Printing screen and system board structure

Abstract

The utility model discloses a printing screen and a system board structure, the printing screen comprises a substrate, the substrate comprises a first surface and a second surface which are arranged oppositely, the substrate is provided with a plurality of through holes penetrating through the first surface and the second surface, and the first surface and the second surface are arranged oppositely. The through holes comprise first through holes located in the corners of the substrate and second through holes located outside the corners of the substrate, and the diameter of the first through holes is smaller than that of the second through holes. According to the utility model, the printing screen plate is applied to form a system board structure with matrix type connecting pads, when the corners of a semiconductor packaging piece warp, the semiconductor packaging piece can be accurately installed on a PCB, and the problem that a short circuit or an open circuit is generated due to the fact that the welding balls of the semiconductor packaging piece are welded to the connecting pads at high temperature is solved.

Description

Technical Field

[0001] The utility model belongs to the technical field of semiconductor package components mounted on PCB boards, and in particular relates to a printed screen board and a system board structure. Background Art

[0002] Ginseng Figure 1 The figure shows a conventional printing stencil 2', which is provided with a plurality of through holes 21' of equal diameter. Solder paste is applied to the PCB 1' through the printing stencil 2', and the solder paste forms pads 11' on the soldering area of ​​the PCB 1'. Then, the semiconductor package 3' is soldered to the PCB 1' (see FIG. Figure 2 If the semiconductor package 3' is large (for example, the size of the semiconductor package 3' is larger than 16*16 mm) or the solder balls 33' of the semiconductor package 3' are densely distributed, during a high-temperature (for example, 260°C) furnace soldering process, the corners of the semiconductor package 3' may warp due to the high temperature because the solder balls 33' of the semiconductor package 3' are connected to the pads of the PCB board 1'. This may cause contact between adjacent solder balls 33' located in the corners of the semiconductor package 3', resulting in short circuits or open circuits among the solder balls 33'.

[0003] Therefore, in order to solve the above technical problems, it is necessary to provide a printing screen and a system board structure. Utility Model Content

[0004] The purpose of the utility model is to provide a printing screen and a system board structure, which can solve the problem of solder ball short circuit or open circuit when the corner of the semiconductor package is warped by reducing the diameter of the through hole at the corner of the printing screen.

[0005] In order to achieve the above-mentioned purpose, the technical solution provided by a specific embodiment of the present invention is as follows:

[0006] A printing screen includes a substrate, the substrate including a first surface and a second surface arranged opposite to each other, the substrate is provided with a plurality of through holes penetrating the first surface and the second surface, the through holes including a first through hole located at a corner of the substrate and a second through hole located outside the corner of the substrate, the diameter of the first through hole being smaller than the diameter of the second through hole.

[0007] In one or more embodiments of the present invention, the distance between the center of the first through hole adjacent to the side edge of the substrate and the side edge of the substrate is smaller than the distance between the center of the second through hole adjacent to the side edge of the substrate and the side edge of the substrate; and / or,

[0008] The distance between the edge of the first through hole adjacent to the side edge of the substrate and the side edge of the substrate is greater than or equal to the distance between the edge of the second through hole adjacent to the side edge of the substrate and the side edge of the substrate.

[0009] In one or more embodiments of the present invention, the corner portion of the substrate is rectangular, and the first through holes are distributed in the corner portion of the substrate in a rectangular array.

[0010] In one or more embodiments of the present invention, the distance between the centers of two adjacent first through holes in each row is greater than or equal to the distance between the centers of two adjacent second through holes in each row; and / or,

[0011] The distance between the centers of two adjacent first through holes in each column is greater than or equal to the distance between the centers of two adjacent second through holes in each column.

[0012] In one or more embodiments of the present invention, the distance between the center of a first through hole not adjacent to the side of the substrate and the center of a second through hole adjacent thereto is equal to the distance between the centers of two adjacent second through holes.

[0013] In one or more embodiments of the present invention, the first through holes are distributed in an m*n array at the corner of the substrate, wherein m is an integer and m≥2, and n is an integer and n≥2.

