JPH09134989A - Cutting of dam bar - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain attachment of dross and improve the quality by irradiating the center of a dam bar with the former half of the laser pulses so as to form a small hole and cutting the entire dam bar with the latter half. SOLUTION: A laser beam which oscillates in a pulse-like manner with its cross section formed into an elongated elliptical shape by a cylindrical lens is used as a laser beam for cutting a dam bar 3. The dam bar 3 is irradiated with a laser beam 50 while an assist gas 60 is emitted from a gas nozzle 61. First, the center of the dam bar 3 is irradiated with the former half of the laser pulses 50 so as to form a small hole 4. Then, the beam mode of the laser beam 50 is changed so as to cover the entire dam bar 3 by a spot 5 of the latter half of the pulses 50, and thus the entire dam bar 3 is cut. A cut portion 6 is formed after the cutting. Thus, melted metal is blown off and attachment of dross 7 is restrained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dam bar cutting method for cutting a dam bar of a semiconductor device, in which a semiconductor chip is mounted on a lead frame and integrally sealed with a resin mold, by laser light.

[0002]

2. Description of the Related Art In a semiconductor device (resin mold type semiconductor device) in which a semiconductor chip is mounted on a lead frame and the lead frame and the semiconductor chip are integrally sealed with a resin mold, a dam bar connects the leads of the lead frame. It plays a role of blocking the resin mold from flowing out between the leads when the lead frame and the semiconductor chip are integrally sealed with the resin mold. The dam bar also has a role of reinforcing each lead. Then, when sealing with the resin mold is completed, the dam bar is cut and removed, and each lead (outer lead) of the lead frame is individually cut off.

Conventionally, this dam bar has often been cut by a punch press. However, recently, as the integration and performance of semiconductor devices have increased, lead frames have become more multi-pin and narrower in pitch. Conventional punch presses are no longer technically compatible. That is, a recent multi-pin, narrow-pitch lead frame has a thickness of 0.1
Since the arrangement pitch of the outer leads is about 0.25 to 0.4 mm and the dimensional accuracy in the final shape is strictly required to be ± 0.01 to ± 0.03 mm, Cutting with such dimensional accuracy is difficult with a punch press.

On the other hand, recently, a method of cutting a dam bar using a laser beam has been developed. According to this method, the dam bar can be cut only by irradiating the dam bar with the laser light, and the laser beam can be narrowed down to an extremely small, so that fine processing is possible.
Moreover, processing with good dimensional accuracy is possible. Since the processing using such laser light is fusing processing, molten metal is generated, but normally, most of this molten metal is blown off and removed by ejecting an assist gas coaxially with the laser light. . In a normal laser processing apparatus, the assist gas is ejected coaxially with a laser beam from the tip of a gas nozzle provided under the processing head.

When the dam bar is cut by using the laser beam as described above, the laser beam having a circular cross-sectional shape is usually narrowed to a small beam diameter and the irradiation position is sequentially shifted while irradiating on a pulse to cut the dam bar. Was there. On the other hand, in the case of a lead frame with many pins and a narrow pitch in which the number of dam bars to be cut is large and the lead interval (the length of the dam bar to be cut) is approximately the same as the plate thickness, the processing speed is improved and at the same time the narrow lead interval (Length of dam bar to be cut) is becoming necessary, and in response to this, as described in Japanese Patent Application Laid-Open No. 4-322454, a pulse-shaped pulse having a slender cross-sectional shape with a cylindrical lens is used. A method has been developed in which a laser beam is used, and the longitudinal direction of the laser beam cross section having an elongated cross section and the longitudinal direction of the lead are made substantially parallel to each other, and the dam bar is cut by one or several laser beam irradiations. According to this method, the dam bar can be cut efficiently at a high processing speed.

Further, in the processing using laser light, in general, a part of the molten metal remains without being removed by the assist gas, and the remaining molten metal adheres to the leads and is cooled and often becomes a dross. . As an example of a method for avoiding the adhesion of the dross, there is a technique described in JP-A-5-55424. In this conventional technique, a minute notch and a clearance hole are provided in advance at a portion of the boundary surface between the dam bar and the lead, which is expected to become a sharp protrusion by being left uncut by the beam spot. Due to the holes, sharp projections are not formed when a few circular beam spots are irradiated, and it is possible to eliminate the attachment of dross that is easily formed at the tip of the projections.

