CN1281965C - Socket and contact of semiconductor package - Google Patents

Abstract

The present invention is directed to a socket connector (TS) having a plurality of contacts to be brought into contact with a plurality of solder balls (S) of a semiconductor package, a socket body (15) in which a mounting hole (11) is provided for each contact (10), a through-hole pierced in a height direction of the socket body and a contact support hole (13) are provided. Each contact (10) is provided with an upright piece (101) extending through the through-hole, a support piece extending from a proximal end side of the upright piece to be inserted into the through-hole and a contact portion (103) formed at a free end portion of the upright piece to be brought into contact with the solder ball. Each contact portion is arranged at a height level such that it projects from the surface of the socket body. A guide projection (14) is provided at a position to face an associated contact portion of each contact. These contact portion and guide projection are set at an interval such that the solder ball may be brought into contact with the both.

Description

Sockets and terminals for semiconductor packages

technical field

The present invention relates to a socket for a semiconductor package provided with terminals for contact with solder balls provided on one side of the semiconductor package, and to these terminals.

Background technique

Recently, because of the miniaturization of information processing equipment and its high-speed performance, reduction of the pitch of integrated circuits has been progressing. Corresponding to this demand, the actual mounting method is changing from the through-hole type to the surface mount type and the arrangement of the terminals is also changing from the peripheral arrangement to the array arrangement. Also, in this array setup, a ball grid array (BGA), in which solder balls are placed in an array on the surface of the package, becomes a major type of package where the terminals can be mounted on the surface of the package .

In the case where a socket is used between a BGA-type integrated circuit and a substrate, the socket is actually installed on the substrate to interchange with the integrated circuit in some cases, plus the burn-in test of the integrated circuit is performed by using a test socket or the case of high frequency testing. In the socket, it is necessary to maintain sufficient electrical conduction by the terminals on the integrated circuit and the terminals formed on the substrate making full contact with all terminals in the socket.

On the other hand, since the density or operating speed of integrated circuits increases, it is required to reduce the self-inductance with respect to high-speed pulses. In order to meet this requirement, it is necessary to make the path length of current through the terminals (intervals between contacts) provided between the terminals of the integrated circuit and the terminals of the substrate as small as possible.

Generally, for example, Japanese Patent Laid-Open No. Hei 8-222335 discloses a socket for a semiconductor package having a plurality of solder balls arranged in an array.

As shown in FIGS. 12 to 14 , the socket 300 disclosed in this application is provided with a terminal 305 . The terminal 305 is punched from a metal plate, and is provided with a base body 302, the base body 302 is provided with sidewalls 301 on both sides thereof and generally has a "U"-shaped cross section, and a substantially "C"-shaped The contact piece 303 has elasticity and protrudes from the bottom of the base body 302 toward the side wall 301 and extends to the top of the base body 302, a contact provided near the end of the contact piece to contact with the solder ball S of the integrated circuit part 303a, a contact part 304 protruding from the bottom end part of the base body 302 in a direction opposite to the contact piece 303;

In such a test socket, in order for the solder ball S to be pressed into contact with the contact portion 303, the contact piece 303 forms a leaf spring having an arc shape. For this reason, it is difficult to shorten the length of the conductive path from the contact portion 303 a to the contact portion 304 . Therefore, it is impossible to reduce the self-inductance. This leads to a problem that high-precision testing or qualification of semiconductor packages cannot be realized in the high-frequency range. In addition, since most of one side of the terminal piece 303 is pressed into contact with the inner wall of the extrusion inlet, it becomes difficult to elastically deform the terminal.

In order to try to solve the above problems, a socket as shown in Fig. 15 has come out. A substantially U-shaped terminal 310 is press-fit and fixed to the socket main body 311, and a portion of the standing piece 313 is elastically deformable except for the fixing portion 312. In this way, when the contact portion 314 is in contact with the solder ball 315 , the upstanding piece 313 is elastically deformed to avoid damage to the solder ball 315 . Also, the height (thickness) of the socket can be reduced.

However, there are still problems to be solved as follows. That is, these problems are related to the requirement to further reduce the height of the socket. In the socket shown in FIG. 15 , the length of the elastically deformable upright piece 313 is shortened due to the existence of the fixing portion 312 . For this reason, there arises a problem that it is difficult to further reduce its height. Also, since the maximum dimension of the terminal 310 is elongated, it is not possible to further reduce the self-inductance.

