Steel mesh opening structure for controlling welding defects of surface-mounted device and design method thereof

Abstract

The invention belongs to the technical field of soldering paste printing, and relates to a steel mesh opening structure for controlling welding defects of a surface-mounted device and a design method of the steel mesh opening structure. The design method comprises the following steps: judging the types of surface-mounted devices, wherein the types comprise a connector with a non-sealing area, a large-size PBGA device which is warped and deformed by heating and a QFP / SOP device of which a lead and a heat sink are not coplanar; obtaining the structural size of the surface-mounted device, and determining a solder opening structure optimization method on the steel mesh based on the type and the structural size of the surface-mounted device; the optimization method comprises the steps of local thinning, internal shrinkage, external expansion, bridging and local thickening treatment. And calculating the opening structure size of the steel mesh based on the structure size of the surface-mounted device and an optimization method, and completing the opening design of the steel mesh. The method solves the problems that a conventional design method easily causes the batch welding defects that tin enters a connector jack in a non-sealing area, large-size PBGA device solder balls which are warped and deformed after being heated are bridged, cold solder joints, QFP and SOP devices with leads not coplanar with heat sinks are less in heat sink tin and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of solder paste printing, and relates to a stencil opening structure for controlling solder joint defects of surface-mounted devices and a design method thereof. Background Art

[0002] Solder paste printing is a core process link in surface mount technology (SMT). Its essence is to accurately transfer the solder paste to the pads of a printed circuit board (PCB) through a stencil (steel mesh), providing a material basis for the mechanical and electrical connections between component pins and PCB pads. This process directly affects the welding quality. As a printing stencil, the steel mesh mainly functions as follows: controlling the solder paste printing position through the opening design to achieve precise alignment; regulating the solder paste release amount through parameters such as the opening area and aspect ratio; and determining the initial shape of the deposited solder paste (such as rectangular, trapezoidal, or custom profile). According to statistics, solder paste printing-induced surface mount solder joint quality problems account for more than 70% of all surface mount solder joint quality problems. The steel mesh thickness, opening size, opening shape, opening inner wall state, etc. directly determine the solder paste printing quality. Solder paste printing defects may cause welding defects such as insufficient solder, solder balls, excessive solder, bridging, ball sockets, and false soldering, seriously affecting the first-pass welding qualification rate of products and even causing major quality problems such as solder joint cracking and short circuit failure.

[0003] The general principles of steel mesh design can be referred to IPC 7525 "Stencil Design Guide" for implementation. This standard lists the recommended steel mesh template thickness ranges, aspect ratio ranges, area ratio ranges, and other general design criteria for common package types such as resistors, capacitors, QFP (Quad Flat Package), and BGA (Ball Grid Array). However, in actual surface mount production, affected by various factors such as PCB (Printed Circuit Board) pad design, layout and wiring design, and component structure, for some special surface-mounted devices, such as connectors with unsealed areas, large-size PBGA (Plastic Ball Grid Array) devices that are warped and deformed by heat, QFP and SOP (Small Outline Package) devices with non-coplanar leads and heat sinks, if the steel mesh is designed only according to the above conventional requirements, welding defects such as solder entering the connector jacks, solder ball bridging, false soldering, and insufficient solder on the heat sink are likely to occur. The welding defects of surface-mounted devices greatly affect the first-pass welding qualification rate of electronic products, resulting in increased post-furnace repair costs, extended production cycles, low production efficiency, and even batch quality problems. Summary of the Invention

[0004] The object of the present invention is to solve the technical problems in the prior art that the steel mesh structure design is prone to welding defects such as solder entering the socket of the connector, solder ball bridging, soldering voids, and less heat-sinked solder, and to provide a steel mesh opening structure for controlling the welding defects of surface-mounted devices and its design method.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In the first aspect, the present invention discloses a design method for a steel mesh opening structure for controlling the welding defects of surface-mounted devices, including:

[0007] Judging the types of surface-mounted devices, the types including connectors 1 with unsealed areas, large-size PBGA devices 4 that are warped and deformed by heat, and QFP / SOP devices 10 whose leads are not coplanar with the heat sink;

[0008] Obtaining the structural dimensions of the surface-mounted devices, and determining the optimization method for the solder opening structure on the steel mesh based on the types and structural dimensions of the surface-mounted devices; the optimization method includes local thinning, inner shrinking, outer expanding, bridging, and local thickening treatments;

[0009] Calculating the opening structure dimensions of the steel mesh based on the structural dimensions of the surface-mounted devices and the optimization method, and completing the steel mesh opening design.

