Doped and Low-Alloyed GoldBonding Wires

Ch Simons, L Scbrdpler and G Herklotz

W C Heraeus, Materials Technology Division, Technology Centre,Heraeusstrasse 12 -14, D-63450, Hanau, Germany

E-mail: Lutz.Schraepler@heraeus.com

Received: 1 February 2000

Fine gold wires with diameters down to 15p.m are used for bonding semiconductor devices. Increasingminiaturization of electronic circuits calls for smaller wire diameters. This requires the development of gold wireswith increased mechanical strength and good bondability to replace thicker wire diameters without detriment to

their functionality. The first gold bonding wires were pure gold, and then doped gold wires of high purity weredeveloped because of their stabilized and improved mechanical characteristics. Nowadays and in the near futurelow alloyed gold wires are increasingly replacing doped gold wires. This paper gives an overview of the mechanicalcharacteristicsof doped and low-alloyed gold bonding wires and their bondability.

Bonding wires serve as electrically conductingconnections in integrated and highly integrated circuits(ICs), in automobile, power, and hybrid components.Usual diameters range from 500rm down to l Sum.Highest demands are made on the reliability and qualityof the wires and resulting wire connections. The mostimportant prerequisites are the uniformity andhomogeneity of the wire and its mechanical and electricalcharacteristics.Thus, bonding wire producers are requiredto achieve the corresponding properties whilst meetingthe highest quality demands.

For automobile, power, and hybrid components so­called thick wires are used from l Ouprn to 500rmdiameter. This wire range is usually made of aluminiumand aluminium alloys in order to keep metal costs as lowas possible.

In ICs and in components where elevated processingtemperatures are possible, gold bonding wires find theiroptimal application. Moreover, the bonding technologyused for gold bonding wires (thermosonic ball-wedgebonding) allows a bonding process about twice as fast asthe technology used with aluminium alloy wires(ultrasonic wedge-wedge bonding). Due to the increasedcost of the metal, gold bonding wires are usually restrictedto diameters from l Ouprn down to l Sum, the so-calledfine and ultra-fine wire ranges.

Gold wire bonding technology began with larger wirediameters within this range. Increasing miniaturization of

c§9' GoldBulletin2000, 33(3)

electronics made a reduction of wire diameters necessary.While chip sizes are still decreasing, their functionality isincreasing and thus the number of electrical connectionsis also increasing. This in turn leads to smaller connectionpads and pitch sizes. In accord with the change inconnection geometries, the most commonly employedwire diameters have been decreasing from ca 30rm to25rm in recent years and will now move towards 20rmwithin the next three years. Simultaneously the technicaldemands on the bonding wires have stayed constant, egthe absolute loads, stresses, and electric currents which thewires have to withstand.

BONDING PROCESS ANDAPPROACH TO GOLD BONDINGWIRE DEVELOPMENT

In order to obtain a more thorough understanding of theimportance of the wire characteristics, the bondingprocess itself (thermosonic ball-wedge bonding) must firstbe briefly considered. Detailed descriptions and schematicdrawings are given in, for example, references 1 - 4. Atthe start of the gold bonding process, a gold ball is meltedby means of an electrical discharge at the wire tip, ie theso-called electric flame-off When melting the ball, theneighbouring wire area is influenced by the heat transferand a heat-affected zone is formed which can determine

89

new higher-strength wire materials. Wire materials withimproved mechanical characteristics allow smallerdiameters to be used at constant stresses. Figure 2 gives anexample of how this happens in principle. A diameterreduction from 30rm to 20rm requires a wire with morethan double the strength in order to withstand constantabsolute stresses, as strength, 0", is proportional to 1/d2,

where d is the wire diameter. In addition, bonding wireswith increased mechanical strength permit the moreprecise and ambitious forming of bonding loops. Whilekeeping wire diameters constant, higher strength wirematerials allow very low and long bonding loops withstable loop geometries. Thus, short circuits between twoneighbouring wires or between the wire and the systemcarrier, which might occur due to the so-called sagging orsweepingof the wire within its loop, can be prevented.

