A storage jig for lead frame processing

Abstract

The application belongs to the technical field of lead frame storage and relates to a storage jig for lead frame processing. The storage jig comprises two symmetrically distributed side plates, a bottom plate and a top plate. The opposite sides of the two side plates are each provided with an equal number of load-bearing strips that are equidistantly distributed. The side plates are each provided with a side support near the position of the adjacent load-bearing strips. The side support is provided with a guide piece. The guide piece is used to centrally guide the lead frame. Without introducing additional equipment, the probability of the lead frame entering the jig in a central state is improved. If the lead frame is offset during the blanking process, the guide piece will not hinder the movement of the lead frame, so that the lead frame can still enter the jig without deformation. The side support and the tendency of the lead frame to move downward under the action of gravity are used to guide the lead frame, so that the lead frame can remain in a central and stationary state in the jig.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lead frame storage, in particular to a storage jig for lead frame processing. BACKGROUND

[0002] Lead frame is a key carrier in the process of semiconductor device packaging. During packaging, lead frame is usually temporarily stored in a special storage jig for transfer to the corresponding process equipment. The storage jig generally stores lead frames in a vertical array form to facilitate push-type feeding. During the feeding process, the mechanical arm pushes the lead frame from one end of the top layer of the storage jig to move it out from the other end, and cooperates with the lifting movement of the jig to realize continuous feeding.

[0003] In the processing procedure of lead frame, there are usually strict requirements for its position accuracy to ensure product quality. However, in actual production, in order to facilitate the automatic loading and unloading of lead frames, and to avoid deformation of the lead frame caused by interference of the jig during pushing and moving, the width of the position for storing lead frames inside the storage jig is usually greater than the width of the lead frame. Although the lead frame has been centered and positioned during storage, the lead frame is still likely to slide and deviate from the center position during the process of the operator transferring the storage jig to other equipment; therefore, after the storage jig is fed to the equipment, the lead frame in the storage jig needs to be centered and corrected, or a centering and positioning mechanism needs to be added to the equipment conveying mechanism to reposition the lead frame. This approach not only increases the length of the conveying mechanism at the front end of the equipment, but also significantly increases the complexity and manufacturing cost of the equipment due to the introduction of additional positioning devices and sensing control systems. SUMMARY

[0004] The present application provides a storage jig for lead frame processing to overcome the shortcomings of the prior art storage jig, in which the width of the storage position inside the storage jig is greater than the width of the lead frame, which causes the lead frame inside the storage jig to deviate from the position during movement.

[0005] Technical solution: A storage jig for lead frame processing, comprising: two side plates symmetrically distributed, a bottom plate and a top plate, the bottom plate and the top plate are fixed between the two side plates, the opposite sides of the two side plates are provided with an equal number of load bars which are equidistantly distributed, the load bars are provided with a load surface for contacting the lead frame, the side plates are provided with a side support near the adjacent load bars, the side support is provided with a guide piece, the lower edges of the side support and the guide piece are in contact with the adjacent load surface, the horizontal distance between the two side supports symmetrically distributed on different side plates gradually increases in the direction from bottom to top, the two edges of the side surface of the guide piece away from the adjacent side plate and the side surface of the adjacent side support away from the adjacent side plate are collinear, and the guide piece is used for guiding the movement of the lead frame.

[0006] Further, the bearing surface and the horizontal plane form an angle, and the horizontal distance between the two bearing surfaces symmetrically distributed on different side plates gradually increases in the direction from bottom to top, so that the lead frame is in line contact with the bearing surface, and the angle between the bearing surface and the horizontal plane is smaller than the angle between the side surface of the side support away from the adjacent side plate and the horizontal plane.

[0007] Further, the bearing surface is coated with a chromium nitride coating to improve surface hardness and smoothness.

[0008] Further, the angle between the bearing surface and the horizontal plane is between 10° and 15°.

[0009] Further, the side plate is provided with a mounting groove near the position of the adjacent bearing strip, and the bearing strip is clamped in the adjacent mounting groove.

[0010] Further, the side support is fixedly connected with a limiting strip for limiting the swing of the lead frame.

