Galanakis, Claire T.

Luijtjes, Neik

MacKay, Colin

Spletter, Phillip J.

English

Haverford College: Haverford Scholarship

Haverford College Haverford Scholarship Faculty Publications Chemistry 1989 Tape automated bonding inner lead bonding with a laser for high performance applications Phillip J. Spletter Claire T. Galanakis Neik Luijtjes Colin MacKay Haverford College Follow this and additional works at: https://scholarship.haverford.edu/chemistry_facpubs Repository Citation Spletter, Phillip J., et al. "Tape automated bonding inner lead bonding with a laser for high performance applications." Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 7.6 (1989): 1967-1970. This Journal Article is brought to you for free and open access by the Chemistry at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact nmedeiro@haverford.edu. Tape automated bonding inner lead bonding with a laser for highperformance applicationsPhillip J. Spletter, Claire T. Galanakis, Neik Luijtjes, Colin MacKay, Yong Jee et al.  Citation: J. Vac. Sci. Technol. B 7, 1967 (1989); doi: 10.1116/1.584658 View online: http://dx.doi.org/10.1116/1.584658 View Table of Contents: http://avspublications.org/resource/1/JVTBD9/v7/i6 Published by the AVS: Science & Technology of Materials, Interfaces, and Processing  Additional information on J. Vac. Sci. Technol. BJournal Homepage: http://avspublications.org/jvstb Journal Information: http://avspublications.org/jvstb/about/about_the_journal Top downloads: http://avspublications.org/jvstb/top_20_most_downloaded Information for Authors: http://avspublications.org/jvstb/authors/information_for_contributors Downloaded 29 Apr 2013 to 128.180.77.165. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvstb/about/rights_and_permissionsTape automated bonding inner lead bonding with a laser for high performance applications PhillipJ. Spletter, Claire T. Galanakis, Neik Luijtjes, Colin MacKay, Yong Jee and Ollie C. Woodard jj,ficroelectronicsand Computer Technology Corporation (MCC). Austin, Texas 78727 (Received 31 May 1989; accepted 24 July 1989) Since 1985, MCC has been developing the use of flashlamp pulsed Nd:Y AG (Y AG) laser technology to bond tape automated bonding (TAB) leads to I/O pads on integrated circuits (I Cs). The I/O pads ha ve 22 micron high gold bumps and the leads are copper plated, in the case described here, with electroless tin. As a result of the work presented here, a methodology has been developed that will result in a high throughput, reliable bonding process that overcomes the limitations of conventional thermo compression (TIC) and thermosonic (TIS) bonding technologies. In addition, because the position of the laser beam is software controlled, the process is very desirable for applications where a broad range of different IC form factors and sizes are to be bonded. Laser bonded devices have been exposed to degrading environments without failure, indicating the long term reliability of the process. I. BACKGROUND II. LASER SELECTiON Laser inner lead bonding was developed because of the re-cognized need for a benign bonding technology that was fast enough ( 100 + bonds/s) to be used in a production environ-ment and could bond small leads ( < 50 microns) on close pitches ( < 100 microns) without damaging the underlying substrate or semiconductor structures. Figure 1 shows a de-tailed drawing of the structure used for bonding develop-ment. Several laser technologies including CO2 , CW Y AG, Q-switched Y AG, and pulsed Y AG were chosen for initial ex-periments because they are mature technologies that are proven in production environments. Today, tape automated bonding (TAB) leads are all bonded with either thermocompression (TIC) or thermo-sonic (TIC) bonding techniques, both of which are mature and reliable methods. TIC bonding uses heat (up to 500°C) and pressure (up to 15000 psi), I With TIC bonding, either all of the leads are bonded simultaneously (gang) or individ-ually (single point) . TIS is another method of single-point bonding that uses heat, pressure and ultrasonic energy. Both of these bonding technologies have limitations. TIC gang bonding is commonly optimized for one specific integrated circuit (IC) type, and to change to different sized parts re-quires a different set of tooling which is time consuming to replace and reoptimize. This limitation can be overcome on many rcs by using single point bonding which moves a me-chanical bond tip to the appropriate location where bonding occurs. However, on many future res, the high number of 1/0 will require that the I/O pads be spaced so close that they will interfere with the bond tip when adjacent