AU458552B2 - Improvements in or relating to bonding - Google Patents

Description

23.862 no This invention relates to methods of and apparatus for bonding workpieces together.

In apparatus used for bonding electronic devices, such as integrated-circuit chips to substrates,

one of the principal problems is aligning the integrated- circuit chipsio -bonding tips, er apertures in complazt- banding meabpsr prior to engaging the chips with the tips 9P spep'vp and aligning the chips with a pattern on a substrate. Various optical systems have been used to facilitate such alignment, but these suffer from

various limitations and are thus not entirely satisfactory.

One of the most common systems employs a half-silvered mirror suspended between a bonding tip

and a plane on which either chips or a substrate lie.

An optical axis of a microscope is obliquely directed

to the half-silvered mirror and an image of both the

tip and a chip or substrate on the plane over the mirror

appear simultaneously in the microscope eyepiece.

However, the oblique viewing causes perspective distortions which make accurate alignment difficult. Additionally, because the mirror is suspended between.

the tip and the workpiece with which the tip must be

engaged, the mirror must be mounted on a swing-away

mechanism which adds complexity to the bonding apparatus.

Problems associated with half-silvered mirrors

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vvay be reduced to a certain extent by the use of beam- splitter prism combinations which are suspended similarly to the mirror but which direct an image of both a tip and a work piece perpendicularly to a bonding axis on which it is suspended These combinations however require very accurate alignment to the bonding axis and thus supporting mechanism, in addition to being capable of swinging away from the bonding axis, must have adjustments provided thereon to achieve precise planar alignment of the prism. Bonding axis" is defined herein as an axis passing through the bonding tip and along;which relative moi ment of the bonding tip towards a workpiece is caused to occur in order to effect bonding.

Systems employing reference reticles have also been used to accomplish thedesired alignments. Such systems are described in United States Patent No. 3,477,630.

These systems, while performing admirably when properly adjusted, do suffer from the limitation that accurate alignment of a mechanism which positions a movable head requires very precise ad justment with respect to the reference recticle in order to achie the necessary degree of accuracy.

Even more complex systems have been utilised employing rapidly oscillated mirrors to project images of work- pieces and bonding tips simultaneously to an operator while an operator tries to achieve alignment therebetween. Obviously, systems that use oscillating mirrors suffer from problems associat with their inherent

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complexity.

Except for the reference reticle system, the above described systems have the common shortcoming

that some optical element of the apparatus must be

temporarily suspended between the bonding tip and the

workpiece. In order to provide space for these elements, the bonding tip must travel extensive distances in

accomplishing either engagement with the workpiece for

pickup purposes or for bonding purposes. Extensive travel of the bonding tip results in difficulty in

providing precise positioning of the bonding tip

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23.862 no According to one aspect of this invention apparatus

for bonding first and second workpieces together includes to means for aligning said workpieces in desired relation to each other, a bonding head for applying bonding energy

said workpieces to bond them together, said bonding head

including a transparent bonding tip, and said apparatus further workpieces including means for inspecting at least one of said

via said transparent tip along a bonding axis (as hereinbefore

defined) to inspect alignment of said one workpiece. According to another aspect of this invention there utilis- is provided a method of bonding two workpieces together a trans- ing bonding apparatus including a bonding head having work- parent bonding tip for applying bonding energy to said

pieces to bond them together, the method including aligning

said workpieces in desired relation to each other, inspecting adapt- alignment of at least one of said workpieces with means axis ed to inspect it via said transparent tip along a bonding to said (as hereinbefore defined), and applying bonding energy

workpieces to bond them together. The bonding energy may be applied to said workpieces

via a transparent compliant bonding medium. A compliant bondin.g mediiM is a medium which serves

to distribute bonding energy more uniformly to the workpieces. of our Reference is directed to the description and claims U.K. Patent No. 1239631 which relates to the bonding of work-

pieces using a compliant..onding medium. preferably said one workpiece is held adjacent said

tip portion after alignment by a reduction in air pressure

transmitted through the tip portion to said workpiece.

