Methods to reduce welding bubbles on LGA pad core boards

Abstract

The present invention discloses a method for reducing welding bubbles of LGA pad core board, comprising the following steps: S1, preparation work: preparing the core board, printed board, steel mesh and welding equipment to be welded; S2, printing solder paste: printing solder paste at the position corresponding to each pad of the printed board through the mesh on the steel mesh, wherein a partition is provided in the mesh on the steel mesh, so that an exhaust groove is formed on the solder paste printed on the printed board; S3, placing the core board: placing the core board on the printed board to ensure that the core board is aligned with the pad; S4, welding operation: welding the core board and the printed board through the welding equipment. The present invention can effectively reduce bubbles in the solder joint by arranging a partition inside the mesh of the steel mesh so that an exhaust groove is formed on the solder paste printed on the printed board, thereby effectively ensuring the product yield and stability, reducing product scrap and rework, and reducing production costs.

Description

Technical Field

[0001] The invention relates to the technical field of integrated circuits, and more particularly to a method for reducing welding bubbles of an LGA pad core board. Background Art

[0002] There are three ways to connect a universal core board to a baseboard: 1) board-to-board connector; 2) gold finger; 3) stamp hole / LGA pad. Board-to-board connector is the mainstream form of universal core boards at present, but its pin function is limited by the number of pins of the connector. The more functional pins there are, the more difficult the testing, welding, installation and other processes will be, and there is also a certain risk of stability. The gold finger form is less used. The stamp hole / LGA pad is the new trend in the welding of universal core boards. However, the LGA solder joint form core boards have poor bubble discharge effect and many bubbles in the solder joints during welding, which leads to reduced product yield and stability, and then increases production costs. Summary of the invention

[0003] In view of this, the present invention provides a method for reducing welding bubbles of LGA pad core board, the purpose of which is to solve the problems existing in the prior art.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] A method for reducing LGA pad core board welding bubbles, comprising the following steps:

[0006] S1, preparation: prepare the core board, steel mesh, printed circuit board and welding equipment required for welding, wherein a partition is provided inside the mesh holes on the steel mesh;

[0007] S2, printing solder paste: printing solder paste at positions corresponding to pads of the printed circuit board through the mesh holes on the steel mesh, so that exhaust grooves are formed on the solder paste printed on the printed circuit board;

[0008] S3, placing the core board: placing the core board on the printed circuit board, ensuring that the core board is aligned with the pad;

[0009] S4, welding operation: welding the core board and printed circuit board through welding equipment.

[0010] Preferably, the partition is in the shape of a straight line or a cross.

[0011] Preferably, a cylinder is provided at the middle position of the partition; the diameter of the cylinder is greater than the width of the partition.

[0012] Preferably, the mesh area on the steel mesh is 76.56% of the pad area.

[0013] Preferably, the partition and the cylinder are an integrally formed structure; the area of ​​the partition and the cylinder is 20%-50% of the mesh area on the steel mesh.

[0014] Preferably, the area of ​​the mesh on the steel mesh after removing the partition and the cylinder is 69.69% of the mesh area.

[0015] Preferably, the welding equipment is a reflow oven.

[0016] Preferably, during the welding process, the preheating time, constant temperature time, reflow time and peak temperature of the reflow oven are adjusted according to the welding effect.

[0017] Preferably, before welding, the welding position is wiped with alcohol.

[0018] Compared with the prior art, the present invention has achieved the following technical effects:

[0019] 1) The present invention forms an exhaust groove on the solder paste printed on the printed circuit board by arranging a partition inside the mesh of the steel mesh. During welding, the solder paste in a molten state on both sides flows to the exhaust groove in the middle to form a solder joint, so that the volatiles generated during the welding process can be discharged from the solder joint, thereby effectively reducing the bubbles in the solder joint, and further effectively ensuring the product yield and stability, reducing product scrap and rework, and reducing production costs;

[0020] 2) The present invention arranges a cylinder in the middle of the partition so that a circular groove is formed in the middle of the exhaust groove during the solder paste printing process, which can reduce the amount of tin in the center of the solder paste, thereby further improving the exhaust effect and reducing bubbles inside the solder joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A flow chart of a method for reducing welding bubbles of LGA pad core board of the present invention;

