Multilayer circuit board welding method

Abstract

The present invention discloses a multilayer circuit board welding method, which belongs to the technical field of circuit board welding, including: forming a plurality of high-temperature solder balls on the surface of a first circuit board for connecting to a second circuit board, the high-temperature solder balls are formed by high-temperature solder paste; printing a plurality of medium-temperature solder pastes on the surface of a second circuit board for connecting to the first circuit board, the plurality of medium-temperature solder pastes corresponding to the plurality of high-temperature solder balls one by one; combining the first circuit board with the second circuit board and heating them through a reflow furnace, the heating temperature of the reflow furnace is sufficient to melt the medium-temperature solder paste while the high-temperature solder balls are not melted, so that the melted medium-temperature solder paste is combined with its corresponding high-temperature solder balls to form a mixed solder joint, and the solder pads on the first circuit board are connected to the solder pads on the second circuit board through the mixed solder joint. The multilayer circuit board welding method provided by the present invention can reduce the volume of bubbles in the solder joint, prevent the problem of solder voids, and will not affect the reliability and strength of the solder joint.

Description

Technical Field

[0001] The invention relates to the technical field of circuit board welding, and in particular to a multi-layer circuit board welding method. Background Art

[0002] Circuit boards are an important part of electronic devices, and they are also called motherboards. Conventional single-layer motherboards are difficult to meet the needs of complex functional modules and form design, so it is necessary to expand the layout space in the vertical direction. At present, motherboard design usually adopts a "three-in-one" solution, which includes an upper motherboard, a middle motherboard, and a lower motherboard arranged in sequence.

[0003] The existing "three-in-one" motherboard production steps are: first install the middle-layer motherboard on the lower-layer motherboard, and then install the upper-layer motherboard on the middle-layer motherboard, and use different solder pastes to weld the different motherboards together to achieve reliable connection and electrical performance.

[0004] The specific steps are as follows: first, print the high-temperature solder paste onto the lower motherboard through a steel mesh template, then mount components on the lower motherboard, then install the middle motherboard onto the lower motherboard, and pass the lower motherboard and the middle motherboard through a high-temperature solder paste reflow furnace together, so that the high-temperature solder paste melts and wets the components and the pads on the motherboard, forming an alloy layer to achieve electrical performance. Next, print the high-temperature solder paste onto the upper motherboard through a steel mesh template, then mount components on the upper motherboard, then pass the upper motherboard through a high-temperature solder paste reflow furnace, so that the high-temperature solder paste melts and wets the components and the pads on the upper motherboard, forming an alloy layer to achieve electrical performance. Next, print the medium-temperature solder paste on the middle motherboard, mount the upper motherboard onto the middle motherboard, and after installing the fixture, control the upper motherboard, the middle motherboard, and the lower motherboard to pass through a medium-temperature solder paste reflow furnace, so that the medium-temperature solder paste melts and wets the pads on the upper motherboard and the middle motherboard, forming an alloy layer to achieve electrical performance.

[0005] However, because the upper motherboard and the middle motherboard have been through a high-temperature solder paste reflow furnace before being combined, the upper motherboard and the middle motherboard are seriously warped and deformed. When the upper board and the middle motherboard are combined, the average height of the solder joint reaches 65um, and the required medium-temperature solder paste volume is large. The pad area is 0.125mm 2 When the corresponding average medium temperature solder paste volume is 0.0160mm 3 The medium-temperature solder paste is doped with flux, and the organic matter in the flux is cracked by the high temperature of the reflow furnace to produce bubbles. If the volume of the medium-temperature solder paste is large, the bubbles will not be able to escape in time, resulting in welding voids. Reducing the amount of solder paste will cause the solder joint to have a small waist and reduce the reliability strength.

[0006] Therefore, a multi-layer circuit board welding method is urgently needed to solve the above problems. Summary of the invention

[0007] The object of the present invention is to provide a multi-layer circuit board welding method, which can reduce the volume of bubbles in the solder joints, prevent the problem of solder voids, and will not affect the reliability and strength of the solder joints.

