CA1075832A

CA1075832A - Micro-surface welding

Info

Publication number: CA1075832A
Application number: CA274,143A
Authority: CA
Inventors: Joseph Funari
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1976-03-16
Filing date: 1977-03-16
Publication date: 1980-04-15
Also published as: BE851928A; ZA771367B

Abstract

MICRO-SURFACE WELDING
ABSTRACT OF THE DISCLOSURE
This invention relates to a method and apparatus for wire bonding a variety of metals in the interconnection of semiconductor chips to electronic package substrate circuitries. A pair of electrically conducting bonding tip members are provided which are electrically isolated from one another and which are constructed of a material having a high resistivity. The wire is positioned beneath the tip members and a load is applied to the members to force the wire against a land on the substrate. A voltage source is provided to apply a voltage between the tip members. Activitation of the voltage source results in current flow through the tips and through the wire, in series, causing heating of the tip members and of the section of wire beneath them. Diffusion bonding will initiate before a significant amount of oxidation has had time to occur.

Description

l9 BACKGROUND OF THE INVENTION
In recent years, the field of electronic packaging 21 has progressively employed smaller circuit components.
22 Electronic functions which ten years ago would have been 23 served by arrays of discrete transistors, capacitors and 24 resistors, mounted on printed circuit cards, are now provided by integrated circuit chips mounted on ceramic 26 substrate modules. This progress in miniaturization has 27 placed greater demands upon the circuit joining tech-~8 nologies involved. For example, special techniques are 29 re~luircd for bonding very small diameter wires, such as, .00l inches in diameter, to the very small pads or cir-31 cuit terminations on the integrated circuit chips and to ~N975021 -1-. ' .' .
~.,, ~ . . ~ :

:
. ~
. : ; .

-: .::
:: .; ~ : : . :, ~
;':, .. -; :

10~7S8~;~

the lands or circuit terminations on the module substrates,

2 for the purpose oE the interconnection of the chip and module

3 circuitries. In addition, it has become desirable to be able

4 to produce changes in either the chip or the module circuits, including the fabrication of circuit "cross-overs" by the 6 bonding of short wire lengths to interconnect two pads of an 7 integrated circuit chip or two lands of a module substrate.
8 From a metallurgical viewpoint, there are three types 9 of bonding mechanisms conventionally employed in the inter-connection of integrated circuit chip pads -to the lands of 11 the module substrate. They are: (a) bonding of solder 12 fusion, (b) bonding by metallic diffusion at elevated 13 temperature, and (c) bonding by room temperature diffusion.
14 In electronic packaging, an example of the first mechanism (a) would be where molten solder is used to join substrate 16 lands to the pads of integrated circuit chips which are 17 mounted in a "flip-chip" or circuit-side down orientation.
18 This is a very economical means for interconnecting integrated 19 circuit chip and substrate circuitries. However, there are some instances wherein it is not convenient to use the 21 "flip-chip" orientation. Where a severe requirement for 22 heat transfer from chip to substrate exists, back-bonding 23 the integrated circuit chip to the substrate in a circuit-24 side up orientation is o~ten necessary. Back-bonding of the integrated circuit chip to the substrate is also 26 necessary in some application of unusually large integrated 27 circuit chips for which cyclic-temperature environmental 28 conditions couid cause a solder joint fatigue problem.
29 Where back-bonding of integrated circuit chips is employed, it is necessary 10~5~33,'~
1 to bo~d a conducting element at its ends to each chip 2 pad and substrate land where an interconnection is de-3 sired. The conducting elemen-t could be a wire, a circuit 4 line on a decal, or any similar element providing the interconnection Eunction.
6 Where it is necessary -to bond a wire to a chip pad 7 and to a substrate land in order to in-terconnect them, 8 any of the aforementioned mechanisms (a), (b) or (c) : 9 could be employed. ~owever, the two types of wire bond- ~ , ing processes conventionally used involve only the latter ; 11 two mechanisms (b) and (c). The process involving mechan-12 ism (b) is called thermo-compression wire bonding. In ` 13 this process, illustrated in Fiy. 1, a ball is first 14 formed on the end of the wire to be bonded by melting the end with a hydrogen flame. The balled end of the wire is ;~ 16 then forced against a metal land or pad under a pre-17 determined load imposed by the hollow tip through which 18 the wire passes. This tip and/or the substrate land or 19 chip pad are heated by appropriate means and the heat passes from these elements to the wire ball bringing the 21 ball to an elevated temperature. The load causes the 22 hot ball to plastically upset and the plastic spreading 23 of the ball over the land results in fragmentation of , 24 oxide films, and any other films, at the ball-land inter-face, in the contact between clean ball metal and clean 26 land metal~ in the codiffusion of the metals across the 27 interface, and in the alloying of the metals causing a , 28 high strength bond at the original interface.
29 The conventional wire bonding ~rocess involving mechanism (c) is called ultrasonic bonding and is illus-31 trated in E'ig. 2. In this process, the end of the wire ; ~N975021 -3-.