Especially with regard to the reduction of the height of the socket, if the part of the upright piece 313 which can be elastically deformed is shortened because of further reducing the height of the socket, the elasticity will be reduced so that the load on the solder ball 315 will increase, resulting in damage to the solder ball. 315 questions.

Reducing the height of sockets As a technique, Japanese Patent Publication No. 2001-167857 describes a terminal strip in which a spring terminal is formed by several cantilevers with ball-shaped terminal holding portions.

In the terminal strip of this publication, since all the cantilever arms are provided on one plane with respect to the terminal strip, it is impossible to secure the advantage of further reduction in height, but there arises a problem that it is impossible to arrange it at a high density. That is, since several cantilevers are arranged horizontally on the plane, the area occupied by each elastic terminal will increase. As a result, it is impossible to have a high-density arrangement.

Also, generally, since the shape of the terminal itself is complicated, and one ball terminal utilizes several cantilevers, this brings about a problem of long installation man-hours and increased costs.

Contents of the invention

The object of the present invention is to provide a socket for semiconductor packaging, which can reduce self-induction and obtain high-density arrangement by reducing the height of the socket, and which also takes into account the reliability of cost reduction and contact of the terminal contact portion .

The present invention adopts the following structure to solve the above-mentioned problems. A socket for a semiconductor package according to the present invention includes terminals contacting solder balls provided on one side of the semiconductor package and a socket body provided with mounting holes for mounting the respective terminals. Each mounting hole is provided with a through hole piercing the socket body in the height direction and a terminal supporting hole. Each terminal is provided with an upright portion extending along the through hole, a support portion extending from a proximal end portion of the upright portion inserted into the through hole, and a free end formed on the upright portion to be in contact with a solder ball contact part. The height of each contact portion is such that it protrudes from the surface of the socket body. A guide protrusion is provided on the surface of the socket body at a position facing the contact portion of each terminal. The contact portion and the guide bump are arranged at a certain interval so that the solder ball can contact with both the contact portion and the guide bump.

According to the present invention, the installation hole of the terminal is provided with the through hole and the support hole, and the terminal is provided with an upright portion extending along the through hole and a support portion extending along the upright portion to be inserted into the support hole. Therefore, the upstanding portion is supported to the supporting portion so that it can be elastically deformed. As a result, it is possible to effectively utilize the entire length of the upright portion as a part that can be elastically deformed. Thus, it is possible to reduce self-inductance by further reducing the height of the socket. Also, since the upstanding portions extend along the through holes, it is possible to achieve a high-density arrangement compared to the case where the terminals are arranged in a horizontal manner. Also, guide projections are provided at the contact portions facing each of the terminals provided on the upper surface of the socket body. The contact portion and the guide bump are arranged at a certain interval so that solder balls can contact with both the contact portion and the guide bump. Since the guide projections are formed on the socket body, the terminal itself can be formed in a simple structure, thereby making it possible to reduce the cost.

Preferably, the size of the above-mentioned through hole is such that the upstanding portion can be displaced inside the through hole. Therefore, since the upright portion is displaceable inside the through hole, freedom of movement of the upright portion as a whole is allowed without restriction, so that the independence of each upright portion is enhanced. Thus, it is possible to form a terminal which is durable by utilizing the length of the effective elastic portion as the entire length of the upstanding portion.

Preferably, a slope having a downward slope toward the through hole is formed on the above-mentioned guide projection, and the surface of the contact portion of the terminal forms a curved surface and moves away from the slope as it extends toward the free end of the upright portion. extended in the direction. With the inclined and curved surfaces arranged in such a manner, they act as guide surfaces to smoothly guide the solder balls to desired relative positions. In particular, since the curved surface of the contact portion can be easily slid over by the spherical solder ball, this enables a low-friction contact to be obtained. Therefore, it is possible to achieve an improvement in the reliability of the contact and a reduction in the load on the solder ball.

Preferably, an anti-falling projection used to prevent falling off from the supporting hole is provided on the supporting portion. It is possible to reliably fix the supporting part in the supporting hole through the anti-falling protrusion.