The invention will now be described by way of 458.552

23 8 62 example with reference to the accompanying drawings,in which:

FIG. 1 is a perspective view of an apparatus in accordance with the invention;

FIG. 2 is a partially sectioned elevation view on a larger scale of the bonding head of the apparatus of FIG. 1;

FIGS. 3 to 6 are views showing the various stages of alignment of first and second workpieces;

FIG. 7 is a partially sectioned elevation view of a bonding head in another apparatus in accordance with the

invention; and

FIGS. 8 to 11 are views showing the various stages of alignment of first and second workpieces. The invention will be described in connection with the bonding of beam-lead integrated-circuit chips to conduct-

ive elements on ceramic substrates. However, it is to be understood that the invention will find application in the

bonding of many types of small articles where accurate

alignment

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between parts is important. Referring now to FIG. 1, a bonding machine includes a movable head assembly 22, a base 24, a

conventional positioner assembly 26, and an electronic

optical system including a television camera 27 and a

monitor 28. Even though the machine 20 is illustrated with an electronic optical system, a conventional

microscope could be used with equal effectiveness.

Operation of the machine 20 can best be understood by a description of one complete bonding

cycle. A beam-lead integrated-circuit 30 is illustrated

on a screen 32 of the monitor 28. The image on the screen illustrates leads 36 of the chip 30 being well

aligned with conductive elements 38 w..hich are patterns

-eneated on a ceramic substrate

Te monitor 28 shows an image of chip engaed with a bondingaperture 42 -of/a compliant member 44. The compliant member 44 is transparent and for this

reason the substrate 40 below the ccnrliart member is

visible through the eptiea system-2.

One example of a material suitable for use as

the compliant member 44 is polyimide film and one example of such a film is marketed by E. I. DuPont-DeNemours and

Company of Wilmington,- Delaware under the/traleneme

"Kapton". By way of example, a film thickness of 0.005

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inch has been found to be suitable for chips which are approximately 0.002 inch thick. For increased visibility, the compliant member 44 may be coloured. Sensing the colour is done readily with a microscope but an electronic optical system must have colour sensing capability to take advantage of the additional visibility which colour imparts to the compliant member 44. The chip 30 is held in engagement with the 4 compliant ,member y vacuum developed within a bonding head The inner structuire of the head 46 is more clearly illustrated in FIG. 2. A vacuum chamber 48 is formed in the 'pff 46 and is connected to a vacuum source 49 through a port 50 and tubing 52 (see FIG. 2 In order that the -eei system -e/an receive images of the chip 340, the substrate 40 and the compliant member 44, -he vacuum chamber 48 is constructed so that the walls through which an optic] axis 54 passes are transparent. It will be noted that the optical axis 54 is also the bonding axis of the machine 20. The uppermost wall is a clear glass or quartz disc 56. Directly below the disc 56 is a clear glass or quartz engagement portion or bonding tip 58. The tip 58 is provided with an aperture or -vacuum

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23.862 port 60 through which the force of vacuum within the chamber 48 can be transmitted to the chip The chip 30 is shown in "engagement" with the tip 58 under the influence of the forces of the vacuum source 49. Although the chip 30 is being held compressively against the tip 58, there is no actual contact therewith because the thickness of the compliant member 44 is greater than the thickness of a body portion 61 of the chip The tip 58 is advantageously provided with a projection 62 to localise bonding forces to a small region of 40. This localisation is often desirable when a number of the chips 30 are bonded on-e&e- thelsubstrates 40. The projection 62 will fit into a small space on the substrate 40 during bonding and the likelihood is reduced of disturbing another one of the chips which has previously been bonded nearby on the substrate. Conventional cartridge heaters 63 are also provided in the head 44 in order to provide for heating of the chip 30 priorc to and during bonding. Bonding is accomplished by lowering the movable head as!sembly 22 along conventional slide assemblies 64. After the assembly 22 is lowered, a contvntinne-4 4mpliant, bond is produced between the-

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description of the use of polyimide as a compliant bonding member is incladed in the. specification of copending British application No. 47,394'70kSerial No. 1315510).