[0014] In one or more embodiments of the present invention, the ratio of the diameter of the first through hole to the diameter of the second through hole is 0.7-0.84.

[0015] In one or more embodiments of the present invention, the ratio of the diameter of the first through hole to the diameter of the second through hole is 5:6; or,

[0016] The ratio of the diameter of the first through hole to the diameter of the second through hole is 3:4; or,

[0017] The ratio of the diameter of the first through hole to the diameter of the second through hole is 7:9.

[0018] Another specific embodiment of the present invention provides a technical solution as follows:

[0019] A system board structure includes a PCB, pads, and a semiconductor package. The PCB has a soldering area on its upper surface, and a plurality of pads are located in the soldering area. The semiconductor package has a plurality of solder balls on its lower surface, and the semiconductor package is connected to the pads via the solder balls.

[0020] The pads include a first pad located at a corner of the welding area and a second pad located outside the corner of the welding area. The diameter of the first pad is smaller than that of the second pad.

[0021] In one or more embodiments of the present invention, the distance between the center of the first pad adjacent to the edge of the welding area and the edge of the welding area is smaller than the distance between the center of the second pad adjacent to the edge of the welding area and the edge of the welding area; and / or,

[0022] The distance between the edge of the first pad adjacent to the edge of the welding area and the edge of the welding area is greater than or equal to the distance between the edge of the second pad adjacent to the edge of the welding area and the edge of the welding area.

[0023] Compared with the prior art, the printed stencil and system board structure of the present invention are designed such that the diameter of the first through-holes located at the corners of the printed stencil is smaller than the diameter of the second through-holes outside the corners. Solder paste or flux is applied to the PCB through the printed stencil to form a rectangular array of pads on the PCB. This solves the problem of solder balls on a semiconductor package being in a warped state during high-temperature soldering. When soldering to pads on the PCB, the solder balls do not contact each other due to deformation of the semiconductor package, causing short circuits or open circuits.

[0024] The utility model further solves the problem of the solder balls at the corners of the semiconductor package shifting in the high-temperature warping state by designing the center of the first through hole adjacent to the side of the printed screen to be offset to the side. The relatively offset pads on the PCB can be more accurately aligned with the solder balls on the semiconductor package for soldering.

[0025] The utility model has a simple structure and improves the quality of the entire system board structure without changing the original design of the semiconductor package and the PCB board;

[0026] The printed stencil and system board structure of the present invention are suitable for mounting various semiconductor packages on PCB boards, and the semiconductor packages include BGA, EMMC, QFN, LGA, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 It is a schematic diagram of the steel mesh structure in the prior art;

[0029] Figure 2 A schematic diagram of a system board structure in the prior art;

[0030] Figure 3 This is a front view of the printing screen in Example 1 of the present utility model;

[0031] Figure 4 This is an enlarged view of a portion A of the printing screen in Example 1 of the present utility model;

[0032] Figure 5 This is a front view of the system board structure in Example 2 of the present utility model;

[0033] Figure 6 This is a top view of the system board structure in Example 2 of the present utility model. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the following will be combined with the drawings of the embodiments of the present invention to clearly and completely describe the technical solutions of the embodiments of the present invention. Obviously, the embodiments described 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 making creative efforts should fall within the scope of protection of the present invention.

[0035] Example 1:

[0036] like Figure 3 As shown, a printing screen in this embodiment includes a substrate 10, the substrate 10 includes a first surface and a second surface arranged opposite to each other, and a plurality of through holes penetrating the first surface and the second surface are provided on the substrate 10, the through holes including a first through hole 21 located at a corner 20 of the substrate and a second through hole 11 located outside the corner 20 of the substrate, wherein the diameter of the first through hole 21 is smaller than the diameter of the second through hole 11.

[0037] The printed stencil in this embodiment is applicable to the field of semiconductor packaging technology. Based on the printed stencil in this embodiment, array-arranged pads are formed in the welding area of ​​the PCB board. Since the diameter of the first through hole 21 at the corner of the printed stencil is smaller than the diameter of the second through hole 11 outside the corner, the diameter of the first pad at the corner of the welding area on the PCB board is smaller than the diameter of the remaining second pads in the welding area.