Preferably, the socket body is formed in a planar shape, the contact portion of each terminal is provided on its surface, the proximal end portion of each terminal is provided on the substrate mounting surface and the proximal end portion is separated from the substrate. protruding extension on the mounting surface. With this arrangement, since the proximal end portion of each terminal protrudes from the substrate mounting surface of the socket body, it is possible to utilize the proximal end portion as a terminal. For example, it is possible to employ a structure in which the socket is mounted on the circuit substrate and the proximal end portion of each terminal is in contact with the terminal provided on the surface of the circuit substrate. Thus, it is possible to effectively utilize the entire length of the upstanding portion. This also helps reduce height.

It is possible to employ a structure in which the surface of the socket body is provided with a recessed portion having the same depth, and the contact portion of each terminal is exposed in the recessed portion. The recessed portion is provided on one side of the upper surface of the socket body so as to expose the contact portion of each terminal, thus giving more freedom of displacement of each contact portion. Therefore, it is possible to make the via hole smaller.

Preferably, a U-shaped bent portion is formed between the upright portion and the support portion of the terminal. This curved portion exhibits the effect of enabling the elastic deformation of the upright portion and the effect of making the support portion approximately parallel to the upright portion. In this way, it is possible to obtain a structure that facilitates mounting of the support portion to the support hole.

On the other hand, according to a terminal of the present invention, which is used to be mounted on the socket body and to be in contact with a plurality of solder balls provided on one surface of the semiconductor package, the terminal can be formed by processing a conductive metal plate and set There is an upright portion, a contact portion formed at the free end of the upright portion in contact with the solder ball, a support portion extending from the proximal end portion of the upright portion so that the upright portion is supported to the socket body, and a support portion arranged on the support The U-shaped bend between the upper part and the upright part.

According to the terminal of the present invention, it is provided with an upstanding portion and a supporting portion. The support portion extends from the proximal portion of the upright portion. Therefore, the upstanding portion is supported to the supporting portion so that it can be elastically deformed. As a result, it is possible to effectively utilize the entire length of the upright portion as the elastically deformable portion. Thus, it is possible to reduce self-inductance by further reducing the height of the socket. Since the U-shaped bent portion is formed between the support portion and the upright portion, the upright portion is supported by the support portion so as to be elastically deformable while maintaining a space between the support portion and the upright portion.

Preferably, the above-mentioned contact portion forms a curved surface extending in a direction further away from the solder ball as it extends towards the free end of the upright portion. Since the curved surface of the contact portion can be easily slid over by the spherical solder ball, it is possible to obtain a low-friction contact. In this way, it is possible to achieve improved contact reliability of the contact portion and reduced load on the solder balls.

It is possible to employ a structure in which the terminal is provided with a reaction portion formed between the contact portion bent into an inverted U shape and the free end of the upright portion, and the free end is in contact with the socket body and supported to On the socket body, a reaction force is generated in the reaction part. With this arrangement, it is possible to increase the contact force of the contact portion with respect to the solder ball.

Description of drawings

By following the description referring to the accompanying drawings, the composition and mode of the structure of the present invention can be better understood, and the operation of the present invention can be better understood. In the accompanying drawings, the same numerals indicate the same parts and components. In the accompanying drawings:

Figure 1 is a plan view of a test socket according to the present invention;

Fig. 2 is a sectional view along line X-X in Fig. 1;

Fig. 3 is a sectional view along the Y-Y line in Fig. 1;

4 is an enlarged perspective view of a terminal according to the present invention;

5 is a view of a terminal according to the present invention, wherein (a) is a plan view, (b) is a front view, and (c) is a right side view thereof;

Figure 6 is a plan view of a socket body according to the present invention;

Fig. 7 is a sectional view along line X1-X1 in Fig. 6;

Fig. 8 is a sectional view along line Y1-Y1 in Fig. 6;

Fig. 9 is a partial perspective view of a test socket according to the present invention;

10 is a perspective view of the substrate mounting surface side of the test socket according to the present invention;

11 is a partially enlarged cross-sectional view of a test socket according to another embodiment of the present invention;

Fig. 12 is a sectional view of a conventional test socket;

Figure 13 is a front view of a terminal installed in a conventional test socket;

Figure 14 is a side view of a terminal mounted on a conventional test socket; and

Fig. 15 is a partially enlarged sectional view of a conventional test socket.

