Multilayer circuit board, solder structure and method for manufacturing the same

Abstract

The application provides a multilayer circuit board, which comprises a connecting layer and at least two flexible substrates stacked. The flexible substrate comprises two circuit layers, and the flexible substrate is divided into a soldering area and a non-soldering area. The soldering areas of two adjacent flexible substrates are oppositely arranged, and the non-soldering areas of two adjacent flexible substrates are oppositely arranged. Each circuit layer comprises a first pad in the soldering area, and the first pads of two adjacent flexible substrates oppositely arranged are oppositely arranged. The connecting layer is connected between the non-soldering areas of two adjacent flexible substrates. The connecting layer and the soldering areas of two adjacent flexible substrates jointly form a cavity. The multilayer circuit board provided by the application is convenient for layer-by-layer soldering. The application also provides a preparation method of a soldering structure, which can solve the problem of deformation of the circuit board after soldering. In addition, the application also provides a soldering structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of circuit board welding, in particular to a multilayer circuit board, a welding structure and a preparation method thereof. BACKGROUND

[0002] When a multilayer circuit board needs to be welded to a carrier board, conductive paste is usually formed on the pads of the carrier board first, and then the outer pads of the multilayer circuit board are attached to the conductive paste to obtain an intermediate body. Finally, the intermediate body is hot-pressed by a hot-pressing head to complete the welding of the carrier board and the circuit board. However, before welding, the hot-pressing head can only hot-press the intermediate body from the surface of the multilayer circuit board away from the carrier board, which results in a relatively thick hot-pressing thickness of the hot-pressing head, and the hot-pressing temperature during welding needs to be increased to ensure the welding effect.

[0003] Moreover, the thermal conductivity of the dielectric layer of the existing multilayer circuit board is low, which also requires an increase in the hot-pressing temperature during welding to ensure the melting of the conductive paste. However, if the welding temperature is too high, the dielectric layer may be deformed, thereby affecting the electrical performance of the product after welding. SUMMARY

[0004] Therefore, the present application provides a multilayer circuit board facilitating layer-by-layer welding.

[0005] The present application also provides a preparation method of a welding structure, which can solve the problem of deformation of the circuit board after welding.

[0006] In addition, the present application also provides a welding structure.

[0007] The present application provides a multilayer circuit board, which comprises a connecting layer and at least two flexible substrates stacked along a first direction. The flexible substrate comprises two circuit layers. The flexible substrate is divided into a welding area and a non-welding area along a second direction perpendicular to the first direction. The welding areas of two adjacent flexible substrates are oppositely arranged in the first direction, and the non-welding areas of two adjacent flexible substrates are oppositely arranged in the first direction. Each circuit layer comprises a first pad located in the welding area, and the oppositely arranged first pads of two adjacent flexible substrates are oppositely arranged in the first direction. The connecting layer is connected between the non-welding areas of two adjacent flexible substrates, and the connecting layer and the welding areas of two adjacent flexible substrates jointly form a cavity. The projection of the first pad in the first direction is located in the cavity.

[0008] In the present application, the connection layer in the multi-layer circuit board and the welding area of the flexible substrate jointly form a cavity, so that the welding area of the flexible substrate can be bent around the non-welding area, thereby facilitating the avoidance of a welding space during welding of the multi-layer circuit board. Therefore, the hot press head can be placed in the welding space during welding, rather than on the side of the multi-layer circuit board away from the welding body. Thus, the multi-layer circuit board provided by the present application can be welded layer by layer during welding. Therefore, the multi-layer circuit board provided by the present application is convenient for layer-by-layer welding.

[0009] In some possible embodiments, the flexible substrate further comprises a dielectric layer connected between the two circuit layers, and the flexible substrate is provided with a through hole penetrating the first pad and the two circuit layers.