1 Usual description in the Far East

1 Low Doped Gold Bonding WIres / High PurityGold Bonding WIres]

As a result of its softness and adhesive qualities and itsinertness in most environments, gold offers many goodcharacteristics for a bonding process. The earliest bondingwires were therefore made of pure gold. 'Pure gold' doesnot necessarily contain constant impurity levels, however,and it easily recrystallizes during the drawing process.Consequently, the strength obtained is neither constantnor sufficiently high, especially if smaller diameters arerequired. Thus, at a very early stage, gold materials weredeveloped with defined levels of doping additions. It isnecessary to start with high purity gold having 99.999%purity as the basic material in order to obtain reproduciblecharacteristics and meet the quality required. The maindopants were alkali, alkaline earth and rare earth elements(1, 3, 6 - 9). The best known element of this group is

The most important tool used to tailor mechanical andelectrical characteristics of bonding wires has been andstill is the chemical composition of the wire material (5).Though optimized microstructures of the same material,created by appropriate drawing and annealing steps, leadto materials with increased strength, the ductility andelongation characteristics simultaneously diminish. Thiseffect leads to problems in forming the loop geometryrequired and in achieving a faultless bonding joint. Bothprocesses need appropriate plasticity and softness of thewire material.

COMPOSITION OF GOLD BONDINGWIRES

20 25 30Wire diam eter [urn]

800 ,-- - -,-- - --r-- - - r-- - -y-- - -,

e 700t---+--------1-----+----+-------1::E- 600 -j----f- ---t----t----j-------j.cbn 500 +-----+-- --+-----+---+----------jc~ 400 +-----f---- -I----I----f-----1

'"~ 300 t-- - -+- - - f-- --f---+------1Ow;c 200+-----f----f-----+--~

100+-----f----f------+------+--------j10 15 35

Figure 2 Constant loads at reduced wirediameters demand higherwirestrengths

the loop size and this will be discussed below. The ball isthen friction-welded to the chip side bond pad by amechanical and ultrasonic impact and forms a trumpet­like bond, a so-called ball bond. The ball geometry and itshardness/strength determine its deformation duringbonding and whether a faultless joint to the chip pad isachieved. Subsequently the wire loop is formed. In thisprocess various movements of the bonding tool arepossible, some of which are complex. In order to permitthese movements, the wire has to show appropriatestrength and plasticity. At the loop end the wire is joinedto the carrier side pad. A mechanical and ultrasonicimpact leads to a second bond of flattened circular shape,the so-calledwedge bond. Finally the wire is torn off at apredetermined weakened point of the wedge bond andthe procedure is repeated beginning with melting the goldball. The bondloop obtained is illustrated in Figure 1.After completing the bonding process, the chip and wireloops are injection moulded with a polymer. In thisprocess the wire loops are stressedboth by the injection ofthe polymer and by the shrinkage due to subsequentcooling of the polymer.

The requirement of capability to withstand thehigh stresses described above and, moreover, thecontinually decreasing pad and pitch sizes which requiresmaller wire diameters compel the bonding wire producerto optimize his fine-wire drawing technology and to create

Figure 1 Bond loop

90 (ei9' Gold Bulletin 2000, 33(3)

WIRECHARACTERISTICS

_ low alloved ,. ~Cnh3J1C<:..J do jed , purelow doped , I'igh purity

f----

1----

f----,

f--i I III

I

1 Wire Material Strengthening

Figure 3 permits a direct comparison of the various dopedand alloyed classes of gold bonding wires. The strength­elongation curves represent various microstructural statesgenerated by a cold working process and an annealingtreatment. The high strength / low elongation ratio relatesto the hard drawn wire state, and the low strength / highelongation ratio describes the recrystallized condition.Typically the curves follow the slopes shown, as anincrease in elongation results in a decrease in strength.Thus, different levels of strength are shown over a wideelongation range which adequately covers all the technicalrequirements of a successful bonding process. In order tosimplify the comparison the 4% elongation point is quite

lower temperature, and also for low and long loops,and for fine pitch/fine size bonding.The amount of alloying elements is restricted mainlyby the following considerations:

Alloying elements increase electrical resistivity(see below).Some alloying elements lead to non-conformingball geometries and to oxidation problemsduring the ball melting of the bonding processand decrease the corrosion stability in the longterm application. These are in particular non­precious metals such as Ti, Zr, Fe, Ni, Co, etc.More highly strengthened materials usually showa higher hardness which might become aproblem during the bonding process becausegood ball deformation and an appropriate softball are necessary to create a stable bond jointwithout damaging the substrate material.