~0758~

1 to be bonded is positioned under a bonding tip by means 2 oE a wire-guide hole in the tip. ~ load exerted on the 3 tip causes the wire to be forced against the subs-trate 4 land or against the chip pad. A hori~ontal cyclic dis-placement of the tip rela-tive to the substrate is pro-6 duced at an ultrasonic frequency Eor a predetermined 7 length of time. This ultrasonic motion scrubs the wire-8 land interface surfaces, fragmen-ting oxide films and any g other films to produce clean-me-tal to clean-metal contact at the interface surfaces and producing a shallow room 11 temperature diffusion and alloying of -the surface metals.
12 This results in a metallic bond between the wire and the 13 land or pad. It is believed that frictional heat devel-14 oped by the ultrasonic motion at the interface surfaces - 15 probably results in diffusion occurring at somewhat 16 higher than room temperature.
17 The two conventional categories of wire bonding meth-18 ods ~b) and (c) described above have exhibited several 19 disadvantages, however~ and the need to eliminate these has led to the superior micro-surface welding process 21 which will be described later. With both of the conven-22 tional wire bonding methods (b) and (c), the practical 23 use of metallurgies for both the wire and the land or ` 24 pad is limited to gold and aluminum and some alloys of ~ 25 these metals. The success in ultrasonic wire bonding ,j ``, 26 with gold and aluminum appears to be a consequence of 27 some chemical or metallurgical surface characteristic.
28 Although the reasons for this characteristic are not 29 generally agreed upon, they appear to be due to a lack of an oxide on the surface of the gold and the high 31 chemical and/or metallurgical reactivity of the aluminum :. ~ '' . . :.

1~7583~

1 surface, once ~he al-lminum oxide film has been fragmen-ted.
2 Metals other than gold and aluminum do not appear to have 3 such characteristics to establish good alloying and high 4 strength bonding at the interface at the room temperature conditions of conventional ultrasonic bonding. Ultrasonic 6 bonding also presents the problem of being sensitive to 7 machine vibration and dirt and sensitive to operator set-up.
8 When an attempt is made to use conventional thermo-9 compre~sion bonding, with its elevated temperature, with wire metals other than gold, the high rate of oxide forma-11 tion appears to offset the benefits of scrubbing and oxide 12 fragmentation in providing clean metal contact. In addition, 13 there is the necessity of heating the substrate and of using 14 hydrogen to melt the balled end of the wire. There are some wire ~onding equipments which combine features such as the 16 elevated temperature and ultrasonic scrubbing of thermo-17 compression and ultrasonic bonding. However, this com-18 bination of features does not permit the use of wires of 19 metals other than gold to be used for wire bonding to a practically significant degree.
21 Other prior art techniques for wire bonding or welding 22 are disclosed in U. S~ Patent 2,160,659 issued May 30, 1939 23 to F. R. Hensel; U. S~ Patent 2,253,375 issued August 19, 24 1941 to P. E. Henninger; and U. S. Patent 3,089,020 issued May 7, 1963 to R. P~ Hurlebaus. Patent 2,160,659 uses 26 highly conductive metals such as silver, copper, and other 27 highly conductive alloys mixed with carbide for a combination 28 of wear and a large flow of current through the tips. The 29 tips employed have a very low resistivity and the bonding is done by the passage of heavy current through the wire to 31 melt the wire. This type of resistance welding technique 32 where the resistance of the wire ~ EN975021 5 .
' . ~ .