leads are bonded. A laser beam, with a precisely controlled amount of ener-gy that is very small in diameter, and no mechanical compo-nents in the bond site, is ideal for bonding leads on close pitches. In addition, sInce its position and the laser param-eters can be controlled by a computer, a broad range of dif-ferent part types can be bonded without time consuming tooling changeovers. CO2 was quickly excluded because radiation at that wave-length (10.6 microns) is reflected by the metals of interest such as copper, gold, and tin. This reflectivity is traditionally overcome by coating the metals to be join cd with an organic material such as flux to absorb the energy and transfer the heat via thermal conduction. 2 The use of additional coatings that must be removed after bonding is very undesirable, par-ticularly on devices that havc close pitches which make thor-F.!eciroles. Sn Plating S" Plated eu Lead 2.4 mils, 1.2 mlls SI Subslrate Pulsed Nd:YAG Laser Beam 330 mil. x 330 mils, 27 mils FI(;, L Cross section of bond structure llsed for laser bonding development. 1967 J. Vac. Sci. Technol. El7 (6), NovlDec 1989 0734-211X/89/0619S7-<14$01.00 Cf) 1989 American Vacuum Society 1967 Downloaded 29 Apr 2013 to 128.180.77.165. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvstb/about/rights_and_permissions1968 Spletter et al.: Tape automated bonding ough cleaning extremely difficult. Failure to dean residual organics can degrade the reliability of the circuit. Nd:Y AG lasers emit radiation with a fundamental wave-length of 1.06 microns. Copper and gold do not absor? much, energy at that wavelength (1.4% and 1.5%, respectlVely),' whereas tin absorbs 53.6%.4 Thus, for this application, Y AG lasers seemed the most appropriate. Q-switched Y AG lasers have high peak power for short periods of time (kilowatts for nanoseconds). As such, they can easily ablate material so they are often used for cutting and drilling and are not satisfactory for bonding. CW Y AG lasers have constant power output for the dura-tion of the pulse. However, experiments showed the energy could not be controlled well enough to prevent damage to the IC surrounding the bond site. When enough power was applied to initiate melting, the pulse could not be terminated fast enough so that heat was continuously applied to the bond site. Sufficient heat was conducted to the structures adjacent to the bond site to damage the chip. Flashlamp pulsed Y AG proved to be the most satisfactory laser technology for this application. With this technology, the pulse width can be short enough to allow melting with-out damaging the surrounding IC. In addition, with the ex-ponential rise and fall of the pulse that is characteristic to fiashlamp pulsed lasers, the power is distributed in a fashion that is most suitable for bonding. Y AG lasers can also be frequency doubled to get a wave-length that is more readily absorbed by the metals of interest, especially gold and copper. However, the conversion efficen-des of the doubling crystals are so low that the laser must be Q-switched to get enough power, but as is the case at the fundamental wavelength, the pulse width is too short to be useful for bonding. With frequency doubled CW lasers, the output power is too low to be useful and the pulse width is too long. Frequency doubling of fiashlamp pulsed Y AG la-sers has not been reduced to practice as of yet so the technol-ogy was not considered for this application. As a result, because of their availability and/or absorption and metallurgical characteristics, the optimal combination of materials and laser technology for this application is bonding tin plated copper leads to gold bumps with a fiash-FIG. 2. SEM photograph of typical laser honds (note localized hond zone and lack ofmclting on gold bump away from the bond interfacc). J. Vac. Sci. Technol. B, Vol. 7, No.6, Nov/Dec 1989 1963 FIG. 3. Cross section of tin plated copper lead bonded to a gold bump with pulsed Nd:Y AG lascr. lamp pulsed Nd:YAG laser. This combination produces consistent reliable bonds by taking advantage of the local-ized, concentrated energy of the laser beam and the absorp-tion and bonding properties of tin in conjunction with cop-per and gold. Photographs of typical bonds and a typical cross section are shown in Figs. 2 and 3, respectively. The laser that was used for bonding development was a 50 W avg. power pulsed Y AG from Carl Haas GmbH & Co in Schramberg, West Germany. It was fitted to a model 44 laser trimmer with linear motor beam positioning from Electro Scientific Industries in Portland, Oregon. III. BONDING PHENOMENA A cross section of a bond that shows the