# Patent

## Title
Ball bonding metal wire bond wires to metal pads

## Abstract

An apparatus, and methods therefor, relates generally to an integrated circuit package. In such an apparatus, a platform substrate has a copper pad. An integrated circuit die is coupled to the platform substrate. A wire bond wire couples a contact of the integrated circuit die and the copper pad. A first end of the wire bond wire is ball bonded with a ball bond for direct contact with an upper surface of the copper pad. A second end of the wire bond wire is stitch bonded with a stitch bond to the contact.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims benefit to U.S. provisional patent application No. 62/158,453, filed May 7, 2015, which is incorporated by reference herein in its entirety for all purposes.


FIELD
The following description relates to integrated circuits (“ICs”). More particularly, the following description relates to ball bonding metal wire bond wires to metal pads.
BACKGROUND
Microelectronic assemblies generally include one or more ICs, such as for example one or more packaged dies (“chips”) or one or more dies. One or more of such ICs may be mounted on a circuit platform, such as a wafer such as in wafer-level-packaging (“WLP”), printed board (“PB”), a printed wiring board (“PWB”), a printed circuit board (“PCB”), a printed wiring assembly (“PWA”), a printed circuit assembly (“PCA”), a package substrate, an interposer, or a chip carrier.
Conventional interconnecting of an IC to a circuit platform has one or more issues with respect to dual finish substrates for bonding of wire bond wires, such as a BVATM wire bond wires for example. Some substrates may have organic solderability preservatives (“OSPs”) as a surface finish to a PCB to ensure solderability. Along those lines, OSPs are used as a surface finish to a PCB to resist oxidation thereof and to ensure solderability to copper pads thereof of low melting point interface layer materials for subsequent interconnection. An OSP finish is a transparent organic complex film that coats onto a copper surface to prevent corrosion.
Copper pads of a PCB may have thereon a layer of nickel followed by a layer of palladium or other combination of interface layers. Such metal interface layers on such copper pads may be used to ensure proper interconnection, including interconnection at a lower temperature, as well as providing a migration barrier for some applications. Effectively, such one or more interface layers may melt at a lower melting point than copper, so as to provide interconnections via soldering with a lower thermal budget. Furthermore, such one or more interface layers may more readily adhere to wire bond wires. In the past, these additional interface layers have allowed for stitch bonding to a PCB using wire bond wires, where such stitch bonding provides a reliable interconnection and is performed within a thermal budget of such a PCB.
Accordingly, it would be desirable and useful to provide bonding of wire bond wires to pads of a substrate platform that avoids having to have one or more interface layers.
BRIEF SUMMARY
An apparatus relates generally to an integrated circuit package. In such an apparatus, a platform substrate has a copper pad. An integrated circuit die is coupled to the platform substrate. A wire bond wire couples a contact of the integrated circuit die and the copper pad. A first end of the wire bond wire is ball bonded with a ball bond for direct contact with an upper surface of the copper pad. A second end of the wire bond wire is stitch bonded with a stitch bond to the contact.
A method relates generally to an integrated circuit package. In such a method, obtained is a platform substrate having a copper pad and having an integrated circuit die coupled to the platform substrate. A wire bond wire is wire bonded for coupling a contact of the integrated circuit die and a surface of the copper pad. The wire bonding includes ball bonding with a ball bond a first end of the wire bond wire for direct contact with the surface of the copper pad.
A method relates generally to an integrated circuit package. Obtained is a platform substrate having a copper pad and having an integrated circuit die coupled to the platform substrate. A wire bond wire is wire bonded for coupling a contact of the integrated circuit die and a surface of the copper pad. The wire bonding includes: removing an organic solderability preservative layer from the surface of the copper pad for exposing at least a portion of the surface for direct contact of a ball bond therewith; and ball bonding with the ball bond a first end of the wire bond wire for the direct contact with the surface of the copper pad.
Other features will be recognized from consideration of the Detailed Description and Claims, which follow.