~C~75133Z

1 creates the heat cannot be used with very small diameter 2 wire because burn ou-t or blow up of the wire will occur.
3 Patent 2,253,375 teaches the use of welding 4 electrodes composed oE molybclenum powders and the elec-trodes have a relatively low resistivity in the order of 6 27 micro-ohm centimeters. The electrodes are eneryized by means of a -trans~ormer. When welding with this type 8 of arrangement, power is continuously being pulled from g the transformer which results in burning out of the wire ; 10 and in order to prevent this it becomes necessary -to 11 employ some sort of cooling means.
12 Patent 3,089,020 shows the use of a pair of elec-13 trodes connected to a capacitor discharge circuit whereby 14 the capacitor discharge pulse is passed serially through one electrode, the wire, the component the wire is being 16 bonded to, back through the wire, and through the other 17 electrode to effect the bond. This welding technique 18 is used to bond to printed circuit type metallic foil 19 components; however, it cannot be used when interconnec-tion bonding integrated circuit chips and lands. The 21 passage of current through the integrated chip would 22 blow the chip up and the lands or pads being intercon-23 nected are mounted spaced apart on a substrate of ceramic 24 or insulator material.
SUMMARY OF THE INVENTION ~ -26 The present invention makes it possible to wire 27 bond a great variety of metals to chips and lands or pads 28 on integrated circuit modules by providing a novel and ~9 improvecl welding technique wherein the temperature a-t the interEace or bonding region is brought very close to the 31 meltin~ point of the wire in a very short period of time.
~N975021 -6-:~75832 1. To accomplish this, a bon~in(J rnecilan:i.sm is provlded 2 whicll comprises two parallel electr.ically conducting 3 members electrical.ly isolated from each other by a thin 4 layer of insulation. The wire to be bonded is posi.-tioned under the tips of these members by way of a wire-guide 6 hole in one of the members. There is provided an appro~
7 priate electrical voltage source, such as, a capacitor 8 charging and discharging circuit, whereby a voltage is 9 applied between the two tip members. A load is exerted on the tip members causing the wire to be forced agai.nst 11 a land or chip on a substrate and the heated section of 12 the wire, which is that section under the tips, will 13 complete an electrical circuit of the tip members and the 14 voltage source. When the voltage source i5 activated to apply the tip voltage, current will flow through the tips 16 and through the heated section of the wire, in series, 17 causing heating of the tip members and of the heated 18 section of the wire. In this manner, the heated section 19 of the wire which is at the bonding interface is used as the heat source to that interface region.
21 In the welding technique of the present invention, 22 the heat is immediately available for the bonding re-23 actions, which involve plastic upset at elevated tempera-24 ture and metallic diffusion, and the heat does not, as in conventional prior art bonding, have to travel from 26 heat sources, such as the heated tip and the heated 27 substrate, and into the wire and i.nto the land or chip.
28 In the present improved technique, the interface region 29 is raised a:Lmost instantaneously -to a temperature just below the melting temperature of the wire and, therefore, 31 diffusion bonding is initiated before a significant amount i83~