Detailed ways

Although the present invention may allow various forms of embodiments, a specific embodiment is shown in the drawings and will be described in detail below, and it should be understood that the disclosure of the present invention is only considered as an example of the spirit of the present invention, and is not intended to be The invention is limited insofar as explained and described.

FIG. 1 is a plan view of a socket (test socket) of a semiconductor package according to an embodiment of the present invention. Fig. 2 is a cross-sectional view along line X-X in Fig. 1 . Fig. 3 is a sectional view along line Y-Y in Fig. 1 . Fig. 4 is an enlarged perspective view of a terminal.

The test socket TS shown in these figures is a ball grid array type test and qualification socket of a semiconductor package 1 having a plurality of solder balls S arranged in an array on its surface (lower surface) side.

The test socket TS is provided with several terminals 10 arranged in an array, and arranged in a manner corresponding to the solder ball S of the semiconductor package 1, and a socket body (housing) 15 provided with several mounting holes 11, so that The above-mentioned corresponding terminals 10 are installed in these installation holes 11. The mounting hole 11 is provided with a through hole 12 piercing through the height direction of the socket body 15 and a supporting hole 13 for the terminal 10 . The supporting hole 13 is independently provided beside the through hole 12 and opens on the substrate mounting surface 15 b of the socket body 15 .

Then, each terminal 10 is provided with an upright portion 101 extending along the through hole 12 and a support portion 102 extending from the upright portion 101 to be inserted into the support hole 13 . At the free end (upper end) of the standing portion 101 is formed a contact portion 103 for contacting the solder ball S. As shown in FIG. The support portion 102 extends from the proximal end portion (lower end portion) of the upright portion 101 .

Each contact portion 103 is provided at such a height that it protrudes from the surface 15 a of the socket body 15 . A guide projection 14 is provided on the surface 15 a of the socket body 15 at a position facing the contact portion 103 of each terminal 10 . The contact portion 103 and the guide bump 14 are arranged at a certain interval so that the solder ball S can contact both of them.

A slope 141 extending downwards toward the through hole is provided on each guide protrusion 14 . A surface of the contact portion 103 at a position facing the slope 141 forms a curved surface, and extends in a direction away from the slope 141 as it extends toward the free end (upper end) of the upright portion 101 . More specifically, it forms an arcuate surface that expands toward the slope 141 .

The curved surface of the contact portion 103 and the inclined surface 141 are arranged in this way to obtain such an effect that they serve as guide surfaces to smoothly guide the solder ball S to a desired relative position. In particular, since the curved surface of the contact portion 103 can be easily slid by the solder ball S having a spherical surface, this enables low-friction contact. Therefore, it is possible to obtain improved contact reliability and low-load contact with the solder ball S. FIG.

Regarding the through hole 12 of the mounting hole 11, it is dimensioned such that the upright portion 101 can be displaced inside it. That is, as shown in FIGS. 6 to 8 , the shape of the through hole 12 itself is much larger than the cross-sectional shape of the upright portion 12 . The uprights 101 are suitable for easy displacement inside the through hole 12, thus allowing the uprights 101 to move freely as a whole without restriction, so as to enhance the independence of each upright 101.

Therefore, it is possible to form a terminal having high durability by utilizing the entire length of the upright portion 101 as the length of the effective elastic portion. Also, like this, elasticity is just increased, therefore, once contacting solder ball S, the load on solder ball S will be reduced, simultaneously, just may overcome the solder ball height along the thickness (height) direction of socket body 15. inconsistency.

An anti-falling protrusion 105 for preventing falling off from the supporting hole 13 is disposed on the supporting portion 102 . The supporting part 102 is reliably fixed in the supporting hole 103 through the anti-falling protrusion 105, and at the same time, the supporting part 102 can be easily installed in the supporting hole 103, which also contributes to the ease of manufacture.

As shown in FIGS. 6 to 8, the socket body 15 is formed in a planar shape of synthetic resin having insulating properties. The contact portion 103 of each terminal 10 is provided on one side of the surface 15a of the body 15 (see FIG. 9 ). The proximal end portion 104 of the terminal 10 is provided on one side of the substrate mounting surface 15b (see FIG. 10). Each proximal end portion 104 protrudes from the substrate mounting surface 15b.

