JP2003224162A - Ultrasonic flip chip mounting method and its device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce damage to an IC chip and a circuit board caused by ultrasonic vibration. <P>SOLUTION: In this ultrasonic flip chip mounting method, a sharp-pointed bump formed on an electrode of an IC chip is positioned at an electrode of the circuit board for ultrasonic bonding. A pressurizing force of the bump for the electrode of the circuit board is increased over time, meanwhile the amplitude of ultrasonic vibration applied to a joint part is decreased in proportion to the increase of the pressurizing force. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION When an IC chip is mounted on a circuit board as a single body (bare chip), the bumps formed on the electrodes of the IC chip are pressed against the electrodes of the circuit board while ultrasonic vibration is applied to bond them. The present invention relates to an ultrasonic flip chip mounting method and device.

[0002]

2. Description of the Related Art In recent years, in mobile information communication related products and the like, there is a strong demand for higher performance by further miniaturization, weight reduction, and higher frequency of circuit mounting boards and cost reduction. Therefore, flip-chip mounting, which enables direct mounting of the IC and the circuit board, is effective. Among them, metal diffusion bonding using ultrasonic vibration has characteristics such as low connection resistance, high bonding strength and short-time bonding, and is attracting more and more attention.

A conventional ultrasonic flip chip mounting method will be described with reference to FIGS. 4, 5 and 6. FIG. 4 shows a method for forming bumps using a wire bonding method for electrodes of an IC chip. First, in FIG. 4A, a ball 203 is formed on the tip of the gold wire 202 held by the capillary 201 by an electric discharge action, and the capillary 201 is moved in the direction of arrow A to move the ball 203 to the electrode of the IC chip 204. 205 is pressed and joined.

For this joining, there is a method of thermocompression bonding or a method of applying ultrasonic vibration thereto. Further, as shown in FIG. 4B, the gold wire 202 is torn off by moving the capillary 201 together with the gold wire 202 in the direction of arrow A, and bumps 206 are formed on the electrodes 205. The bump 206 thus formed has a rivet-like shape with a sharp tip 206A.

FIG. 5 shows a main part of a conventional general ultrasonic flip chip mounting apparatus. FIG. 6 is a diagram for explaining the joining process. First, the IC chip 204 on which the bumps 206 are formed by the method described above is sucked and held on the tip 101 of the bonding tool by the negative pressure generated by using the air flow path 10 with the surface on which the bumps 206 are formed facing down.

Further, a circuit board 104 is mounted on the stage 103.
And the bump 206 formed on the IC chip 204
After aligning the electrode 105 on the circuit board 104 with the electrode 105, the joining tool 101 is lowered in the direction of arrow D by the pressing means 1. As a result, the tip 206A of the bump 206 contacts the electrode 105, and the tip 206A is slightly crushed.
Here, the ultrasonic oscillator 106 transfers electric energy to the vibrator 1
07, and the vibrator 107 converts the electric energy into mechanical ultrasonic vibration.

Further, the ultrasonic vibration is generated by the ultrasonic horn 10.
8 transmits as a longitudinal wave in the direction of arrow c, and a predetermined ultrasonic vibration is applied to the welding tool 101 connected or formed thereto. In this way, the circuit board 104 is attached to the joint.
The ultrasonic vibration is applied in a direction parallel to the surface of the circuit board, and at the same time, a pressing force is applied in the direction of arrow D to press the surface of the circuit board 104 perpendicularly. Further, by heating this bonding portion to a predetermined temperature, the bump 206 is formed in the bumps 206 shown in FIGS.
While being deformed as described above, the electrode 105 is joined.

[0008]

At the initial stage of joining, that is, the stage of FIG. 6A, the circuit board 10 is to be solved.
Even if ultrasonic vibration in the direction parallel to the surface of 4 is applied,
Excessive amplitude component of ultrasonic vibration is absorbed by the bending deformation of the thin protruding portion of the bump 206.