[0010] In some possible embodiments, the side wall of the through hole is provided with a conductive part connecting the two circuit layers. The present application further provides a preparation method of a welding structure, comprising the following steps:

[0011] providing the multi-layer circuit board;

[0012] providing a welding body, wherein the welding body comprises a welding body and a second pad fixed to a surface of the welding body;

[0013] forming conductive paste on the second pad, and stacking the multi-layer circuit board and the welding body in the first direction, so that the first pad of the flexible substrate closest to the welding body is attached to the conductive paste in the first direction;

[0014] bending the welding area of the flexible substrate except the flexible substrate closest to the welding body around the non-welding area in a direction away from the welding body, and hot pressing the attachment of the first pad and the conductive paste, so that the flexible substrate closest to the welding body is welded to the welding body;

[0015] forming the conductive paste on another first pad of the flexible substrate after welding, releasing the next flexible substrate adjacent to the welded flexible substrate, so that the first pad of the flexible substrate after deformation recovery is attached to the conductive paste on the previous flexible substrate;

[0016] welding the flexible substrate after deformation recovery to the previous flexible substrate by hot pressing, until each adjacent two flexible substrates are welded to each other.

[0017] In some possible embodiments, the flexible substrate further comprises a dielectric layer connected between the two circuit layers, and the flexible substrate is provided with a through hole penetrating the first pad and the two circuit layers, and the conductive paste also fills in the through hole after each hot pressing.

[0018] In some possible implementations, the sidewall of the through hole is provided with a conductive part connecting the two circuit layers, and the conductive paste is also welded to the conductive part after each hot pressing.

[0019] In some possible implementations, the preparation of the multilayer circuit board comprises: forming an isolation layer and the connecting layer on each circuit layer of the flexible substrate, the isolation layer being arranged at the welding area and covering the corresponding first pad; stacking another flexible substrate on the isolation layer and the connecting layer and pressing, the isolation layer being arranged between the welding areas of the two adjacent flexible substrates; and removing the isolation layer to form the cavity.

[0020] In some possible implementations, the isolation layer is a release layer.

[0021] In some possible implementations, the connecting layer is a glue layer or a prepreg.

[0022] In the present application, the connecting layer and the welding area of the flexible substrate in the multilayer circuit board jointly form a cavity, so that the welding area of the flexible substrate can be bent around the non-welding area. When welding the multilayer circuit board, the flexible substrates except the one closest to the welding body can be bent around the non-welding area to avoid the welding space. Therefore, the hot pressing head can be placed in the welding space instead of being placed on the side of the multilayer circuit board away from the second pad, that is, the hot pressing thickness is equivalent to the thickness of one flexible substrate, thereby facilitating the reduction of the hot pressing thickness of the hot pressing head. Subsequently, after forming new conductive paste on the flexible substrate after welding, the next flexible substrate is released one by one and the welding of the flexible substrate is carried out one by one to weld the two adjacent flexible substrates. Each time when welding the two adjacent flexible substrates, the hot pressing thickness is still equivalent to the thickness of one flexible substrate. Therefore, the present application facilitates the reduction of the hot pressing thickness, thereby being capable of reducing the hot pressing temperature during welding. Therefore, the preparation method provided by the present application can solve the problem of deformation during the welding of the multilayer circuit board. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A cross-sectional view of the stacked flexible substrate, isolation layer and connecting layer provided by an embodiment of the present application;

[0024] Figure 2 A cross-sectional view of the pressed Figure 1 flexible substrate and the removed isolation layer to form a cavity to obtain an intermediate product;

[0025] Figure 3 A cross-sectional view of the multilayer circuit board obtained by an embodiment of the present application;

[0026] Figure 4 A cross-sectional view of the welding body provided by an embodiment of the present application;

[0027] Figure 5 for hot pressing using a hot press head Figure 4 the solder body and Figure 3 a cross-sectional view of the flexible substrate of the multilayer circuit board shown in FIG. 1, taken behind a first pad of the flexible substrate nearest the solder body;

[0028] Figure 6 for Figure 5 a cross-sectional view of the flexible substrate shown in FIG. 1, taken behind another first pad of the flexible substrate on which conductive paste is formed;

[0029] Figure 7 for hot pressing using a hot press head Figure 6 the flexible substrate and Figure 5 a cross-sectional view of the conductive paste shown in FIG. 1;

[0030] Figure 8 a cross-sectional view of a solder structure obtained by an embodiment of the present application.