1 of oxidation has had time to occur.
2 A~ important aspect of the present invention is the 3 material which make up the bonding tips. The two con-4 ducting elements of the tip should be made of high re-sistivity material. The minimum desired resistivity is 6 in the order of 100 micro-ohm centimeters which is ob-7 tained by using titanium carbide. The use of this high 8 resistivity material for the tips, as well as o-ther g materials of much higher resistivity, accomplishes two things. The first is ~hat the flow of current passing 11 through the tips and the heated section of -the wire, in 12 series, heats the tip elements as it heats the heated 13 section of wire and the heated tip elements do not, 14 therefore, tend to draw off the heat generated in the heated section of wire.
16 The second beneficial feature of the high resis-17 tivity tip material is in its contribution to the con-18 tact resistance between the tip elements and the heated 19 section of the wire. The heated section of the wire is heated primarily because of this contact resistance due 21 to the power generated by the current passing through 22 the contact resistance and not because of the bulk 23 resistivity of the wire itself. A large voltage drop 24 occurs at the tip interface with the wire which limits the flow of current through the wire and prevents the 26 wire from overheating and blowing up. Also, a heat 27 spike occurs at the interface between the tips and the 28 wire and as a result the wire is not re-flowed into a 29 glob but instead it is fused in a precise spot under the tip which makes for better fusion. The bonds will appear 31 under the tips in the same shape and size as the shape ~N975021 -8-83~

1 and size of the tips.
2 It is, then, a prim~ry object of the present inven-3 tion to provide a novel and improved method and apparatus 4 for wire bonding a variety of metals in the interconnec-tion of semiconductor chips to electronic package sub-6 stra-te circuitries.
7 A Eurther object of the present invention is to pro-8 vide a novel and improved method and apparatus for wire g bonding wherein the interface bonding region is brought to a temperature very close to the melting temperatu~e 11 of the wire in a very short period of time.
12 Ano-ther object of the present invention is to pro-13 vide a novel and improved wire bonding apparatus compris-14 ing bonding tips constructed of high resistivity mater-ial to provide substantial contact resistance between , 16 the tips and wire.
17 A still further object of the present invention is 18 to provide a novel and improved apparatus for bonding 19 wire to a substrate which comprises electrically isolated bonding tips of high resistivity material and circuit 21 means for passing current through the tips and wire in 22 series.
23 The foregoing and other objects, features and 24 advantages of the invention will be apparent from the following more particular description of a preferred 26 embodiment of the invention as illustrated in the 27 accompanying drawings.
28 BRIEF DESCRIPTION OF T~IE DRAWINGS
29 FIG. l is a schematic illustration of the prior art thermo-compression wire bonding mechanism.
31 FIG. 2 is a schematic illustration of the prior art 1075!33Z

1 ultrasonic wire bondln~ mechanism.
2 FIG. 3a is a schematic illustration of the micro-3 welding apparatus of the present invention.
4 FIG. 3b is a schematic illustration lookiny at the welding tip end of the apparatus of FIG. 3a.
6 FIG. 4 is a diagram showing the equivalent electri-7 cal circuit of the apparatus shown in FIGS. 3a and 3b.
8 FIG. 5 is a chart showing a comparison of the wire 9 and substra-te materials bonded by the present micro-welding apparatus and the prior art thermo-compression 11 and ultrasonic bonding apparatus.

._ 13 Referring to FIGS. 3a and 3b, there is shown the 14 micro-surface welding apparatus of the present invention.
The apparatus comprises a pair of juxtaposed electrically 16 conducting bonding tip members 10 and 11 which are elec-17 trically isolated from each other by a thin sheet of 18 insulation material 12, such as, mica, glass, or the likeu 19 As will be further described, the bonding tip members 10 and 11 are constructed of a material having a high resis-21 tivity. The bonding tip assembly is fixed to a support 22 arm 13 at-tached to an end block member 14 which is pivot-23 ally mounted on a support shaft 15. Attached to the end 24 of block 14 is a counter-balancing weight member 16.
To lower and raise the bonding tip assembly into and 26 out of contact with a wire 17 positioned between the ends 27 of -the tips and a metallized circuit land 18, or the 28 like, on a substrate to which the wire is to be bonded, 29 there is provided a motor driven eccentric cam 19 which operates against a cam follower arm 20. The cam follower 31 arm is fixecl to one end of a link arm 21 and the other ~N975021 -10-, ~7S83Z