[0007]

In order to increase the number of pins and the pitch of the lead frame for higher integration and higher performance of the semiconductor device, it is necessary to improve the strength of the resin mold. Therefore, the resin mold must be thick enough. In addition, the dam bar is provided near the resin mold side wall to minimize the amount of resin protrusion. When cutting the dam bar of such a semiconductor device by laser light, in order to avoid interference between the resin nozzle and the tip of the gas nozzle that ejects the assist gas,
Since the gas nozzle must be held higher than the height of the resin mold, it becomes impossible to sufficiently approach the surface of the dam bar (lead frame) to be cut. As a result, the assist gas ejected from the gas nozzle is dispersed before reaching the surface of the dam bar, the pressure of the assist gas on the surface of the dam bar is weakened, the molten metal cannot be sufficiently removed, and the amount of dross attached increases.

Further, when the thickness of the resin mold changes depending on the type of semiconductor device, the gas nozzle must be appropriately retracted according to the thickness of the resin mold, and the distance between the tip of the gas nozzle and the surface of the dam bar must be kept constant. Can not be. As a result, the pressure of the assist gas on the surface of the dam bar cannot be kept constant, and it becomes impossible to blow off the molten metal under stable conditions. Further, the laser beam irradiation conditions cannot be kept constant, so that it is difficult to maintain appropriate cutting conditions.

Further, in order to keep the gas nozzle higher than the height of the resin mold, the assist gas flows down along the side wall of the resin mold and is blocked by the lead frame to change its flow direction. In addition, a flow in the opposite direction to the resin mold is induced. As a result, especially when the amount of molten metal is large,
A part of the molten metal flows in that direction by the assist gas flowing in the horizontal direction, and dross grows in the direction opposite to the resin mold from the side wall surface of the lead from which the dam bar is removed. This is an elongated needle-shaped or rod-shaped dross, unlike a normal drop-shaped dross (hereinafter referred to as a mustache dross).

Since this beard loss is not firmly attached to the leads, there is a risk that it easily rises or rises, contacts adjacent leads, and electrically shorts the leads. Also, since the size of the beard dross is considerably larger than that of a normal drop-shaped dross, it may adhere irregularly on the board when the semiconductor device is mounted on the electronic circuit board. There is also a risk of electrical short circuit. On the other hand, if it is attempted to forcibly remove such beard dross, there is a risk that the multi-pin, narrow-pitch lead frame with weak rigidity will be deformed or twisted, and in the worst case, the lead may be broken. Moreover, since there are many places to be removed in the lead frame having many pins, it can be said that it is almost impossible to completely and surely remove the beard loss. As described above, the beard loss causes a remarkable defect in the semiconductor device.

By the way, in the case of using a laser beam having an elongated cross-sectional shape as disclosed in Japanese Patent Laid-Open No. 4-322454, it is melted by one pulsed laser beam as compared with a laser beam having a circular cross-sectional shape. The portion, that is, the molten metal to be removed is increased. Therefore, there is a concern that the amount of dross generated from the molten metal during laser cutting will also increase. Therefore, when the dam bar of a semiconductor device having a thick resin mold is cut by using such a laser beam having an elongated cross-sectional shape, not only the usual dross but also the above-described beard dross is likely to occur. Therefore, even if machining can be efficiently performed with good dimensional accuracy using this conventional technique, if shaving loss or the like occurs, good dimensional accuracy will be impaired by removing it, and the required dimensional accuracy will be reduced. Can't be maintained.

On the other hand, in the prior art disclosed in Japanese Patent Laid-Open No. 5-55424, since a circular spot is used, the amount of molten metal is relatively small and no beard loss occurs, but notches or escape holes are formed. Since the number of steps for forming is increased, the time required for cutting is required by that much, and the efficiency is reduced. Moreover, there is a problem that the manufacturing cost is increased.