[0010] A further improvement lies in:

[0011] When the surface-mounted device is a connector with an unsealed area, the optimization method for the solder opening structure on the steel mesh determined based on the type and structural dimensions of the surface-mounted device is:

[0012] The thickness of the steel mesh is set to the lower limit of the conventional requirements; if the lower limit of the conventional requirements differs greatly from the steel mesh thickness of other components on the entire board, the steel mesh adopts the optimization methods of local thinning and inner shrinking of the steel mesh opening.

[0013] When the surface-mounted device is a large-size PBGA device 4 that is warped and deformed by heat, the optimization method for the solder opening structure on the steel mesh determined based on the type and structural dimensions of the surface-mounted device is:

[0014] Obtaining the law of thermal warping and deformation of the large-size PBGA device 4 that is warped and deformed by heat, and accordingly inner shrinking and outer expanding the steel mesh opening structure, increasing the solder volume at the upward warping part and reducing the solder volume at the downward warping part, so as to avoid soldering voids, bridging, and ball socket defects caused by warping and deformation.

[0015] When the surface-mounted device is a QFP / SOP device 10 whose leads are not coplanar with the heat sink, the optimization method for the solder opening structure on the steel mesh determined based on the type and structural dimensions of the surface-mounted device is:

[0016] An optimized method of locally thickening the stencil opening is adopted at the heat sink 11 of the QFP / SOP device 10 where the lead and the heat sink are non-coplanar, increasing the solder printing height and volume, compensating for the non-coplanarity of the heat sink and the lead, thereby avoiding the defect of less solder on the heat sink.

[0017] When the surface mount device is the connector 1 with a non-sealed area, the opening structure size of the stencil is calculated based on the structural size and optimization method of the surface mount device as follows:

[0018] The stencil opening structure 2 is set as a rectangle, the opening width is the same as the width of the PCB pad 3 of the connector, the outer edge L2 of the opening is consistent with the outer edge of the PCB pad 3 of the connector, and the inner edge L1 of the opening is calculated by the following formula:

[0019] L1 = L - a

[0020] Wherein, L is the inner edge of the connector lead; a is a constant, and the value range is 0.1mm - 0.2mm.

[0021] Chamfering is performed on the four sides of the rectangle, and the chamfering radius is 0.05mm - 0.1mm.

[0022] When the surface mount device is the large-size PBGA device 4 with thermal warping deformation, the opening structure size of the stencil is calculated based on the structural size and optimization method of the surface mount device as follows:

[0023] The large-size PBGA device 4 is divided into a central area 5, four corner areas 6 and other areas 7;

[0024] The stencil opening structure 2 is set as a circle, and the diameters of the stencil opening structures 2 corresponding to the central area 5, the four corner areas 6 and the other areas 7 are calculated by the following formula:

[0025] D1 = b1 × D,

[0026] D2 = b2 × D

[0027] D3 = D

[0028] Wherein, D1 represents the diameter of the stencil opening corresponding to the four-corner warping-down area 6; D2 represents the diameter of the stencil opening corresponding to the central warping-up area 5; D3 represents the diameter of the stencil opening corresponding to the other areas 7; D represents the diameter of the pad of the large-size PBGA device 4; b1 and b2 are constants, which are determined through the following steps:

[0029] Through experiments or dynamic warping measurement, obtain the thermal warping deformation law of the large-size PBGA device 4;

[0030] If the thermal deformation law of the large-size PBGA device 4 is that the four corners warp up, the selection range of b1 is 110% - 130%, and the selection range of b2 is 70% - 90%;

[0031] If the law of thermal deformation of the large-size PBGA device 4 is that the four corners are warped downward, the selection range of b1 is 70%-90%, and the selection range of b2 is 110%-130%.