1 end of the link arm ls pivotally mounted at 22 to the 2 block 14. When the cam Eollower coacts with the high 3 portion of the cam, support arm 13 will be pivoted 4 clockwise to raise the bonding tip assembly out of con-tact with -the wire. When the cam follower coaets wlth 6 the low portion of the eam, support arm 13 will be `~ 7 allowed to pivot eounter-elockwise by virtùe of its 8 weight and the bonding tip assembly will drop into eon-9 taet with the wire. The eounter-balancing weight member 16, whieh may be made adjustable, ic~ provided to control 11 the eontact pressure exerted on the wire by the bonding 12 tips.
13 The souree of electrieal power for the bonding 14 tips is preferably a eapaeitor eharging-diseharging eir-lS euit. As shown in FIG~ 3a, the eireuit eomprises a 16 D. C. potential source or battery 23, a eapaeitor 24, a 17 eapacitor charging resistor 25, and a single pole -18 double throw switch 26. The eireuit is eonneeted to the 19 bonding tips by way of eonduetor leads 27 and 28. Lead 27 is eonneeted to a metallie screw 29 mounted in the 21 front tip portion 13a of support arm 13. The serew 29 22 makes electrieal eontact with the bonding tip 10 and the 23 support arm front tip portion 13a whieh extends to the 24 bonding tip 10 is eonstrueted of a non-eonducting plastie material. Lead 28 is eonneeted to a metallie serew 30 26 whieh is fastened to the main portion of support arm 13 27 whieh extends rearwardly from the bonding tip 11. This 28 portion of the support arm is construeted of a eondueting 29 metal and it makes eleetrieal eontaet with the bonding tip 11. When switeh 26 is connee-ted to its terminal 31, ' 31 eapacitor 24 will become charged and when the switch is L'N975021 -11-' ..
: . . .

~758~

1 connected to its terminal 32, capacitor 24 will discharge 2 and current will flow through bonding tip 10, wire 17, 3 and bonding tip 11 in series to effect bonding of the 4 wire when the tips are down in bonding position in con-tact with the wire.
6 There is provided a wire clamping, Eeeding and 7 cutting mechanism. Referring to both FIGS. 3a and 3b, 8 the mechanism comprises a pair of clamp members 33 and 9 34 arranged one on each side of the support arm 13 and in proximity to the bonding tips 10 and 11. The elamp 11 member 33 is mounted on a pivot member 35 which is 12 movable horizontally by a suitable cam mechanism, not 13 shown. Clamp member 34 is mounted on a fixed pivot 14 member 36. The bottom ends of the clamp members are curved toward each other to provide a gap 37 whieh re-16 ceives the wire 17. The inner surfaces of the upper 17 ends of the elamp members are biased against a pivot 18 rod 38 by means of a U-shaped spring 39 the ends of 19 whieh press against the outer surfaces of the upper ends of the clamp members so that normally the gap 37 21 is maintained suffieiently open so as to not grip or 22 elamp the wire therein. To grip or elamp the wire to 23 feed same, the pivot member 35 is moved against elamp 24 member 33 to reduee the gap 37 whereby the ends of the elamp members will grip the wire.
26Referring to FIG. 3a, the wire clamping and feed-27 ing mechanism is pivoted cloekwise and eounter-elockwise 28 by means o~ an eccentrie eam 40 whieh coacts with a 29 follower arm 41 pivotally mounted at 42. Connected to the end of the follower arm is a slideable pusher arm 31 43 which bears against the clamp member assembly and ~:N975021 -12-~37~83Z