However, after that, the bonding progresses while increasing the pressing force in the direction of arrow D, and the shape of the bump 206 becomes as shown in FIG.
When the bump 206 is in the crushed state, the area of the horizontal cross section of the bump 206 at any height becomes large, and the above-described flexural deformation of the bump 206 almost does not occur.

At this time, the ultrasonic vibration causes the bonding surface between the bump 206 and the electrode 105 of the circuit board 104, the bonding surface between the bump 206 and the electrode 205 of the IC chip 204, and further the bonding between the main body of the circuit board 104 and the electrode 105. Surface and IC
A shearing force is applied to the joint surface between the body of the chip 204 and the electrode 205.

In addition to the shearing force, a force that the axial center of the bump 206 inclines in the height direction gives a moment to each of the bonding surfaces at the four bonding surfaces. Then, the peeling force and the pressing force are repeatedly applied.

Therefore, when the pressure applied in the direction of arrow D increases and the bump 206 is crushed, the excessive amplitude component of the ultrasonic vibration becomes a risk factor that damages both electrode parts of the IC chip 204 and the circuit board 104. In addition, the bonding surface of the bump 206 and the electrode 105, which has advanced with great care, and the bonding surface of the bump 206 and the electrode 205, which have been bonded in advance, are also adversely affected.

In the prior art, the adverse effect of the above-mentioned excessive amplitude component of the ultrasonic vibration is that of the welding tool 101 shown in FIG.
There is no choice but to avoid it due to the sliding action of the suction surface and the suction surface which is the upper surface of the IC chip 204. A frictional force is generated between the attracted surface and the attracted surface due to the pressing force in the direction of arrow D. Since this frictional force depends on the friction coefficient of both the attracted surface and the attracted surface and the applied pressure, the work IC
It was difficult to control according to the number of bumps on the chip 204 or the shape and size of the bumps 206.

Further, since the friction coefficient changes due to wear due to repeated joining, it is difficult to keep the friction force constant for a long period of time, and further, the bump is crushed as the pressing force in the arrow D direction increases. , While the proportion of the action of absorbing the excessive amplitude component of the ultrasonic vibrations that depends on the slip of the suction surface increases, the frictional force of the suction surface increases with the increase of the pressing force and becomes difficult to slip. Both show relative changes in the opposite direction to the ideal over the course of time as the joining progresses.

In other words, the phenomenon of ultrasonic bonding generally found in the past, in which the amount of slippage of the suction surface does not decrease even if the applied pressure increases, is a phenomenon that the suction surface does not contact the suction surface despite the increase in the friction force. The IC chip 204, the electrode portion of the circuit board 104, and the bumps 2 can be moved by a force that can cause slippage.
This means that a mechanical load (damage) is given to the joint surface between 06 and the electrode.

The present invention has been made in order to solve the above-mentioned problems, and includes the electrodes 205 of the IC chip 204 and the electrodes 105 of the circuit board 104, as well as the bumps 206 and both electrodes 20.
It is an object of the present invention to provide a highly reliable ultrasonic flip chip mounting method and apparatus by realizing a bonding with a low defect rate by reducing the damage given to the bonding surface with 5, 105.

[0017]

SUMMARY OF THE INVENTION The present invention is an ultrasonic flip chip mounting method in which bumps having sharp tips formed on electrodes of an IC chip are aligned with electrodes of a circuit board and ultrasonically bonded. As a first aspect, the pressing force of the bump with respect to the electrode of the circuit board is increased with the passage of time, while the amplitude of ultrasonic vibration applied to the joint is decreased in accordance with the increase of the pressing force. An ultrasonic flip chip mounting method is provided.

As a second aspect, the ultrasonic flip-chip mounting method according to the first aspect is provided, in which the amplitude of the ultrasonic vibration applied to the bonded portion is controlled based on the applied pressure.

