TW202125125A - Mask for ball array and manufacturing method thereof in which internal protrusions do not peel off from the mask body even when the mounting head of the ball loader is rubbed during the ball loading operation - Google Patents

Abstract

An object of the present invention is to develop a mask for ball array in which internal protrusions do not peel off from the mask body even when the mounting head of the ball loader is rubbed during the ball loading operation, an impact from the outside is caused by the field operator during mask handling, or a mask washing such as a high-pressure rinsing is performed. To achieve the object, the mask for ball array includes: a mask body provided with a plurality of openings; and a plurality of protrusions that protrude beyond the openings from the back of the mask body on the side opposite to the electrodes of the substrate, and are separated and independent from each other. The plurality of separated and independent protrusions are constituted by: internal protrusions that are formed on the side that does not contact the substrate by using a first plating layer; and external protrusions that are formed on the surface of the internal protrusions by covering the peripheral side surfaces of the internal protrusions and the underside of the substrate with a second plating layer, so as to become the side opposite to the electrodes of the substrate. The internal protrusion has a mushroom shape composed of a trunk and an umbrella-shaped part connected to the head of the trunk and provided with a larger diameter than the trunk. The mask body is formed integrally with the outer protrusions by the second plating layer. The outer protrusions are formed by covering the trunk and the umbrella-shaped part of the mushroom-shaped internal protrusions.

Description

Mask for ball array and manufacturing method thereof

The present invention relates to a ball array mask for loading conductive balls and a manufacturing method thereof.

Conventionally, a well-known mask for a ball array is characterized by including: a mask body, which is formed by plating and forming a plurality of openings through which the mounted conductive balls are inserted; and a plurality of protrusions, which are formed by plating and are not conductive ball implants. The entrance surface is the part of the back of the mask body on the side opposite to the electrode of the substrate that protrudes beyond the opening, and is separated and independent from each other; the above-mentioned multiple independent protrusions are composed of the following: internal protrusions, using the first plating layer Formed on the side that is not in contact with the substrate; and an external protrusion, on the surface of the internal protrusion, a second plating layer different from the first plating layer is formed to cover the peripheral side surface of the internal protrusion and the bottom surface of the substrate side, so as to oppose the electrode of the substrate Side; The mask body is formed integrally with the external protrusions using the second plating layer, and the external protrusions are formed into an R shape with a circular edge portion as viewed from the side (for example, refer to the first patent document). [Advanced Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 6500138 (the first item in the scope of patent application)

[Problems to be solved by the present invention]

The conventional mask for a ball array is manufactured according to the manufacturing method shown in FIG. 7, for example. First, a photoresist is laminated on the SUS base material 11, and the required exposure and development processes are performed, and a photoresist layer 12 for forming a protrusion with a thickness t1 is formed on the SUS base material 11 (refer to Fig. 7(a)). The photoresist layer 12 for forming protrusions is, for example, a hollow cylindrical shape. Next, for example, in order to make the cylindrical inner protrusion 13a a predetermined height (thickness) t2, the entire SUS base material 11 is plated to form a first plated layer 13 with a thickness t2 (see Fig. 7(b)). Here, since the relationship between the thickness t1 of the photoresist layer 12 for forming protrusions and the thickness t2 of the first plating layer 13 is t1>t2, the first plating layer 13 is not plated to exceed the thickness t1 of the photoresist layer 12 for forming protrusions. Next, after the formation of the first plating layer 13 is completed, the photoresist layer 12 for forming protrusions is removed (refer to FIG. 7(c)). Next, all the first plating layers 13 other than the cylindrical inner protrusions 13a are removed, and only the cylindrical inner protrusions 13a obtained by the first plating layer 13 are left on the SUS base material 11 (see Fig. 7(d)). Next, the photoresist 14 for opening part formation is pasted on the SUS base material 11 (refer FIG. 7(e)). The required exposure and development processes are performed, and a plurality of photoresist layers 14a for forming openings are formed between the cylindrical inner protrusions 13a on the SUS base material 11 (refer to FIG. 7(f)). Next, in order to make the mask body 15 a predetermined thickness, a second plating layer 15 is formed by plating on the SUS base material 11 and the cylindrical inner protrusion 13a (see Fig. 7(g)). Then, when the photoresist layer 14a for forming a plurality of openings is removed, it becomes the second plating layer 15 in which the plurality of openings 15a through which conductive balls are inserted are formed between the cylindrical inner protrusions 13a (see Fig. 7(h)). The second plating layer 15 is formed to cover the peripheral side surface of the mask body and the inner protrusion 13a and the lower surface of the substrate side, forming an outer protrusion 15b on the side opposite to the electrode of the substrate. Finally, the cylindrical inner protrusion 13a formed by the first plating layer 13 and the cylindrical outer protrusion 15b formed by the second plating layer 15 and the mask body are peeled off from the SUS base material 11 (refer to Fig. 7(i)). According to the above, the mask for the ball array is completed. The portion formed by the plurality of openings 15a through which the conductive balls are inserted is a ball mounting area. The front end portion of the cylindrical outer protrusion 15b of the completed ball array mask, and the peripheral edge portion thereof, as shown in Fig. 8, is not an angular shape but a round R shape.