BRIEF DESCRIPTION OF THE DRAWING(S)
Accompanying drawing(s) show exemplary embodiment(s) in accordance with one or more aspects of exemplary apparatus(es) or method(s). However, the accompanying drawings should not be taken to limit the scope of the claims, but are for explanation and understanding only.
FIG. 1 is a block diagram of a cross-sectional side view depicting a portion of an exemplary integrated circuit package (“assembly).
FIG. 2 is a block diagram of a cross-sectional side view depicting a portion of another exemplary assembly.
FIG. 3 is a flow diagram depicting an exemplary process flow in accordance with the assemblies of FIGS. 1 and 2.
FIG. 4A is a table diagram depicting exemplary pre-baking and post-baking of the assemblies of FIGS. 1 and 2 for ball bonding wire bond wires.
FIG. 4B is a ball shear diagram depicting exemplary ball shear strengths for ball bondings of the examples of wire bond wires of FIG. 4A.
FIGS. 5-1 through 5-4 are a sequence of diagrams depicting exemplary operations of the process flow of FIG. 3, where FIGS. 5-1 through 5-3 are top-down views depicting an exemplary portion of an in-process platform substrate, and where FIG. 5-4 is an offset or alternate position view with respect to the top-down view of FIG. 5-3.
FIGS. 6-1 and 6-2 are respective map scan diagrams depicting exemplary cross-sections of map scans for a ball bond using copper-only wire bond wire and a ball bond using a lead coated copper wire bond wire, respectively.
FIG. 7 is a spectrum graph depicting an exemplary spectrum analysis of the ball bonds of FIGS. 6-1 and 6-2.
FIG. 8 is a histogram diagram depicting an exemplary mapped sum spectrum for the spectrum analysis of FIG. 7.