Thus, in general, the use of higher alloyed goldbonding wire materials is severely restricted.

20IG8 12elongation [%]

4

GOO~ 550c!: 500~ 450

t 400~ 350~ 300~ 250~ 200

150100

0

Figure 3 Strength-elongation diagram ofgoldbonding wires

3 LowAlloyed Gold Bonding Wires with HighestStrength

The latest developments have brought low-alloyed goldbonding wires on to the market. These gold alloysusually contain up to 1% alloying elements, in somecases up to 5% (14, 15). The most important alloyingelements are the precious metals Ag, Pd, Pt, and alsoCu (10, 16 - 18). Other elements are also used, butpreferably at lower concentrations to minimize thedanger of corrosion (19). The alloying elements areaccompanied by doping elements from the groupsmentioned above. These wires are now starting to beused in Europe and already have a market share ofabout 30% in Asia. The advantages of these wires areobvious: the great increase in strength enables muchbetter workability even with difficult bondinggeometries. With an additional increase in hightemperature strength they reveal a higher thermalstability and thus a wider field of application. Thesewires are especially suitable for new bondingtechniques using higher speed, higher frequency and

beryllium. Thus, the oldest standard wire material is aberyllium low-doped wire. The dopants are usually in therange up to about 50 ppm.

2 Enhanced Doped Gold Bonding Wires withHigher Strength / Pure Gold Bonding Wires]

In order to meet the need for wires of greater strength,doped wire materials were developed at a later stage byusing appropriate combinations of several dopingelements up to about 100 ppm (7, 10). Dopant elementsused were those of the alkali, alkaline earth and rare earthgroups as well as elements of Groups 3 and 4 of thePeriodic System. Here the composition of dopingelements plays an important role. For example, in anumber of patents the following elements arerecommended: K, Be, Mg, Ca, Ba, La, Ce, Zr, Ga, In,Ge, Pb, Cu, Ag, Pt, Pd, B (1, 7, 11 - 13). Exact data areusually not available as these belong to the proprietaryknowledge of the bonding wire producers. Also, exactanalytical data are difficult to obtain as the individualdopant element may be present at levels of under 10ppm. Reliable comparative measurements in this rangeare difficult to obtain due to lack of reliable calibrationstandards and the presence of small additional impuritiesarising from the melting and casting processes. Thesemore highly doped gold materials are used for moredifficult loop geometries and offer better strengths and ahigher thermal stability. A major part of current bondingwire consumption is covered by enhanced doped / puregold wires.

(ei9' GoldBulletin2000, 33(3) 91

useful as it represents a WIre condition that is oftenrequested.Doping of pure gold material is assumed mainly tostabilize its grain boundaries. Thus, a stabilization ofstrength characteristics is obtained even with low dopingconcentrations. This effect is enhanced by usingappropriate concentrations and combinations of dopingelements. As the characteristic doping elements (alkali,alkaline earth and rare earth elements) have very low oreven zero solubility in gold (20, 34) in the solid state, theyshould precipitate out as intermetallic compounds on thegrain boundaries during solidification, thus stabilizingand pinning the grain boundaries and hinderingdislocation movement in subsequent deformationprocesses. The work hardening effect and therecrystallization temperature are increased leading toincreased strength. During annealing, grain growth isrestricted by the large number of grain nuclei and bypinning or hindering the movement of grain boundariesresulting in a finer grain structure with stabilized grainboundaries. Thus the higher strength is also effective inthe recrystallized state. However, exact metallurgicalinvestigations of the effect and location of dopingelements in gold have not been carried out and this fielddeserves further intensive work (2, 3,5,21, 34).

With an enhanced doped / pure gold wire materialthe improvement in tensile strength is about 20% at thesame elongation levels compared with low doped / highpurity gold materials. So a tensile strength of ca260 MPais achieved at 4% elongation whereas low doped / highpurity materials reach ca210 MPa.