An object of the present invention is to provide a dambar cutting method capable of efficiently cutting a dambar by using a laser beam and suppressing the adhesion of dross to obtain a highly accurate and good quality semiconductor device. It is to be.

[0014]

To achieve the above object, according to the present invention, a dam bar in a semiconductor device in which a semiconductor chip is mounted on a lead frame and integrally sealed with a resin mold is used to discharge an assist gas from a gas nozzle. However, in the dam bar cutting method of irradiating and cutting a pulsed laser light having an elongated cross-sectional shape, a small hole is formed in the center of the dam bar in the first half of each pulse of the laser light with which the dam bar is irradiated, There is provided a dambar cutting method characterized by cutting the entire dambar in the latter half.

The present invention has been proposed in order to suppress the attachment of dross as much as possible when the dam bar is cut by using a pulsed laser beam having an elongated cross-sectional shape. As a result of detailed observations of the phenomenon of melting, scattering, and exclusion of the molten metal during cutting through the dam bar cutting experiment in No. 1, how was the molten metal cut when cutting the dam bar using a laser beam with an elongated cross-sectional shape? It was found that proper blowing is important for suppressing dross adhesion.

That is, in the present invention configured as described above, by forming a small hole in the center of the dam bar in the first half of each pulse of the laser beam applied to the dam bar, the assist gas can pass through the small hole. , The phenomenon that the flow of the assist gas is dammed by the dam bar (lead frame) and the direction of the flow of the assist gas as in the conventional method, that is, the case where the dam bar is cut by irradiating the pulse laser beam while releasing the assist gas. It is possible to avoid a phenomenon in which is changed in the horizontal direction and in the opposite direction to the resin mold, and it is possible to continuously and reliably blow the assist gas until the cutting of the dam bar at one place is completed. Therefore, the molten metal is sufficiently blown off by the reliable and effective assist gas, and the adhesion of dross is suppressed.

Further, by forming a small hole in the center of the dam bar as described above, when the whole dam bar is cut in the latter half of each pulse of the laser light, the amount of molten metal to be removed is reduced, and the assist gas is used. The amount of dross attached without being blown off is also reduced. As described above, it is possible to obtain a highly accurate and high quality semiconductor device with less dross attached.

In the dam bar cutting method, it is preferable that the first half of the pulse of the laser light is in a mode suitable for processing a small hole, and the second half of the pulse is in a mode suitable for cutting the entire dam bar. Switch the beam mode continuously.

Alternatively, the pulse waveform of the laser light may be changed so that the peak output becomes low in the first half of the pulse of the laser light and becomes high in the second half of the pulse.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Next, a first embodiment of a dam bar cutting method according to the present invention will be described with reference to FIGS.

FIG. 1 is a view for explaining the dam bar cutting method of this embodiment. In the semiconductor device 100, a semiconductor chip (not shown) is mounted on a lead frame, terminals are connected, and then sealed with a resin mold 2, and a dam bar 3 is provided between each lead 1. The dam bar 3 connects the leads 1 and plays a role of preventing the resin mold 2 from flowing out between the leads 1 when the lead frame is sealed with the resin mold 2. The dam bar 3 also has a role of reinforcing each lead 1. Then, when the sealing with the resin mold 2 is completed, the dam bar 3 is cut and removed, and the lead 1 is removed.
(Outer leads) are separated individually.

As the laser light used for cutting the dam bar 3 in this embodiment, a laser light (pulse laser light) whose cross-sectional shape is converted into an elongated elliptical shape by a cylindrical lens (see FIG. 4) and which oscillates in a pulse shape (pulse laser light) is used. Use the laser beam 5 while emitting the assist gas 60 from the gas nozzle 61.
Irradiate 0 to the dam bar 3.

First, in the first half of each pulse of the laser beam 50, as shown in FIGS. 1A and 1B, the laser beam 50 is irradiated to the center of the dam bar 3 to make a small hole 4. At this time, the laser beam 50 has a beam mode (described later) that allows the small holes 4 to be formed. Then, in the latter half of each pulse of the laser light 50, as shown in FIG.
The beam mode of 0 is changed so that the entire dam bar 3 can be covered with the spot 5, and the entire dam bar 3 is cut. After the cutting, the cutting portion 6 is formed. In FIG.
The arrow view from the DD direction of (c) is shown.