[0008] As conceived above, the technical solution adopted by the present invention is:

[0009] A multi-layer circuit board welding method is used to weld a first circuit board and a second circuit board, comprising the following steps:

[0010] S1. forming a plurality of high-temperature solder balls on a surface of the first circuit board for connecting to the second circuit board, wherein the high-temperature solder balls are formed of high-temperature solder paste;

[0011] S2. Printing a plurality of medium-temperature solder pastes on a surface of the second circuit board for connecting to the first circuit board, wherein the plurality of medium-temperature solder pastes correspond one to one to the plurality of high-temperature solder balls;

[0012] S3. Combine the first circuit board and the second circuit board and heat them in a reflow furnace, wherein the heating temperature of the reflow furnace is sufficient to melt the medium-temperature solder paste while not melting the high-temperature solder balls, so that the melted medium-temperature solder paste is combined with the corresponding high-temperature solder balls to form mixed solder joints, and the solder pads on the first circuit board are connected to the solder pads on the second circuit board through the mixed solder joints.

[0013] Optionally, in the mixed solder joint, the volume ratio of the medium-temperature solder paste is 50% to 65%, and the volume ratio of the high-temperature solder ball is 35% to 50%.

[0014] Optionally, step S1 includes:

[0015] S11, printing a plurality of first high-temperature solder pastes on a plurality of connecting pads of the first circuit board, wherein the connecting pads correspond to the first high-temperature solder pastes one by one, and the connecting pads are located on a surface of the first circuit board for connecting to the second circuit board;

[0016] S12, controlling the first circuit board to be heated by a reflow furnace so that each of the first high-temperature solder pastes melts and wets the corresponding connecting pad, and the first high-temperature solder paste forms the high-temperature solder ball after cooling.

[0017] Optionally, in step S11, a plurality of first high-temperature solder pastes are printed on a plurality of connecting pads of the first circuit board through a first steel mesh template, the first steel mesh template having a plurality of first mesh holes corresponding one-to-one to the plurality of first high-temperature solder pastes, and the shape of the first mesh holes is a fish-mouth shape.

[0018] Optionally, the fish mouth angle of the first fish mouth shaped mesh is 85° to 95°, the thickness of the first steel mesh is 0.06 mm, and the area of ​​the first mesh is 0.114 mm.2 , the volume of the first mesh is 0.0068mm 3 .

[0019] Optionally, while printing a plurality of the first high-temperature solder pastes, a second high-temperature solder paste is printed on the surface of the first circuit board, and the second high-temperature solder paste is used for SMT mounting.

[0020] Optionally, in step S2, a plurality of medium-temperature solder pastes are printed on the surface of the second circuit board for connecting to the first circuit board through a second steel mesh template, and the second steel mesh template has a plurality of second mesh holes corresponding one-to-one to the plurality of medium-temperature solder pastes, and the shape of the second mesh holes is a fish-mouth shape.

[0021] Optionally, the fish mouth angle of the second fish mouth-shaped mesh is 55° to 65°, the thickness of the second steel mesh is 0.08 mm, and the area of ​​the second mesh is 0.115 mm. 2 , the volume of the second mesh is 0.0092mm 3 .

[0022] Optionally, the melting point of the high temperature solder ball is 217°C.

[0023] Optionally, before step S2, the multi-layer circuit board welding method further includes the following steps:

[0024] printing a third high temperature solder paste on a third circuit board;

[0025] The third circuit board is combined with the second circuit board and heated by a reflow furnace to connect the third circuit board and the second circuit board, wherein the third circuit board and the first circuit board are located on both sides of the second circuit board.

[0026] The beneficial effects of the multi-layer circuit board welding method provided by the present invention are as follows:

[0027] First, a plurality of high-temperature solder balls are formed on the first circuit board, and then a plurality of medium-temperature solder pastes are printed on the surface of the second circuit board used for connecting to the first circuit board, and the plurality of medium-temperature solder pastes correspond to the plurality of high-temperature solder balls one by one. Then, the first circuit board and the second circuit board are combined and heated through a reflow furnace to melt the medium-temperature solder paste on the second circuit board, while the high-temperature solder balls on the first circuit board will not melt, thereby forming a mixed solder joint comprising high-temperature solder balls and medium-temperature solder paste. The mixed solder joint is used to connect the pads of the first circuit board and the pads of the second circuit board. Due to the presence of the high-temperature solder balls, the amount of medium-temperature solder paste is reduced while ensuring the amount of solder paste required for combining the first circuit board with the second circuit board. The high-temperature solder balls will not melt during the heating of the medium-temperature solder paste, so no bubbles will be generated, thereby reducing the volume of bubbles in the mixed solder joint. Moreover, the high-temperature solder balls will not melt and collapse, so that the first circuit board will not be close to the molten medium-temperature solder paste, which is conducive to the escape of bubbles generated by the high-temperature decomposition of organic matter in the flux in the medium-temperature solder paste, avoiding the stagnation of bubbles in the mixed solder joint to form voids, and will not affect the reliability and strength of the mixed solder joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of a multi-layer circuit board welding method provided by an embodiment of the present invention;