In a third aspect, the pressure applied to the bumps is controlled based on the passage of time, while the amplitude of ultrasonic vibration applied to the joint is determined based on the passage of the time associated with the pressure applied. An ultrasonic flip-chip mounting method according to the first aspect is provided.

As a fourth aspect, the amplitude of the ultrasonic vibration applied to the joint is controlled based on the deformation amount of the bump which changes with the increase of the pressing force. An ultrasonic flip chip mounting method as described is provided.

As a fifth aspect, any one of the first to fourth aspects is characterized in that the amplitude of ultrasonic vibration applied to the joint is reduced substantially continuously in accordance with the increase in the pressing force. And an ultrasonic flip-chip mounting method described above.

As a sixth aspect, the ultrasonic flip-chip mounting method according to any one of the first to fifth aspects, in which the pressure applied to the bump on the electrode of the circuit board increases substantially continuously over time. I will provide a.

As a seventh aspect, the bump is a ball formed by a discharge action at the tip of the bonding wire.
The ultrasonic flip-chip mounting method according to any one of the first to sixth aspects, which is formed by tearing a wire after joining the electrode of the C chip by applying pressure or applying ultrasonic vibration to the pressure. provide.

As an eighth aspect, there is provided an ultrasonic flip chip mounting device for aligning ultrasonically bonded bumps formed on electrodes of an IC chip and having sharp tips to electrodes of a circuit board. A stage for mounting and holding the IC chip, a bonding tool for aligning and holding the IC chip with the bump on the electrode of the circuit board, and a pressing force applied to the bump on the electrode of the circuit board via the bonding tool. Pressurizing means for generating, ultrasonic horn for transmitting ultrasonic vibration of the vibrator to the welding tool, ultrasonic oscillator for driving the vibrator, and pressure applied by the pressing means increases with time. On the other hand, a controller for controlling the ultrasonic oscillator so as to decrease the amplitude of the ultrasonic vibration in accordance with the increase of the pressing force is provided. To provide a flop mounting apparatus.

[0025]

1 is a perspective view of an ultrasonic flip-chip mounting apparatus showing an embodiment of the present invention. In FIG. 1, the circuit board 104 is placed on the stage 103 and held by a holding means (not shown) so as not to be displaced. Further, the IC chip 204 is suction-held on the suction surface of the welding tool 101 provided on the ultrasonic horn 108 by the negative pressure generated by using the air flow path 10.

Electrodes 105 on the circuit board 104 and bumps 206 (not shown) formed on the lower surface of the IC chip 204.
After aligning, the controller 2 issues a command to the pressurizing means 1 and the IC chip 204 descends as the welding tool 101 descends.

Eventually, the tip of the bump 206 comes into contact with the electrode 105, and then the pressurizing means 1 further increases the pressing force and a command is issued from the control section 2 to the oscillator 106, which causes the oscillator 106 to generate electric energy of ultrasonic vibration. Oscillator 107
Output to.

The vibrator 107 converts the input electric energy into mechanical ultrasonic vibration and outputs it to the ultrasonic horn 108. The ultrasonic horn 108 transmits this ultrasonic vibration to the bonding tool 101, and pressure is applied to the bump 206 and the electrode 105 in the arrow D direction and ultrasonic vibration parallel to the surface of the circuit board 104 is applied.

FIG. 2 shows the relationship between the applied pressure, the amplitude of ultrasonic vibration, and time at this time. First, at the time point Ta when the tip of the bump 206 contacts the surface of the electrode 105, the pressure Pa per bump is zero. After that, the tip of the sharp bump is increased by a predetermined amount (about 20
μm) It collapses. As shown by the pressure characteristic Q, this time T
In the state of b pressure Pb, all the bumps come into contact with the electrodes 105 and the tips are slightly crushed, and the preparation for starting ultrasonic bonding is completed. This Pb is the first set load.

