Wire bonding method

A method, which is for forming accurate low wire loop shapes or short wire loop shapes which are stable and which have a high shape retention force in devices in which height differences between first and second bonding points are small and the wiring distance is short, including the steps of bonding a ball formed at the end of the wire extending out of the capillary to the first bonding point, raising the capillary while delivering the wire, moving the capillary toward the second bonding point, and then raising the capillary diagonally upward.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a bonding method and more particularly to
 a wire bonding method which is suitable for devices in which the height
 difference between the first and second bonding points is small and the
 wiring distance is short.
 2. Prior Art
 FIGS. 4(a) and 4(b) show prior art semiconductor device assembly bonding
 processes.
 In these processes, a pad 2a (which is a first bonding point) on a
 semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (which is a
 second bonding point) on the lead frame 1 are connected by a wire 3; and
 FIG. 4(a) shows a triangular wire loop (of the bonded wire 3), and FIG.
 4(b) shows a trapezoidal wire loop.
 These bonding processes are disclosed in, for example, Japanese Patent
 Application Laid-Open (Kokai) No. H4-318943 and Japanese Patent
 Application Publication (Kokoku) Nos. H1-26531, H5-60657 and H6-101490.
 In particular, Japanese Patent Application Laid-Open (Kokai) No. H4-318943
 discloses both triangular and trapezoidal loops in FIG. 3 and in column 1,
 lines 31 through 41. Japanese Patent Application Publication (Kokoku) No.
 H1-26531 discloses a triangular wire loop in FIG. 3(c) and in column 3,
 line 3 through line 25. Japanese Patent Application Publication (Kokoku)
 No. H5-60657 discloses a triangular wire loop in FIG. 2(e) and in column
 3, line 30 through column 4, line 40. Japanese Patent Application
 Publication Kokoku) No. H6-101490 discloses a trapezoidal wire loop in
 FIG. 1 and in column 4, lines 39 through column 5, line 22.
 The triangular loop of FIG. 4(a) is formed by the process shown in FIG.
 5(a) and FIG. 6. As seen from FIG. 5(a), a capillary is moved from point A
 to G through points B, C and F to form the triangular wire loop.
 More specifically, as shown in FIG. 6, in step (a), the capillary 4 is
 lowered with a clamper (not shown) which holds the wire 3 maintained in an
 open state, and a ball formed on the tip end of the wire 3 is bonded to
 the first bonding point A, after which the capillary 4 is raised to point
 B, delivering the wire 3. Next, in step (b), the capillary 4 is moved
 horizontally to point C in the opposite direction from the second bonding
 point G. A loop forming operation in which a capillary is moved in the
 opposite direction from a second bonding point is generally called a
 "reverse operation". As a result of this reverse operation, the wire 3
 assumes a shape that is inclined from point A to point C, and a kink 3a is
 formed in one portion of the wire 3. The wire 3 delivered in this process
 and extending from point A to point C forms a neck height part 31 shown in
 FIG. 4(a).
 Next, in step (c), the capillary 4 is raised to point F, delivering the
 wire 3; and the clamper (not shown) is closed. When the clamper is closed,
 no wire 3 is delivered from the capillary 4 even if the capillary 4 is
 subsequently moved. Next, in step (d), the capillary 4 is positioned at
 the second bonding point G by being moved circularly or by being lowered
 after a circular-arc movement, thus bonding the wire 3 to the second
 bonding point G.
 On the other hand, the trapezoidal loop shape shown in FIG. 4(b) is formed
 by the process shown in FIG. 5(b) and FIG. 7. As seen from FIG. 5(b), the
 capillary is moved from point A to G through points B, C, D, E and F.
 More specifically, as shown in FIG. 7, in step (a), the capillary 4 is
 lowered with the clamper (not shown) which holds the wire 3 maintained in
 an open state, and the ball formed on the tip end of the wire is bonded to
 the first bonding point A, after which the capillary 4 is raised to point
 B, delivering the wire 3. Next, in step (b), the capillary 4 is moved
 horizontally to point C in the opposite direction from the second bonding
 point G. As a result, the wire 3 assumes a shape that is inclined from
 point A to point C, and a first kink 3a is formed in one portion of the
 wire 3. The wire 3 delivered in this process and extending from point A to
 point C forms the neck height part 31 shown in FIG. 4(b).
 Next, in step (c), the capillary 4 is raised to point D, delivering the
 wire. Afterward, in step (d), the capillary 4 is again moved horizontally
 to point E in the opposite direction from the second bonding point G, i.
 e., a reverse operation is performed. As a result, the wire 3 assumes a
 shape that is inclined from point C to point E, and a second kink 3b is
 formed in one portion of the wire 3. The wire 3 delivered and extends from
 point C to point E forms the trapezoidal length part 32 shown in FIG.