[0029] Figure 2 is a partial structural schematic diagram of a first steel mesh template provided by an embodiment of the present invention;

[0030] Figure 3 is a partial structural schematic diagram of a second steel mesh template provided by an embodiment of the present invention;

[0031] Figure 4 is a schematic diagram of a first circuit board printed with a first high-temperature solder paste provided by an embodiment of the present invention;

[0032] Figure 5 is a schematic diagram of a third circuit board printed with a third high-temperature solder paste provided by an embodiment of the present invention;

[0033] Figure 6 The second circuit board and the third circuit board provided in the embodiment of the present invention are connected schematically. Figure 1 ;

[0034] Figure 7 Schematic diagram of the connection between the second circuit board and the third circuit board provided in the embodiment of the present invention Figure 2 ;

[0035] Figure 8 is a schematic diagram of connection between a first circuit board and a second circuit board provided by an embodiment of the present invention;

[0036] Fig. 9 is a schematic diagram of a "three-in-one" structure provided by an embodiment of the present invention;

[0037] Fig.10 is a distribution diagram of indium in a solder joint provided by an embodiment of the present invention;

[0038] Fig.11 This is a red ink test result diagram of a solder joint provided by an embodiment of the present invention.

[0039] In the figure:

[0040] 10, first steel mesh template; 101, first mesh; 20, second steel mesh template; 201, second mesh;

[0041] 1. First circuit board; 2. Second circuit board; 3. Third circuit board; 4. First high-temperature solder paste; 5. Third high-temperature solder paste; 6. Low-temperature solder paste; 7. High-temperature solder ball. DETAILED DESCRIPTION

[0042] In order to make the technical problems solved by the present invention, the technical solutions adopted and the technical effects achieved clearer, the technical solutions of the present invention are further described below in conjunction with the accompanying drawings and through specific implementation methods. It is understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for the convenience of description, only the parts related to the present invention are shown in the accompanying drawings, not all.

[0043] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0044] In the description of the present invention, unless otherwise clearly specified and limited, the terms "connected", "connected", and "fixed" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0045] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0046] In the description of this embodiment, the terms "upper", "lower", "right", etc., and the like, are based on the directions or positions shown in the accompanying drawings, and are only for the convenience of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first" and "second" are only used to distinguish in the description and have no special meaning. In this embodiment, unless otherwise specified, "multiple" specifically refers to two or more.

[0047] In the prior art, there are two main factors that cause solder voids during the bonding process of the upper motherboard and the middle motherboard: 1. In order to ensure the plump shape of the solder joint, the amount of medium-temperature solder paste required for the welding of the upper motherboard and the middle motherboard is relatively large, and the amount of flux is increased (the volume ratio of the alloy component to the flux in the medium-temperature solder paste is 1:1); 2. During the melting process of the medium-temperature solder paste, the distance between the upper motherboard and the middle motherboard is relatively small, and the upper motherboard is close to the solder paste, which causes the bubbles generated by the high-temperature decomposition of organic matter in the flux in the reflow furnace to be blocked from escaping, leaving more bubbles in the solder joint.

[0048] The present embodiment provides a multi-layer circuit board welding method for welding a first circuit board and a second circuit board, which can reduce the volume of bubbles in the solder joints, prevent the problem of solder voids, and will not affect the reliability and strength of the solder joints. The first circuit board and the second circuit board in the present embodiment are two circuit boards that need to be combined. For example, the first circuit board can be an upper main board, and the second circuit board can be an intermediate main board.

[0049] like Figure 1 As shown, the multi-layer circuit board welding method includes the following steps:

[0050] S1, forming a plurality of high-temperature solder balls 7 on a surface of a first circuit board 1 for connecting to a second circuit board 2;

[0051] In step S1 , the surface of the first circuit board 1 used for connecting with the second circuit board 2 can be understood as the surface of the first circuit board 1 facing the second circuit board 2 after the first circuit board 1 and the second circuit board 2 are connected.