[0032] When the surface-mount device is a QFP / SOP device 10 with non-coplanar leads and heat sink, the opening structure size of the stencil is calculated based on the structural size and optimization method of the surface-mount device as follows:

[0033] The thickness of the stencil opening structure 2 corresponding to the heat sink 11 of the QFP / SOP device 10 with non-coplanar leads and heat sink is calculated by the following formula:

[0034] H2 = H1 + c

[0035] Wherein, H1 is the normal thickness of the stencil opening structure 2; H2 is the increased thickness corresponding to the heat sink 11 on the stencil opening structure 2; c is a constant, and the value range is 0.05mm - 0.10mm.

[0036] The stencil opening structure 15 corresponding to the heat sink pad 14 is meshed; wherein, the value range of the inner shrinkage width c1 is 0.15mm - 0.3mm, and the value range of the bridging width c2 is 0.2mm - 0.3mm;

[0037] The four corners of the stencil opening structure 15 at the heat sink pad are chamfered, and the chamfer radius value range is 0.05mm - 0.1mm.

[0038] In a second aspect, the present invention discloses a stencil opening structure for controlling welding defects of surface-mount devices, which is obtained by using the above design method of the stencil opening structure for controlling welding defects of surface-mount devices.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The present invention discloses a design method of a stencil opening structure for controlling welding defects of surface-mount devices. According to the structural characteristics of actual non-standard surface-mount devices or PCB design characteristics, the optimization of the stencil opening structure is realized through methods such as local thinning, inner shrinking, outer expanding, bridging, and thickening treatment, solving the problems of batch welding defects such as tin entering the plug holes of connectors with non-sealed areas, solder ball bridging and virtual soldering of large-size PBGA devices with thermal warping and deformation, and less solder at the heat sink of QFP and SOP devices with non-coplanar leads and heat sink, improving the first-pass welding qualification rate of products. And this method does not change the original process route, can simply and efficiently reduce the post-furnace repair rate of products, reduce the quality cost, reduce the quality risk, and improve the product reliability. It can be compatible with the original device structure or PCB pad size, can be applied to normal production and repair and modification, and has strong universality. The structure is simple and convenient for automated production. Brief Description of the Drawings

[0041] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can be obtained based on these drawings.

[0042] Figure 1 Schematic diagram of the stencil opening structure of the connector with a non-sealed area in the present invention;

[0043] Figure 2 Schematic diagram of the layout of the large-size PBGA device area in the present invention;

[0044] Figure 3 Schematic diagram of the stencil opening structure of the large-size PBGA device with thermal warping deformation in the present invention;

[0045] Figure 4 Schematic diagram of the stencil opening structure of the QFP / SOP device with non-coplanar leads and heat sink in the present invention;

[0046] Figure 5 Schematic diagram of the grid processing structure of the stencil opening of the QFP / SOP device with non-coplanar leads and heat sink in the present invention.

[0047] Wherein: 1 - Connector with a non-sealed area; 2 - Locally thinned stencil opening structure; 3 - PCB pad of the connector; 4 - Large-size PBGA device; 5 - Central area; 6 - Corner areas; 7 - Other areas; 8 - PCB pad of the large-size PBGA device; 9 - Retracted / expanded stencil opening structure; 10 - QFP / SOP device; 11 - Heat sink; 12 - Leads; 13 - Locally thickened stencil opening structure; 14 - Heat sink pad; 15 - Stencil opening structure at the heat sink pad. Detailed Embodiments

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0049] Accordingly, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0050] It should be noted that like reference numerals and letters denote like items in the following drawings. Therefore, once an item is defined in one drawing, it is not necessary to further define and explain it in subsequent drawings.

[0051] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper", "lower", "horizontal", "inner", etc. are used to indicate the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, terms such as "first", "second", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance.

[0052] In addition, if the term "horizontal" appears, it does not mean that the component is required to be absolutely horizontal, but it can be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and it does not mean that the structure must be completely horizontal, but it can be slightly inclined.