[0011] Further, the side plate is provided with two sliding grooves near the position of the adjacent side support, the side support is limited to slide in the adjacent sliding groove, two threaded rods are slidingly and rotationally connected in the side plate, the side support is threadedly connected with the adjacent threaded rod, the threaded rod is provided with a limiting groove, and the side plate is slidingly connected with a limiting block near the position of the adjacent threaded rod, and the limiting block limits the adjacent threaded rod through the limiting groove.

[0012] Further, the bearing strip is made of elastic material, the guide piece is fixedly connected with the adjacent side support through an elastic piece, and the side of the elastic piece away from the adjacent side plate is coplanar with the side of the side support away from the adjacent side plate.

[0013] Further, two side plates are slidingly connected with a sliding strip near the side of the adjacent guide piece, the sliding strip and the top plate are fixedly connected with two elastic strips symmetrically distributed, the bottom plate and the top plate are fixedly connected with two supporting strips symmetrically distributed away from the sliding strip, and the elastic strips and the supporting strips are used for keeping the position of the lead frame.

[0014] Further, the minimum horizontal distance between the two elastic strips is smaller than the minimum horizontal distance between the two side supports symmetrically distributed on different side plates.

[0015] Those skilled in the art can understand that the present application has at least the following beneficial effects:

[0016] 1. The guide piece is used for centering and guiding the lead frame, so that without introducing additional equipment, on the one hand, the probability of the lead frame entering the jig in the centered state is improved, and on the other hand, in the case of offset during the lead frame blanking process, the guide piece can avoid hindering the movement of the lead frame, so that the lead frame can still enter the jig without deformation, and the side support and the downward movement of the lead frame due to gravity are used to guide the lead frame, so that the lead frame can remain in a centered and stationary state in the jig.

[0017] 2. The contact area of the lead frame and the bearing surface is reduced by the inclined bearing surface, which can reduce the area of the lead frame subjected to abrasion during the feeding and discharging process, thereby reducing the probability of metal debris.

[0018] 3. The position of the side support is changed by rotating the threaded rod, so that the side support and the adjacent bearing surface slide relative to each other, thereby changing the position of the lead frame in contact with the bearing surface, thereby improving the utilization rate of the bearing surface, increasing the number of times the bearing surface is subjected to abrasion, and prolonging the service life of the bearing surface. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of the three-dimensional structure of the present application;

[0020] Figure 2 is a schematic diagram of the three-dimensional structure of the side plate and the top plate of the present application;

[0021] Figure 3 is a schematic diagram of the three-dimensional structure of the bearing strip and the side support of the present application;

[0022] Figure 4 is a schematic diagram of the three-dimensional structure of the side support and the guide piece of the present application;

[0023] Figure 5 is a schematic diagram of the three-dimensional structure of the guide piece and the limiting strip of the present application;

[0024] Figure 6 is a schematic diagram of the three-dimensional structure of the side support and the threaded rod of the present application;

[0025] Figure 7 is Figure 6 is an enlarged view of position A in FIG. 6;

[0026] Figure 8 is a schematic diagram of the three-dimensional structure of the bottom plate and the sliding strip of the present application;

[0027] Figure 9 is Figure 8 is an enlarged view of position B in FIG. 6.

[0028] In the figure: 1, side plate, 2, bottom plate, 3, top plate, 4, bearing strip, 401, bearing surface, 402, mounting groove, 5, side support, 501, sliding groove, 6, guide piece, 7, limiting strip, 8, threaded rod, 801, limiting groove, 9, limiting block, 10, sliding strip, 11, elastic strip, 12, support strip. DETAILED DESCRIPTION

[0029] In order to make the technical means, creative features, purposes and effects of the present application easy to understand, the present application will be further described below in conjunction with specific embodiments.

[0030] Example 1

[0031] The present embodiment provides a storage jig for lead frame processing to solve the problem that the storage jig in the prior art is wider than the width of the lead frame, causing the lead frame in the storage jig to easily deviate in position during movement.