localized heating unique to laser bonding is shown in FIg. 4. This sample, which has been preferentially etched to highlight the grain structure of the copper, shows three distinct regions oftem-perature distribution in the bond. It should be noted that the copper lead was formed by electrodepositing (ED) copper. In the as-deposited state, ED copper has a very distinct grain structure. The top region, marked "A", contains copper that has been completely melted as evidenced by the lack of any grain structure. The middle region, marked "B", was heated FIG. 4. Cross section of tin-plated copper lead bonded to a gold bump. Copper has bcen preferentially etched to show dilference in grain structure indicated high temperature gradient in bonding process. Region A has been completely melted, region B has recrystalized and region C has not been altered. The compounds containing till have been displaced to region D. Downloaded 29 Apr 2013 to 128.180.77.165. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvstb/about/rights_and_permissions1969 Spiatter et al.: Tape automated bonding \""" ........... ___ .- '073C (All MP). 1013 (C:.J MP) ~ / 45CC (Sole,,) I ! i .i ~ ?3'C ISN MELTING POINT) ! ! ______ ?SC '. \ \ ~ K""All!CU Alloy (sage) FIG. 5. Notational time-temperature relationship of reactions during bond formation (not to scale). enough to recrystallize the grain structure but not melt it completely. Region "e" of the lead is virgin ED copper, indicating that the region stayed below any recrystallization temperatures. The bond is at the interface between region "C" and the bump. It is postulated that the bond is formed as follows: The tin absorbs a high percentage of the incident laser energy but, due to its low vapor pressure, does not evaporate. Rather, the molten tin enhances thermal coupling and produces inter-metallic compounds with gold and copper. The compounds, once formed, melt at temperatures below the melting pt)ints of the elements in independent states. See, c.g., a copper-tin phase diagram. The molten copper and tin flows around the outside of the lead and initiates melting ofthe gold on top of the bump. The molten material also decreases the thermal contact resistance in the interface between the copper lead and the gold bump so that conductive heat transfer is maxi-mized. In this way, atomic copper and gold are introduced into liquid phases early in the laser pulse/temperature cycle so that full alloying is accelerated and enhanced while being contained in a small region. With molten material in the bond interface, the laser ener-gy is funy coupled to the lead and the bump so that a com-plete metallurgical bond is formed. Finally, solidification oc-curs from the inside out by conduction of the heat to the outside where convection carries the heat away. Thus, the gold and copper alloy solidifies first. Any ternary com-pounds at this stage are still molten and are displaced to the joint exterior by the advancing solidification front because the solubility oftin in copper and gold is low. The net result is that any copper/gold/tin intermetallic compounds solidify on the exterior of the copper/ gold alloy bond. In this loca-tion they do not have any effect on bond strength or reliabil-ity. This area, which is very tin rich, is marked "0" in Fig. 4. A notional time vs. temperature profile, shown in Fig. 5, illustrates the various stages of reaction taking place during the bond formation process. A simple experiment gave further indication of the local-ized nature of laser bonding. A 10 micron high bump was located over a seven micron thick layer of silicon dioxide. J. Vac. Sci. Techno!. B, Vol. 7, No.6, Nov/Dec 1989 Cia"""" lBS: Lesd a"",k In Span lBP: Le.d e1t!8'" Pad LS: I.ea!I Sopo,., .... ""'" Bump BF:Bu",!,F""""", BL:Bu"'l'UIt SC: __ Cmlering 1969 Dage Frecima 8T~~ MICf'(ltesster sc Holding by V8<;uum FIG. 6. Method and equipment for determining bond quality and locations of possible failures in the chiplbump/bond/Jead structure. Under the silicon dioxide was a diode with a constant cur-rent applied. The voltage change in the diode, a function of temperature, was monitored and recorded with a storage oscilloscope during a bonding cycle. The measured voltage change in the diode corresponded to approximately a 100 °e temperature rise. IV. BOND QUALITY AND RELIABiliTY After bonding, the leads are mechanically stressed to de-tennine the quality of the bond. Figure 6 schematically shows the set-up. The IC is held to the stage with vacuum and the TAB lead frame is clamped to the fixture with a portion of the lead exposed so that a sman hook is used to lift the lead until some portion