That is, the near base end portion 104 of the terminal 10 protrudes from the substrate mounting surface 15b of the socket body 15, so that the near base end portion 104 can be directly used as a terminal. In this case, the test socket TS is provided on the circuit substrate 20 such that the proximal end portion 104 of each terminal 10 is in contact with the terminal 21 provided on the surface of the circuit substrate 20 . Therefore, it is possible to effectively utilize the entire length of the upright portion 101 . This also contributes to the reduction in height.

The surface 15a of the socket body 15 is provided with a recessed portion 16 having the same depth, and at least the contact portion 103 of each terminal 10 is exposed in the recessed portion 16 . The recessed portion 16 is provided on one side of the surface 15 a of the socket body 15 so as to expose the contact portion 103 of each terminal 10 and possibly give more freedom of displacement of each contact portion 103 . Therefore, it is possible to make the via hole smaller.

As shown in FIGS. 4 and 5 , a U-shaped bent portion 106 is formed between the upright portion 102 and the support portion 102 of the terminal 10 . The bent portion 106 is used to make the upright portion 101 elastically deformable and make the supporting portion 102 substantially parallel to the upright portion 101 . In this case, the manner of elastically deforming the upright portion 101 includes, for example, that the contact portion 103 is displaced in a horizontal or vertical direction when the upright portion 101 itself is curved.

The interval between the support hole 13 and the through hole 12 in the substrate mounting surface 15b of the socket body 15 is smaller than the interval between the upright portion and the support portion 102 . The reason why the interval is provided in this way is to increase the degree of freedom of the upright portion 101 by preventing the upright portion 101 from coming into contact with the inner wall of the through hole 12 .

Since the solder ball S is spherical, when the contact portion 103 facing the slope 141 comes into contact with the solder ball S, the contact portion will move away from the slope 141 due to the elasticity of the upright portion 101 . In this case, the contact portion 103 has a natural reaction force which tries to bring the contact portion 103 closer to the slope 141 . Therefore, by utilizing the elasticity of the terminal, it is possible to improve the reliability of securing the contact portion 103 with the solder ball S. FIG.

As shown in FIG. 10, when each terminal 10 is installed in the relevant mounting hole 11, the proximal end portion 104 of each terminal 10 slightly protrudes from the substrate mounting surface 15b of the socket body 15 to form a contact with the circuit substrate. The contact portion where the terminal 21 of 20 contacts.

In this embodiment, since the supporting portion 102 is press-fitted into the supporting hole 13 when the terminal 10 is installed in the mounting hole 11 of the socket body 15 , the upright portion 101 can be slightly displaced up and down. Therefore, it is possible to absorb the inconsistency in the position of the proximal end portion (tail portion) 104 forming the contact portion with the circuit substrate 20 .

Incidentally, FIGS. 9 and 10 are perspective views of the test socket TS in the case where the terminals 10 are installed in all the mounting holes 11 of the body 15 . In addition to this, FIG. 10 is a perspective view showing one side of the substrate mounting surface for ease of understanding.

Fig. 11 is an enlarged sectional view of main parts according to another embodiment of the present invention.

In this embodiment, the shape of the terminal 10A is somewhat changed. Other than that, its structure is roughly the same as the terminal structure of the above-mentioned embodiment. Therefore, the same reference numerals are used to designate the same elements and explanations thereof are omitted.

Like the terminal 10 described above, the terminal 10A is provided with an upright portion 101 , a support portion 102 , a contact portion 103 , a proximal end portion 104 , an anti-dropping protrusion 105 and a U-shaped bending portion 106 . In addition, the terminal 10A is further provided with a reaction portion 108 . The reaction portion 108 is formed by bending the contact portion 103 into an inverted U shape and extending the free end 109 of the upright portion 101 to the inner wall of the through hole. The free end 109 is arranged to be in contact with the inner wall of the through hole so as to have a reaction force.

When the solder ball S is disposed between the contact portion 103 and the slope 141 , the terminal 10A will be elastically deformed along the dotted line in FIG. 11 . In this case, the reaction force of the reaction part 108 is also added as a reaction force due to the elastic deformation of the upright part 101 and the U-shaped bending part 106 . Therefore, in the case where the contact force between the terminal and the solder ball S is smaller than the required contact force, this structure can be adopted.

Incidentally, the present invention is not limited to the test socket of the ball fence array type as described above, but can be applied to any socket of a semiconductor package, such as a CSP (Chip Scale Package) type socket.