The conventional ball array mask produced in this way can improve the bonding strength even if the inner protrusion surface formed by the first plating layer is chemically treated with hydrochloric acid, etc., and the ball loader's mounting head may be rubbed during the ball loading operation or due to the mask of the field operator. When an impact from the outside is caused by the treatment, or when the mask is cleaned such as high-pressure rinsing, there has been a problem that the internal protrusion formed on the side that does not come into contact with the substrate peels off from the mask body.

The present invention is formed to solve the above-mentioned problems, and provides high-pressure washing even when the ball loader's mounting head rubs during the ball loading operation, or the impact from the outside is caused by the mask processing of the field operator. When the mask is cleaned such as cleaning, the inner protrusions formed on the side that is not in contact with the substrate will not peel off from the mask body and the ball array mask and its manufacturing method. [Means to solve the problem]

The ball array mask according to the present invention includes: a mask body, which is formed by plating and forming a plurality of openings through which the mounted conductive balls are inserted; The back of the mask body on the side opposite to the electrode of the substrate protrudes beyond the above-mentioned opening, and is separated and independent from each other; among them, separate and independent plural protrusions are composed of the following: internal protrusions, which are not in contact with the substrate by the first plating layer And external protrusions, formed on the inner protrusion surface with a second plating layer different from the first plating layer covering the peripheral side of the internal protrusions and the underside of the substrate side, and become the side facing the electrode of the substrate; the internal protrusions are the trunk and Mushroom-shaped umbrella-shaped part that is connected to the head of the trunk and has a larger diameter than the trunk; the mask body is formed integrally with the external protrusions by the second plating layer; the external protrusions cover the trunk and the umbrella of the mushroom-shaped internal protrusions Shaped part formed.

In addition, the mushroom-shaped internal protrusion is a hook that prevents the internal protrusion from falling off at the connecting portion of the trunk and the umbrella-shaped portion, and the hook of the internal protrusion is caught on the middle part of the external protrusion to prevent the internal protrusion from falling off.

In addition, according to the method of manufacturing a ball array mask of the present invention, the array mask includes: a mask body formed by plating and forming a plurality of openings through which the mounted conductive balls are inserted; and a plurality of protrusions using the plating layer Formed, not the conductive ball implantation surface but the back of the mask body on the side opposite to the electrode of the substrate protruding from the part other than the above-mentioned opening, and are separated and independent from each other; wherein the separated and independent plural protrusions are composed of the following: internal protrusions, The first plating layer is formed on the side that is not in contact with the substrate; and the outer protrusions are formed on the surface of the inner protrusions to cover the peripheral side surfaces of the inner protrusions and the underside of the substrate side with a second plating layer different from the first plating layer, so as to face the electrodes of the substrate Side; internal protrusion, a mushroom-shaped part of the trunk and an umbrella part that is connected to the head of the trunk and has a larger diameter than the trunk; the main body of the mask is formed integrally with the external protrusion by the second plating layer; the external protrusion, covering Formation of the trunk and umbrella-shaped parts of mushroom-shaped internal protrusions; the above-mentioned manufacturing method includes: forming a photoresist layer of thickness t1 on the SUS base material; the mushroom-shaped internal protrusion exceeds the thickness t1 of the photoresist layer for internal protrusion formation Protruding, the step of forming the first plating layer by plating the SUS base material to a predetermined height t2 thicker than the thickness t1 of the photoresist layer; after the formation of the first plating layer is completed, the first plating layer other than the mushroom-shaped protrusions is removed, the SUS mother The step of leaving the mushroom-shaped internal protrusions produced by the first plating layer on the material; the step of removing the photoresist layer for forming the internal protrusions left around the mushroom-shaped internal protrusions; the SUS base material forms a plurality of openings between the mushroom-shaped internal protrusions The step of forming the photoresist layer for the part; the step of forming the second plating layer by plating the SUS base material and the mushroom-shaped internal protrusions so that the mask body has a specified thickness; after the second plating layer is formed, the plural openings are removed A step of forming a photoresist layer for the part; and a step of peeling the mushroom-shaped inner protrusions formed by the first plating layer and the outer protrusions formed by the second plating layer and the plating layer of the mask body from the SUS base material. [Effects of the invention]