[0052] It should be noted that the first circuit board 1 has a plurality of pads, a portion of which is used to connect the first circuit board 1 and the second circuit board 2, and another portion of which is used for SMT (Surface Mounted Technology) mounting, wherein SMT mounting is a circuit assembly technology that mounts pinless or short-lead surface-mounted components on the surface of a printed circuit board (PCB) or other substrates, and assembles them by soldering through methods such as reflow soldering or dip soldering. The high-temperature solder balls 7 in this embodiment are formed on the pads used to connect the first circuit board 1 and the second circuit board 2.

[0053] The high temperature solder ball 7 in this embodiment is formed by high temperature solder paste, that is, the melting temperature of the high temperature solder ball 7 is the same as the melting temperature of the high temperature solder paste and is greater than the melting temperature of the medium temperature solder paste.

[0054] It should also be noted that the "connection" in this embodiment is a general term for mechanical connection and electrical connection, that is, the first circuit board 1 and the second circuit board 2 can be electrically connected through the solder pad and the solder paste provided on the solder pad.

[0055] S2, printing a plurality of medium-temperature solder pastes on the surface of the second circuit board 2 for connecting to the first circuit board 1, wherein the plurality of medium-temperature solder pastes correspond one to one to the plurality of high-temperature solder balls 7;

[0056] In step S2 , the surface of the second circuit board 2 used for connecting to the first circuit board 1 is a surface where the second circuit board 2 faces the first circuit board 1 when the second circuit board 2 and the first circuit board 1 are stacked.

[0057] It should be noted that, similar to the first circuit board 1, the surface of the second circuit board 2 is also provided with a plurality of pads. For example, some or all of the plurality of pads are used to connect the second circuit board 2 with other circuit boards. The second circuit board 2 may be provided with pads for SMT mounting, or the second circuit board 2 may not be provided with pads for SMT mounting, which is not limited in this embodiment. The medium-temperature solder paste in this embodiment is printed on the pads.

[0058] In this embodiment, a plurality of medium-temperature solder pastes correspond one-to-one to a plurality of high-temperature solder balls 7 , so as to facilitate the corresponding contact between the medium-temperature solder paste and the high-temperature solder balls 7 in the subsequent sequence.

[0059] S3. Combine the first circuit board 1 and the second circuit board 2 and heat them in a reflow furnace. The heating temperature of the reflow furnace is sufficient to melt the medium-temperature solder paste while not melting the high-temperature solder ball 7. The melted medium-temperature solder paste is combined with its corresponding high-temperature solder ball 7 to form a mixed solder joint. The solder pad on the first circuit board 1 is connected to the solder pad on the second circuit board 2 through the mixed solder joint.

[0060] In step S3 , the combination of the first circuit board 1 and the second circuit board 2 means that the first circuit board 1 and the second circuit board 2 are buckled together, and the medium-temperature solder paste can contact the corresponding high-temperature solder ball 7 .

[0061] It should be noted that solder paste for welding is divided into high temperature solder paste, medium temperature solder paste and low temperature solder paste 6, wherein the components of high temperature solder paste, medium temperature solder paste and low temperature solder paste 6 are different. The main components of high temperature solder paste are tin, silver and copper; the main components of medium temperature solder paste are tin, silver and bismuth; the main components of low temperature solder paste 6 are tin and bismuth. The melting point of high temperature solder paste is 210℃-227℃; the melting point of medium temperature solder paste is 172℃; the melting point of low temperature solder paste 6 is 138℃.

[0062] Optionally, in step S3, the heating temperature of the reflow furnace is controlled to be greater than 172°C and less than 210°C, so that the medium-temperature solder paste can be melted while the high-temperature solder ball 7 will not be melted. After the medium-temperature solder paste is melted, it contacts with the high-temperature solder ball 7, for example, it can be attached to the high-temperature solder ball 7. After the medium-temperature solder paste is melted, it also contacts with the pad where the high-temperature solder ball 7 is located and the pad where the medium-temperature solder paste is located, so that the cooled medium-temperature solder paste and the high-temperature solder ball can form a mixed solder joint with a larger volume.

[0063] It should also be noted that the pads on the first circuit board 1 are connected to the corresponding pads on the second circuit board 2 via mixed solder joints.