[0032] Referring to Figures 1 to 5 A storage jig for lead frame processing, comprising: two symmetrical side plates 1, a bottom plate 2 and a top plate 3, the bottom plate 2 and the top plate 3 are fixed between the two side plates 1 by bolts, the opposite sides of the two side plates 1 are provided with an equal number of bearing strips 4 which are equally distributed, the number of bearing strips 4 is determined according to the vertical distance between the bottom plate 2 and the top plate 3, which is not limited here; the upper side of the bearing strip 4 is provided with a bearing surface 401 for contacting the lead frame, the position of the side plate 1 close to the adjacent bearing strip 4 is provided with a side support 5, the front side of the side support 5 is provided with a guide piece 6, the lower edges of the side support 5 and the guide piece 6 are in contact with the adjacent bearing surface 401, the horizontal distance between the opposite sides of the two side supports 5 which are symmetrically distributed on different side plates 1 gradually increases in the direction from bottom to top, that is, the opposite sides of the two side supports 5 are inclined surfaces, the included angle of the inclined surface with respect to the horizontal plane is 75°; the two edges of the front side of the guide piece 6 close to the side surface of the adjacent side support 5 are collinear, and the guide piece 6 is used to guide the movement of the lead frame.

[0033] The above arrangement can achieve the following effects: the guide piece 6 is used to centrally guide the lead frame, so that on the premise of not introducing additional equipment, on the one hand, the probability of the lead frame entering the jig in a centered state is improved, and on the other hand, in the case of deviation during lead frame blanking, the guide piece 6 can avoid hindering the movement of the lead frame, so that the lead frame can still enter the jig without deformation, and the side support 5 and the tendency of the lead frame to move downward under gravity are used to guide the lead frame, so that the lead frame can remain in a centered and stationary state in the jig.

[0034] It should be noted that in the present embodiment, the relationship between the side support 5 and the adjacent guide piece 6 can be considered as fixed.

[0035] The height of the contact edge between the side support 5 and the bearing surface 401 is equal to the height of the lower side edge of the lead frame in the blanking position. The following describes three cases: the lead frame is centered before entering the jig, the lead frame is slightly skewed before entering the jig, and the lead frame is greatly skewed before entering the jig.

[0036] If the lead frame is centered before entering the jig, the lead frame can directly enter between the two side plates 1 and rest on the upper side of the two bearing surfaces 401 during the blanking process, and the left and right sides of the lead frame are in contact with the two side supports 5 respectively. During the movement of the jig, even if the lead frame is shaken, it can still return to the centered state under the guidance of the two side supports 5.

[0037] If the lead frame is not centered before entering the jig and is slightly skewed, as the lead frame moves into the jig, one corner of the lead frame first contacts the guide piece 6 on one side, and under the guidance of the guide piece 6, the lead frame gradually returns to the normal position and enters between the two side supports 5.

[0038] If the lead frame is not centered before entering the jig and is greatly skewed, as the lead frame moves into the jig, the corner of the lead frame contacts the guide piece 6, and then the corner of the lead frame starts to slide along the side surface of the guide piece 6. Subsequently, the corner of the lead frame contacts the inclined surface of the side support 5 and continues to slide along the inclined surface of the side support 5 (the guide piece 6 does not completely block the movement of the lead frame, which prevents the lead frame from being squeezed and deformed due to the inability to move one end). During the sliding process of the lead frame, the lead frame has a tendency to self-align under the guidance of gravity and the side support 5 and the guide piece 6. Finally, the lead frame is placed on the upper side of the two bearing surfaces 401 in an inclined state. In this state, when the jig is moved, the jig is slightly shaken to make the lead frame slide downward along the side support 5 and eventually return to the centered state.

[0039] Example 2

[0040] This embodiment is based on Example 1, and reduces the contact area between the lead frame and the bearing surface 401 to reduce the degree of wear during the feeding and blanking of the lead frame.

[0041] Referring to Figures 3 to 5, the included angle between the bearing surface 401 and the horizontal plane is present, and the horizontal distance between the two bearing surfaces 401 symmetrically distributed on the different side plates 1 gradually increases in the direction from bottom to top, so that the lead frame is in line contact with the bearing surface 401, and the included angle between the bearing surface 401 and the horizontal plane is smaller than the included angle between the side surface of the side support 5 away from the adjacent side plate 1 and the horizontal plane; the bearing surface 401 is coated with a chromium nitride coating to improve the surface hardness and smoothness, reduce the friction between the lead frame and the bearing surface 401, and further reduce the wear rate of the bearing surface 401; the included angle between the bearing surface 401 and the horizontal plane is between 10° and 15°, which keeps the lead frame and the bearing surface 401 in line contact, and keeps the included angle between the lead frame and the bearing surface 401 less than 45° at all times, reduces the horizontal support component force provided by the bearing surface 401 to the lead frame, and further reduces the influence of the inclination angle of the bearing surface 401 on the adjustment of the lead frame action by the side support 5.