According to the present invention, the ball array mask is composed of: internal protrusions, which are formed on the side not in contact with the substrate by the first plating layer; The underside of the substrate is formed on the side facing the electrode of the substrate; among them, even if the ball loader's loading head rubs during the ball loading operation, or the impact from the outside is caused by the masking process of the field operator, or high voltage is applied When the mask is cleaned such as rinsing, it is possible to obtain the effect that the internal protrusions formed on the side not in contact with the substrate will not peel off from the mask body.

In order to explain the present invention in more detail, this description is explained based on the attached drawings. In addition, in each figure, the same or corresponding parts are denoted by the same reference numerals, and repetitive descriptions are omitted without simplification as appropriate.

The first embodiment Fig. 1 is a cross-sectional view showing the manufacturing method of the ball array mask in the first embodiment of the present invention in the order of steps, and Fig. 2 shows the mask for the ball array in the first embodiment of the present invention in the order of steps A plan view of the manufacturing method. Fig. 3 is a cross-sectional view showing a step in the process of manufacturing a mask for a ball array in the first embodiment of the present invention, and Fig. 4 is a cross-sectional view showing a mask for a ball array in the first embodiment of the present invention. FIG. 5 is a plan view showing a step in the middle of the manufacturing method of the cover, and FIG. 5 is an enlarged sectional view showing the main part of the columnar protrusion of the ball array mask in the first embodiment of the present invention. In addition, in this first embodiment, a cylindrical protrusion is taken as an example. However, the shape of the protrusion may be a polygon, an ellipse, an elongated rectangle, or an elongated horizontally extending protrusion (protrusion), and the shape is not limited to a cylindrical shape. .