[0053] In the description of the embodiments of the present invention, it should also be noted that unless otherwise clearly specified and limited, if terms such as "set", "installed", "connected", "connected" are used, they 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 directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0054] The present invention will be further described in detail below with reference to the accompanying drawings:

[0055] The embodiments of the present invention disclose a design method for the stencil opening structure for controlling the soldering defects of surface-mounted devices, including:

[0056] Step 1, determining the types of surface-mounted devices, where the types include the connector 1 with a non-sealed area, the large-size PBGA device 4 that is warped and deformed by heat, and the QFP / SOP device 10 whose leads are not coplanar with the heat sink;

[0057] See Figure 1, when the surface-mounted device is the connector 1 with a non-sealed area, the method for optimizing the solder opening structure on the stencil based on the type and structural dimensions of the surface-mounted device is as follows:

[0058] For the connector 1 with a non-sealed area, the thickness of the stencil is set at the lower limit of the conventional requirements. If there is a large difference in the stencil thickness from that of other components on the whole board, the stencil adopts the optimization methods of local thinning and shrinking the stencil opening to reduce the solder printing area and volume, thereby preventing solder from entering the sockets of the connector.

[0059] See Figure 2 and Figure 3 , when the surface-mounted device is the large-size PBGA device 4 that is warped and deformed by heat, the method for optimizing the solder opening structure on the stencil based on the type and structural dimensions of the surface-mounted device is as follows:

[0060] Obtain the law of warping and deformation of the large-size PBGA device 4 that is warped and deformed by heat, and accordingly shrink and expand the stencil opening structure. Increase the solder volume at the upward warping part and reduce the solder volume at the downward warping part, thereby avoiding soldering defects such as virtual soldering, bridging, and ball socket defects caused by warping and deformation.

[0061] See Figure 4 , when the surface-mounted device is the QFP / SOP device 10 whose leads are not coplanar with the heat sink, the method for optimizing the solder opening structure on the stencil based on the type and structural dimensions of the surface-mounted device is as follows:

[0062] Adopt the optimization method of locally thickening the stencil opening at the heat sink 11 of the QFP / SOP device 10 whose leads are not coplanar with the heat sink, increase the solder printing height and volume, compensate for the non-coplanarity of the heat sink and the leads, thereby avoiding the defect of less solder at the heat sink.

[0063] Step 2: Obtain the structural dimensions of the surface-mounted device, and determine the method for optimizing the solder opening structure on the stencil based on the type and structural dimensions of the surface-mounted device; the optimization methods include local thinning, shrinking, expanding, bridging, and local thickening treatments; calculate the opening structure dimensions of the stencil based on the structural dimensions and optimization methods of the surface-mounted device to complete the stencil opening design.

[0064] When the surface-mounted device is the connector 1 with a non-sealed area, the opening structure dimensions of the stencil calculated based on the structural dimensions and optimization methods of the surface-mounted device are as follows:

[0065] Set the stencil opening structure 2 as a rectangle, the opening width is the same as the width of the PCB pad 3 of the plug-in component, the outer edge L2 of the opening is consistent with the outer edge of the PCB pad 3 of the plug-in component, and the inner edge L1 of the opening is calculated by the following formula:

[0066] L1 = L - a

[0067] Among them, L is the inner edge of the connector lead; a is a constant, and its value range is 0.1 mm - 0.2 mm.

[0068] Chamfering is performed on the four sides of the rectangle, and the chamfering radius is 0.05 mm - 0.1 mm.

[0069] When the surface-mounted device is a large-size PBGA device 4 with thermal warping deformation, the opening structure size of the stencil is calculated based on the structural size and optimization method of the surface-mounted device as follows:

[0070] The large-size PBGA device 4 is divided into a central region 5, four-corner regions 6, and other regions 7;

[0071] The stencil opening structure 2 is set to be circular, and the diameters of the stencil opening structures 2 corresponding to the central region 5, four-corner regions 6, and other regions 7 are calculated by the following formula:

[0072] D1 = b1 × D,

[0073] D2 = b2 × D

[0074] D3 = D

[0075] Among them, D1 represents the stencil opening diameter corresponding to the four-corner downward warping region 6; D2 represents the stencil opening diameter corresponding to the central upward warping region 5; D3 represents the stencil opening diameter corresponding to other regions 7; D represents the pad diameter of the large-size PBGA device 4; b1 and b2 are constants, which are determined through the following steps:

[0076] Through experiments or dynamic warping measurement, obtain the thermal warping deformation law of the large-size PBGA device 4;

[0077] If the thermal deformation law of the large-size PBGA device 4 is that the four corners are upward warped, the selection range of b1 is 110% - 130%, and the selection range of b2 is 70% - 90%;

[0078] If the thermal deformation law of the large-size PBGA device 4 is that the four corners are downward warped, the selection range of b1 is 70% - 90%, and the selection range of b2 is 110% - 130%.