[0042] The above arrangement can achieve that the lead frame is supported by the inclined bearing surface 401, reducing the contact area between the lead frame and the bearing surface 401, which can reduce the area of the lead frame that is abraded during the feeding and discharging process, and reduce the area of the edge of the lead frame that needs to bear the support force, thereby enabling the lead frame to have a narrower edge during production, and further improving the utilization rate of the overall material of the lead frame.

[0043] Embodiment 3

[0044] This embodiment provides the function of replaceable bearing strip 4 on the basis of embodiment 2, to prolong the service life of the storage fixture and reduce production cost.

[0045] During the feeding and discharging process of the lead frame, the lead frame will slide relative to the storage fixture. As the number of sliding increases, the position of the fixture in contact with the lead frame will be abraded, resulting in an increase in the surface roughness of the contact position of the fixture and the lead frame. In this case, during the relative sliding of the lead frame and the fixture, both the lead frame and the fixture will be abraded and generate metal debris. Since the processing procedure of the lead frame has a high requirement for the surface cleanliness, the prior art needs to replace the fixture in time after it is abraded to a certain extent to control the generation of metal debris, which undoubtedly increases the production cost.

[0046] Referring to Figure 2 and Figure 3 , the side plate 1 is provided with a mounting groove 402 near the adjacent bearing strip 4, and the bearing strip 4 is clamped in the adjacent mounting groove 402, so that the bearing strip 4 can be freely disassembled and replaced without replacing the entire storage fixture, thereby reducing the production cost.

[0047] Embodiment 4

[0048] This embodiment is further optimized on the basis of Embodiment 3.

[0049] Referring to Figures 3 to 5 , the side support 5 is fixedly connected with the limiting strip 7, and the limiting strip 7 is used to limit the swing of the lead frame; the bearing strip 4, the adjacent side support 5, the adjacent guide piece 6 and the adjacent limiting strip 7 jointly constitute a set of support units, and taking two sets of support units on the lower side of the two side plates 1 as an example, the minimum distance between the limiting strip 7 and the bearing strip 4 which are not in the same set is smaller than the minimum distance between the two side supports 5, so as to limit the maximum inclination angle of the lead frame in the jig, so that the lead frame cannot fall off between the two bearing strips 4 within the inclination angle range.

[0050] The above arrangement can realize that the lead frame entering the jig is guided and limited by the limiting strip 7, when the lead frame enters the jig in a skewed state, the limiting strip 7 limits the lead frame from excessive skewing and guides the lead frame in combination with the side support 5, so that the lead frame tends to be in a central state between the two side supports 5; during the process of transferring the jig, the maximum skewing angle of the lead frame is limited by the limiting strip 7, so as to avoid the lead frame from falling off between the two bearing strips 4.

[0051] Embodiment 5

[0052] This embodiment is further optimized on the basis of Embodiment 4, so as to prolong the service life of a single bearing strip 4.

[0053] Referring to Figure 2 , Figure 3 , Figure 6 and Figure 7 , the positions of the side plate 1 close to the adjacent side support 5 are each provided with two slip grooves 501 which are symmetrically distributed in front and back, the side support 5 is limited to slide up and down in the adjacent two slip grooves 501, two threaded rods 8 are slidingly and rotationally connected in the side plate 1, the side support 5 is threadedly connected with the adjacent threaded rod 8, the upper portion of the threaded rod 8 is provided with a limiting groove 801, the positions of the side plate 1 close to the adjacent threaded rod 8 are each slidingly connected with a limiting clamping block 9, the projection of the limiting clamping block 9 on the horizontal plane is U-shaped, and the limiting clamping block 9 limits the adjacent threaded rod 8 through the limiting groove 801, so that the threaded rod 8 can only rotate but cannot slide up and down.

[0054] The above arrangement can realize that the position of the side support 5 is changed by the rotation of the threaded rod 8, so that the side support 5 and the adjacent bearing surface 401 slide relatively, and then the position of the lead frame in line contact with the bearing surface 401 is changed, so as to improve the utilization rate of the bearing surface 401, increase the number of times of bearing wear of the bearing surface 401, and prolong the service life of the bearing surface 401.