1 connecte~ to the assembly is a biasincJ spring 44.
2 Briefly, in the sequence of operation of the various 3 mechanisms, at the start of a cycle the clamp members 4 33 and 34 are in a closed or clamping position and the support arm 13 is tilted counter-clockwise from a hori-6 zontal position by approximately 4 degrees. 'rhe operator 7 threads the wire 17 from a supply spool 45, through a 8 suitable guide hole 46 in the support arm 13, in between g the clamp members 33 and 34, by manually opening them, then through a wire guide groove 47 in the bonding tips 11 11, and into a position beneath the bonding tips 10 and 12 11. Cam 19 is now operated to bring the bonding tips 13 down to approximately .002 inches above the wire and sub-14 strate and the operator further positons the end of the wire onto the precise spot on the substrate at which it 16 is to be bonded.
17 Next, the bonding tips are moved down into contact 18 with the wire, the circuit is activated to discharge 19 the capacitor and pass current through the bonding tips and wire in series to effect the bond, the clamp members 21 are opened, and the bonding tips are returned upward to 22 a full horizontal position to be in position for a wire 23 cut-off operation. With one end of the wire now bonded, 24 the operator moves the substrate to the next bonding position. The wire moves freely in its threaded path 26 as it is pulled by the substrate. The bonding tips are 27 again moved down to .002 inches and the operator places 28 the wire and substrate in the desired position. The 29 clamp members are now closed, the bonding tips moved into contact with the wire, and the circuit discharged 31 to bond the other end of the wire. While the tips are . . . ~
.. ~ .

~7583~

1 still in contact with the wire and the clamp rnembers 2 are closed, cam 40 operates to move the clamp members 3 counter-clockwise or rearwardly and the wire is broken 4 by the sharp bottom edge 48 of the bonding -tip 11.
~he bonding tips are now returned upward and the closed 6 clamp members are pivoted clockwise to feed wire into a precise position beneath the tips in preparation for 8 another bonding cycle.
g Although the above described mechanisms are known and available in commercial equipment, they have been 11 shown and described to illustrate one type of wire bond-12 ing equipment to which the principles of the present in-13 vention may be applied.
14 As was previously mentioned, the important aspect of the present invention resides in the fact that the 16 bonding tips 10 and 11 are constructed of a material 17 having a substantially high resistivity compared to the 18 resistivity of the wire being bonded. It was found 19 that the minimum resistivity which will give the de-sired fusion results is in the range of 100-170 micro-21 ohm centimeters which is the resistivity range of 22 titanium carbide. In comparison, the resistivity of 23 gold wire used for bonding is approximately 2.35 micro-24 ohm centimeters. In addition to titanium carbide, excellent results were obtained by constructing the tips 26 from carbon and its alloys which have a resistivity in 27 the range of 4000-S000 micro-ohm centimeters. Other 28 materials which it is anticipated will not only give 29 cJood results but additionally will increase the life oE the tips are a combination of carbide and graphite 31 having a resistivity of 3200 micro-ohm centimeters and ., . , :
~. .

10758;~Z

1 up, silicon carbide having a resistivity of 200,000 2 micro-ohm centimeters and up, a zirconium boride silicon 3 carbide composition having a resistivity range of 170 -4 200 micro-ohm centimeters, and silicon crystals having a rasistivity range of 4000-5000 micro-ohm centimeters.
; 6 Referring now to FIG. 4, th~re is shown an equiva-7 lent electrical circuit for the apparatus shown in 8 FIGS. 3a and 3b. The circuit values shown are for a g welding cycle that was made to bond .001 inch gold wire to a gold surface or land using titanium carbide bonding 11 tips. Power of 4.5 volts was used to deliver a current 12 spike of 0.5 amps for 250 milli-seconds. Heat generated 13 at the interface of the bonding tips and the gold wire 14 was approximately 800 C. RO represents a 5 ohm load on the D.C. power supply 23. Capacitor 24 had a value of 16 32000 micro-farads. Rl and R2 are the resistances of 17 the wire leads from the power source 23 to the titanium 18 carbide bonding tips and they were equal at 200 micro-19 ohms each. R3 and R4 represent the material resistivity of the titanium carbide bonding tips which for each tip 21 was 105 micro-ohm centimeters. R5 and R6 represent the 22 contact resistance between the bonding tips and the gold 23 wire and these were each 4.5 ohms. R7 represents the 24 resistance of the length of gold wire from bonding tip to bonding tip that was welded and this was .03 micro-26 ohms. It was found that in order to effect the desired 27 contact resistance at the interface of the bonding tips 28 and the wire being bonded, the resistance ratio between 29 the wire material and the bonding tip material, R3 o~r-R4' should be a minimum of 1 to 11.
31 To illustrate two other examples of -the many .. . . .