[0031] Explanation of Main Element Symbols

[0032] multilayer circuit board 100 flexible substrate 10

[0033] circuit layer 11 first pad 111

[0034] dielectric layer 12 via 121

[0035] conductive portion 13 connection layer 20

[0036] isolation layer 30 solder body 40

[0037] solder body 41 second pad 42

[0038] conductive paste 50 solder structure 200

[0039] solder region 101 non-solder region 102

[0040] cavity 201 first direction X

[0041] second direction Y hot press head 60

[0042] solder space 601 intermediate body 103

[0043] The following detailed description will further describe the present application in conjunction with the above-described drawings. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments.

[0045] It is to be understood that where an element such as a layer, region or substrate is described as being "on" another element, it can be directly on the other element or intervening elements can also be present. Where an element such as a layer, region or substrate is described as being "connected" to another element, it can be directly connected to the other element or intervening elements can also be present. Where an element such as a layer, region or substrate is described as being "positioned on" another element, it can be directly on the other element or intervening elements can also be present.

[0046] Embodiment 1

[0047] The present application provides a method for preparing a solder structure. The order of steps of the method can be changed, some steps can be omitted or combined according to different requirements. The method comprises the following steps:

[0048] Step 1: referring to Figure 1 , a flexible substrate 10 is provided. The flexible substrate 10 comprises two circuit layers 11 stacked along a first direction X. The flexible substrate 10 is divided into a solder area 101 and a non-solder area 102 along a second direction Y perpendicular to the first direction X. The flexible substrate 10 further comprises a dielectric layer 12 connected between the two circuit layers 11. Each circuit layer 11 comprises a first pad 111 located in the solder area 101. Exemplarily, the material of the circuit layer 11 can be copper, and the material of the dielectric layer 12 can be one of polyimide or polyethylene terephthalate.

[0049] In some embodiments, the flexible substrate 10 is provided with a through hole 121 penetrating the dielectric layer 12 and the first pad 111 of the two circuit layers 11. The sidewall of the through hole 121 is provided with a conductive part 13 connecting the two circuit layers 11.

[0050] Exemplarily, the flexible substrate 10 provided with the conductive part 13 can be obtained by drilling, electroplating, exposure and development, etching on a double-sided copper-clad substrate.

[0051] Then, an isolation layer 30 and a connecting layer 20 are formed on one circuit layer 11 of a flexible substrate 10. The isolation layer 30 is provided in the solder area 101 and covers the corresponding first pad 111. The connecting layer 20 is provided in the non-solder area 102.

[0052] In some embodiments, the isolation layer 30 can be a release layer, such as a release paper or a release film. In this embodiment, the isolation layer 30 is a release paper, such as a Gelasin release paper, a CCK release paper or a silicone release paper. By setting the isolation layer 30 as a release layer, it is convenient to remove the isolation layer 30 in the subsequent step.

[0053] In another embodiment, the isolation layer 30 can be a pyrolytic adhesive layer, which can be removed by heating in the subsequent step.

[0054] In some embodiments, the connecting layer 20 can be an adhesive layer, such as an epoxy adhesive layer or a vinyl adhesive layer. In this case, the adhesive can be directly coated to form the connecting layer 20.

[0055] In another embodiment, the connecting layer 20 can be a prepreg.

[0056] Then, another flexible substrate 10 is stacked on the isolation layer 30 and the connecting layer 20. The soldering areas 101 of the two adjacent flexible substrates 10 are oppositely arranged in the first direction X. The non-soldering areas 102 of the two adjacent flexible substrates 10 are oppositely arranged in the first direction X. The oppositely arranged first pads 111 of the two adjacent flexible substrates 10 are oppositely arranged in the first direction X. The isolation layer 30 is located between the soldering areas 101 of the two adjacent flexible substrates 10. The connecting layer 20 is located between the non-soldering areas 102 of the two adjacent flexible substrates. The two flexible substrates 10, the isolation layer 30 and the connecting layer 20 are laminated so that the connecting layer 20 is connected to the non-soldering areas 102 of the two adjacent flexible substrates 10. When the connecting layer 20 is an adhesive layer, the connecting layer 20 can connect the two adjacent flexible substrates 10 without heating. When the connecting layer 20 is a prepreg, the connecting layer 20 can connect the two adjacent flexible substrates 10 by hot pressing.