Obviously, the greatest increase in strength is obtainedby the low alloyed gold materials. In the cast state, thesematerials already show much finer grain structures thancast doped materials because the higher impurity levelsprovide numerous nuclei for grain formation at manylocations in the solidifYing melt (22). Figure 4 illustrates acomparative example. As the commonly employedalloying elements usually have a significant solubility in thegold, a solid solution strengthening effect is also obtained.Work hardening is therefore increased by solid solutionhardening and grain refining effects, leading to muchhigher strengthening levels and also to increasedrecrystallization temperatures (22). Precipitationhardening, which gives stronger effects than those of thedoping elements discussed above, can be obtained, forexample by titanium (23, 24). However, this approach isnot yet commonly used because it is generated mainly byalloying elements that increase the corrosion risk and oftengive ball hardnesses which are too high. The strengths oflow-alloyed gold materials are around 20% higher thanthose of enhanced doped / pure gold, ie an increase from

92

Figure 4 Cast strands ofdopedgold(Figure 4A) and lowalloyedgold(Figure 4B), scales 1mm

ca260 MPa (enhanced doped / pure) to ca310 MPa (low­alloyed) at the 4% elongation level.

2 High Temperature StrengthGold bonding wires usually have to withstandtemperature exposures of up to ca 150°C, and in recentautomotive engine applications up to 200°C. Thebonding process itself is usually carried out with heatedsubstrate materials up to 150°C, in some cases up to250°C, in order to improve the welding of wire and padmaterial at the bonding point. After bonding a mouldingpolymer covers the complete system. Moulding polymersreach temperatures of up to ca 200°C. Thus, shrinkageoccurs during cooling and stresses the wire especiallyat itsweakest locations. In addition, some applications requirestability for long term exposure at 200°C. As thetemperature exposure will lead to a decrease in strength,knowledge of the high temperature strength is necessaryto enable every user to choose the most suitable wire. Thehigh temperature strength of gold bonding wire materialsis therefore compared in Figure 5. By analogy with theeffects described in Section 1 above, a considerableincrease in high temperature strength is achieved by

(ei9' GoldBulletin2000, 33(3)

Figure 5 Hightemperature strength ofgoldbonding wires after250°C exposurefOr 20sec

enhanced doped / pure gold wires, but here again thehighest strength values are obtained with low-alloyed wirematerials. The reasons for this behaviour are directlyconnected with the enhanced basic strength and themicrostructure stabilizing influence of the doping /alloying elements described above.

o Tensile Strength [MPa]o Elongation [%]

15pm15

18

o

3

1 2 ~2....c

9 .2~co

6 ~

enha nced low alloyeddoped I pure

low doped Ihigh purit y

.---f--

f--

- f--

f-- n n nf--

o

250

50

300

£~ 200..ctilE150Vi.!!.;;; 100c~

3 HeatAffectedZoneThe melting of a gold ball at the beginning of everybonding process influences the adjoining wire area.Thermal conductivity causes a recrystallized length of thewire in the zone near the ball, the so-called heat-affectedzone (5, 10). This zone is disadvantageous because it hasa coarser grain structure and therefore lower strength(Hall-Perch relationship). While bending the wire fromthe ball bond to the required loop height, critical stressesmay occur in the heat affected zone. When forming lowloop geometries, the length of this zone is the mostcritical factor. Figure 6 shows a wire with a melted goldball at the wire tip and its heat affected zone. In Figure 7the gold ball is bonded to the substrate pad, but the

Figure 6 Section through a lowalloyedgoldbonding wirewithmeltedgoldballand heataffictedzone

Figure 7 Photograph ofa bonded lowalloyedgoldwireshowingballbondwith defOrmed balland heataffictedzone

neighbouring heat affected zone is only slightly deformedin order not to overstress the wire.

Figure 8 shows a comparison of heat affected zones ofthe different wire groups. The recrystallization behaviour ofthe wire material determines the length of this zone.According to the explanations in Section 1 above, thelength of this zone is reduced with increasingconcentrationand appropriate type of doping / alloying elements.Minimum heat affected zones well below 100 rm can beobtained by using low-alloyed gold bonding wires. In thisway the formation ofvery low and stable loops is possible.

4 ResistivityAs the electrical resistrvity increases with theconcentration and type of alloying element, control ofthis property may be crucial for some applications.Doping in the range described here leads to negligibleincreases in resistivity. Low-alloyed wires have resistivitiesca 50% higher than doped gold. Figure 9 gives theresistivities of the gold wire materials. For alloying rangesup to 5% the increase in resistivitycan be estimated usingthe Matthiesen rule.