Here, a conventional dam bar cutting method using a laser beam having an elongated cross section will be described with reference to FIG. As shown in FIGS. 2A and 2B, even in this cutting method, cutting is performed by irradiating the dam bar 3 with the elongated elliptical pulsed laser light 50 while discharging the assist gas 60 from the gas nozzle 61. The entire dam bar 3 is covered with the spot 5a, and the entire dam bar 3 is cut without changing the beam mode as in the present embodiment.

In this case, before the dam bar 3 is cut and removed, the assist gas 60 is the assist gas flow 60 in the figure.
As shown by a, the surface is dammed by the surface of the dam bar 3 and its direction is changed, and the flow 60b in the horizontal direction and in the opposite direction to the resin mold 2 is induced by the presence of the resin mold 2. The blowing force of the assist gas flows 60a and 60b is lower than that of the original assist gas 60.

FIG. 3 (a) is an enlarged view of the vicinity of the dam bar shown in FIG. 2 (b), and is a view showing a state where the dam bar is being cut from the state of FIG. 2 (b). 3 (b) shows a state in which the cutting of the dam bar is completed. Due to the presence of the assist gas flow 60a, the degree of progress of melting and removal at the surface side edge of the dam bar 3 is accelerated as shown in FIG. 3 (a), and the cross-sectional shape of the dam bar 3 during cutting becomes close to a semicircle. . When the cross-sectional shape of the dam bar 3 becomes a semi-circle, a flow 60c of assist gas is formed along the arc surface thereof, and the molten metal 3A also scatters in the direction of the assist gas 60c, but the blowing force of the assist gas is low. Does not change. Therefore, as shown in FIG. 3B, after the dam bar 3 is cut, a large amount of dross 7a adheres to the edge portion on the back surface side of the protruding portion (uncut portion of the dam bar) 3B desired for the cut portion 6a. Further, due to the presence of the assist gas flow 60b shown in FIG. 2B, the dross 7a attached to the edge portion of the protruding portion 3B farther from the resin mold 2 becomes large and often becomes a mustard loss. In the description of FIGS. 2 and 3, the same members as those in FIG. 1 are designated by the same reference numerals.

On the other hand, in the present embodiment, as shown in FIG. 1A, a small hole 4 is formed in the center of the dam bar 3 in the first half of each pulse of the laser beam 50, so that the structure shown in FIG. As described above, even when the dam bar 3 is not completely cut, part of the assist gas 60 can pass through the hole 4, and the assist gas 60 is sprayed almost straight on the dam bar 3. In this case, as in the conventional case shown in FIG. 2, the flow of the assist gas is blocked by the surface of the dam bar 3 and its direction is changed, or the flow is in the horizontal direction and in the direction opposite to the resin mold 2. No flow is induced. Therefore, the molten metal is sufficiently blown off by the reliable and effective assist gas 60, the adhesion of the dross 7 is suppressed, and even if the dross 7 adheres, the amount thereof can be significantly reduced.

Further, by forming a small hole 4 in the center of the dam bar 3, the amount of molten metal to be removed is reduced when cutting the entire dam bar 3 in the latter half of each pulse of the laser light 50, and the assist gas 60 is used. Therefore, the amount of dross 7 that is unavoidably attached and cannot be blown off is reduced.

Next, a method of generating laser light will be described with reference to FIG. In FIG. 4 showing an apparatus for generating a laser beam and irradiating the dam bar, a laser beam 11 (having a substantially circular cross section) emitted from a laser oscillator 10 has an elongated cross section by a convex cylindrical lens 12 and a concave cylindrical lens 13. The laser light 50 is converted into an elliptical shape, is turned by the bending mirror 14 toward the semiconductor device 100, and is condensed by the condenser lens 15.
Is irradiated to the dam bar 3. As a means for converting the laser light 11 into an elongated cross-sectional shape, a method other than the method using the cylindrical lens as described above can be used.