[0057] Step two: referring to Figure 1 and Figure 2 , the isolation layer 30 is removed so that the connecting layer 20 and the soldering areas 101 of the two adjacent flexible substrates 10 together form a cavity 201, obtaining an intermediate body 103. At this time, the projection of the first pad 111 in the first direction X is located in the cavity 201. The soldering area 101 can be bent around the non-soldering area 102.

[0058] Step three: referring to Figure 3 , the steps of forming the isolation layer 30 and the connecting layer 20, laminating, and removing the isolation layer 30 are repeated on at least one flexible substrate 10 of the intermediate body 103, obtaining a multi-layer circuit board 100, so that the multi-layer circuit board 100 includes at least three flexible substrates 10. In this embodiment, the multi-layer circuit board 100 includes three flexible substrates 10 and two connecting layers 20. The three flexible substrates 10 and the two connecting layers 20 together form two cavities 201.

[0059] In another embodiment, step three can be omitted, so that the multi-layer circuit board 100 includes only two flexible substrates 10.

[0060] In another embodiment, referring to Figure 3The multilayer circuit board 100 can be provided directly. The multilayer circuit board 100 includes the connection layer 20 and at least two flexible substrates 10 stacked along the first direction X. The flexible substrate 10 includes two circuit layers 11. The flexible substrate 10 is divided into a solder region 101 and a non-solder region 102 along a second direction Y perpendicular to the first direction X. The solder regions 101 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The non-solder regions 102 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. Each circuit layer 11 includes a first pad 111 located at the solder region 101. The oppositely arranged first pads 111 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The connection layer 20 is connected between the non-solder regions 102 of two adjacent flexible substrates 10. The connection layer 20 and the solder regions 101 of two adjacent flexible substrates 10 jointly form a cavity 201. The cavity 201 is arranged such that the solder region 101 can be bent around the non-solder region 102. The projection of the first pad 111 along the first direction X is located within the cavity 201.

[0061] Step four: referring to Figure 4 , a solder body 40 is provided. The solder body 40 can be one of a rigid circuit board, a chip or a flexible circuit board, for example. The solder body 40 includes a solder body 41 and a second pad 42 fixed to one surface of the solder body 41. The material of the second pad 42 can be copper, for example.

[0062] Step five: referring to Figure 5 , a conductive paste 50 is formed on the second pad 42. The conductive paste 50 can be one of a tin paste or a copper paste, for example. The multilayer circuit board 100 and the solder body 40 are stacked along the first direction X such that a first pad 111 of a flexible substrate 10 closest to the solder body 40 is attached to the conductive paste 50 along the first direction X. The solder regions 101 of the flexible substrates 10 other than the flexible substrate 10 closest to the solder body 40 are bent around the non-solder regions 102 in a direction away from the solder body 40, thereby leaving a solder space 601 on a side of the flexible substrate 10 closest to the solder body 40 away from the second pad 42. The order of the step of attaching the first pad 111 to the conductive paste 50 and the step of deforming the solder regions 101 around the non-solder regions 102 can be exchanged. A hot press head 60 is placed at the solder space 601 to hot press the attachment of the first pad 111 and the conductive paste 50, such that the flexible substrate 10 closest to the solder body 40 is soldered to the solder body 40.

[0063] In the welding, the hot press head 60 is placed at the avoided welding space 601 and only hot presses the one flexible substrate 10 to be welded, that is, the conductive paste 50 to be welded and the hot press head are respectively placed on the opposite sides of one flexible substrate 10, so that the hot pressing thickness of the hot press head 60 is equivalent to the thickness of one flexible substrate 10. Therefore, in the welding of the multilayer circuit board 100 of the present application, the hot press head 60 does not need to be placed on the surface of the multilayer circuit board 100 away from the second pad 42, so that in the welding of the multilayer circuit board 100 of the present application, the hot press head 60 does not need to hot press all the flexible substrates 10 at the same time. The arrangement and welding method of the multilayer circuit board 100 of the present application are beneficial to reduce the hot pressing thickness of the hot press head 60, so that the present application can reduce the hot pressing temperature, and thus can solve the deformation problem of the multilayer circuit board 100 when welded to the welding body 40.

