layer flexible PCB. The technical solution is as follows:

[0030] The current (conductor) flowing back and forth in the multi-layer flexible PCB is divided into an even number of equal parts, into conductors in the first current direction and conductors in the second current direction, where the first current direction is the opposite direction of the second current direction, and rearranged to obtain Figure 1 The N×N distribution method with multiple offset arrangements shown in the figure, in which the conductors in the first current direction and the conductors in the second current direction are arranged alternately in the layer width direction (horizontal direction) and layer thickness direction (vertical direction) of the multi-layer flexible PCB. For example, it can be a 2×2 distribution, a 4×4 distribution, etc. The 4×4 distribution can be divided into three configurations: A, B, and C. Among them, 2×2 means a 2-layer flexible PCB with 2 conductors laid on each layer; 4×4 means a 4-layer flexible PCB with 4 conductors laid on each layer. "·" means the current direction is perpendicular to the paper and outward, and "×" means the current direction is perpendicular to the paper and inward.

[0031] like Figure 1 As shown, the three configurations of 4×4 distribution are as follows:

[0032] Configuration A: The arrangement of the first layer of wires is “·×·×”, the arrangement of the second layer of wires is “×·×·”, the arrangement of the third layer of wires is “·×·×”, and the arrangement of the fourth layer of wires is “×·×·”;

[0033] Configuration B: The arrangement of the first layer of wires is “·××·”, the arrangement of the second layer of wires is “×··×”, the arrangement of the third layer of wires is “·××·”, and the arrangement of the fourth layer of wires is “×··×”;

[0034] Configuration C: The arrangement of the first layer of wires is “·××·”, the arrangement of the second layer of wires is “×··×”, the arrangement of the third layer of wires is “×··×”, and the arrangement of the fourth layer of wires is “·××·”.

[0035] Apply the infinite straight wire model to calculate, using the trace spacing d1 (horizontal wire spacing), d2 (vertical wire spacing), the distance D to the test point, and the number of wires per layer K. Calculate the magnetic field generated by each wire at point P, decompose it, add it up, and then express the magnetic field at the test point P as:

[0036]

[0037] The distance from each wire to point P is i represents the i-th conductor in the transverse direction (layer width direction), j represents the j-th conductor in the longitudinal direction (layer thickness direction), μ0 represents the vacuum magnetic permeability, and I represents the current in each conductor.

[0038] According to the general process capability of flexible PCB, 2-layer, 4-layer, and 8-layer are more suitable choices; the trace spacing d1 is set in the layer width direction, and the general process is 10mil; the spacing d2 is set in the layer thickness direction, and the general process is 3mil. Using numerical calculation, the magnetic field modulus M generated by transmitting 100mA current at point P when the distance D to the test point changes from 1 to 10mm is obtained. sum As shown in Table 1.

[0039] Table 1

[0040]

[0041] All of the above configurations can cancel the magnetic field to 1 nT at a distance of 1 cm from the test point. Among them, the 4×4 configuration C scheme has the best symmetry and therefore the strongest magnetic field cancellation effect.

[0042] During the flexible PCB design process, the design of a magnetically compatible current transmission solution can be completed by selecting the appropriate number of layers, line width, and layer spacing (i.e., the conductor spacing d1 in the layer width direction) and layer spacing (i.e., the conductor spacing d2 in the layer thickness direction) based on the test environment requirements, cost, and volume factors. Furthermore, the electrical connections are designed based on the transmission object. Layer spacing is the distance between adjacent conductors on the same layer, and layer spacing is the distance between adjacent layers.

[0043] According to one embodiment of the present invention, a method for transmitting high current on a flexible PCB in a weak magnetic environment mainly includes three steps:

[0044] The first step is to design the routing topology according to actual needs: Figure 1 A 2×2 and three 4×4 cancellation configurations are presented. According to theoretical calculations, Structure C has the strongest symmetry and the best cancellation effect. In some extremely weak magnetic field detection applications, such as biomagnetic measurements of heart and brain magnetism, where signal levels range from pT to fT, Structure C with a higher number of layers is more advantageous. Configurations with a higher number of layers offer better cancellation, but most current flexible PCB processes only support 2 to 12 layers. Therefore, while meeting the magnetic compatibility requirements of the signal being measured, choosing fewer layers can better control costs and reduce design and manufacturing complexity.

[0045] The second step is to plan the PCB lamination structure: for example, after selecting a 4-layer flexible PCB, determine the line width and trace spacing, plan the lamination structure, and determine the PCB geometry according to the measurement environment. Figure 2 As shown, a four-layer flexible PCB configuration, configuration C, is selected. Four conductors are laid on each layer. The conductors on the top and bottom layers are defined in sequence as "+," "-," "-," and "+." The conductors on the middle two layers are defined as "-," "+," "+," and "-." The "+" and "-" are connected at the head and tail ends to complete the conductor definition. "+" indicates the positive direction of current propagation, and "-" indicates the return path of the current, which is opposite to the propagation direction. Multiple vias are provided to connect the conductors of different layers without interfering with each other. Each layer is bonded together using an insulating material such as polyimide, and a protective layer is laid on the surface to form a laminated structure, which determines the total thickness of the flexible PCB.