FIG. 5 shows an example of the beam mode in the cross section of the laser beam 11 emitted from the laser oscillator 10. However, FIG. 5 shows the beam mode in the VV cross section of FIG. 4, P shows the power density, and D shows the coordinates of the cross section direction of the laser light. Further, the beam mode in the cross section of the laser light 50 also conforms to this. In the first half of each pulse of the laser light 50, as shown in FIG. 5A, the single mode is suitable for making a small hole 4 in the center of the dam bar 3 by the laser light 50. Further, in the latter half of each pulse of the laser light 50, as shown in FIG. 5B, a multi-mode suitable for cutting the entire dam bar 3 is set. Then, as described above, the laser light used in the present embodiment can be supplied by continuously switching between the single mode and the multimode in the first half and the second half of each pulse of the laser light 50.

The beam mode switching as described above is performed by
For example, it can be realized by switching the mirrors in the laser oscillator to those having different curvatures. Further, the pulse width of the laser light is approximately 0.1 to 0.5 msec.
Since the frequency and the frequency are about 100 Hz, the switching of the beam mode is actually performed instantaneously.

According to this embodiment as described above, a small hole 4 is formed in the center of the dam bar 3 in the first half of each pulse of the laser beam 50, so that the flow of assist gas is blocked at the surface of the dam bar 3 as in the conventional case. Therefore, the molten metal can be blown off reliably and effectively by blowing the assist gas 60 without changing the direction of rotation or inducing a flow in the direction opposite to the resin mold 2 in the horizontal direction. Therefore, the adhesion of the dross 7 is suppressed, and the amount thereof can be significantly reduced.

Since a small hole 4 is made in the center of the dam bar 3 in the first half of each pulse, the amount of molten metal to be removed is reduced when cutting the entire dam bar 3 in the latter half of the pulse, and the dross 7 is removed. The adhered amount of is reduced.

Next, a second embodiment of the dam bar cutting method according to the present invention will be described with reference to FIGS.

In this embodiment, the peak output is lowered in the first half of each pulse of the laser light, and the peak output is raised in the latter half of each pulse. FIG. 6 is a diagram showing an apparatus for generating a laser beam and irradiating the dam bar. However, for simplicity, FIG.
4, the same members as those in FIG. 4 are designated by the same reference numerals,
The path from the generation of the laser beam 11 to the irradiation of the laser beam 50 on the dam bar is almost the same as in FIG. In FIG.
The laser light 21 emitted from the laser oscillator 20 (having a substantially circular cross-sectional shape) has a divergence angle that is almost parallel to a predetermined laser output. By increasing this laser output, the divergence angle of the laser light becomes large and becomes the laser light 22. This phenomenon is a well known phenomenon.

FIG. 7 shows an example of the laser output waveform in the cross section of the laser beams 21 and 22 emitted from the laser oscillator 20. However, FIG. 7 shows a laser output waveform in the VII-VII cross section of FIG. 6, where W is the laser output, τ is the pulse width, and f is the laser repetition frequency. As shown in FIG. 7, the laser output is reduced in the first half of each pulse of the laser light (waveform A in the figure) to generate the laser light 21 having a divergence angle that is almost parallel, and in the latter half of each pulse, the laser light 21 is generated. The output is reduced (waveform B in the figure) to generate the laser beam 22 having a large divergence angle. As a result, the spot becomes smaller in the first half of each pulse of the laser light and a small hole 4 can be formed in the center of the dam bar 3, and the spot becomes larger in the latter half of each pulse of the laser light to cut the entire dam bar 3. You can As a result, the laser light used in this embodiment can be supplied.

The change in the laser output as described above can be realized, for example, by switching the electric power applied to the excitation lamp in the laser oscillator.

As described above, by lowering the peak output in the first half of each pulse of the laser light and increasing the peak output in the latter half of the pulse, the same effect as in the first embodiment can be obtained.