[0079] When the surface-mounted device is a QFP / SOP device 10 with non-coplanar leads and heat sink, the opening structure size of the stencil is calculated based on the structural size and optimization method of the surface-mounted device as follows:

[0080] The thickness of the stencil opening structure 2 corresponding to the heat sink 11 of the QFP / SOP device 10 with non-coplanar leads and heat sink is calculated by the following formula:

[0081] H2 = H1 + c

[0082] Among them, H1 is the normal thickness of the stencil opening structure 2; H2 is the thickened thickness corresponding to the heat sink 11 on the stencil opening structure 2; c is a constant, and the value range is 0.05 mm - 0.10 mm.

[0083] See Figure 5 , perform meshing on the stencil opening structure 15 at the heat sink pad corresponding to the heat sink pad 14; among them, the value range of the inner shrinkage width c1 is 0.15 mm - 0.3 mm, and the value range of the bridging width c2 is 0.2 mm - 0.3 mm;

[0084] The four corners of the stencil opening structure 15 at the heat sink pad are chamfered, and the value range of the chamfer radius is 0.05 mm - 0.1 mm.

[0085] The present invention relates to a stencil opening structure and a design method for reducing soldering defects of surface-mounted devices. Aiming at the problem that designing a stencil according to conventional requirements is likely to cause various soldering defects of non-standard surface-mounted devices, an economical and efficient stencil opening design method is provided. That is, according to the structural characteristics of actual non-standard surface-mounted devices or PCB design characteristics, the stencil opening structure is optimized through methods such as local thinning, inner shrinking, outer expanding, bridging, and thickening treatment to control the solder volume and morphology, thereby reducing its soldering defects. This method can be compatible with different types of solders, and can not change the original PCB design and device selection, and has strong universality.

[0086] The embodiment of the present invention also discloses a stencil opening structure for controlling soldering defects of surface-mounted devices obtained by using the design method of the stencil opening structure for controlling soldering defects of surface-mounted devices described above.

[0087] Set solder openings on the stencil template, and control the solder printing area, height, volume, and morphology by limiting the opening size, shape, and thickness, thereby reducing its soldering defects.

[0088] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for designing a steel mesh opening structure for controlling welding defects of surface mount devices, characterized in that: include: Determining the types of surface mount devices, the types including connectors (1) with non-sealed areas, large-sized PBGA devices (4) that are warped by heat, and QFP / SOP devices (10) whose leads are not coplanar with the heat sink; Obtaining the structural dimensions of the surface mount device, and determining a method for optimizing the solder opening structure on the steel mesh based on the type and structural dimensions of the surface mount device; the optimization method includes local thinning, inward shrinkage, outward expansion, bridging and local thickening processing; The opening structural dimensions of the steel mesh are calculated based on the structural dimensions of the surface mount device and the optimization method, and the steel mesh opening design is completed.

2. The method for designing a steel mesh opening structure for controlling surface mount device welding defects according to claim 1, characterized in that: When the surface mount device is a connector (1) having a non-sealed area, the method for optimizing the solder opening structure on the steel mesh based on the type and structural dimensions of the surface mount device is: The thickness of the steel mesh is set to the lower limit of the conventional requirements; if the lower limit of the conventional requirements is greatly different from the thickness of the steel mesh of other components on the entire board, the steel mesh is optimized by local thinning and shrinking the steel mesh opening.

3. The method for designing a steel mesh opening structure for controlling surface mount device welding defects according to claim 1, characterized in that: When the surface mount device is a large-sized PBGA device (4) that is warped and deformed due to heat, the method for optimizing the solder opening structure on the steel mesh based on the type and structural size of the surface mount device is: The heat warping deformation law of a large-size PBGA device (4) that is warped under heat is obtained, and accordingly, the steel mesh opening structure is shrunk inwards and expanded outwards, the amount of solder is increased in the upward warping part, and the amount of solder is reduced in the downward warping part, thereby avoiding cold solder joints, bridges and ball-socket defects caused by the warping deformation.