The method of manufacturing the mask for the ball array in the first embodiment of the present invention will be described based on FIGS. 1 to 5. In addition, the cross-sectional view of Figure 1 shows a case where two internal protrusions 3a and two external protrusions 5a are formed on the left and right sides of the plurality of openings 5a, and the plan view of Figure 2 shows that internal protrusions 3a and 3a are formed on the left and right sides of the plurality of openings 5a. When there is one external protrusion 5a each. First, the SUS base material 1 is laminated with a photoresist, and the required exposure and development processing are performed. On the SUS base material 1, a photoresist layer with a thickness t1 (for example, a photoresist thickness of 30μm (micrometer)) for forming internal protrusions is formed 2 (Refer to Figure 1 (a) and Figure 2 (a)). The photoresist layer 2 for forming internal protrusions has, for example, a hollow cylindrical shape. Next, for example, the mushroom-shaped internal protrusion 3a protrudes beyond the thickness t1 of the internal protrusion forming photoresist layer 2 to form a predetermined height (thickness) t2 (for example, the protrusion height 40 μm) thicker than the thickness t1 of the photoresist layer 2, The entire plating layer is performed on the SUS base material 1 to form the first plating layer 3 projecting the head of the thickness t2 (refer to FIG. 1(b) and FIG. 2(b)). Here, because the relationship between the thickness t1 of the photoresist layer 2 for forming internal protrusions and the thickness t2 of the first plating layer 3 is t1>t2, the first step is to protrude beyond the thickness t1 of the photoresist layer 2 for forming internal protrusions. The plating layer of the plating layer 3. Thereby, although the trunk part 3b is cylindrical, the mushroom-shaped internal protrusion 3a forms the umbrella part 3c with a larger diameter than the trunk part 3b because the head part protrudes, and the whole shape becomes a mushroom shape. The connection part of the trunk part 3b of the mushroom-shaped internal protrusion 3a and the umbrella-shaped part 3c becomes the hook part 3d for fall prevention with an L-shaped cross section. Therefore, after the formation of the first plating layer 3 is completed, unlike the conventional manufacturing method, the first plating layer 3 except for the mushroom-shaped internal protrusions 3a is completely removed, and only the mushroom-shaped interior of the first plating layer 3 is left on the SUS base material 1. At the same time as the protrusions 3a, the photoresist layer 2 for forming internal protrusions is left (refer to FIGS. 1(c), 2(c), 3, and 4). After that, swelling peeling (amine) + dissolving peeling is performed to remove the photoresist layer 2 for internal protrusions left around the mushroom-shaped internal protrusions 3a (see FIGS. 1(d) and 2(d)). Then, the photoresist 4 for opening part formation is stuck on the SUS base material 1 (refer 1st (e) figure). The required exposure and development processing are performed, and a plurality of photoresist layers 4a for forming openings are formed between the mushroom-shaped internal protrusions 3a on the SUS base material 1 (refer to Figs. 1(f) and 2(f)). Next, the mask body 5 is plated to a predetermined thickness on the SUS base material 1 and the mushroom-shaped internal protrusion 3a to form the second plated layer 5 (see Figures 1(g) and 2(g)). The second plating layer 5 is plated so as to cover the SUS base material 1 and the trunk portion 3b and the umbrella portion 3c of the mushroom-shaped internal protrusion 3a. As a result, the cross-sectional L-shaped drop-off preventing hook 3d of the connecting portion between the trunk portion 3b and the umbrella portion 3c, which is the mushroom-shaped internal protrusion 3a, is caught on the cylindrical outer portion of the longitudinal cross-sectional Ω shape formed by the second plating layer 5. The middle detail of the protrusion 5b will not peel off from the mask body (refer to Fig. 5). Then, when the photoresist layer 4a for forming a plurality of openings is removed, it becomes the second plating layer 5 in which the plurality of openings 5a through which the conductive balls are inserted are formed between the mushroom-shaped internal protrusions 3a (see Fig. 1(h)). Finally, from the SUS base material 1, the mushroom-shaped internal protrusions 3a formed by the first plating layer 3 and the cylindrical external protrusions 5b having a longitudinal cross-sectional Ω shape formed by the second plating layer 5 and the mask body are peeled off (refer to Figure 1(i)) . Based on the above, the ball array mask is completed. The part forming the plurality of openings 5a through which the conductive balls are inserted is the ball mounting area. As shown in Fig. 5, the front end of the cylindrical outer protrusion 5b of the completed ball array mask has a circular Ω shape in longitudinal section. Therefore, even when performing high-pressure rinsing, etc., the cross-section of the connecting portion between the trunk portion 3b and the umbrella portion 3c, which is the mushroom-shaped internal protrusion 3a, has an L-shaped drop-off prevention hook portion 3d because it is stuck in the second plating layer. The middle part of the cylindrical outer protrusion 5b with a longitudinal section Ω-shaped formed by 5 will not peel off from the mask body. In addition, the shape of the protrusion may be a polygon, an ellipse, an elongated rectangle, or an elongated horizontally extending protrusion, and the shape is not limited to a cylindrical shape.