[0064] The multi-layer circuit board welding method provided in this embodiment first forms a plurality of high-temperature solder balls 7 on the first circuit board 1, and then prints a plurality of medium-temperature solder pastes on the surface of the second circuit board 2 used for connecting to the first circuit board 1, and the plurality of medium-temperature solder pastes correspond one to one to the plurality of high-temperature solder balls 7. After that, the first circuit board 1 and the second circuit board 2 are combined and heated through a reflow furnace so that the medium-temperature solder paste on the second circuit board 2 melts, while the high-temperature solder balls 7 on the first circuit board 1 do not melt, thereby forming a mixed solder joint containing the high-temperature solder balls 7 and the medium-temperature solder paste, and the mixed solder joint is used to connect the solder pads of the first circuit board 1 and the solder pads of the second circuit board 2. Due to the presence of the high-temperature solder ball 7, the amount of medium-temperature solder paste is reduced while ensuring the amount of solder paste required for the combination of the first circuit board 1 and the second circuit board 2. The high-temperature solder ball 7 will not melt during the heating of the medium-temperature solder paste, so no bubbles will be generated, thereby reducing the volume of bubbles in the mixed solder joints. In addition, the high-temperature solder ball 7 will not melt and collapse, so that the first circuit board 1 will not be close to the molten medium-temperature solder paste, which is conducive to the escape of bubbles generated by the high-temperature decomposition of organic matter in the flux in the medium-temperature solder paste, avoiding the stagnation of bubbles in the mixed solder joints to form voids, and will not affect the reliability and strength of the mixed solder joints.

[0065] Optionally, in the mixed solder joint, the volume ratio of the medium temperature solder paste is 50% to 65% to have better connection reliability and prevent the formation of voids. For example, the volume ratio of the medium temperature solder paste is 50%, 55%, 60%, 65%. Correspondingly, the volume ratio of the high temperature solder ball 7 in the mixed solder joint is 35% to 50%, and the volume ratio of the high temperature solder ball 7 is 35%, 40%, 45%, 48%, 50%.

[0066] Optionally, step S1 includes:

[0067] S11, printing a plurality of first high-temperature solder pastes 4 on a plurality of connecting pads of the first circuit board 1, the connecting pads corresponding to the first high-temperature solder pastes 4 one by one, and the connecting pads are located on a surface of the first circuit board 1 for connecting to the second circuit board 2;

[0068] In step S11 , the position of the first high-temperature solder paste 4 is determined according to actual needs.

[0069] S12, controlling the first circuit board 1 to be heated by the reflow furnace so that each first high-temperature solder paste 4 melts and wets the corresponding connection pad, and the first high-temperature solder paste 4 forms a high-temperature solder ball 7 after cooling.

[0070] Optionally, in step S11, a plurality of first high-temperature solder pastes 4 are printed on a plurality of connection pads of the first circuit board 1 by means of a first stencil template 10. The first stencil template 10 has a plurality of first mesh holes 101 corresponding to the plurality of first high-temperature solder pastes 4 one by one, and each first mesh hole 101 is used for printing a first high-temperature solder paste 4.

[0071] For example, Figure 2 As shown, in this embodiment, the shape of the first mesh 101 is a fish-mouth shape. The fish-mouth-shaped first mesh 101 can shorten the distance that bubbles move, and help the bubbles generated when organic matter in the flux is decomposed to escape quickly, thereby reducing the volume of the solder joint voids and improving the welding quality.

[0072] In some optional embodiments, the fish mouth angle of the first fish mouth shaped mesh 101 is 85° to 95°. Figure 2 As shown, the fish mouth angle of the first mesh 101 is 90°.

[0073] Optionally, the thickness of the first steel mesh is 0.06 mm, and the area of ​​the first mesh hole 101 is 0.114 mm 2 , the volume of the first mesh 101 is 0.0068mm 3 , so that the volume of the formed high temperature solder ball 7 is 0.0068mm 3 It should be noted that, unless otherwise specified, the area of ​​the pads increased in this embodiment is 0.125 mm 2 .

[0074] Optionally, while printing a plurality of first high temperature solder pastes 4, a second high temperature solder paste is printed on the surface of the first circuit board 1, and the second high temperature solder paste is used for SMT mounting. Printing the first high temperature solder paste 4 and the second high temperature solder paste simultaneously can improve efficiency.

[0075] Optionally, in step S2, a plurality of medium-temperature solder pastes are printed on the surface of the second circuit board 2 for connecting to the first circuit board 1 through a second steel mesh template 20, and the second steel mesh template 20 has a plurality of second mesh holes 201 corresponding one-to-one to the plurality of medium-temperature solder pastes, and the shape of the second mesh holes 201 is a fish-mouth shape.