[0022] Figure 2-Figure 9 Schematic diagram of meshes with partitions of different shapes and angles;

[0023] Figure 10-Figure 17 Schematic diagram of solder paste with exhaust slots of different shapes and angles on a printed circuit board;

[0024] Figure 18-Figure 20 The mesh size is Figure 5 When the shape is shown, the weld spot images when the area of ​​the mesh after removing the partition and the cylinder is 69.69%, 71.53% and 68.25% of the mesh area respectively;

[0025] Fig.21 The mesh size is Figure 7The shape shown is the weld spot image when the area of ​​the mesh after removing the partition and the cylinder is 60.06% of the mesh area;

[0026] Figure 22-Figure 23 The mesh size is Figure 2 When the shape is shown, the area of ​​the mesh after removing the partition and the cylinder is 74.62% and 65.52% of the mesh area respectively;

[0027] Fig.24 The mesh size is Figure 3 When the shape is shown, the area of ​​the mesh after removing the partition and the cylinder is 74.62% of the mesh area;

[0028] In the figure: 1, mesh; 2, partition; 3, exhaust groove; 4, cylinder; 5, welding point; 6, bubble. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Example

[0031] The invention discloses a method for reducing welding bubbles of LGA pad core board, comprising the following steps:

[0032] S1, preparation: prepare the core board, steel mesh, printed circuit board and welding equipment required for welding, wherein a partition plate 2 is arranged inside the mesh 1 on the steel mesh;

[0033] S2, printing solder paste: printing solder paste at positions corresponding to pads of the printed circuit board through the mesh 1 on the steel mesh, so that exhaust grooves 3 are formed on the solder paste printed on the printed circuit board;

[0034] S3, placing the core board: placing the core board on the printed circuit board, ensuring that the core board is aligned with the pad;

[0035] S4, welding operation: welding the core board and printed circuit board through welding equipment.

[0036] The above technical solution forms an exhaust groove 3 on the solder paste printed on the printed circuit board by arranging a partition 2 inside the mesh 1 of the steel mesh. During welding, the molten solder paste on both sides flows to the middle exhaust groove 3 to form a solder joint, which can enable the volatiles generated during the welding process to be discharged from the solder joint, thereby effectively reducing the bubbles in the solder joint, and further effectively ensuring the product yield and stability, reducing product scrap and rework, and reducing production costs.

[0037] In this embodiment, the partition 2 is in a straight line or a cross shape, so that a straight line or a cross-shaped exhaust groove 3 is formed on the solder paste to facilitate the exhaust of volatiles from the solder joint.

[0038] In this embodiment, a cylinder 4 is provided in the middle position of the partition 2; the diameter of the cylinder 4 is greater than the width of the partition 2; by providing the cylinder 4, a circular groove is formed in the middle of the exhaust groove 3; after adding the exhaust groove 3, the closer the two semicircular solder pastes on both sides of the exhaust groove 3 are to the middle position, the more tin there is. By providing a circular groove in the middle position of the exhaust groove, the amount of tin at the center position can be reduced, thereby better reducing the bubbles inside the solder joint.

[0039] The design process of mesh 1 on the steel mesh: 1) The mesh 1 on the conventional steel mesh is designed according to the pad diameter designed by simulation data. Generally, the mesh 1 area on the steel mesh is shrunk in a certain proportion of the pad area, and the shape of mesh 1 is consistent with the shape of the pad; 2) After welding, X-ray irradiation is used to observe the welding condition and the bubble condition of the solder joint to see if there is any regularity. The multimeter is used to measure the continuity of the solder joint to understand the formation process of the bubble and sort out the method to reduce the bubble; 3) Theoretical analysis shows that the bubble is the substance that needs to be volatilized into the air and eliminated after the chemical reaction is completed when the solder paste enters the reflow oven and the solder paste is heated to a molten state. The current solder joint and steel mesh design is fixed in the Z-axis direction, and there is no elimination channel in the xy plane solder joint, so an exhaust slot 3 is added to try to solve it; 4) An exhaust slot 3 is added to the internal xy plane during the formation of the solder joint (a straight or cross-shaped partition 2 is set on the mesh 1 of the steel mesh).

[0040] In this embodiment, the area of ​​the mesh 1 on the steel mesh is 76.56% of the area of ​​the pad.