[0064] In some embodiments, after hot pressing, the conductive paste 50 arranged on the second pad 42 also fills the through hole 121 of the flexible substrate 10 closest to the welding body 40 in the first direction X, thereby improving the welding stability of the flexible substrate 10 closest to the welding body 40 and the welding body 40.

[0065] In some embodiments, after hot pressing, the conductive paste 50 arranged on the second pad 42 also welds the conductive part 13 in the through hole 121 of the flexible substrate 10 closest to the welding body 40, thereby further improving the welding stability of the flexible substrate 10 closest to the welding body 40 and the welding body 40.

[0066] Step six: refer to Figure 6 The conductive paste 50 is formed on the other first pad 111 of the flexible substrate 10 after welding.

[0067] Step seven: refer to Figure 6 And Figure 7 The next flexible substrate 10 adjacent to the welded flexible substrate 10 is released, so that the first pad 111 of the deformed flexible substrate 10 is attached to the conductive paste 50 on the previous flexible substrate 10.

[0068] Then the deformed flexible substrate 10 is welded to the previous flexible substrate 10 by hot pressing. In the hot pressing, the newly formed conductive paste 50 and the hot press head are also respectively placed on the opposite sides of one deformed flexible substrate 10, and the hot press head 60 can still be placed in the avoided welding space 601, so that the hot pressing thickness is equivalent to the thickness of one flexible substrate 10. Therefore, the hot pressing temperature for welding the next flexible substrate 10 can also be reduced, and thus the deformation problem of the flexible substrate 10 after welding can also be solved.

[0069] Step eight: refer to Figure 8The steps of forming the conductive paste 50, releasing the flexible substrate 10 and welding are repeated until each two adjacent flexible substrates 10 are welded to each other, and a welded structure 200 is obtained.

[0070] When the multilayer circuit board 100 only includes two flexible substrates 10, step eight can be omitted.

[0071] In some embodiments, after hot pressing, the conductive paste 50 on the first pad 111 of the upper flexible substrate 10 also fills the through hole 121 of the lower flexible substrate 10, so as to improve the welding stability of the two adjacent flexible substrates 10.

[0072] In some embodiments, after hot pressing, the conductive paste 50 on the first pad 111 of the upper flexible substrate 10 also welds the conductive part 13 in the through hole 121 of the lower flexible substrate 10, so as to further improve the welding stability of the two adjacent flexible substrates 10.

[0073] In the present application, the welding area 101 of the flexible substrate 10 and the connecting layer 20 in the multilayer circuit board 100 together form a cavity 201, so that the welding area 101 of the flexible substrate 10 can be bent around the non-welding area 102. When welding the multilayer circuit board 100, the flexible substrates 10 except the one closest to the welding body 40 can be bent around the non-welding area 102 to avoid the welding space. Therefore, the hot pressing head 60 can be placed in the welding space 601 during welding, instead of being placed on the side of the multilayer circuit board 100 away from the second pad 42, that is, the hot pressing thickness is equivalent to the thickness of one flexible substrate 10, thereby facilitating the reduction of the hot pressing thickness of the hot pressing head 60. Subsequently, after forming new conductive paste 50 on the welded flexible substrate 10, the next flexible substrate 10 is released one by one and the welding of the flexible substrate 10 is carried out one by one to weld two adjacent flexible substrates 10. Each time when welding two adjacent flexible substrates 10, the hot pressing thickness is still equivalent to the thickness of one flexible substrate 10. Therefore, the present application is beneficial to reduce the hot pressing thickness, thereby being able to reduce the hot pressing temperature during welding. Therefore, the preparation method provided by the present application can solve the problem of deformation of the multilayer circuit board 100 during welding.