Example 2 Fig. 6 is a plan view showing a step in the middle of the manufacturing method of the ball array mask in the second embodiment of the present invention. In the second embodiment of the present invention, even when the loading head of the ball loader is rubbed during the ball loading operation, or the impact from the outside is caused by the mask processing of the field operator, or the mask is subjected to high-pressure washing, etc. During cleaning, in order to prevent the internal protrusions formed on the side not in contact with the substrate from peeling off from the mask body, the horizontal cross-sectional shape of the internal protrusions 3a is, for example, the shape shown in FIG. 6. That is, as shown in FIG. 6, the inner protrusion 3a has as large a joint area as possible in the contact portion with the second plating layer 5 and the outer protrusion 5b to improve the anchoring effect. First, in Figure 6(a), the horizontal cross-sectional shape of the internal protrusion 3a forms two horizontal slits 3e from the peripheral edge 2 to the center, and the two horizontal slits 3e do not reach the center. In Fig. 6(b), the horizontal cross-sectional shape of the internal protrusion 3a forms four vertical and horizontal slits 3f from the peripheral edge 4 to the center, and the four vertical and horizontal slits 3f do not reach the center. In addition, Fig. 6(c) shows that the horizontal cross-sectional shape of the inner protrusion 3a is formed into a doughnut shape, and the center portion 3g of the inner protrusion 3a is hollowed out. As described above, the horizontal cross-sectional shape of the inner protrusion 3a forms two horizontal slits 3e from the peripheral edge 2 to the center to the middle (Figure 6(a)), and four vertical and horizontal slits 3f from the peripheral edge 4 to the center to the midway (No. 6 (b) Figure), forming a donut shape hollowed out in the center (Figure 6(c)), both of which maximize the joints between the inner protrusion 3a, the second secondary plating layer 5, and the contact portion of the outer protrusion 5b The area, because the internal protrusion 3a can be prevented from falling off by improving the anchoring effect, even if the ball loader's loading head rubs during the ball loading operation, or the impact from the outside caused by the mask processing of the field operator, or high pressure During mask cleaning such as rinsing, the internal protrusions 3a will not peel off from the mask body.

1: SUS base material 2: Photoresist layer for forming internal protrusions 3: The first coating 3a: Mushroom-shaped internal protrusion 3b: Internal protruding torso 3c: Umbrella protruding inside 3d: Hook for preventing internal protrusions from falling off 3e: 2 horizontal slits protruding inside 3f: 4 vertical and horizontal slits protruding inside 3g: The central part of the internal protrusion 4: Photoresist for forming openings 4a: Photoresist layer for forming plural openings 5: The second plating layer (mask body) 5a: Plural openings 5b: cylindrical (longitudinal section Ω shape) external protrusion 11: SUS base material 12: Photoresist layer for protrusion formation 13: The first coating 13a: Cylindrical internal protrusion 14: Photoresist for opening formation 14a: Photoresist layer for forming plural openings 15: The second plating layer (mask body) 15a: Plural openings 15b: Cylindrical external protrusion

[Figure 1] is a cross-sectional view showing the manufacturing method of the ball array mask in the first embodiment of the present invention in the order of steps; [Figure 2] is a plan view showing the manufacturing method of the ball array mask in the first embodiment of the present invention in the order of steps; [Figure 3] is a cross-sectional view showing a step in the middle of the manufacturing method of the ball array mask in the first embodiment of the present invention; [Figure 4] is a plan view showing a step in the middle of the manufacturing method of the ball array mask in the first embodiment of the present invention; [Figure 5] An enlarged cross-sectional view showing the main part of the columnar protrusion of the ball array mask in the first embodiment of the present invention; [FIG. 6] A plan view showing a step in the middle of the manufacturing method of the mask for the ball array in the second embodiment of the present invention; [Figure 7] is a cross-sectional view showing the manufacturing method of the conventional ball array mask in the order of steps; [Figure 8] An enlarged cross-sectional view showing the main part of the columnar protrusion of the conventional ball array mask;

1: SUS base material

2: Photoresist layer for forming internal protrusions

3: The first coating

3a: Mushroom-shaped internal protrusion

3b: Internal protruding torso

3c: Umbrella protruding inside

4: Photoresist for forming openings

4a: Photoresist layer for forming plural openings

5: The second plating layer (mask body)

5a: Plural openings

5b: cylindrical (longitudinal section Ω shape) external protrusion

t1: Thickness of photoresist

t2: Thickness of inner protrusion coating

Claims (5)