[0076] Alternatively, if Figure 3 As shown, the fish mouth angle of the second fish mouth shaped mesh 201 is 55° to 65°. Preferably, the fish mouth angle of the second fish mouth shaped mesh 201 is 60°, so that the amount of solder paste formed meets the requirements, and the distance of bubble movement can be shortened, which helps the bubbles generated when the organic matter in the flux is decomposed to escape quickly, thereby reducing the volume of the solder joint void and improving the welding quality. In this embodiment, the thickness of the second steel mesh is 0.08mm, and the area of ​​the second mesh 201 is 0.115mm 2 , the volume of the second mesh 201 is 0.0092mm 3 It can be seen that in this embodiment, the volume of the medium-temperature solder paste of each solder joint on the middle layer circuit board is reduced from 0.016mm in the prior art. 3 Reduced to 0.0092mm 3 , which reduces the total amount of flux from medium-temperature solder paste, and simultaneously reduces the bubbles generated by the decomposition of organic matter in the flux during reflow soldering, thereby reducing the volume of empty solder joints.

[0077] Optionally, the high temperature solder ball 7 in this embodiment has a melting point of 217° C. and can withstand a maximum temperature of 235-240° C.

[0078] In some optional embodiments, one side of the second circuit board 2 has a first circuit board 1, and the other side of the second circuit board 2 has a third circuit board 3. At this time, before step S2, the multi-layer circuit board welding method also includes the following steps: the third circuit board 3 in this embodiment is specifically the lower main board.

[0079] Printing a third high temperature solder paste 5 on the third circuit board 3;

[0080] The third circuit board 3 is combined with the second circuit board 2 and heated by a reflow furnace to connect the third circuit board 3 and the second circuit board 2 . The third circuit board 3 and the first circuit board 1 are located on both sides of the second circuit board 2 .

[0081] Next, this embodiment provides a complete welding method for a multi-layer circuit board:

[0082] First, print the first high temperature solder paste 4 on the pad of the first circuit board 1 to obtain Figure 4 In the structure shown, the first high temperature solder paste 4 is used to connect with the second circuit board 2. In this process, the high temperature solder paste for SMT mounting can also be printed on other pads of the first circuit board 1. Then, the first circuit board 1 is heated by the reflow furnace to melt the first high temperature solder paste 4 and infiltrate the pads of the first circuit board 1 to form high temperature solder balls 7 on the pads. In addition, the third high temperature solder paste 5 is printed on the pads of the third circuit board 3 to obtain the following structure: Figure 5 In the structure shown in the figure, the third high temperature solder paste 5 is used to connect with the second circuit board 2. In this process, the high temperature solder paste for SMT mounting can also be printed on other pads of the third circuit board 3. Then, the second circuit board 2 is combined with the third circuit board 3, that is, Figure 6 The second circuit board 2 and the third circuit board 3 are aligned as shown. Specifically, the plurality of pads on the second circuit board 2 correspond to the plurality of third high-temperature solder pastes 5 on the third circuit board 3 one by one. The second circuit board 2 and the third circuit board 3 are clamped by a fixture and then heated by a reflow furnace. The temperature heated by the reflow furnace is sufficient to melt the third high-temperature solder paste 5 so that the melted solder paste infiltrates the pads on the third circuit board 3 and the pads on the second circuit board 2. The structure after welding is as shown in FIG. Figure 7 Next, a medium-temperature solder paste is printed on the pads of the second circuit board 2 facing away from the third circuit board 3 to obtain the following Figure 8 The structure shown in FIG. 1 is then combined with the first circuit board 1 on one side of the second circuit board 2, and then formed into a circuit board as shown in FIG. Fig. 9 The "three-in-one" structure shown.

[0083] The solder joint microstructure and welding strength reliability analysis of the "three-in-one" structure formed by welding the multi-layer circuit board provided by the present embodiment are performed to determine whether the "three-in-one" structure formed by welding meets the welding strength requirements.

[0084] 1. Use X-ray to detect solder voids and determine that the maximum area ratio of solder voids is 15%, which is less than 25%, meeting the IPC standard and can be accepted;

[0085] 2. The IMC (interface metal co-compound or intermetallic) thickness of the upper layer (i.e. the first circuit board 1) and the lower layer (i.e. the third circuit board 3) is measured by slicing, and is 1.24um to 2.93um, which meets the product standard of 1um to 5um;

[0086] 3.SEM (Scanning electron microscope) and EDS (Energy Dispersive Spectrometer) confirm that the trace element In (indium) in the medium-temperature solder paste is evenly distributed without aggregation; among them, Fig.10 The distribution diagram of indium in the solder joint is given, and it can be seen that the indium is distributed relatively evenly.