[0038] In the prior art, the through-holes on the printed stencil are all of equal diameter. However, if the semiconductor package is large, has a large number of solder balls, or has a dense distribution of solder balls, during high-temperature soldering, the semiconductor package may warp at its corners due to the high temperature, as the semiconductor package is connected to the PCB pads via the solder balls. Furthermore, the pads themselves may overflow to a certain extent at high temperatures, causing contact between the solder balls at the corners of the semiconductor package, leading to solder ball shorts or open circuits. However, the printed stencil in this embodiment can precisely control the pad size: because the diameter of the first through-hole 21 is smaller than the diameter of the second through-hole 11, the diameter of the first pad formed at the corner of the soldering area is smaller than the diameter of the remaining second pads in the soldering area.

[0039] When a semiconductor package corner warps, the smaller diameter of the first pads at the corners of the PCB soldering area and the use of solder paste or other flux materials in these corners prevents overflow of the first pads. This reduces the likelihood of solder balls contacting each other at the corners, ensuring soldering quality. This embodiment is suitable for high-density semiconductor packaging structures, meeting the needs of high-end electronic products.

[0040] like Figure 4 As shown, in order to more clearly illustrate the technical solution, this embodiment uses the through hole 11A shown by the dotted line to demonstrate the size and position of the through hole in the unimproved prior art, and then compares and illustrates the size and position of the first through hole 21 in this embodiment. It can be understood that the diameter of the first through hole 21 is smaller than the diameter of the second through hole 11, and for the first through hole 21 adjacent to the side of the substrate 10: the center of the first through hole 21 adjacent to the side of the substrate is offset toward the side of the substrate 10, especially for the first through hole 21 located closest to the corner 20 of the substrate, its center is offset toward the vertex of the substrate closest to the first through hole 21.

[0041] Furthermore, for the through holes adjacent to the side of the substrate 10 , the distance between the center of the first through hole 21 adjacent to the side of the substrate 10 and the side of the substrate 10 is smaller than the distance between the center of the second through hole 11 adjacent to the side of the substrate 10 .

[0042] Furthermore, for the through holes adjacent to the side of the substrate 10 , the distance between the edge of the first through hole 21 adjacent to the side of the substrate 10 and the side of the substrate 10 is greater than or equal to the distance between the edge of the second through hole 11 adjacent to the side of the substrate 10 and the side of the substrate 10 .

[0043] Furthermore, the ratio of the diameter of the first through hole 21 to the diameter of the second through hole 11 is 0.7-0.84.

[0044] It can be understood that the ratio of the diameter of the first through hole 21 to the diameter of the second through hole 11 is controlled within the range of 0.7 to 0.84. This can avoid the problem that the first pad and the solder ball at the corners of the PCB board are disconnected after the PCB board and the semiconductor package are welded due to the pad volume being too small. If the semiconductor package is combined with the PCB board in a warped state, it can also avoid the problem that the solder balls connected to the first pad at the corner come into contact with each other due to the first pad being too large, thereby causing the solder balls to short-circuit or open.

[0045] Preferably, the ratio of the diameter of the first through hole 21 to the diameter of the second through hole 11 is 5:6. For example, the diameter of the second through hole 11 is 0.3 mm, and the diameter of the first through hole 21 is 0.25 mm.

[0046] Preferably, the ratio of the diameter of the first through hole 21 to the diameter of the second through hole 11 is 3:4. For example, the diameter of the second through hole 11 is 0.4 mm, and the diameter of the first through hole 21 is 0.3 mm.

[0047] Preferably, the ratio of the diameter of the first through hole 21 to the diameter of the second through hole 11 is 7:9. For example, the diameter of the second through hole 11 is 0.45 mm, and the diameter of the first through hole 21 is 0.35 mm.

[0048] like Figure 4 As shown, the corner portion 20 of the substrate in this embodiment is rectangular, and the first through holes 21 are distributed in the corner portion 20 of the substrate in a rectangular array.