(ei9' GoldBulletin2000, 33(3) 93

Figure 9 Electrical resistivity ofgoldbonding wires

BONDABILITYAND RELIABILITY

Figure 8 Heatafficted zones ofgoldbonding wires (wire diameter25J1m, balldiameter 40Jlm)

The bondability of a wire is mainly described by themelting of a gold ball, the deformation process of the ballduring bonding, and the formation of a stable bond joint.In addition to the influence of the bonding equipment,ball melting and deformation are determined by the wire,whereas the bond joint is determined by both wire andpad. The pad surface and layer characteristics play animportant role, ie cleanliness and roughness of the pad

LowAlloyed

6560

Enh ancedDoped/Pure

W ire Type Low Doped/High Purity

Hardness Hv 55

on the equipment. Using defined vanation of theseparameters, a process window for the most suitableparameters can be achieved for every wire type on everybonding machine.

The resulting ball hardness is a function both of theseparameters and of the wire material. The hardness andstrength of the ball are determined by its chemicalcomposition and the microstructure. Thus the dopingand alloying elements and the ball melting processdetermine hardness and strength. High strength wirematerials result in slightly higher strength and hardness ofthe wire balls under comparable melting parameters ascan be seen from Table 1. Higher hardnesses than those ofthe low-alloyed gold wire balls are limited by the dangerof chip cratering, ie the impact of a ball that is too hardmay damage the chip.

After melting the ball, the subsequent bonding processbegins by pressing the ball onto the chip pad. Here the ballis deformed under the parameters of static bonding forceand ultrasonic power impact. A typical deformed ball afterball bonding is illustrated in Figure 7. The balldeformation is given as the diameter of the bonded ball inthe bonding plane. By varying the parameters of bondingforce and ultrasonic impact a processwindow for a definedball deformation ratio (ie bonded ball diameter: meltedball diameter) can be achieved for different wire types asshown in Figure 10. Low-alloyed wires show less sensitivityto variations in these parameters and therefore allow awider process window for the bonding machine thandoped wires. Low doped / high purity and enhanceddoped / pure wires do not differ very much and aretypified by the curves in Figure lOA).

The bond joint generated has to reveal a pore-freematerial junction between wire and pad in order to givefaultless function. Nevertheless, during extended workingat higher temperatures diffusion processes occur betweenthe gold of the wire and the aluminium of the pad,leading to the formation of various AuxAly intermetallicphases in the interface area combined with the generationof Kirkendall voids mainly at the interface between theintermetallic phases (1, 25 - 29). Both intermetallicphases and Kirkendall voids influence the strength of thebonding joint and determine the long-term reliability ofthe electronic component. Whereas the intermetallicphases may lead to a strengthening effect in the bondjoint, the Kirkendall voids may increase to the size of

Table 1 Average BallHardness ofDifferent 1Jpes ofGoldWire

low alloyedenha nceddoped I pure

low doped Ihigh purity

_ .---

~-- Hea t Affected Zone

-

0-

-

D~--

180.. ,;;,160

~ ~1 40Jl ~ 120.::: ~ 100~ 1l 80eo t:l 60c ..j ~ 40

20o

surface, appropriate geometries and construction of thelayers and their chemical composition and physicalcondition. The melting of a gold ball has to create a ballof good conformity, ie the ratio between ball width andball height, and high reproducibility. Although dopingand alloying elements can affect the ball geometrydisadvantageously (5), the exact spherical shape may beless important as good bonding can also be achieved withnon-spherical balls. The main factors determining highreproducibility and appropriate geometry are the meltingtime and electric current during the electric flame offTypical values for ad = 25rm wire with a ball size of 1.4d- 2.4d might be 1.5 - 2 ms and 20 - 25 rnA, depending

.§ 0.05....E 0.04EE 0.03

D~.c2.

002 ~D D0:~ 0.01 1-1'"'5 0.00 '-,~ low doped I enhanc ed low alloyed

high puri ty doped I pure

94 (ei9' GoldBulletin2000, 33(3)

Figure 10 BalldefOrmation ofgoldbonding wires vsbondingparameters ultrasonic power and bondfOrce (wirediameter 25J1m, balldiameter 40Jlm); fOr dopedgoldbonding wire(Figure lOA) and lowalloyedgoldbonding wire(Figure lOB)

I~

II

-~

"",.,.".,.-

Figure 11 Section through a temperature exposed (J50°C, 500h)lowalloyedgoldballbond

such as AU4Al are dominant. Our own results with long­term temperature exposures show slight advantages of areduced intermetallic phase thickness for bonds with low­alloyed wires (Table 2). Accordingly, the size and quantityof Kirkendall voids should be reduced for the low alloyedwire bond. The first Kirkendall voids in visible pore sizeshave been detected for exposures at ISO°C and toooi, fora doped wire bond, whereas a low alloyed wire bondshows visible pores only after more challenging exposuresat 200°C and SOOh.