[0055] Referring to Figure 4 and Figure 5, the thickness of the bearing surface 401 of the bearing strip 4 is greater than the thickness of the rest of the bearing strip 4, so that the bearing surface 401 remains flat when the bearing strip 4 is pressed by the adjacent side support 5; the guide piece 6 is fixed to the adjacent side support 5 by an elastic piece, and the side of the elastic piece away from the adjacent side plate 1 is coplanar with the side of the side support 5 away from the adjacent side plate 1; after the bearing surface 401 is pressed by the adjacent side support 5 and the angle is changed, the angle of the lower edge of the guide piece 6 relative to the horizontal plane increases, at which time the guide piece 6 can swing and change the position of contact with the bearing surface 401.

[0056] When the degree of wear of the position where the bearing surface 401 contacts the adjacent side support 5 is such that the bearing strip 4 cannot continue to be used normally (since the lead frame and the bearing surface 401 are in line contact, only the area corresponding to a line on the bearing surface 401 is worn out each time), the two threaded rods 8 on the side plate 1 are rotated, the threaded rods 8 drive the adjacent side support 5 downward through threaded transmission, the side support 5 presses the adjacent bearing strip 4 to bend, and at the same time the side support 5 slides downward along the adjacent bearing surface 401, changing the position where the bearing surface 401 contacts the adjacent side support 5; during the process of the side support 5 sliding downward along the adjacent bearing surface 401, the side support 5 drives the adjacent guide piece 6 to move downward, the guide piece 6 slides along the adjacent bearing surface 401 under the driving action of the elastic piece and the side support 5, and the elastic piece of the guide piece 6 is bent and deformed to always maintain contact with the adjacent bearing surface 401.

[0057] Example 6

[0058] This example is based on example 5, and prevents the lead frame from slipping out of the jig during the transfer process.

[0059] Referring to Figure 1 , Figure 2 , Figure 8 and Figure 9The front sides of the two side plates 1 are jointly and slidably connected with a sliding strip 10, the sliding strip 10 is jointly and fixedly connected with two elastic strips 11 which are symmetrically distributed left and right, the left and right sides of the elastic strip 11 are provided with two upper and lower grooves, in the up-down direction, the four grooves on the facing sides of the two elastic strips 11 are located between the four grooves on the opposite sides of the two elastic strips 11, in the left-right direction, the four grooves on the opposite sides of the two elastic strips 11 are located between the four grooves on the facing sides of the two elastic strips 11, for controlling the bending position of the elastic strip 11, and the elastic strip 11 is in a bent force storage state in the drawing; the rear sides of the bottom plate 2 and the top plate 3 are jointly and fixedly connected with two support strips 12 which are symmetrically distributed left and right, the elastic strip 11 and the support strip 12 are jointly used for keeping the position of the lead frame; the minimum horizontal distance between the two elastic strips 11 and the two support strips 12 in the horizontal direction is less than the minimum horizontal distance between the two side support pieces 5 which are symmetrically distributed on different side plates 1, and the maximum horizontal distance of the projections of the two elastic strips 11 on the horizontal plane is greater than the minimum horizontal distance between the two side support pieces 5 which are symmetrically distributed on different side plates 1.

[0060] The above arrangement can realize that the two directions of the lead frame are limited by the elastic strip 11 and the support strip 12 respectively, so as to avoid that the lead frame slips out of the jig during the transfer of the jig.

[0061] It should be known that the jig in the drawing of the present application is in a state of being placed on a plane, and the jig as a whole is in a suspended state during the transfer of the jig.

[0062] During the blanking process of the lead frame, the width of the lead frame is equal to the minimum distance between the two side support pieces 5 in the left-right direction, and is less than the minimum distance between the middle parts of the two elastic strips 11 in the left-right direction, in this state, the lead frame can smoothly enter between the two side plates 1, but the lead frame cannot continue to move backward when it moves backward to the state of contacting the support strip 12, and the limit position of the backward movement of the lead frame is limited by the support strip 12.