DETAILED DESCRIPTION
In the following description, numerous specific details are set forth to provide a more thorough description of the specific examples described herein. It should be apparent, however, to one skilled in the art, that one or more other examples or variations of these examples may be practiced without all the specific details given below. In other instances, well known features have not been described in detail so as not to obscure the description of the examples herein. For ease of illustration, the same number labels are used in different diagrams to refer to the same items; however, in alternative examples the items may be different.
FIG. 1 is a block diagram of a cross-sectional side view depicting a portion of an exemplary integrated circuit package (“assembly”) 100. In assembly 100, an integrated circuit (“IC”) die 102 is coupled to a package substrate 101, such as a laminate layered circuit board or other laminate substrate. An underside surface of an IC die 102 may be coupled to an upper surface 110 of platform substrate 101, and an upper surface 111 of an IC die 102 may have thereon a contact 105.
A wire bond wire 103, which may be of a bond via array, may be coupled to contact 105. Along those lines, an end of wire bond wire 103 may be bonded to contact 105 of IC die 102 with a stitch bond 107, as is known. As is known, contact 105 may have a lower copper layer, a nickel layer on such lower copper layer, and a palladium layer on such nickel layer. In another known implementation, contact 105 may have a lower copper layer, an aluminum layer on such lower copper layer, and a titanium-nitride layer on such aluminum layer.
Another end of wire bond wire 103 may be directly bonded for direct contact to a pad having a copper upper surface 108, namely “copper pad” 106, of platform substrate 101 with a ball bond 104. Copper pad 106 may be a solid copper pad or may have an upper surface layer of copper.
Ball bond 104 may be formed directly on a bare upper copper surface 108 of copper pad 106. In other words, a ball bond 104 of material from wire bond wire 103 feed wire may be in direct contact with a bare upper copper surface 108 of copper pad 106 for forming a ball bond 104 in direct contact with such copper surface 108. In other words, by having a completely bare upper copper surface 108, such upper copper surface 108 may have no intervening layer between such ball bond 104 and such upper copper surface 108 when forming such ball bond 104. This metal-on-metal direct contact between ball bond 104 formed of wire bond wire 103 material has a limited thermal budget for metal-to-metal soldering without exceeding a thermal limit of platform substrate 101, which may include one or more conductive layers.
Therefore, expenses associated with having one or more interface layers for solderability and thermal budget may be avoided. Even though pad 106 is illustratively depicted as being partially recessed in platform substrate 101 and partially above an upper surface 110 of platform substrate 101, in other implementations pad 106 may be disposed on upper surface 110 or completely recessed in platform substrate 101.
Wire bond wire 103 in this example is a copper wire. However, in another implementation, wire bond wire 103 may be a PCC wire.
Optionally, an OSP layer or like anti-corrosion layer 109 may be on platform substrate 101, and thus may be on an upper copper surface 108 prior to forming ball bond 104. Thus, a “bare” upper copper surface 108 may include an anti-corrosion layer of OSP or the like. Such an anti-corrosion layer 109 may be removed at least locally to have such upper copper surface 108 be in direct contact with a lower area of such ball bond 104. Thus, a portion of a bare upper copper surface 108 may include an anti-corrosion layer 109, and another portion of such a bare upper copper surface 108 may be completely exposed for a metal-to-metal direct interconnection with ball bond 104. Such completely exposed portion may be revealed by having an anti-corrosion layer 109 thereof removed with and by formation of ball bond 104. Optionally, such an anti-corrosion layer 109 may be removed prior to forming ball bond 104.
FIG. 2 is a block diagram of a cross-sectional side view depicting a portion of another exemplary assembly 100. Assembly 100 of FIG. 2 is the same as that of FIG. 1, except a ball bond 207 is used to interconnect an end of wire bond wire 103 to contact 205. Contact 205, like pad 106, has no intervening metal interface layer. This ball bonding to an IC die 102 may be used in instances where a thermal budget of IC die 102 is approximately 160 to 165° C. maximum.
With simultaneous reference to FIGS. 1 and 2, a copper or PCC wire bond wire 103 may be directly ball bonded to a copper or copper-OSP surface 108 of a pad 106 of a platform substrate 101. This may be used to eliminate having to use a dual finish platform substrate for wire bonding wires, including without limitation bond via array wire bond wire applications. In other words, there is/are no intermediate layer or layers used, whether Al, Au, or NiPd for example, between an upper surface of such copper or copper-OSP pad of a platform substrate and a lower or contact surface of a ball bond of a copper or PCC wire bond wire to such pad.
BVATM type wire bond wires were developed for use with eNiG and eNePiG surface finished substrates. However, as Cu-OSP finished substrates are becoming more prevalent for cost reduction, having the ability to interconnect BVATM wire bond wires to such Cu-OSP substrates facilitates use of BVATM wire bond wires. Advantageously, using a process described below for Cu-OSP finished substrates, there may be a short time between OSP burn-off exposing an upper surface area of a Cu pad and wire bonding to such exposed upper surface area. This short exposure time may reduce likelihood of exceeding a thermal budget of an underlying platform substrate 101, and thus may reduce the likelihood of damage to such underlying platform substrate 101 due to heating thereof.