[0074] Embodiment 2:

[0075] With reference to Figure 3In an embodiment of the present application, a multilayer circuit board 100 is provided. The multilayer circuit board 100 comprises a connecting layer 20 and at least two flexible substrates 10 stacked along a first direction X. In this embodiment, the multilayer circuit board 100 comprises two connecting layers 20 and three flexible substrates 10. The flexible substrate 10 comprises two circuit layers 11. The flexible substrate 10 is divided into a soldering area 101 and a non-soldering area 102 along a second direction Y perpendicular to the first direction X. The soldering areas 101 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The non-soldering areas 102 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. Each circuit layer 11 comprises a first pad 111 located in the soldering area 101. The oppositely arranged first pads 111 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The connecting layer 20 is connected between the non-soldering areas 102 of two adjacent flexible substrates 10. The connecting layer 20 and the soldering areas 101 of two adjacent flexible substrates 10 jointly form a cavity 201, so that the soldering area 101 can be bent around the non-soldering area 102. The projection of the first pad 111 along the first direction X is located in the cavity 201.

[0076] In the present application, the connecting layer 20 and the soldering area 101 of the flexible substrate 10 in the multilayer circuit board 100 jointly form a cavity 201, so that the soldering area 101 of the flexible substrate 10 can be bent around the non-soldering area 102, thereby facilitating the avoidance of a soldering space 601 when soldering the multilayer circuit board 100, so that the hot press head 60 and the conductive paste 50 are located on opposite sides of only one flexible substrate 10 before soldering. Therefore, the multilayer circuit board 100 provided by the present application can realize layer-by-layer soldering during soldering. Therefore, the multilayer circuit board provided by the present application facilitates layer-by-layer soldering.

[0077] In some embodiments, the flexible substrate 10 further comprises a dielectric layer 12 connected between the two circuit layers 11. The flexible substrate 10 is provided with a through hole 121 penetrating the first pad 111 of the dielectric layer 12 and the two circuit layers 11. The through hole 121 facilitates the filling of the conductive paste in the through hole 121 after the multilayer circuit board 100 is soldered, thereby improving the soldering stability of the flexible substrate 10.

[0078] In some embodiments, the side wall of the through hole 121 is provided with a conductive part 13 connecting the two circuit layers 11. The conductive part 13 facilitates the conduction of different circuit layers 11 of the same flexible substrate 10, and further improves the soldering stability of the flexible substrate 10 after the multilayer circuit board 100 is soldered.

[0079] In some embodiments, the connecting layer 20 is a glue layer, which facilitates the connection of the non-soldering areas 102 of two adjacent flexible substrates 10.

[0080] In another embodiment, the connecting layer 20 can be a prepreg.

[0081] The materials of the elements in this embodiment are the same as those of the corresponding elements in Embodiment 1, and thus will not be repeated.

[0082] Embodiment 3

[0083] With reference to Figure 8 In an embodiment, a solder structure 200 is provided. The solder structure 200 includes a solder body 40, a multilayer circuit board 100, and a conductive paste 50.

[0084] The multilayer circuit board 100 includes a connection layer 20 and at least two flexible substrates 10 stacked along a first direction X. The flexible substrate 10 includes two circuit layers 11. The flexible substrate 10 is divided into a solder region 101 and a non-solder region 102 along a second direction Y perpendicular to the first direction X. The solder regions 101 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The non-solder regions 102 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. Each circuit layer 11 includes a first pad 111 located in the solder region 101. The oppositely arranged first pads 111 of two adjacent flexible substrates 10 are oppositely arranged along the first direction X. The connection layer 20 is connected between the non-solder regions 102 of two adjacent flexible substrates 10. The connection layer 20 and the solder regions 101 of two adjacent flexible substrates 10 jointly form a cavity 201. A projection of the first pad 111 along the first direction X is located in the cavity 201.

[0085] The solder body 40 includes a solder body 41 and a second pad 42 fixed to a surface of the solder body 41. The multilayer circuit board 100 and the solder body 40 are stacked along the first direction X. A first pad 111 of a flexible substrate 10 closest to the solder body 40 along the first direction X is soldered to the second pad 42 through the conductive paste 50. The first pads 111 of every two adjacent flexible substrates 10 are soldered to each other through the conductive paste 50.

[0086] In some embodiments, the flexible substrate 10 further includes a dielectric layer 12 connected between the two circuit layers 11. The flexible substrate 10 is provided with a through hole 121 penetrating the first pad 111, the dielectric layer 12, and the two circuit layers 11. The conductive paste 50 is also filled in the through hole 121.