[0087] 4. Fig.11 This is the result diagram of the red ink test on the solder joint. The red ink experiment confirmed the welding effect and strength: no red ink entered the solder joint, and there was no poor welding; the fracture mode was mainly concentrated on the joint surface between the circuit board (the first circuit board 1, the second circuit board 2 or the third circuit board 3) and the solder pad, and the solder joint strength was greater than the bonding force between the circuit board and the solder pad, meeting the reliability requirements.

[0088] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A multi-layer circuit board welding method for welding a first circuit board and a second circuit board. It is characterized in that The steps include: S1. forming a plurality of high-temperature solder balls on a surface of the first circuit board for connecting to the second circuit board, wherein the high-temperature solder balls are formed of high-temperature solder paste; S2. Printing a plurality of medium-temperature solder pastes on a surface of the second circuit board for connecting to the first circuit board, wherein the plurality of medium-temperature solder pastes correspond one to one to the plurality of high-temperature solder balls; S3, combining the first circuit board with the second circuit board and heating them in a reflow furnace, wherein the heating temperature of the reflow furnace is greater than 172° C. and less than 210° C., so as to melt the medium-temperature solder paste while not melting the high-temperature solder balls, so that the melted medium-temperature solder paste is combined with the corresponding high-temperature solder balls to form mixed solder joints, and the solder pads on the first circuit board are connected with the solder pads on the second circuit board through the mixed solder joints; In the mixed solder joint, the volume ratio of the medium-temperature solder paste is 50% to 65%, and the volume ratio of the high-temperature solder ball is 35% to 50%; In step S2, a plurality of medium-temperature solder pastes are printed on a surface of the second circuit board for connecting the first circuit board through a second steel mesh template, the second steel mesh template has a plurality of second mesh holes corresponding to the plurality of medium-temperature solder pastes one by one, and the shape of the second mesh holes is a fish mouth shape; the fish mouth angle of the second mesh holes in the fish mouth shape is 55°~65°, the thickness of the second steel mesh is 0.08mm, the area of ​​the second mesh holes is 0.115mm², and the volume of the second mesh holes is 0.0092mm³; A plurality of first high-temperature solder pastes are printed on a plurality of connecting pads of the first circuit board through a first steel mesh template, and the first high-temperature solder paste forms the high-temperature solder balls after cooling. The first steel mesh template has a plurality of first mesh holes corresponding one by one to the plurality of first high-temperature solder pastes, and the shape of the first mesh holes is fish-mouth-shaped; the fish-mouth angle of the first mesh holes in the fish-mouth shape is 85°~95°, the thickness of the first steel mesh is 0.06mm, the area of ​​the first mesh holes is 0.114mm², and the volume of the first mesh holes is 0.0068mm³.

2. The multi-layer circuit board welding method according to claim 1, It is characterized in that Step S1 includes: S11, printing a plurality of first high-temperature solder pastes on a plurality of connecting pads of the first circuit board, wherein the connecting pads correspond to the first high-temperature solder pastes one by one, and the connecting pads are located on a surface of the first circuit board for connecting to the second circuit board; S12, controlling the first circuit board to be heated by a reflow furnace so that each of the first high-temperature solder pastes melts and wets the corresponding connecting pad, and the first high-temperature solder paste forms the high-temperature solder ball after cooling.

3. The multi-layer circuit board welding method according to claim 2, It is characterized in that While printing the plurality of the first high-temperature solder pastes, a second high-temperature solder paste is printed on the surface of the first circuit board, and the second high-temperature solder paste is used for SMT mounting.

4. A multi-layer circuit board welding method according to any one of claims 1 to 3, It is characterized in that The melting point of the high temperature solder ball is 217°C.

5. The multi-layer circuit board welding method according to any one of claims 1 to 3, It is characterized in that Before step S2, the multi-layer circuit board welding method further includes the following steps: printing a third high temperature solder paste on a third circuit board; The third circuit board is combined with the second circuit board and heated by a reflow furnace to connect the third circuit board and the second circuit board, wherein the third circuit board and the first circuit board are located on both sides of the second circuit board.

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