In the following description, specific values are provided for purposes of clarity by way of example. However, it should be understood that these and/or other values may be used in other implementations, as may vary from application-to-application.
FIG. 3 is a flow diagram depicting an exemplary process flow 300 in accordance with the above-described assemblies 100 of FIGS. 1 and 2. Accordingly, process flow 300 is further described with simultaneous reference to FIGS. 1 through 3.
At 301, optionally an organic solderability preservative (“OSP”) layer 109 may be removed from surfaces 108 of copper pads 106 for subsequent ball bonding. Along those lines, at least a portion of an upper surface of a surface 108 may be exposed or otherwise laid bare for a direct contact between a ball bond and a temporary flux layer. After removal of an OSP layer 109, if present, at 301 a flux may be applied to a platform substrate 101. Along those lines, such flux may be applied to microbump contacts (not shown) of such platform substrate 101, as is known.
At 302, an IC or IC die 102 may be coupled to such platform substrate 101 by means of a flip-chip attachment for example. Accordingly, microbumps (not shown) may be used for a flip-chip attachment, as is known. At 303, a reflow of assembly 100 may be performed. Even though the example of a flip-chip coupling of IC die 102 to platform substrate 101 is described, other types of interconnecting an IC die to micro contacts may be used.
At 304, a surface activation of surfaces 108 of pads 106 of platform substrate 101 may be performed. For example, a flux, such as a water soluble flux (e.g., Flux WSF 808) may be applied to surfaces 108 followed by baking such assembly 100, including platform substrate 101, for approximately 5 to 10 minutes in an oven having a temperature of approximately 70 degrees Celsius. At 305, excess flux, generally flux not baked onto surfaces 108, may be rinsed away. For example, approximately a 5 minute rinse with approximately 70° C. deionized (“DI”) water may be used.
At 306, wire bond wires 103 may be attached by forming respective ball bonds 104 for wire bonding to surfaces 108 of pads 106. Ball bonding of wire bond wires 103 to platform substrate 101 may be performed generally in a range of approximately 150 to 175° C. for a maximum temperature. Along those lines, thermal assist ultrasonic bonding may be used for forming ball bonds 104 for wire bond bonding at 306.
Ultrasonic waves assisted by heat may be used to break-up oxidized copper on a surface 108 of a pad 106 to expose bare Cu. Along those lines, power may be increased approximately 20 to 30 percent more than conventional thermal assist ultrasonic bonding. Likewise, time and force of such thermal assist ultrasonic bonding may be increased over conventional levels therefor.
Exposed bare Cu promotes forming a quality intermetallic layer between a ball bond 104 and pad 106. Thermal assist ultrasonic bonding with low pressure may be used for such bonding at 306 for forming ball bonds 104. Along the above lines, ball bonds 207 may likewise be formed provided thermal budget and pressure constraints are met for forming such ball bonds 207 in accordance with the description herein.
In some implementations, a K&S iConn wire bond bonder may be used with a bare Cu laminate substrate without OSP for platform substrate 101 and with various wire bond wires. Examples of wire bond wires that may be used include Heraeus 2 mil PdSoft (i.e., Pd coated Cu wire), Heraeus 2 mil MaxSoft (i.e., bare Cu wire), and/or Tanaka 2 mil CLR-1A (i.e., Pd coated Cu wire with an Au flash). An SU-64250-97X5G10-RU34 capillary may be used for applying wire bond wire at a bonding temperature of approximately 170° C.
With the above general understanding borne in mind, various configurations for ball bonding of wire bond wire 103, namely bare copper wire, a lead coated copper wire, or lead coated copper wire with a flash gold plating, to copper pads are generally described below.
FIG. 4A is a table diagram depicting examples of pre-baking and post-baking of assemblies 100 for ball bonding wire bond wires as described herein. For this table diagram, a ball shear of a 100 gf (gram force) minimum 409 was used; all temperatures listed in Table 400 are in degrees Celsius.
FIG. 4B is a ball shear diagram depicting examples of ball shear strengths 411 through 414 for ball bondings of the examples of wire bond wires 103 of FIG. 4A. Along those lines, ball shear strengths 411 through 414 for such two different types of PCC wire bond wires 103H and 103T of FIG. 4A for directly bonding to Cu pads are described. With simultaneous reference to FIGS. 1, 2, 4A and 4B, these examples of wire bonding are further described.
Table 400 generally indicates that ball shear and wire pull requirements may be met for some applications. Along those lines, Heraeus 2 mil PdSoft wire bond wires 103H and Tanaka 2 mil CLR-1A wire bond wires 103T may each be ball bonded after pre-baking 401 assemblies 100, namely before wire bond wire ball bonding. Heraeus 2 mil PdSoft wire bond wires 103H and Tanaka 2 mil CLR-1A wire bond wires 103T may then be ball bonded to pads 106 of a pre-baked platform substrate 101 followed by a post baking of assemblies 100, namely respectively post-bakes 402 and 403 after ball bonding wire bond wires. All ball shear strengths 411 through 414 are generally within a range of approximately 150 to 210 gfs along a y-axis 410 in units of gfs.
For pre-baking 401, wire bond wire 103H ball bond 104 may have a ball shear strength 411. For pre-baking 401, wire bond wire 103T ball bond 104 may have a ball shear strength 413. These ball shear strengths 411 and 413 are similar to one another. For post-baking 402, such wire bond wire 103H ball bond 104 may have a ball shear strength 412. For post-baking 403, such wire bond wire 103T ball bond 104 may have a ball shear strength 414. These ball shear strengths 412 and 414 are similar to one another; however, both are slightly less than ball shear strengths 411 and 413.
FIGS. 5-1 through 5-4 are a sequence of diagrams depicting exemplary operations of a process flow 300 of FIG. 3. FIGS. 5-1 through 5-3 are top-down views depicting an exemplary portion of an in-process platform substrate 101. With additional reference to FIGS. 1, 2, and 3, FIGS. 5-1 through 5-4 are further described.