[0049] The first through holes 21 are distributed in the substrate corner 20 in an m*n array, wherein m is an integer and m≥2, and n is an integer and n≥2.

[0050] Preferably, the first through holes 21 are distributed in an m*m array at the substrate corner 20. It will be appreciated that those skilled in the art can set the values ​​of m and n based on the total number of through holes on the substrate 10 and the area of ​​the abnormal region. (The abnormal region in this embodiment refers to the area at the corner of the semiconductor package where contact between pads is likely to occur.)

[0051] For example, if the number of pads in the PCB soldering area is 23*23 (i.e., there are 23*23 through-holes on the substrate 10), the abnormal area is the 2*2 pads located at the substrate corners 20. Therefore, 3*3 first through-holes 21 can be set at each of the four substrate corners 20, and the remaining through-holes outside the substrate corners 20 can be set as second through-holes 11. If the abnormal area is larger, 4*4 first through-holes 21 or 5*5 first through-holes 21 can be set at each of the four substrate corners 20.

[0052] like Figure 4 As shown, further, for the first through hole 21 not adjacent to the side of the substrate 10: the distance between the center of the first through hole 21 not adjacent to the side of the substrate and the center of the adjacent second through hole 11 is equal to the distance between the centers of the two adjacent second through holes 11. It can be understood that if the first through holes 21 are distributed in the corner portion 20 of the substrate in an m*m array shape, and m is greater than or equal to 3, except for the first through hole 21 adjacent to the side of the substrate 10, the center positions of the remaining first through holes 21 are relative to the existing design (see Figure 4 The through hole 11A indicated by the dotted line does not move.

[0053] Furthermore, the present invention is applicable to semiconductor packages with a size of 16*16 mm or larger, and the present invention is applicable to semiconductor package structures that require soldering through more than 500 pads.

[0054] Example 2:

[0055] like Figure 5 As shown, the system board structure of this embodiment includes a PCB 30, pads, and a semiconductor package 50. The upper surface of the PCB 30 has a welding area 40, and multiple pads are arranged in a matrix within the welding area 40. The lower surface of the semiconductor package 50 has multiple solder balls 51. The solder balls 51 of the semiconductor package 50 are connected to the pads after soldering, so that the semiconductor package 50 is connected to the PCB 30. The multiple solder balls 51 are arranged in a matrix on the lower surface of the semiconductor package 50.

[0056] The system board structure comprises various semiconductor packages 50 soldered to a PCB 30 , wherein the system board structure can be applied to various consumer electronic products, including smart phones, tablet computers, digital cameras, home appliances, etc.

[0057] like Figure 6 As shown, the system board structure includes a first pad 41 located at a corner of a soldering area 40 of a PCB 30 and a second pad 42 located outside the corner of the soldering area 40. The diameter of the first pad 41 is smaller than the diameter of the second pad 42. Furthermore, the material of the pad in this embodiment can be a tin-based alloy (such as Sn-Pb, Sn-Ag-Cu, etc.), a copper-based alloy, a gold-based alloy, or other flux materials.

[0058] It can be understood that by applying solder paste or flux to the welding area 40 on the PCB board 30 through the printed stencil of the present invention, a matrix-arranged pad is formed. Since the diameter of the first through hole 21 on the printed stencil is smaller than the diameter of the second through hole 11, the diameter of the first pad 41 formed at the corner of the welding area 40 on the PCB board 30 in this embodiment is smaller than the diameter of the second pad 42 of the remaining welding areas 40.

[0059] like Figure 6 As shown, for the pads adjacent to the edge of the welding area 40 , the distance between the center of the first pad 41 adjacent to the edge of the welding area and the edge of the welding area 40 is smaller than the distance between the center of the second pad 42 adjacent to the edge of the welding area and the edge of the welding area 40 .

[0060] The distance between the edge of the first pad 41 adjacent to the edge of the welding area and the edge of the welding area 40 is greater than or equal to the distance between the edge of the second pad 42 adjacent to the edge of the welding area and the edge of the welding area 40 .