After that, the pressing force is further increased substantially continuously from the first set load Pb, but in this embodiment, there is a time zone TR in which the pressing force does not increase or decrease. This time T
R is a time zone provided for switching the pressure control system to the high-speed pressure control system by using the pressure Pb as a trigger,
It is not necessary if the control system is not switched from the initial contact between the bump 206 and the electrode 105 to the end of bonding.

After the pressure characteristic Q starts to rise from the first set load Pb toward the second set load Pc, application of ultrasonic vibration is started at time Tc. The reason why the amplitude at this time is the maximum set amplitude Sc is based on the purpose of heating the bonding surface of the bump 206 in a short time by using frictional heat generated by ultrasonic vibration of the bonding surface. At this time, even if there is an excessive amplitude component, it is absorbed by the bending deformation of the thin projecting portion at the tip of the bump, so that there is no fear of serious damage to both electrode parts of the IC chip 204 and the circuit board 104.

Further, in the present embodiment, the amplitude of the ultrasonic vibration has a binary rise at time Tc, but this may be such a rise that the amplitude continuously increases as shown by the amplitude characteristic V. However, although not shown, the amplitude Sc may be set to rise gradually.

Thereafter, the bonding progresses and the bump is deformed and crushed as the applied pressure (characteristic Q) increases, but at the time Td when the second set load Pc is approached, the ultrasonic vibration is changed to the amplitude Sb like the amplitude characteristic A. Switch. Unlike the conventional amplitude characteristic W, the maximum set amplitude Sc is not maintained until the joining is completed, but the amplitude is reduced to suppress the generation of an excessive amplitude component. (Claim 1)

Here, in addition to the method of gradually decreasing the amplitude in two steps or three or more steps like the amplitude characteristic A, the method of decreasing the amplitude at a constant change rate like the amplitude characteristic B and the amplitude characteristic C. It is also effective to use a method of decreasing along the function curve such as a quadratic curve and a method of continuously decreasing the amplitude. (Claim 5)

As described above, the following method can be used as a method of decreasing the amplitude of ultrasonic vibrations in response to an increase in pressure. First, as shown in FIG. 1, a pressure sensor 3 such as a load cell is provided below the pressurizing means, and the applied pressure is fed back to the control unit 2 to control the amplitude of ultrasonic vibration based on the applied pressure. I do. (Claim 2)

Secondly, as shown in FIG. 2, the control of the pressing force is performed based on the time elapsed in the joining process, and the amplitude control of the ultrasonic vibration is performed based on the time associated with this pressing force. . (Claim 3)

Thirdly, as shown in FIG. 1, the collapse amount, which is the most important factor of the deformation amount of the bump, is determined by the position (height) information obtained from the encoder 4 in the pressurizing means 1 which is a motor. The amplitude is fed back to the control unit 2 and the amplitude of ultrasonic vibration is controlled based on this amount of collapse. FIG. 3 shows the relationship between the height characteristic H and the amplitude control of ultrasonic vibration in this case. (Claim 4)

[0038]

According to the present invention, the amplitude of ultrasonic vibration is reduced in accordance with the increase of the pressing force, so that the bumps crushed due to the progress of bonding cannot absorb the excessive amplitude component of ultrasonic vibration. It is possible to prevent damage to the electrode parts of the IC chip 204 and the circuit board 104, improve the yield in the joining process, and realize highly reliable flip chip mounting. (Claim 1)

Further, not only the tack-shaped bumps described in the detailed description of the invention, but also the bumps having a plurality of sharp tips as seen by microscopic eyes formed by any manufacturing method, the same action occurs, and flip chip mounting is performed. It is effective in improving the quality of. (Claim 1)

Here, by controlling the amplitude of the ultrasonic vibration based on the pressing force information, even if the change in the pressing force fluctuates in time due to some factor, the ultrasonic wave that is optimal for the pressing force at that time is The amplitude of vibration is obtained. (Claim 2)

Further, the control of the pressing force is performed based on the passage of time, and the amplitude control of the ultrasonic vibration is performed based on the time associated with the pressing force, whereby the time management of the bonding operation can be easily performed. Furthermore, the controls are simply unified to facilitate control of the entire control system. (Claim 3)