[0041] In this embodiment, the partition plate 2 and the cylinder 4 are an integrally formed structure; the area of ​​the partition plate 2 and the cylinder 4 is 20%-50% of the area of ​​the mesh 1 on the steel mesh.

[0042] In this embodiment, most preferably, the area of ​​the mesh 1 on the steel mesh after removing the partition 2 and the cylinder 4 is 69.69% of the mesh area on the steel mesh, and the exhaust groove 3 formed is a straight-line structure inclined 45° to the right, and a circular groove is formed in the middle position of the exhaust groove 3.

[0043] In this embodiment, the soldering equipment is a reflow oven.

[0044] For some open solder joints (such as non-BTC type device solder joints such as RC, SOP, QFP), increasing the preheating time and constant temperature time, controlling the constant temperature time at 10-105s, and the reflow time at 85s will help reduce bubbles. For non-open solder joints (such as BTC type device solder joints such as QFN and LGA), increasing the peak temperature is effective in reducing bubbles. The peak temperature is controlled at 235°-250° and the time is controlled at 60-90s. In the actual welding process, the preheating time, constant temperature time, reflow time and peak temperature of the reflow oven are adjusted according to the welding effect.

[0045] In this embodiment, before welding, the position to be welded is wiped with alcohol.

[0046] In some other embodiments, during the welding process, a jig is used to press the printed circuit board as needed to prevent the printed circuit board from warping during the welding process and causing poor welding.

[0047] In some other embodiments, the shape and area of ​​the mesh 1 and the partition 2 on the steel mesh can be designed and adjusted according to the welding effect of the project.

[0048] Select meshes of different shapes for experiment:

[0049] 1) Make meshes with different partition shapes, print solder paste and solder the core board; 2) Use x-ray detection equipment to irradiate and image the soldered printed circuit board (such as Figure 18-24 ); 3) judge the obtained image and observe whether there are bubbles 6 in the solder joint 5.

[0050] Method for judging whether there are bubbles 6 in solder joint 5: Under X-ray irradiation, when there are bubbles 6 in solder joint 5, a light-colored area will be formed inside solder joint 5, and the light-colored area represents bubbles 6. By observing the number and size of the light-colored areas in solder joint 5, the size and number of bubbles 6 in solder joint 5 can be determined.

[0051] By comparison Figure 18-24 In the light-colored area, it can be seen that when the mesh is Figure 5 For the shape shown, when the area of ​​the mesh after removing the partition and the cylinder is 69.69% of the mesh area, there are the least bubbles in the weld and the welding effect is the best.

[0052] The above description is only a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any slight modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for reducing LGA pad core board welding bubbles, characterized in that: The following steps are involved: S1, preparation: prepare the core board, steel mesh, printed circuit board and welding equipment required for welding, wherein a partition is provided inside the mesh holes on the steel mesh; S2, printing solder paste: printing solder paste at positions corresponding to pads of the printed circuit board through the mesh holes on the steel mesh, so that exhaust grooves are formed on the solder paste printed on the printed circuit board; S3, placing the core board: placing the core board on the printed circuit board, ensuring that the core board is aligned with the pad; S4, welding operation: welding the core board and the printed circuit board through welding equipment; A cylinder is provided in the middle of the partition; the diameter of the cylinder is greater than the width of the partition; The mesh area on the steel mesh is 76.56% of the pad area; The partition plate and the cylinder are an integrally formed structure; the area of ​​the partition plate and the cylinder is 20%-50% of the mesh area of ​​the steel mesh; The area of ​​the mesh on the steel mesh after removing the partition and the cylinder is 69.69% of the mesh area.

2. A method for reducing LGA pad core board welding bubbles according to claim 1, characterized in that: The partition is in a straight line shape or a cross shape.

3. A method for reducing LGA pad core board welding bubbles according to claim 1, characterized in that: The welding equipment adopts a reflow oven.

4. A method for reducing LGA pad core board welding bubbles according to claim 1, characterized in that: During the welding process, the preheating time, constant temperature time, reflow time and peak temperature of the reflow oven are adjusted according to the welding effect.

5. A method for reducing LGA pad core board welding bubbles according to claim 1, characterized in that: Before welding, wipe the welding position with alcohol.

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