[0087] In some embodiments, a side wall of the through hole 121 is provided with a conductive part 13 connecting the two circuit layers 11. The conductive paste 50 is also soldered to the conductive part 13.

[0088] The above description is merely one specific implementation of the present application, but in practical applications, it cannot be limited to this implementation. Other modifications and changes made by those skilled in the art according to the technical concept of the present application should also be within the scope of protection of the present application.

Claims

1. A multilayer circuit board comprising at least two flexible substrates stacked along a first direction, the flexible substrates comprising two circuit layers, characterized in that, The flexible substrate is divided into a soldering area and a non-soldering area along a second direction perpendicular to the first direction, the soldering areas of two adjacent flexible substrates are oppositely arranged along the first direction, the non-soldering areas of two adjacent flexible substrates are oppositely arranged along the first direction, each circuit layer comprises a first pad located in the soldering area, and the oppositely arranged first pads of two adjacent flexible substrates are oppositely arranged along the first direction; The multilayer circuit board further comprises a connecting layer connected between the non-soldering areas of two adjacent flexible substrates, the connecting layer and the soldering areas of two adjacent flexible substrates jointly form a cavity, and a projection of the first pad along the first direction is located in the cavity.

2. The multilayer circuit board of claim 1, wherein, The flexible substrate further comprises a dielectric layer connected between two circuit layers, and the flexible substrate is provided with a through hole penetrating the dielectric layer and the first pads of the two circuit layers.

3. The multilayer circuit board of claim 2, wherein, The side wall of the through hole is provided with a conductive part connecting the two circuit layers.

4. A method of producing a welded structure, characterized by The method comprises the following steps: providing the multilayer circuit board according to claim 1; providing a soldering body comprising a soldering body and a second pad fixed to one surface of the soldering body; forming conductive paste on the second pad, and stacking the multilayer circuit board and the soldering body in the first direction, so that the first pad of the flexible substrate closest to the soldering body is attached to the conductive paste; bending the soldering areas of the flexible substrates other than the flexible substrate closest to the soldering body around the non-soldering areas in a direction away from the soldering body, and hot-pressing the attachment of the first pad and the conductive paste, so that the flexible substrate closest to the soldering body is soldered to the soldering body; forming the conductive paste on the other first pad of the soldered flexible substrate, releasing the next flexible substrate adjacent to the soldered flexible substrate, so that the first pad of the flexible substrate after deformation recovery is attached to the conductive paste on the previous flexible substrate; soldering the flexible substrate after deformation recovery to the previous flexible substrate by hot-pressing until each two adjacent flexible substrates are mutually soldered.

5. The method of producing a welded structure according to claim 4, wherein The flexible substrate further comprises a dielectric layer connected between two circuit layers, and the flexible substrate is provided with a through hole penetrating the dielectric layer and the first pads of the two circuit layers, and the conductive paste is also filled in the through hole after each hot-pressing.

6. The method of producing a welded structure according to claim 5, wherein The side wall of the through hole is provided with a conductive part connecting the two circuit layers, and the conductive paste is also soldered to the conductive part after each hot-pressing.

7. The method of producing a welded structure according to any one of claims 4 to 6, wherein The preparation of the multilayer circuit board comprises: forming an isolation layer and the connecting layer on the circuit layer of each flexible substrate, the isolation layer is located in the soldering area and covers the corresponding first pad; stacking another flexible substrate on the isolation layer and the connecting layer and pressing, the isolation layer is located between the soldering areas of two adjacent flexible substrates; removing the isolation layer to form the cavity.

8. The method of producing a welded structure according to claim 7, wherein The isolation layer is a release layer.

9. The method of producing a welded structure according to claim 7, wherein The connecting layer is a glue layer or a prepreg.

10. A solder structure comprising a solder body, the solder body comprising a solder body and a second pad affixed to a surface of the solder body, wherein, The solder structure further comprises a multilayer circuit board as claimed in any one of claims 1 to 3, the multilayer circuit board and the solder body are stacked in the first direction, a first pad of a flexible substrate most adjacent to the solder body in the first direction is soldered to the second pad by conductive paste, the first pads of every two adjacent flexible substrates are soldered to each other by the conductive paste.

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