of the structure fails. The loca-tion of the failure and the amount of force that was required to induce the failure are both recorded. Figure 6 also lists the six possible failure mechanisms and the location in the struc-ture where they occur. The most desirable failure is when the lead breaks in span indicating that the bond and the rest of the structure is at least as strong as the copper lead. However, when there are failures elsewhere in the structure they are deemed accepta-ble if their pull strengths are at least 25 grams. Figure 7 (a) shows the cumulative probability offailure vs pull strength while Fig. 7 (b) shows the failure frequency vs pull strength for the possible failure modes. The units for failure frequency are an arbitrary scale from zero to 100 and not percentage. In order to prove the long term reliability of the process that has been discussed in this paper, a series of devices were bonded and subjected to environmental testing. In all, 60 CMOS les with 48 leads on each one were bonded, then divided into six sets often. Each set ofICs was then subjected to one of a number of tests (see Table I). Most tests were performed in accordance with the provi-sions ofMIL-STD-883. Downloaded 29 Apr 2013 to 128.180.77.165. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvstb/about/rights_and_permissions1970 Spletter et al.: Tape automated bonding 100 l ~ 80 :c 11 60 2 Co f! 40 .a ;f 20 C I) 20 40 60 80 100 120 lal Pull Strength (gram) 100 :w II Le." Break in Span :::I 110 .ci III Lead Break "I Pad !. Ii! Lead Seperalion ~ m Bump Fracture s: 60 0 Bump Lin G.> III SUDstrate Crater [ 40 OJ. e .a : 20 0 0 20 40 60 811 100 120 (bl Pull Strength (gram) FIG" 7" (a) Cumulative probability of failure vs pull strength" (b) Pull strength vs failure frequency for various failure modes" TABLE L Environmental tests performed on laser bonded devices" Test Autoclave High temp storage 85/85 Moisture resistance Liq-to-liq temp shock Air-to-air temp cycle Environment Duration 121 "C, 100% KR and 15 psig 96 h 150'C 1000 h 85 'C and 85% K R "- lO'C@2"7%KR to 65'C@95%KR - 55 'C to 125°C - 55 "C to 125 "C 1000h 50cydes 1000 cycles 1000 cycles J. Vac. ScI. Techno!. S, Vol. 7, No.6, Nov/Dec 1989 1970 TABLE It Failure modes and failure forces lor laser bonded devices after environmental testing" Failure mode Lead break in span Lead break @ bond Lead lift from bump Bump lift from die TOTAL Avg force 55"66 gms 39"91 gms 26"81 gms 34"24 gms 39"10 gms % of failure 10081 66"38 16A9 06.32 100.00 The devices were removed from the environments at peri-odic intervals and electrically tested" The entire 100% of the bonds passed the tests. After the environmental testing was complete, the leads were then pull tested. The results are summarized in Table II. As can be seen from Table II, the bonds, and in fact the entire structure, remained very strong during and after expo-sure to the severe environments. V. CONCLUSIONS The process described takes advantage of the unique prop-erties of the localized, concentrated energy in a focused pulsed Y AG laser beam io reliably bond tin plated copper leads to gold bumps on les. The process relies on the rapid alloying of copper and gold with tin as an enhancer of cou-pling the laser energy to the bond site. The tin also causes the copper and gold to melt at lower temperatures than in their elemental state, and promotes conductive heat transfer in the bond without forming the brittle intermetallic com-pounds often found in gold tin alloys formed with conven-tional methods such as soldering" Because ofthe extreme flexibility of a computer controlled laser and laser beam positioning system, the process has very important implications for manufacturing, especially in those environments with a broad product mix. While much work remains before this process is used in high volume manufacturing, the potential and the feasibility of the pro-cess have been established here. 1 A" Keizer and D" Brown, Solid State TcchnoL 30( 3), 59 (1987)" "E Burns and C Zyctz, Electron" Packaging and Pro{l, VoL 21, no" 5, 109 (1981)" 'D. E Gray, American Institute oj Physics Handbook, 3rd edition, (McGraw- Hill, New York, 1972)" 4K Eo Lindquist and A" W" Ewald, Phys. Rev" 135, AI91 (1964)" Downloaded 29 Apr 2013 to 128.180.77.165. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvstb/about/rights_and_permissions

Tape automated bonding inner lead bonding with a laser for high performance applications

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Tape automated bonding inner lead bonding with a laser for high performance applications

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