l~S~3Z

1 materials which were successfully fused or bonded 2 using the present micro-surface welding technique, 3 .0015 inch copper wire was bonded to a chrome surface 4 land using titanium carbide bonding tips powered at 14 volts for 200 milli-seconds. A minimum temperature 6 of 750 C was generated at the interface of the bonding 7 tips and wire. ~lso, .003 inch copper wire which was 8 tin-lead coated was bonded to a tin-lead coated land 9 using titanium carbide bonding tips powered at 10 volts for 200 milli-seconds. A minimum temperature of 400 C
11 was generated at the interface of the bonding tips and 12 the wire.
13 Referring to the chart shown in FIG. 5, there is 14 depicted a comparison of the materials bonding capa-bility of thermal compression bonding TC, represented 16 by a triangle; ultrasonic bonding USB, represented by 17 a circle; and the present micro-surface welding tech-18 nique MSW, represented by a square. All of the various 19 material combinations shown between the surface material and the wire material that was bonded thereto were car-21 ried out and, as shown, it was found that many more 22 material combinations were successfully bonded using the 23 micro-surface bonding technique than was possible using 24 the described prior art techniques. As was previously pointed out, the improved success of the present micro-26 surface bonding technique is attributed to the use of 27 bonding tips which are constructed of a material having 28 a substantially high resistivity which results in a 29 large voltage drop at the interface with the wire being bonded which limits the flow of current through 31 the wire and the temperature at the interface is raised ~N975021 -16-.

1~7S83Z

1 almost instantaneously to ef~ect diffusion bonding 2 before a significant amount of oxidation llas had 3 time to occur. Wire having a diameter of .0005 inches 4 to .005 inches can be bonded to pad or land sizes of .002 inches by .002 inches, and up, and having a thick-6 ness of 500 angstroms, and up. Either evaporated 7 conductor or paste conductor circuits on metallized ceramic substrates may be bonded to.
9 In addition to accommoclatiny a greater number of wire material - land material combinations because of 11 its high heating rate and low oxidation development 12 at the bonding interface, the present micro-surface weld-13 ing technique may be conveniently used to bond insulated 14 wire. ~nder practical land levels, the tip elements can penetrate some types of wire insulation coatings; such 16 as, teflon, extruded polyamide, polyurethane, and varnish, 17 sufficiently to contact the wire and effect the bonding 18 operation. Since the bonding cycle is so short, only 19 the insulation in the heated section is burned away or damaged. It was also observed that the high power -21 density and short cycle conditions permit bonding with-22 out the necessity to heat the substrate. This gives 23 protection from heating effects of any heat sensitive 24 materials or devices on the substrate.
Other advantages of the present bonding technique 26 is that it is very insensitive to floor vibration of 27 the bonding equipment, to the degree of squareness of 28 the bonding surface to the bonding tip axis, to the 29 cleanliness of the bonding land or pad, and to the hard-ness of the land or pad. ~nlike ultrasonic bonding 31 equipment, it is not necessary to tune to a frequency ~N975021 -17-... . .
`: . ' ' .
': . , ' :

~L07583Z

1 or a power input in order to produce good wire bonds.
2 Some substrate heating may be used effectively 3 to enhance bonding results particularly if the bonding 4 land is wide or thick and is able to act as a powerful heat sink.
6 As was described, the preferred source of electri-7 cal power is a charcJed capacitor which is discharged, 8 by the use of an appropriate switch, -through the bonding 9 tip circuit. This arrangement provides high "time-æero"
current and power conditions for rapidly heating the 11 section of wire under the bonding tips. However, it is 12 not intended to exclude the use of any other appropriate 13 type of power source which will produce the desired 14 results.
While the invention has been particularly shown 16 and described with reference to a preferred embodiment 17 thereof, it will be understood by those skilled in the 18 art that various changes in form and details may be made 19 therein without departing from the spirit and scope of the invention.