4. The method for designing a steel mesh opening structure for controlling surface mount device welding defects according to claim 1, characterized in that: When the surface mount device is a QFP / SOP device (10) whose leads are not coplanar with the heat sink, the method for optimizing the solder opening structure on the steel mesh based on the type and structural dimensions of the surface mount device is: An optimization method of locally thickening the steel mesh opening is adopted at the heat sink (11) of a QFP / SOP device (10) whose leads are not coplanar with the heat sink, so as to increase the solder printing height and volume, compensate for the non-coplanarity of the heat sink and the leads, and avoid the defect of insufficient tin in the heat sink.

5. The method for designing a steel mesh opening structure for controlling soldering defects of surface mount devices according to claim 1, characterized in that: When the surface mount device is a connector (1) having a non-sealed area, the opening structural dimensions of the steel mesh are calculated based on the structural dimensions of the surface mount device and the optimization method as follows: The steel mesh opening structure (2) is set to be rectangular, the opening width is consistent with the width of the PCB pad (3) of the plug-in component, the opening outer edge L2 is consistent with the outer edge of the PCB pad (3) of the plug-in component, and the opening inner edge L1 is calculated by the following formula: L1=La Wherein, L is the inner edge of the connector lead; a is a constant, and its value range is 0.1mm-0.2mm.

6. The method for designing a steel mesh opening structure for controlling soldering defects of surface mount devices according to claim 5, characterized in that: The four sides of the rectangle are chamfered, and the chamfer radius is 0.05mm-0.1mm.

7. The method for designing a steel mesh opening structure for controlling soldering defects of surface mount devices according to claim 1, characterized in that: When the surface mount device is a large-sized PBGA device (4) that is warped due to heat, the opening structural dimensions of the steel mesh are calculated based on the structural dimensions of the surface mount device and the optimization method as follows: Dividing the large-size PBGA device (4) into a central area (5), four corner areas (6) and other areas (7); The steel mesh opening structure (2) is set to be circular, and the diameters of the steel mesh opening structure (2) corresponding to the central area (5), the four corner areas (6) and other areas (7) are calculated by the following formula: D1=b1×D, D2=b2×D D3=D Wherein, D1 represents the diameter of the steel mesh opening corresponding to the four corner downward warping areas (6); D2 represents the diameter of the steel mesh opening corresponding to the central upward warping area (5); D3 represents the diameter of the steel mesh opening corresponding to the other areas (7); D represents the pad diameter of the large-size PBGA device (4); b1 and b2 are constants, which are determined by the following steps: Obtaining the thermal warpage deformation law of the large-size PBGA device (4) through experiments or dynamic warpage measurement; If the large-size PBGA device (4) is deformed by heat with four corners warping upward, the selection range of b1 is 110%-130%, and the selection range of b2 is 70%-90%; If the large-size PBGA device (4) is deformed by heat with four corners warping downward, the selection range of b1 is 70%-90%, and the selection range of b2 is 110%-130%.

8. The method for designing a steel mesh opening structure for controlling soldering defects of surface mount devices according to claim 7, characterized in that: When the surface mount device is a QFP / SOP device (10) whose leads are not coplanar with the heat sink, the opening structural dimensions of the steel mesh are calculated based on the structural dimensions of the surface mount device and the optimization method as follows: The thickness of the steel mesh opening structure (2) corresponding to the heat sink (11) of the QFP / SOP device (10) whose leads are not coplanar with the heat sink is calculated by the following formula: H2=H1+c Wherein, H1 is the normal thickness of the steel mesh opening structure (2); H2 is the thickened thickness of the steel mesh opening structure (2) corresponding to the heat sink (11); and c is a constant, with a value range of 0.05 mm-0.10 mm.

9. The method for designing a steel mesh opening structure for controlling soldering defects of surface mount devices according to claim 8, characterized in that: A mesh processing is performed on the steel mesh opening structure (15) at the heat sink pad corresponding to the heat sink pad (14); wherein the value range of the inner shrinkage width c1 is 0.15 mm-0.3 mm, and the value range of the bridge width c2 is 0.2 mm-0.3 mm; The four corners of the steel mesh opening structure (15) at the heat sink pad are chamfered, and the chamfer radius ranges from 0.05 mm to 0.1 mm.

10. A steel mesh opening structure for controlling welding defects of surface mount devices, characterized in that: The method for designing a steel mesh opening structure for controlling welding defects of surface mount devices according to any one of claims 1 to 9 is adopted to obtain the steel mesh opening structure.

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