A substantial amount of heat is required to produce an effective thermocompression bond and it is found desirable to supplement the heating produced by the heater 63 in the head 46 by heating the substrate This is best accomplished by putting conventional heaters 65 in a substrate support 66. After the bond is complete, the head assembly 22 is raised again to the position shown in FIG. 1. The positioner assembly is moved to the right along conventional slide members 68 until a chip tray 70 is positioned oncLe optical axis 54. The compliant member

44 is also indexed so that a subsequent one of the bonding apertures 42 is aligned with the axis 54. It can be seen from FIG. I that the compliant member 44 is a continuous strip which is wound from a supply reel 72 to a takeup reel 74. The takeup reel 74. has torque continuously applied to it by a conventional torque motor 76. Control of the movement of the takeup reel 74 is provided through an indexing sprocket 78. The indexing sprocket 78 is moved in accurately

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controlled 450 increments. Movement of the member 44 is initiated when a cylinder-actuated pin 80 is

withdrawn from one of eight holes 82 in a plate 84.

The pin 80 is controlled by a spring-biased cylinder

86 and the cylinder acts only momentarily to pull the pin out of the hole 82 with which it is engaged. As soon as the pin 80 is free from the hole 82, the plate

84, which is on the same shaft as the sprocket 78,

begins to rotate because of the force exerted on the

sprocket by the member 44. After 450 of rotation the pin 80 snaps into the next one of the holes 82 and

motion of the compliant member 44 stops. Spacing between bonding aperture 42 is such that 450 rotation of the

sprocket 78 results in accurate location of the subsequent

one of the apertures 42 on the optical axis 54.

After a new one of the bonding apertures is

aligned with the optical axis 54, an image, such as that

shown in FIG. 3, appears on the screen 32 of the monitor

28. An operator can see the aperture 42 and the projection

62 of the tip 58 as well as any one of the chips

which is within the field of the camera 27. The chips are not necessarily in an ordered array on the chip

tray 70 and because of this, it is quite likely that

the chip 30 may not be aligned with the aperture 42.

To achieve alignment, an operator moves a

458.55 2 handle 88 of the position-er assembly 26 and rotates the chip tray 70 while viewing the screen 32 until the image shown or, FIG. 4 is achieved. FIG. 4, of course, illustrates the chip 30 being properly aligned with the aperture 42. After the image of FIG. 4 is achieved, the head assembly is lowered and the force of vacuum within the chamber 48 brings the chip 30 into engagement with the head 46 by being drawn against the compliant member 44. After this engagement, the head assembly 22 is raised to the position shown in FIG. 1. The positioner assembly 26 is then moved to the left so that'ene-e-th tbs9a 4-0 is roughly aligned wi-th the optical axis 54. At this time, the operator sees an image similar to that shown. in FIG. on the screen 32. The operator then moves the positioner handle 88 to manipulate the substrate 4.0 while viewing the screen 32 to achieve alignment of the conductive elements 38 with the leads 36. After an image like that shown in FIG. 6 is achieved, bonding of the leads 346 to the elements 38 can take place and a full cycle of operation is thus completed. It is important to~note that the vertical

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small movement of the head assembly 22 can be quite

because there is no need to place complex optical the workpieces. system components between the tip 58 and that the It is desirable to take advantage of the fact can be movement can be small so that great precision of the provided in machine 20. Keeping the movement of the assembly 22 small also ensures that the image supported on aperture 42 and the image of the chip 30 the tray 70 as well as the image of the conductive 66 are elements 38 supported on the substrate holder eptieel-- simultaneously within the depth of field of the

system 27 It is also important to construct the

machine 20 so that the chips 50 and the conductive same elements 38 are supported at substantially the will elevation so that each of the respective images axis be in focus when they are placed on the optical 26. 54 by movement of the positioner assembly Another embodiment of the apparatus illustrated

in FIG.7 includes a ne eemplieit bonding head 146. a The bonding head includes a vacuum chamber 148,

vacuum port 150, a transparent disc 156 and a transparent

bonding tip 158. The head 146 can be used on a bonding

machine similar to that shown in FIG. 1 with the exception

that it is not necessary to use a compliant member.