[0063] During the transfer of the jig, the jig is in a suspended state, at this time, the sliding strip 10 moves downward relative to the side plate 1 under the action of gravity, and the two elastic strips 11 deform and tend to be straight under the action of elastic force, in this state, the maximum horizontal distance of the two elastic strips 11 in the left-right direction is less than the width of the lead frame, so that the forward movement of the lead frame is limited by the two elastic strips 11, and the lead frame is prevented from slipping out of the jig.

[0064] The embodiments of the present embodiment are the preferred embodiments of the present application, and are not limited to the protection scope of the present application, therefore: any equivalent changes made according to the structure, shape, principle of the present application should be covered within the protection scope of the present application.

Claims

1. A storage fixture for lead frame processing, characterized in that, include: Two symmetrically distributed side plates (1), a bottom plate (2), and a top plate (3) are provided. The bottom plate (2) and the top plate (3) are fixed between the two side plates (1). The two side plates (1) are provided with an equal number of equally spaced bearing strips (4) on their opposing sides. The bearing strips (4) are provided with bearing surfaces (401) for contacting the lead frame. The side plates (1) are provided with side supports (5) near the adjacent bearing strips (4). The side supports (5) are provided with guide pieces. (6) The lower edges of the side support (5) and the guide plate (6) are in contact with the adjacent bearing surface (401). The horizontal distance between the two side supports (5) symmetrically distributed on different side plates (1) gradually increases from bottom to top. The two edges of the guide plate (6) that are close to the side of the adjacent side plate (1) and the side of the adjacent side support (5) that are close to the side of the adjacent side plate (1) are collinear. The guide plate (6) is used to guide the movement of the lead frame.

2. A storage fixture for lead frame processing according to claim 1, characterized in that, There is an angle between the bearing surface (401) and the horizontal plane, and the horizontal distance between two bearing surfaces (401) that are symmetrically distributed on different side plates (1) gradually increases from bottom to top, so that the lead frame is in line contact with the bearing surface (401), and the angle between the bearing surface (401) and the horizontal plane is smaller than the angle between the side of the side support (5) away from the adjacent side plate (1) and the horizontal plane.

3. A storage fixture for lead frame processing according to claim 2, characterized in that, The bearing surface (401) is coated with a chromium nitride coating to improve surface hardness and smoothness.

4. A storage fixture for lead frame processing according to claim 2, characterized in that, The angle between the bearing surface (401) and the horizontal plane is between 10° and 15°.

5. A storage fixture for lead frame processing according to claim 1, characterized in that, Each side plate (1) is provided with a mounting groove (402) near the adjacent support strip (4), and the support strip (4) is snapped into the adjacent mounting groove (402).

6. A storage fixture for lead frame processing according to claim 1, characterized in that, The side support (5) is fixedly connected to a limiting strip (7), which is used to limit the swing of the lead frame.

7. A storage fixture for lead frame processing according to claim 6, characterized in that, The side plate (1) is provided with two sliding grooves (501) near the adjacent side support (5). The side support (5) slides within the adjacent sliding grooves (501). The side plate (1) is slidably and rotatably connected to two threaded rods (8). The side support (5) is threadedly connected to the adjacent threaded rod (8). The threaded rod (8) is provided with a limiting groove (801). The side plate (1) is slidably connected to a limiting block (9) near the adjacent threaded rod (8). The limiting block (9) limits the adjacent threaded rod (8) through the limiting groove (801).

8. A storage fixture for lead frame processing according to claim 7, characterized in that, The bearing strip (4) is made of elastic material. The guide piece (6) is fixed to the adjacent side support (5) by an elastic piece. The side of the elastic piece away from the adjacent side plate (1) is coplanar with the side of the side support (5) away from the adjacent side plate (1).

9. A storage fixture for lead frame processing according to claim 8, characterized in that, The two side plates (1) are slidably connected to a sliding strip (10) on the side close to the adjacent guide plate (6). The sliding strip (10) and the top plate (3) are fixed together with two symmetrically distributed elastic strips (11). The bottom plate (2) and the top plate (3) are fixed together with two symmetrically distributed support strips (12) on the side away from the sliding strip (10). The elastic strips (11) and the support strips (12) are used together to maintain the position of the lead frame.

10. A storage fixture for lead frame processing according to claim 9, characterized in that, The minimum horizontal spacing between the two elastic strips (11) is less than the minimum horizontal spacing between the two side supports (5) symmetrically distributed on different side plates (1).

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