Reliability tests prove the bond joints and wirestrength in the bonded condition under exposure todifferent relevant parameters such as temperature andhumidity (30). An important test for the ball bondjoint is the shear test in which a tool moves parallel tothe chip surface contacting the ball bond andshearing it off For the pull test a hook is locatedunder the bond loop and pulls the loop until it tears.Different hook positions relative to the loop canevaluate different wirelloop zones.

Reliability tests after long-term temperature exposureindicate higher shear strength values for the low alloyedwires, as illustrated in Figure I2A. Figure I2B shows thatthe pull strength of low-alloyed wires also lies slightlyabove that of doped wires. Wedge bond testing by thepull test also shows slightly better values for the alloyedwires (Figure I2C). Low-doped / high purity and

17

17

16

16

13 14 15

US-Power [%J

400m N- 350m N- 300m N

13 14 15US-Power [%J

400 mN- 350 mN- 300 mN

12

12

5011

5011

66

64

E 622-c 60.g<'l

E 58...J:.,

560;:;

54I:l:l

52

B

A

(,6

64

E (,22-c.g 60<'l

E.. 58J:.,0 56;:;I:l:l

54

52

Table 2 Thickness oflntermatallic Phase ofTemperature Exposed(J50°C, 500 h, GoldBallBonds

pores and link up leading to a separation between phaseinterfaces, thus weakening or even destroying the bondjoint. Figure 11 demonstrates the generation of anintermetallic phase mainly growing into the wire side ofthe bond after temperature exposure of ISO°C for SOOh.As the phase growth is mainly observed in the direction ofthe wire side, gold-rich compositions of the AuxAly phases

W ire Type Low Doped/High Purity

IntermatallicPhase Thickness 3.8 IJm

Enhance dDoped/Pure

LowAlloyed

3. l lJ m

(ei9' GoldBulletin2000, 33(3) 95

SUMMARY AND FUTURE OUTLOOK

The characteristics ofdoped and low-alloyedgold bondingwires have been elucidated as well as their bondability andreliability. Whereas low doped / high purity gold bondingwires were mainly used in the past, enhanced doped / puregold bonding wires have now become the standardbonding wire material. They will be replaced now and inthe near future by low-alloyed gold wires as these materialsshow many advantages compared with the standard dopedwire materials. A general improvement in wire strength isaccompanied by an improvement in bondability andreliabilityof the bonding joint.

Nevertheless, much work is still needed to developfurther optimized wire alloys. The most restricting obstaclesare the requirement ofelectrical resistivity very close to thatof gold itself, the condition of the pad-side layers whichdemand fairly soft gold alloys in order to withstand thebonding process without any cratering defects, and thenecessary resistance to corrosion by contact with gaseousand solid environments. These demands permit only a fewand very low concentrations ofalloying elements. Thus thedevelopment of more highly alloyed gold wires has not yetreally started to accelerate. The focus will be on the lowalloyed gold wire materials in order to match the differentdemands with the most suitable wire. Furthermore, highlydoped wire materials with enhanced strength might beoptimized in order to combine the qualities ofdoped / pureand low-alloyedwires.

Of special interest may be particle / metal reinforcedwires (31 - 33) which reveal interesting materialscharacteristics. However, the bondability of these wiresstill has to be established.

1000

1000

1000800

800

800

400 GOOTime [hI

400 600Time [hI

400 GOOTime [hI

200

200

I II :

_ Iow alloyeddoped

IIIII

_ low alloyed -doped -

I

-low alloyeddoped _

r

c

A

B

11 .00

Z 10.50

~ 10.00

~ 9.50_ 9.00tS 8.50

8.00

7.50o 200

Figure 12 Reliability tests of25J1m goldwirebonds aftertemperature exposure at 150°e- shear test(Figure12A);pull test, hooknearballbond (Figure 12B);andpull test, hooknearwedge bond (Figure 12C)

enhanced-doped / pure gold wires are shown in one curvedue to the negligible differences in reliability values.