[0061] like Figure 6 As shown, the corners of the welding area 40 in this embodiment are rectangular, and the first pads 41 are distributed in the corners of the welding area 40 in a rectangular array.

[0062] Furthermore, the distance between the centers of two adjacent first pads 41 in each row is greater than or equal to the distance between the centers of two adjacent second pads 42 in each row.

[0063] The distance between the centers of two adjacent first pads 41 in each row is greater than or equal to the distance between the centers of two adjacent second pads 42 in each row. It is understood that for first pads 41 adjacent to the edge of welding area 40, the center of the first pad 41 is offset toward the edge of welding area 40. In particular, for first pads 41 located closest to the vertex of welding area 40, the center of the first pad 41 is offset toward the vertex of welding area 40.

[0064] For first pads 41 not adjacent to the edge of soldering area 40: the distance between the center of a first pad 41 not adjacent to the edge of soldering area 40 and the center of an adjacent second pad 42 is equal to the distance between the centers of two adjacent second pads 42. First pads 41 are distributed in an m*n array at the corners of soldering area 40, where m is an integer and m≥2, and n is an integer and n≥2.

[0065] Preferably, the first pads 41 are distributed in an m*m array at the corners of the soldering area 40. It is understood that if the first pads 41 are distributed in an m*m array at the corners of the soldering area, and m is greater than or equal to 3, then, except for the first pads 41 adjacent to the edge of the soldering area 40, the center positions of the remaining first pads 41 remain unchanged relative to the existing design.

[0066] The ratio of the diameter of the first pad 41 to the diameter of the second pad 42 is 0.7 to 0.84. In this embodiment, the pads at the corners of the soldering area 40 do not contact each other, and when the corners of the semiconductor package 50 warp, the solder balls on the bottom surface of the semiconductor package 50 can accurately connect to the displaced pads.

[0067] Preferably, the ratio of the diameter of the first pad 41 to the diameter of the second pad 42 is 5:6. For example, the diameter of the second pad 42 is 0.3 mm, and the diameter of the first pad 41 is 0.25 mm. Correspondingly, the diameter of the solder ball on the semiconductor package 50 is 0.3 mm.

[0068] Preferably, the ratio of the diameter of the first pad 41 to the diameter of the second through hole 11 is 3:4. For example, the diameter of the second pad 2 is 0.4 mm, the diameter of the first pad 41 is 0.3 mm, and the diameter of the solder ball on the semiconductor package 50 is 0.4 mm.

[0069] Preferably, the ratio of the diameter of the first pad 41 to the diameter of the second through hole 11 is 7:9. For example, the diameter of the second pad 42 is 0.45 mm, the diameter of the first pad 41 is 0.35 mm, and the diameter of the solder ball on the semiconductor package 50 is 0.45 mm.

[0070] like Figure 6 As shown, the corners of the welding area 40 in this embodiment are rectangular, and the first pads 41 are distributed in the corners of the welding area 40 in a rectangular array.

[0071] The first pads 41 are distributed in the corners of the welding area 40 in an m*n array, where m is an integer and m≥2, and n is an integer and n≥2.

[0072] Preferably, the first pads 41 are distributed in an m*m array at the corners of the soldering area. It will be appreciated that those skilled in the art can set the values ​​of m and n based on the total number of pads on the soldering area 40 and the area of ​​the abnormal region. (The abnormal region in this embodiment refers to the corner of the semiconductor package where solder balls are likely to contact each other.)

[0073] For example, if the number of pads in soldering area 40 on a PCB is 23*23 (i.e., a total of 23*23 solder balls are provided on the semiconductor package), the abnormal region is the 2*2 pads located at the corners of soldering area 40. Therefore, 3*3 first pads 41 can be provided at each of the four corners of soldering area 40, and the remaining pads outside the corners of soldering area 40 can be provided as second pads 42. If the abnormal region is larger, 4*4 first pads 41 or 5*5 first pads 41 can be provided at each of the four corners of soldering area 40.