As described above, according to the present invention, the mounting hole of the terminal is provided with a through hole and a support hole, and the terminal is provided with an upright portion extending along the through hole and a support portion extending from the upright portion to be inserted into the support hole. Therefore, the upstanding portion is supported to the supporting portion so that it can be elastically deformed. As a result, it is possible to effectively utilize the entire length of the upright portion as the elastically deformable portion. Thus, it is possible to reduce self-inductance by further reducing the height of the socket.

Also, because the upstanding portion is in such a form that it extends along the through hole, it is possible to achieve a high-density arrangement relative to the case where the terminals are arranged in a horizontal manner. Equally, since the effective elastic portion of the terminal is a lengthwise configuration, the terminal is not completely inserted into the socket body but only its support portion is press-fitted, etc., and it is possible to obtain One degree of freedom, and it is possible to improve the contact reliability of the contact part.

Also, since the contact portion and the guide bump are in contact with a solder ball, and the guide bump is formed in the socket body, the terminal itself can be formed in a simple structure, thereby making it possible to reduce the cost.

While embodiments of the present invention have been shown and described, it is envisioned that various modifications can be devised by those skilled in the art without departing from the scope and basic principles of the invention as appended hereto.

Claims (15)

1. the socket of a semiconductor packages, comprise and contacted several terminals of several solder sphere and a jack body of being located on the side of described semiconductor packages, described jack body is provided with several mounting holes so that terminal separately to be installed, wherein each described mounting hole is provided with through hole and terminal support hole of the short transverse that pierces through jack body, it is characterized in that:

Each terminal is provided with an erection part of extending along through hole, a support portion of extending from the nearly base end part that is inserted into the erection part in the through hole, and free end that is formed on erection part with the contacted contact site of solder sphere;

The set height of each contact site makes it outstanding from the surface of jack body, and guiding projection is located on the surface of jack body and is positioned at the position of facing mutually with the contact site of each terminal; And

Described contact site is arranged so that with certain interval solder sphere can all contact with described guiding projection with described contact site with guiding projection.

2. the socket of semiconductor packages according to claim 1, the size of wherein said through hole is arranged so that erection part can be shifted within it.

3. the socket of semiconductor packages according to claim 1 wherein is formed with an inclined-plane to downslope that has towards through hole on described projection.

4. the socket of semiconductor packages according to claim 1, wherein the surface of the contact site of terminal forms one along the curved surface that extends away from the direction of guiding projection.

5. the socket of semiconductor packages according to claim 1, wherein said support portion comprises one in order to be housed in the slip-off preventing projection in the supported hole.

6. the socket of semiconductor packages according to claim 1, wherein jack body forms a flat shape, and the contact site of described each terminal is given prominence on the surface of jack body.

7. the socket of semiconductor packages according to claim 1, wherein the nearly base end part of each terminal is located on the substrate installation surface of jack body, and each nearly base end part is outstanding from the substrate installation surface.

8. the socket of semiconductor packages according to claim 1, one of them depressed part with same degree of depth is arranged on surface one side of jack body, and the contact site of each terminal is exposed in this depressed part.

9. the socket of semiconductor packages according to claim 1, one of them bend that bends to U-shaped is formed between the erection part and support portion of terminal.

10. the socket of semiconductor packages according to claim 1, wherein each terminal comprises a retroaction portion of extending along contact site.

11. one be used for being installed on the jack body and with one that is arranged on semiconductor packages lip-deep several solder sphere in each contacted terminal, it is characterized in that:

Described terminal comprises an erection part, a free end and the contacted contact site of solder sphere that is formed at erection part, a support portion of extending from the nearly base end part of erection part makes erection part support on the jack body, a U-shaped bend that is arranged between support portion and the erection part.

12. terminal according to claim 11, wherein said contact site forms a curved surface, and when the free end towards erection part extended, this curved surface extended along the direction away from solder sphere.

13. terminal according to claim 11, wherein said terminal are provided with a retroaction portion of extending from contact site.

14. terminal according to claim 11, wherein said terminal comprise a retroaction portion between the free end that is formed on contact site and erection part.

15. terminal according to claim 11, wherein said terminal is provided with a retroaction portion, this retroaction portion is formed between the free end that bending is inverted U-shaped contact site and erection part, described free end contacts with jack body and supports on it, thereby produces reacting force in retroaction portion.

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