It is remarkable that no test reveals a degradation of thebonds. Instead the pull and shear forces obtained increaseslightly at the beginning of the temperature exposurebecause of the strengthening due to interdiffusion inadjoining areas in the contact zone between wire and padand also due to the formation of intermetallic phases.Whereas pull forces remain fairly constant over long-termtemperature exposure, the shear strength increases distinctlywith exposure time for the low alloyedwire bonds.

ABOUTTHEAUTHORS

Dr Christoph Simons was a development project managerin the Technology Centre at WC Heraeus in Hanau untilJanuary 2000. He was mainly engaged in the developmentof and production improvement processes for bondingwires. He is now with Leybold Materials in Hanau.

Lutz Schrapler is the worldwide technical coordinatorfor bonding wires at W C Heraeus, responsible fordevelopment and production technology at all theHeraeus wire production sites.

Dr Gunther Herklotz was head ofdevelopment at theHeraeus Technology Centre until June 1999, responsiblefor electronic and packaging materials, and now acts as aconsultant on these topics.

Continued on page 102

96 (ei9' Gold Bulletin 2000, 33(3)

ABOUTTHEAUTHOR

Dr Mark Humphrey completed his PhD with ProfessorMichael Bruce at the University of Adelaide, Australia, in1987. He then undertook postdoctoral research withProfessor Gerhard Erker at the Universitat Wiirzburg,Germany (1987-1989) and with Professor John Shapleyat the University of Illinois, USA (1989-1990). He wasappointed to a Lectureship in Chemistry at the Universityof New England, Australia, in 1990, moving to theAustralian National University in 1994, where he iscurrently an ARC Australian Senior Research Fellow andReader in the Department of Chemistry. He is the authorof one hundred scientific papers, many dealing with thenonlinear optical properties of organometallic complexes.

REFERENCES

S.R Marder, J.E. Sohn andG.D. Stucky (Eels), Materials fOr Nonlinear Optics: Chemical

Perspectives,ACS Symposium Series 455,ACS, Washington DC,1991

2 H.S. Nalwa andS. Miyata (Eds), Nonlinear Optics of Organic Molecules

CRC Press, Boca Raton, Florida, 1997

3 R.L. Sutherland, Handbook ofNonlinear Optics, Marcel Dekker, New York, NY, 1996

4 J.Zyss, Nonlinear Opt., 1991, 1,3

5 S. Houbrechts, I. Wada, H. Sasabe, J.P.L. Morrall, I.R. Whittall, AM. McDonagh,

Continued frompage96

REFERENCES

1 Avr Report 4,Technologie des Drahtbondens, Germany, August 1991

2 'High Technology BondingTools', Gaiser Tool Company, Ventura, California, 1989

W Kohl, 'The Interaction of Material and Machine Parameters in Thermosonic

Bonding',translation ofa reprint from 'DerElektroniker' Issue 6/1990, AT-Fachverlag,