At 501, Cu bond pads 106 are shown before exposure to heat, such as due to pre-baking platform substrate 101. At 502, such Cu bond pads 106 are shown after exposure to heat, such as due to baking platform substrate 101.
Heat exposed bond pads may have some Cu oxidation 512, as generally denoted by shading, after heat exposure due to pre-baking. This heat exposure may be due to a pre-baking, namely baking prior to ball bonding. Optionally, a platform substrate 101 having such bond pads 106 may be non-plasma treated or may be CF4 plasma treated to sufficiently remove Cu oxidation 512 on such pads 106 for subsequent ball bonding for forming a quality intermetallic layer between a ball bond 104 and pad 106
At 503, bond via array 500 copper wires 103 are ball bonded with corresponding ball bonds 104 to heat exposed bond pads 106 as illustratively depicted in FIG. 5-3. Along those lines, FIG. 5-4 is an offset view with respect to the top-down view of FIG. 5-3 and with non-orthogonal wire bond wires 103 in order to more clearly delineate wire bond wires 103 ball bonded to pads 106 for extending away from an upper surface 110 of platform substrate 101.
FIGS. 5-3 and 5-4 are examples of bare Cu wire bond wires 103 ball bonded to corresponding bare Cu pads 106. Optionally, at 503, bond via array 500 PCC wire bond wires 103 may be ball bonded to heat exposed bond pads 106, where such bond pads 106 are bare Cu pads.
In an example implementation using thermal assist ultrasonic bonding, a tip of 12 thousandths of an inch (“mils”) (i.e., clearance between a bonding tool and a substrate surface before constant velocity (“CV”) is applied), a CV of 3 mils/millisecond (i.e., rate of descent of a bonding head from a tip), an ultrasound (“USG”) pre-bleed of 60 milliamps (i.e., USG level applied during an impact portion of ball bonding; pre-bleed is the amount of ultrasonic energy applied before a bond head touches down onto a substrate surface), and a contact threshold of 40 grams (i.e., equipment's reference force used to sense a bond head touching down onto a bonding surface, which may trigger application of various bonding parameters) were used. For this example implementation, USG current was in a range of approximately 180 to 250 milliamps; USG pre-bleed was in a range of approximately 60 to 120 milliamps; contact threshold was in a range of approximately 40 to 70 grams; bond time was in a range of approximately 20 to 40 milliseconds; and bonding force (“BF”) was in a range of approximately 20 to 70 grams. In such an implementation, 3 scrub cycles were used, with an X scrub amplitude of 4 microns and a scrub frequency of 300 hertz. Bond temperature was in a range of approximately 165 to 175° C. with a forming gas low of approximately 45 to 65 liters per minute (lpm) of a forming gas of approximately 95% N2 and approximately 5% H. Of course, these or other values and/or materials in accordance with the description herein may be used.
FIGS. 6-1 and 6-2 are respective map scan diagrams depicting exemplary cross-sections of map scans for a ball bond 104 using a copper-only wire bond wire 103 and a ball bond 104 using lead-coated copper wire bond wire 103, respectively. Ball bond 104 using Cu only wire bond wire 103 and ball bond 104 using Cu and Pd wire bond wire 103 are each bonded to a bare Cu pad 106 in accordance with the above description. Each of these ball bonds 104 respectively forms a metal-to-metal bond 611 and 612, each of which may include a quality intermetallic layer. Similar cross-sections may be obtained for ball bonds 104 to Cu pads 106 for a Tanaka CLR-1A wire bond wire and a Heraeus PdSoft wire bond wire, namely instances of ball bonds 104 for a PCC wire bond wire 103.
Along the above lines of forming an intermetallic layer, FIG. 7 is a spectrum graph depicting an exemplary spectrum analysis 700 of ball bonds 104 of FIGS. 6-1 and 6-2. Spectrum analysis 700 is kilo-electronvolts (“keV”) along x-axis 701 versus counts per second per electronvolt (“cps/eV”) along y-axis 702. Peaks 711 and 713 are copper or Cu peaks, and peak 712 is a Carbon or C peak. Other peaks are for Br, Pd, Cl, Ca, and P.
FIG. 8 is a histogram diagram depicting an exemplary mapped sum spectrum 800 for spectrum analysis 700 of FIG. 7. Mapped sum spectrum 800 has an x-axis 810 from 0 to 50% weight. Copper 801 and carbon 802 have the first and second amounts of material followed by oxygen 803 and silicon 804. Other elements 805 include Br, Pd, Cl, Ca, and P.
While the foregoing describes exemplary embodiment(s) in accordance with one or more aspects of the invention, other and further embodiment(s) in accordance with the one or more aspects of the invention may be devised without departing from the scope thereof, which is determined by the claim(s) that follow and equivalents thereof. Claim(s) listing steps do not imply any order of the steps. Trademarks are the property of their respective owners.

## Claims

1. An apparatus for an integrated circuit package, comprising:
a platform substrate having a copper pad without having a lower melting point interface layer;
an integrated circuit die coupled to the platform substrate;
a wire bond wire coupling a contact of the integrated circuit die and the copper pad;
a first end of the wire bond wire being ball bonded with a ball bond for direct contact with an upper surface of the copper pad; and
a second end of the wire bond wire being stitch bonded with a stitch bond to the contact;
wherein:
the wire bond wire is of a bond via array;
a first portion of the upper surface of the copper pad has thereon an organic solderability preservative layer;
a second portion of the upper surface of the copper pad is bare for the direct contact of the ball bond formed of the wire bond wire; and
the ball bond is formed with a thermal budget in a range of approximately 150 to 175° C.

2. The apparatus according to claim 1, wherein the wire bond wire is a lead coated copper wire.

3. The apparatus according to claim 1, wherein the wire bond wire is a bare copper wire.