 4(b).
 Next, in step (e), the capillary 4 is raised to point F, delivering an
 amount of wire 3 that forms the inclined part 33 shown in FIG. 4(b); and
 the clamper (not shown) is closed. When the clamper is closed, no further
 wire 3 is delivered even if the capillary 4 is subsequently moved. Next,
 in step (f), the capillary 4 is positioned at the second bonding point G
 by being moved circularly or by being lowered after a circular-arc
 movement, thus bonding the wire 3 to the second bonding point G.
 The triangular loop formation process shown in FIG. 5(a) and FIG. 6 is
 advantageous in that the loop can be formed by a simpler process than the
 trapezoidal loop formation process shown in FIG. 5(b) and FIG. 7, and the
 loop formation is accomplished in a shorter time. However, in cases where
 the height difference between the first bonding point A and the second
 bonding point G is large, or in cases where the first bonding point A and
 the end portion of the semiconductor chip 2 are separated from each other
 by a considerable distance, the wire 3 contacts the semiconductor chip 2
 if the wire is in the triangular loop shape as shown in FIG. 4(a). In such
 cases, contact between the wire 3 and the semiconductor chip 2 is
 prevented by using the trapezoidal loop shape as shown in FIG. 4(b).
 As seen from the above, either a triangular loop or a trapezoidal loop is
 selected depending on the conditions involved. However, in the case in
 which the height difference between the first bonding point A and the
 second bonding point G is small (e.g., 100 .mu.m or less), and the wiring
 distance is short (e.g., 1 mm or less), problems arise when a low wire
 loop shape or short wire loop shape is made by the triangular loop
 formation process or the trapezoidal loop formation process. In
 particular, in cases where the amount of reverse movement in the reverse
 operation is sufficient so that a kink 3a is strongly formed in the wire 3
 as shown in FIG. 6(b) and FIG. 7(b), an accurate triangular loop or
 trapezoidal loop can be formed as shown in FIG. 6(d) and FIG. 7(f);
 however, in cases where a low wire loop shape is to be formed, a large
 amount of reverse movement cannot be performed because the neck height
 part 31 must be low.
 A wire shape in which a triangular wire loop formation process forms a low
 neck height part 31 is shown in FIG. 8, while a wire shape by a
 trapezoidal wire loop formation process that forms a low neck height part
 31 is shown in FIG. 9.
 In these cases, as shown in step (b) in FIG. 8 and in step (b) in FIG. 9,
 the kink 3a formed in steps (c) in FIGS. 6 and 7 cannot be obtained
 sufficiently, and a bent part 3c is formed in step (c) in FIGS. 8 and 9.
 As a result, a triangular loop or trapezoidal loop with an inaccurate
 shape which is bowed upward is formed and the overall height of the wire
 loop is increased in step (d) in FIG. 8 and in step (i) in FIG. 9.
 Unfortunately, when the wire is formed with a bend in it initially or when
 a bend is formed in the wire during the wire loop forming operation, there
 is no way for removing such bends. As a result, bends and bows are
 generated in the bonded wires.
 SUMMARY OF THE INVENTION
 Accordingly, the object of the present invention is to provide a wire
 bonding method which makes it possible to form accurate low wire loop
 shapes and short wire loop shapes that are stable and that have a high
 shape retention force for devices which have a small height difference
 between first and second bonding points and which have a short wiring
 distance.
 The above object is accomplished by unique steps of the present invention
 which are taken in a wire bonding method in which a wire that passes
 through a capillary is connected between a first bonding point and a
 second bonding point by the capillary; and in the present invention, the
 capillary is raised while the wire is delivered from the tip end of the
 capillary after a ball formed on the tip end of the wire extending from
 the tip end of the capillary has been bonded to the first bonding point,
 the capillary is then moved toward the second bonding point, and then the
 capillary is subsequently raised directly upward or upward along an
 inclined path.
 The above object is further accomplished by unique steps of the present
 invention which are taken in a wire bonding method in which a wire that
 passes through a capillary is connected between a first bonding point and
 a second bonding point by the capillary; and in the present invention, the
 capillary is raised while the wire is delivered from the tip end of the
 capillary after a ball formed on the tip end of the wire extending from
 the tip end of the capillary has been bonded to the first bonding point,
 the capillary is then moved toward the second bonding point, and the
 capillary is subsequently raised directly upward or upward along an
 inclined path, and further, the capillary is lowered to a position which
 is above the second bonding point but located slightly on the first
 bonding point side of the second bonding point, the capillary is then
 moved to a point directly above the second bonding point, and then the
 capillary is lowered so that the wire is bonded to the second bonding
 point.