Stuttgart, Germany, 1991

4 Merkblatt DVS 2810 Drahtbonden, (instructionalleaJIet, wire bonding), Germany,

1992

5 S.Tomiyama andY. Fukui, Gold Bull., 1982, 15(2), 43

6 W Kohl, 'Metallurgical Aspects ofWire Bonding', printed byHeraeus, 1990

7 ASTM-F72

8 Japanese Patent Application 5-179375 A

9 Japanese Patent Application 5-179376 A

10 I.H. Rarnsey, Solid State TechnolofJ, 1973, Oct, 43-47

11 German Patent Application DE3936281 Al

12 European Patent Application 0743679 A2

13 German Patent 1608 161

14 US Patent 5491 034

15 Japanese Patent6-112251

16 European Patent 0288 776

17 German Patent Application DE19733954Al

18 European Patent Application 0761831 Al

19 Japanese Patent Application 52-051867 A

102

M.G. Humphrey andA Persoons, Nonlinear Opt., 1999,22, 165

6 I.R. Wbittall, M.G. Humphrey, A Persoons andS.Houbrechts, Org.anOirImrllics, 1996,

15,5738

7 S. Houbrechts, C. Boutton, K. Clays, A. Persoons, I.R Whittall, R.H. Naulty, M.P.

Cifuentes andM.G. Humphrey,] Nonlinear Opt. Phyi Mater., 1998,7,113

S. Houbrechts, K. Clays, A Persoons, V. Cadierno, M.P. Gamasa, J. Gitoeno, I.R

Whittall andM.G. Humphrey, SPIE Proc: Int. Soc. Opt. Eng., 1996,2852,98

9 AM. McDonagh, N.I. Lucas, M.P. Cifuentes, M.G. Humphrey, S.Houbrechts andA

Persoons,] Organomet. 2000. 605,193

10 R.H. Naulty, M.P. Cifuentes, M.G. Humphrey, S. Houbrechts, C. Boutton, A

Persoons, GA. Heath, D.C.R. Hockless, B. Luther-Davies andM. Samac,] Chem.

Soc; Dalton Trans., 1997,4167

11 I.R. Whittall, M.G. Humphrey, S. Houbrechts, J. Maes, A Persoons, S.Schmid and

D.C.RHockless,] Organomet. Cbem., 1997,544,277

12 I.R Wbittall, M.G. Humphrey, M. Samac, B. Luther-Davies andD.C.R Hockless,]

Organomet. Cbem, 1997,544,189

13 I.R Whittall, M.G. Humphrey, M. Samac andB. Luther-Davies, Angew. Cbem., Int.

Ed. Engl, 1997,36,370

14 J. Vicente, M.T. Chicote, M.D. Abrisqueta, P.G. Jones, M.G. Humphrey, M.P.

Cifuentes, M.Samac andB. Luther-Davies, 2000,19,2968

15 J. Vicente, M.-I. Chicote, P. Gonzilez.-Herreto, P.G. Jones, M.G. Humphrey, M.P.

Cifuentes, M.Samac andB. Luther-Davies, Inorg. Cbem, 1999,38,5018

16 H. Zheng, W j, M.L.K. Low, G. Sakane, I. Shibahara andX. Xin,] Chem Soc.,

Dalton Trans., 1997,2357

17 I. Kamata, I. Kawasaki, I. Kodzasa, H. Ushijima, H. Matsuda, F. Mizukami, Y. Nakao,

Y. Fujii andY. Usui, Synth. Met., 1999, 102,1560

20 I.B. Massalski, 'Binary Alloy Phase Diagrams', 2nd Ed., ASM International, Ohio,

USA, 1990

21 Merkblatt DVS 2807 Teil1, Zusatzwerkstoffe fur Mikroverbindungen, Bonddrahte und

Bandchen, (instructionalleaJIet, Addition Materials for Microjoints, Bonding Wires and

Ribbons), Germany, 1992

22 D.Ott andCh.J. Raub, GoldBull., 1981, 14 (2), 69

23 D.M. Jacobson, M.R Harrison, S.P.S. Sangha, Gold Bull, 1996,29 (3), 95

24 G. Humpston andD.M. Jacobson, Gold Bull, 1992,25 (4), 132

25 W Kohl, S. Weber and A Bischoff, 'Metallographic Investigation of the Au-AI

lntermetallic Phase Formation, Metallography Conference ofDeutsche Gesellschafi fur

Metallkunde, Garmisch-Partenkirchen, Germany, 1988

26 V. Koeninger and E. Fromm, 'Einflussc von Alterung und Kontarnination auf die

Degradation von Gold-Aluminium-Ballbonds', (Influence ofAgeing andCantarnination

on the Degradation of Gold-Aluminum Ball Bonds), Verbindungstechnik in der

Elekronik undFeinwerktechnik, Dusseldorf, DVS-Verlag, 1996, 8 (4), 193

27 E.M. Philofsky, Solid-State Electronics, 1970, 13,1391

28 G.Majni andG.Ottaviani,] 1979,47,583

29 P.K Iootner, B.P. Richards and R.B. Yates, and Engineering

International, 1987,3,177

30 MI1-STD-883E

31 A.Russell, K Xu, S.Chumbley,J. Parks and]. Harringa, Gold Bull, 1998,31, (3), 88

32 M.Judge, New Scientist, 1997, March 8th,pp37

33 M.Poniatowski andM.Clasing, GoldBull., 1972,5(2),34

34 C.WCorti, Gold Bull, 1999,32 (2), 39

CS9' Gold Bulletin 2000, 33(3)