[0074] It can be seen from the above technical solutions that the present invention has the following beneficial effects:

[0075] The diameter of the first through-holes at the corners of the printed stencil is set to be smaller than the diameter of the second through-holes outside the corners. Solder paste or flux is applied to the PCB through the printed stencil to form a rectangular array of pads on the PCB. This solves the problem of the solder balls on the semiconductor package being in a warped state during high-temperature soldering. When soldering to the pads on the PCB, the solder balls do not contact each other due to the deformation of the semiconductor package, causing a short circuit or open circuit.

[0076] The utility model further solves the problem of the solder balls at the corners of the semiconductor package shifting in the high-temperature warping state by designing the center of the first through hole adjacent to the side of the printed screen to be offset to the side. The relatively offset pads formed on the PCB can be more accurately aligned with the solder balls on the semiconductor package for soldering.

[0077] The utility model has a simple structure and improves the quality of the entire system board structure without changing the original design of the semiconductor package and the PCB board;

[0078] The printed stencil and system board structure of the present invention are suitable for mounting various semiconductor packages on PCB boards, and the semiconductor packages include various BGA, EMMC, QFN, LGA, etc.

[0079] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the present invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be encompassed within the present invention. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0080] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A printing screen, comprising a substrate, characterized in that: The substrate includes a first surface and a second surface arranged opposite to each other, and a plurality of through holes are provided on the substrate that penetrate the first surface and the second surface. The through holes include a first through hole located at a corner of the substrate and a second through hole located outside the corner of the substrate. The diameter of the first through hole is smaller than the diameter of the second through hole.

2. The printing screen according to claim 1, wherein: The distance between the center of the first through hole adjacent to the side edge of the substrate and the side edge of the substrate is smaller than the distance between the center of the second through hole adjacent to the side edge of the substrate and the side edge of the substrate; and / or, The distance between the edge of the first through hole adjacent to the side edge of the substrate and the side edge of the substrate is greater than or equal to the distance between the edge of the second through hole adjacent to the side edge of the substrate and the side edge of the substrate.

3. The printing screen according to claim 1, wherein: The corner portion of the substrate is rectangular, and the first through holes are distributed in the corner portion of the substrate in a rectangular array.

4. The printing screen according to claim 3, wherein: The distance between the centers of two adjacent first through holes in each row is greater than or equal to the distance between the centers of two adjacent second through holes in each row; and / or, The distance between the centers of two adjacent first through holes in each column is greater than or equal to the distance between the centers of two adjacent second through holes in each column.

5. The printing screen according to claim 3, wherein: The distance between the center of the first through hole not adjacent to the side of the substrate and the center of the second through hole adjacent to the first through hole is equal to the distance between the centers of the two adjacent second through holes.

6. The printing screen according to claim 3, wherein: The first through holes are distributed in the corner of the substrate in an m*n array shape, wherein m is an integer and m≥2, and n is an integer and n≥2.

7. The printing screen according to claim 1, wherein: The ratio of the diameter of the first through hole to the diameter of the second through hole is 0.7 to 0.

84.

8. The printing screen according to claim 1, wherein: The ratio of the diameter of the first through hole to the diameter of the second through hole is 5:6; or, The ratio of the diameter of the first through hole to the diameter of the second through hole is 3:4; or, The ratio of the diameter of the first through hole to the diameter of the second through hole is 7:

9.

9. A system board structure, characterized in that: The device comprises a PCB, a pad and a semiconductor package, wherein the upper surface of the PCB has a welding area, a plurality of pads are located in the welding area, and the lower surface of the semiconductor package has a plurality of solder balls, and the semiconductor package is connected to the pad via the solder balls; The pads include a first pad located at a corner of the welding area and a second pad located outside the corner of the welding area. The diameter of the first pad is smaller than that of the second pad.

10. The system board structure according to claim 9, wherein: The distance between the center of the first pad adjacent to the edge of the welding area and the edge of the welding area is smaller than the distance between the center of the second pad adjacent to the edge of the welding area and the edge of the welding area; and / or, The distance between the edge of the first pad adjacent to the edge of the welding area and the edge of the welding area is greater than or equal to the distance between the edge of the second pad adjacent to the edge of the welding area and the edge of the welding area.

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