The head 146 is designed to be used in a so-called

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23.8 62 no0 "hard tip" mode of operation. Provision must be made for

compensating for different thicknesses of leads 36, to thereby

equalize bonding forces simulataneously applied to all of leads 36.

The tip 158 includes a projection 162 and an

aperture 160. The aperture 160 is large enough so that the

body portion 61 of one of the chips 30 will fit into the

aperture with some clearance around sides of the body portion.

The relative size of the aperture 160 to the body portion 61 i.

shown in FIG. 9. The projection 162 serves substantially the

same purpose as the projection 62 (FIG. 2). Some heat is transmitted through the tip 158 from

cartridge heaters 163. Of course, it is still desirable to prc

heating from sources other than the head 146. In using the head 146, an operator usually first

encounters a situation in which the aperture 160 is misaligned

with respect to one of the chips 30 such as that shown in

FIG. 8. This is somewhat analogous to the situation shown in

FIG. 3. The ship 30 is moved into proper alignment with

the aperture 160 while being viewed along an ioptical-bonding

axis 164 through a system which takes advantage of the transp

of the disc 156 and the tip 158. FIG. 9 represents an image

that an operator sees after proper aliqnment of the chip 30 an aperture 160 is achieved.

FIG 10 illustrates the conductive elements 38 of

a substrate 40 as they appear to an operator prior to final al

ment of the substrate and the chip 30. The substrate 40 is

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carefully moved until the conductive elements 38 are properly

aligned with the leads 36. FIG. Ii illustrates an image that an operator sees

during bonding of the leads 36 to the elements 38. One of the

very significant advantages of apparatus according to the invent-

ion is that is may permit an operator to visually determine the extent of "squash out" of the leads 36 while bonding is proceed-

ing and to respond to excessive or insufficient "squash-out"

the likelihood of getting good quality bonds is greatly

enhanced.

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Claims (1)

1. 23,862 /7 The claims defining the invention are as follows:

   1. Apparatus for bonding first and second workpieces together, said apparatus including means for aligning said

   workpieces in desired relation to each other, a bonding head

   for applying bonding energy to said workpieces to bond them

   together, said bonding head including a transparent bonding

   tip, and said apparatus further including means for inspectinc at least one of said workpieces via said transparent tip alone

   a bonding axis (as hereinbefore defined) to inspect alignment of said one workpiece.

   2. A method of bonding two workpieces together util- izing bonding apparatus including a bonding head having a

   transparent bonding tip for applying bonding energy to said

   workpieces to bond them together, the method including align- ing said workpieces in desired relation to each other, inspec- ting alignment of at least one of said workpieces with means

   adapted to inspect it via said transparent tip along a bonding axis (as hereinbefore defined), and applying bonding energy to said workpieces to bond them together.

   3. A method as claimed in claim 2 wherein said work- pieces are inspected with said means during application of

   bonding energy.

   4. A method as claimed in claim 2 or 3 wherein said bonding energy is applied to said workpieces via a transparent compliant bonding medium.

   A method as claimed in claim 2 or 3 wherein said

   tip directly contacts one of said workpieces during applicatio

   of bonding energy.

   .A method as claimed in any one of claims 2 to

   wherein said one workpiece is held adjacent said tip after -16- 4 58.55

   23.862 alignment by a reduction in air pressure transmitted through

   said tip to said workpiece.

   7. A method of bonding two workpieces together sub-

   stantially as herein described with reference to the accompany-

   ing drawings.

   8. Workpieces bonded together by a method as claimed

   in any one of claims 2 to 7.

   9. Apparatus for bonding workpieces together substant-

   ially as herein described with reference to the accompanying

   drawings.

   DATED this THIRTIETH day of JANUARY, 1974

   WESTERN ELECTRIC COMPANY, INCORPORATED

   Patent Attorneys for the Applicant SPRUSON FERGUSON

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