~N975021 -18-, . . . . . . .
: . . , :" ,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Welding apparatus for bonding a metal lead to metallized circuitry on a substrate which comprises:
a pair of electrically conducting bonding tip members electrically isolated from each other, said tip members being constructed of a material having a substantially higher resistivity than the resistivity of said lead;
means for positioning said lead in contact with said tip members and said metallized circuitry; and circuit means for passing a single current pulse serially through said tip members and lead to effect bonding of said lead whereby a limited amount of current is passed through said lead to prevent damage thereto.

2. Welding apparatus as defined in claim 1 wherein said tip members are constructed of a material having a resistivity of at least 100 micro-ohm centimeters.

3. Welding apparatus as defined in claim 2 wherein said circuit means comprises capacitor charging and discharging circuit means.

4. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed of titanium carbide.

5. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed from carbon and its alloys.

6. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed from a combination of carbide and graphite.

7. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed of silicon carbide.

8. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed from a zirconium boride silicon carbide composition.
.

9. The welding apparatus set forth in claim 3 wherein said bonding tip members are constructed from silicon crystals.

10. Welding apparatus as defined in claim 1 wherein:
the resistivity of said tip members and said lead is such that the resistance ratio between the material of said lead and the material of said bonding tip members is at least 1 to 11;
said resistance ratio creating a substantial voltage drop at the interface of said bonding tip members and lead which results in rapid heating at said interface with a limited amount of current flow through said lead to prevent damage thereto.

11. The welding apparatus set forth in claim 10 wherein said bonding tip members are constructed from titanium carbide and said lead and metallized circuitry are constructed from gold.

12. The welding apparatus set forth in claim 10 wherein said bonding tip members are constructed from titanium carbide, said lead from copper and said metallized circuitry from chrome.

13. The welding apparatus set forth in claim 10 wherein said bonding tip members are constructed from titanium carbide, said lead from tin-lead coated copper and said metallized circuitry being tin-lead coated.

14. The welding apparatus set forth in claim 3 wherein said lead is constructed of aluminum and said metallized circuitry is constructed of either aluminum, copper, gold, or chrome.

15. The welding apparatus set forth in claim 3 wherein said lead is con-structed of gold and said metallized circuitry is constructed of either aluminum, copper, titanium, gold, chrome, indium antimony, tin-lead, nickel paste, palladium, ternary paste, silver-palladium paste, or molybdenum paste.

16. The welding apparatus set forth in claim 3 wherein said lead is con-structed of copper and said metallized circuitry is constructed of either aluminum, copper, gold, chrome, tin-lead, nickel paste, ternary paste, or silver-palladium paste.

17. The welding apparatus set forth in claim 3 wherein said lead is con-structed of gold plated copper and said metallized circuitry is constructed of either aluminum, copper, gold, chrome, tin-lead, nickel paste, ternary paste, or silver-palladium paste.

18. The welding apparatus set forth in claim 3 wherein said lead is teflon coated and constructed of either aluminum, gold, copper, or gold plated copper and said metallized circuitry is constructed of either copper or gold.

19. The welding apparatus set forth in claim 3 wherein said lead is con-structed of platinum iridium and said metallized circuitry is constructed of either gold or tin-lead.

20. The method of bonding a metal lead to metallized circuitry on a sub-strate comprising the steps of;
positioning said lead over a portion of said metallized circuitry to which the lead is to be bonded;
actuating a pair of electrically conducting bonding tip members to maintain said lead in interface contact with the tips of the bonding members and said portion of the metallized circuitry, said bonding tip members being electrically isolated from each other and constructed of a material having a substantially higher resistivity than the resistivity of said lead; and passing a single current pulse serially through said bonding tip members and lead to effect bonding of said lead whereby a limited amount of current is passed through said lead to prevent damage thereto.

21. The method of claim 20 wherein said bonding tip members are constructed of a material having a resistivity of at least 100 micro-ohm centimeters;
and said single current pulse is passed by discharging a charged capacitor.

22. The method set forth in claim 20 wherein the resistance ratio between the material of said lead and the material of said bonding tip members is at least 1 to 11.