Further, by controlling the amplitude of ultrasonic vibration based on the amount of deformation of the bump, information closest to the purpose of control can be fed back, and control that is not affected by differences in the material of the bump or variations in physical properties. Can be realized. (Claim 4)

Further, by decreasing the amplitude of the ultrasonic vibration almost continuously in accordance with the increase of the pressing force, the influence of the amplitude fluctuation on the diffusion bonding itself can be appropriately controlled rather than decreasing it stepwise. (Claim 5)

[Brief description of drawings]

FIG. 1 is a perspective view of an ultrasonic flip-chip mounting apparatus showing an embodiment of the present invention.

FIG. 2 is a diagram showing an example of pressure control and ultrasonic vibration amplitude control.

FIG. 3 is a diagram showing an example of bump height information and amplitude control of ultrasonic vibration.

FIG. 4 is a sectional view showing a bump forming method.

FIG. 5 is a diagram showing a main part of a conventional ultrasonic flip chip mounting device.

FIG. 6 is a diagram illustrating a conventional joining process.

[Explanation of symbols]

1 Pressurizing means 2 control unit 3 Pressure sensor 4 encoder 10 air flow paths 101 welding tool 103 stage 104 circuit board 105 electrode 106 ultrasonic oscillator 107 oscillator 108 Ultrasonic horn 204 IC chip 205 electrode 206 bump

Claims (8)

[Claims] 1. An ultrasonic flip-chip mounting method in which bumps having sharp tips formed on electrodes of an IC chip are aligned with the electrodes of a circuit board and ultrasonically bonded to the electrodes of the circuit board. An ultrasonic flip-chip mounting method characterized in that the pressure applied to the bumps is increased with the passage of time, while the amplitude of ultrasonic vibration applied to the joint is decreased in accordance with the increase in the pressure applied. 2. The amplitude of ultrasonic vibration applied to the joint portion is controlled based on the applied pressure.
Ultrasonic flip chip mounting method. 3. The pressure applied to the bumps is controlled based on the passage of time, while the amplitude of ultrasonic vibration applied to the joint is controlled based on the passage of the time associated with the applied pressure. The ultrasonic flip-chip mounting method according to claim 1, wherein 4. The ultrasonic flip-chip mounting according to claim 1, wherein the amplitude of ultrasonic vibration applied to the bonded portion is controlled based on the deformation amount of the bump which changes with the increase of the pressing force. Method. 5. The ultrasonic flip according to any one of claims 1 to 4, wherein the amplitude of ultrasonic vibration applied to the joint is reduced substantially continuously in accordance with the increase in the pressing force. Chip mounting method. 6. The ultrasonic flip-chip mounting method according to claim 1, wherein the pressure applied to the bump on the electrode of the circuit board increases substantially continuously over time. 7. The bump is bonded to a ball formed by a discharge action at the tip of a bonding wire by pressure or ultrasonic vibration applied to the electrode of the IC chip, and then bonded.
The ultrasonic flip-chip mounting method according to claim 1, wherein the ultrasonic flip-chip mounting method is formed by tearing a wire. 8. An ultrasonic flip-chip mounting device for ultrasonically bonding a bump having a sharp tip formed on an electrode of an IC chip to an electrode of a circuit board by positioning the bump on the circuit board. A stage for holding, a bonding tool for holding the IC chip with its bump aligned with the electrode of the circuit board, and a pressurizing force for generating a pressing force to the electrode of the circuit board on the bump via the bonding tool. Means, an ultrasonic horn for transmitting ultrasonic vibration of a vibrator to the welding tool, an ultrasonic oscillator for driving the vibrator, and a pressure applied by the pressurizing means that increases with the passage of time. An ultrasonic flip-chip mounting device, comprising: a controller that controls the ultrasonic oscillator so as to reduce the amplitude of the sonic vibration according to the increase in the pressing force.