 In the above methods, the direction in which the capillary is raised when
 the capillary is raised upward along an inclined path is a direction that
 is more or less oriented along the chamfer surface of the capillary.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 One embodiment of the present invention will be described with reference to
 FIGS. 1 and 2.
 First, in step (a) shown in FIG. 1, which is the same as that used in a
 conventional method, is performed. In other words, the capillary 4 is
 lowered with a clamper (not shown) that holds the wire 3 maintained in an
 open state, and the ball formed on the tip end of the wire 3 is bonded to
 the first bonding point A. After this, the capillary 4 is raised to point
 B, delivering the wire 3.
 Next, the process that characterizes the present embodiment is performed.
 In step (b), the capillary 4 is moved from point B to point C in the
 direction of the second bonding point G. As a result, a kink 3a is formed
 in one portion of the wire 3. The wire 3 delivered during the movement of
 the capillary 4 from point A to point C will form the neck height part 31
 shown in step (f).
 Next, in step (c), the capillary 4 is raised from point C to point D along
 a path that is inclined upward in the direction of the second bonding
 point G, delivering the wire 3. The length of wire 3 that is delivered out
 in this operation from point C to point D will form the inclined portion
 33 shown in step (f). It is desirable that the track along which the
 capillary 4 is raised from point C to point D be a linear track as
 indicated by the solid line arrow or a circular-arc track as indicated by
 the two-dot chain line arrow. The reasons for this will be described with
 reference to FIGS. 3(a) and 3(b).
 As shown in FIG. 3(a), the capillary 4 is formed with a chamfer surface 4a,
 a corner edge 4b and a hole 4c. The corner edge 4b is where the chamfer
 surface 4a joins with the hole 4c. Depending on the type of capillary 4
 used, the capillary 4 may have a rounded corner edge 4b, or it may have a
 chamfer surface 4a that is formed in a multiple number of steps. The
 embodiment is described with reference to the commonly used type of
 capillary shown in FIG. 3(a) which has a rounded corner edge. Generally,
 the chamfer angle .theta. formed by the chamfer surface 4a is about
 90.degree. to 120.degree..
 When the wire 3 is delivered, the wire 3 contacts the chamfer surface 4a
 and corner edge 4b as shown in FIG. 3(b); and as a result, the wire 3 is
 rubbed by the corner edge 4b, and any bend or kink in the wire 3 can be
 removed. Since no stress should be applied to the wire 3 after the removal
 of such bends or kinks, it is desirable that the chamfer surface 4a be
 more or less oriented along the wire 3. Thus, by moving (or raising) the
 capillary 4 along the direction of the chamfer surface 4a, the wire 3 is
 rubbed; and any bend and kink formed in the wire 3 during bonding or after
 bonding is removed.
 After the step (c) in FIG. 1 is completed, the clamper (not shown) is
 closed. When the clamper is closed, no further wire 3 is delivered out
 even if the capillary 4 should subsequently move.
 Next, in step (d), the capillary 4 is positioned at point E by being caused
 to perform a circular-arc movement or by being caused to perform a
 circular-arc movement, and then lowered.
 Furthermore, in step (e), the capillary 4 is moved to point F which is
 slightly above the second bonding point G. As a result of this movement
 from point E to point F, the wire 3 is stretched so that tension is
 applied to the wire 3, thus causing the downward bow of the wire shape 34
 formed in step (d) to be absorbed, thus forming a wire shape 35 with an
 ideal wire length.
 Finally, in step (f), the capillary 4 is lowered and positioned at the
 second bonding point G, and the wire 3 is bonded thereto.
 The operation of the capillary from point D to point E and the operation
 from point F to the second bonding point G have no direct connection with
 the gist of the present invention; therefore, it goes without saying that
 operations similar to those disclosed in the prior art may be performed or
 operations of various other types may be performed. However, by taking the
 step (e) to move the capillary from point E to F as in the present
 invention, an ideal wire shape 35 is obtained as described above.
 In step (c) shown in FIG. 1, the capillary 4 is raised upward along an
 inclined path; however, it is also possible to simply raise the capillary
 4 directly upward. However, by raising the capillary 4 along an inclined
 path as in the shown embodiment, the wire 3 is rubbed as described above,
 and bends and kinks in the wire 3 during bonding or after bonding can be
 removed.
 As seen from the above, according to the present invention, after the ball
 at the tip end of the wire extending from the tip end of the capillary has
 been bonded to the first bonding point, the capillary is raised,
 delivering the wire; and then the capillary is moved toward the second
 bonding point and is raised directly upward or upward along an inclined
 path. Accordingly, accurate low wire loop shapes or short wire loop shapes
 which are stable and which have a high shape retention force can be formed
 in devices in which the height difference between the first bonding point
 and the second bonding point is small and in which the wiring distance is
 short.