[0046] Finally, the electrical connection method is designed according to the structure or function of the transmission object: the connection method is designed according to the actual object to be connected. For example, for unpackaged bare chips that meet magnetic compatibility requirements, wire bonding is required for connection. Figure 2 As shown, after the "+" and "-" wires are connected to each other through vias, mounting pads are set on the flexible PCB to form an electrical connection with the wires, and then connected to the chip through wire bonding and other methods to complete the electrical connection design.

[0047] In summary, the present invention discloses a method for transmitting large currents on a flexible printed circuit board in a weak magnetic environment. This method includes designing a routing structure, decomposing the current loop into an array form, and arranging it with high symmetry; planning the flexible PCB laminate structure, designing the line width, routing spacing, geometric dimensions, etc. according to the determined routing structure, and connecting the current conductors of different layers using vias; completing the electrical connection according to the structure and function of the transmission object, connecting the current with the completed routing design to structures such as pads and connectors to form an electrical connection and complete the current transmission. The present invention can realize the application of large currents in a weak magnetic environment, reduce the magnetic field of a current of hundreds of milliamperes to the nanotesla level, and at the same time have the functions of complex structural design and electrical connection.

[0048] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any skilled artisan may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be as set forth in the appended claims.

Claims

1. A method for transmitting high current on a flexible printed circuit board in a weak magnetic environment, characterized in that: The following steps are involved: Divide the conductors running back and forth in the multi-layer flexible PCB into an even number of equal parts, into conductors in a first current direction and conductors in a second current direction, wherein the first current direction is the opposite direction of the second current direction; Arranging the conductors in the first current direction and the conductors in the second current direction into an N×N distribution pattern for canceling magnetic fields, wherein the conductors in the first current direction and the conductors in the second current direction are alternately arranged in both the layer width direction and the layer thickness direction of the multilayer flexible PCB; The N×N distributed conductors in the multi-layer flexible PCB are used to achieve high current transmission and reduce the magnetic field.

2. The method according to claim 1, characterized in that The N×N distribution is a 2×2 distribution, indicating a 2-layer flexible PCB with 2 conductors laid on each layer.

3. The method according to claim 1, characterized in that The N×N distribution is a 4×4 distribution, indicating a 4-layer flexible PCB with 4 conductors laid on each layer.

4. The method according to claim 3, characterized in that The 4×4 distribution is one of the following three configurations, where “·” and “×” represent the first current direction and the second current direction, respectively: Configuration A: The arrangement of the first layer of wires is "·×·×", the arrangement of the second layer of wires is "×·×·", the arrangement of the third layer of wires is "·×·×", and the arrangement of the fourth layer of wires is "×·×·"; Configuration B: The arrangement of the first layer of wires is "·××·", the arrangement of the second layer of wires is "×··×", the arrangement of the third layer of wires is "·××·", and the arrangement of the fourth layer of wires is "×··×"; Configuration C: The arrangement of the first layer of wires is "·××·", the arrangement of the second layer of wires is "×··×", the arrangement of the third layer of wires is "×··×", and the arrangement of the fourth layer of wires is "·××·".

5. The method according to claim 1, wherein The magnetic field at the measured point P is calculated using the following formula: Among them, D i,j is the distance from each wire to point P, i represents the i-th horizontal conductor, j represents the j-th vertical conductor, μ0 represents the vacuum magnetic permeability, I represents the current in each conductor; d1 represents the horizontal conductor spacing, d2 represents the vertical conductor spacing, D represents the distance to the test point, and K represents the number of conductors in each layer.

6. The method according to claim 1, characterized in that Based on the requirements of the test environment as well as cost and volume factors, the number of layers, line width, layer width spacing, and layer thickness spacing of the flexible PCB are selected, and the electrical connections are designed according to the transmission object to form a magnetically compatible current transmission solution.

7. The method according to claim 6, characterized in that By setting up multiple vias, the wires of different layers of the flexible PCB are connected without interfering with each other. Insulating materials are used to bond each layer, and a protective layer is laid on the surface to form a laminated structure, thereby determining the total thickness of the flexible PCB.

8. The method according to claim 7, characterized in that After the wires are connected to each other through vias, mounting pads are set on the flexible PCB to form an electrical connection with the wires, and then connected to the chip through wire bonding to complete the electrical connection.

9. A flexible printed circuit board in a weak magnetic environment, characterized in that: The multi-layer flexible PCB comprises a plurality of conductors running back and forth in the multi-layer flexible PCB, each conductor being divided into conductors with a first current direction and conductors with a second current direction, wherein the first current direction is opposite to the second current direction; The conductors in the first current direction and the conductors in the second current direction are arranged in an N×N distribution pattern to cancel out the magnetic field, wherein the conductors in the first current direction and the conductors in the second current direction are alternately arranged in the layer width direction and the layer thickness direction of the multilayer flexible PCB.

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