[0039]

According to the present invention, since a small hole is formed in the center of the dam bar in the first half of each pulse of the laser light, the assist gas can be reliably and effectively blown until the dam bar is completely cut. Adhesion of dross can be suppressed.

Further, since a small hole is formed in the center of the dam bar, the amount of molten metal to be removed when cutting the entire dam bar in the latter half of each pulse of laser light is reduced, and the amount of dross adhered can be reduced. .

Therefore, according to the present invention, it is possible to obtain a highly accurate and high quality semiconductor device with less dross attached.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a dambar cutting method according to a first embodiment of the present invention, in which (a) is a diagram showing the dambar periphery in the first half of each pulse of laser light with which the dambar is irradiated, and (b) is FIG. 6A is a cross-sectional view taken along line BB of FIG. 7A and showing a laser light irradiation state, FIG. 7C is a view showing the dam bar and its periphery in the latter half of each pulse of the laser light with which the dam bar is irradiated, and FIG. It is a sectional view of the D-D direction of c), and is a figure showing the state after irradiation of laser light is completed.

2A and 2B are views for explaining a conventional dam bar cutting method, in which FIG. 2A is a view showing the dam bar and its periphery, and FIG. 2B is a sectional view taken along line BB of FIG. 2A.

FIG. 3A is an enlarged cross-sectional view of the vicinity of the dam bar during cutting of the dam bar, and FIG. 3B is a cross-sectional view showing a state in which cutting of the dam bar is completed.

FIG. 4 is a diagram showing an apparatus for generating a laser beam and irradiating the dam bar.

5A and 5B are diagrams showing an example of beam modes in a laser beam cross section, wherein FIG. 5A shows a single mode in the first half of each pulse of laser light, and FIG. 5B shows a multimode in the second half of each pulse of laser light. .

FIG. 6 is a view for explaining the dam bar cutting method according to the second embodiment of the present invention, and is a view showing an apparatus for generating a laser beam and irradiating the dam bar.

FIG. 7 is a diagram showing an example of a laser output waveform in a cross section of laser light.

[Explanation of symbols]

 1 Lead 2 Resin Mold 3 Dam Bar 4 (Small) Hole 5 Spot 6 Cutting Section 7 Dross 10 Laser Oscillator 11 Laser Light 12 Convex Cylindrical Lens 13 Concave Cylindrical Lens 14 Bending Mirror 15 Condensing Lens 20 Laser Oscillator 21, 22 Laser Light 50 Laser light 60 Assist gas 61 Gas nozzle 100 Semiconductor device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiya Nagano 650 Kazunachi-cho, Tsuchiura-shi, Ibaraki Hitachi Construction Machinery Co., Ltd.Tsuchiura factory Inside the Tsuchiura Factory (72) Inventor Shigeyuki Sakurai 650 Kintatemachi, Tsuchiura City, Ibaraki Hitachi Construction Machinery Co., Ltd. Inside the Tsuchiura Factory (72) Inventor, Yasushi Minomoto 650 Kintatecho, Tsuchiura City, Ibaraki Hitachi Construction Machinery Engineering Co., Ltd. Ring Co., Ltd.

Claims (3)

[Claims] 1. A dam bar in a semiconductor device in which a semiconductor chip is mounted on a lead frame and integrally sealed with a resin mold, while discharging assist gas from a gas nozzle,
In a dam bar cutting method of irradiating and cutting a pulsed laser beam having an elongated cross-sectional shape, a small hole is formed in the center of the dam bar in the first half of each pulse of the laser light with which the dam bar is irradiated, and in the latter half of each pulse. A method for cutting a dam bar, which comprises cutting the entire dam bar. 2. The dambar cutting method according to claim 1, wherein a mode suitable for machining a small hole is provided in a first half of each pulse of the laser light, and a mode suitable for cutting the entire dambar is provided in a latter half of each pulse. As described above, the dam bar cutting method is characterized in that the beam mode of the laser light is continuously switched. 3. The dambar cutting method according to claim 1, wherein the pulse output of the laser light is such that the peak output is low in the first half of each pulse of the laser light and high in the second half of each pulse. A dambar cutting method characterized by changing a waveform.