A mask for a ball array, which is characterized in that: Including: a mask body, formed by plating, and forming a plurality of openings through which the mounted conductive balls are inserted; and a plurality of protrusions, formed by plating, not the conductive ball implantation surface but the above-mentioned mask on the side opposite to the electrode of the substrate The back of the cover body protrudes beyond the above-mentioned opening and is separated and independent from each other; The above-mentioned separate and independent plural protrusions are composed of the following: Internal protrusions are formed on the side not in contact with the above-mentioned substrate by using the first plating layer; and The external protrusion is formed on the surface of the internal protrusion by covering the peripheral side surface and the underside of the substrate side of the internal protrusion with a second plating layer different from the first plating layer, and becomes the side facing the electrode of the substrate; The above-mentioned internal protrusion is a mushroom-shaped part of the trunk and an umbrella-shaped part that is connected to the head of the trunk and has a larger diameter than the trunk; The mask body is formed integrally with the external protrusion by the second plating layer, and the external protrusion is formed to cover the trunk portion and the umbrella portion of the mushroom-shaped internal protrusion. The mask for a ball array as described in item 1 of the scope of patent application is characterized by: Mushroom-shaped internal protrusions are hooks that prevent the internal protrusions from falling off by connecting the trunk and the umbrella-shaped parts, and the hooks of the internal protrusions are caught on the middle part of the external protrusions to prevent the internal protrusions from falling off. A method for manufacturing a mask for a ball array, wherein: The array mask includes: a mask body, which is formed by plating and forms a plurality of openings through which the mounted conductive balls are inserted; and a plurality of protrusions, which is formed by plating, and is not the surface where the conductive balls are implanted but opposes the electrodes of the substrate. The back of the mask body on the side protrudes from the part other than the opening and is separated and independent from each other; the separated and independent plural protrusions are composed of: internal protrusions formed on the side that is not in contact with the substrate by the first plating layer; and external protrusions , On the surface of the internal protrusion, a second plating layer different from the first plating layer is formed to cover the peripheral side surface of the internal protrusion and the underside of the substrate side, and become the side opposite to the electrode of the substrate; The head of the trunk part is connected to a mushroom-shaped part with a larger diameter than the trunk part; the mask body is formed integrally with the external protrusion by the second plating layer; the external protrusion covers the mushroom-shaped internal protrusion The above-mentioned trunk and the above-mentioned umbrella-shaped part are formed; The above manufacturing method includes: A step of forming a photoresist layer for forming internal protrusions with a thickness of t1 on the SUS base material; The mushroom-shaped internal protrusions protrude beyond the thickness t1 of the photoresist layer for forming the internal protrusions, and the step of forming the first plating layer by plating the SUS base material to a predetermined height t2 thicker than the thickness t1 of the photoresist layer; After the formation of the first plating layer is completed, the step of removing the first plating layer other than the mushroom-shaped protrusions and leaving the mushroom-shaped internal protrusions generated by the first plating layer on the SUS base material; The step of removing the photoresist layer for forming the internal protrusions left around the mushroom-shaped internal protrusions; A step of forming a plurality of photoresist layers for forming openings between the mushroom-shaped internal protrusions on the SUS base material; The step of forming a second plating layer by plating a layer on the SUS base material and the mushroom-shaped internal protrusions so that the mask body has a specified thickness; After the formation of the second plating layer is completed, the step of removing the photoresist layers for forming the plurality of openings; and A step of peeling the mushroom-shaped inner protrusions formed by the first plating layer, the outer protrusions formed by the second plating layer, and the plating layer of the mask body from the above-mentioned SUS base material. A mask for a ball array, which is characterized in that: Including: a mask body, formed by plating, and forming a plurality of openings through which the mounted conductive balls are inserted; and a plurality of protrusions, formed by plating, not the conductive ball implantation surface but the above-mentioned mask on the side opposite to the electrode of the substrate The back of the cover body protrudes beyond the above-mentioned opening and is separated and independent from each other; The above-mentioned separate and independent plural protrusions are composed of the following: Internal protrusions are formed on the side not in contact with the above-mentioned substrate by using the first plating layer; and The external protrusion is formed on the surface of the internal protrusion by covering the peripheral side surface and the underside of the substrate side of the internal protrusion with a second plating layer different from the first plating layer, and becomes the side facing the electrode of the substrate; The internal protrusion has a horizontal cross-sectional shape with an enlarged joint area in a contact portion with the second plating layer and the external protrusion; The mask body is formed integrally with the external protrusion by the second plating layer; the mask body and the external protrusion formed by the second plating layer have a horizontal cross-sectional shape that increases the joint area in the contact portion with the internal protrusion . The ball array mask described in item 4 of the scope of patent application is characterized in that: The horizontal cross-sectional shape of the inner protrusion is a shape with 2 horizontal slits from the periphery 2 to the center to the middle, and 4 vertical and horizontal slits from the periphery 4 to the center to the middle, forming a donut shape with a hollowed-out center. Any of these shapes increases the joint area and improves the anchoring effect.

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