Study of wire bonding looping formation in the electronic packaging process using the three-dimen

Available online at https://www.360docs.net/doc/a63702509.html,

Finite Elements in Analysis and Design40(2004)263–286

https://www.360docs.net/doc/a63702509.html,/locate/?nel Study of wire bonding looping formation in the electronic packaging process using the three-dimensional

?nite element method

D.S.Liu?,Y.C.Chao,C.H.Wang

Department of Mechanical Engineering,National Chung Cheng University,160,

San-Hsing,Ming-Hsiung,Chia-Yi,621,Taiwan,ROC

Received18December2001;received in revised form17September2002;accepted8October2002 Abstract

This paper presents a three-dimensional?nite element model of the thermosonic wirebond looping process. The gold wire heat a ected zone load-displacement relationship was investigated using precision micro-force tensile tests.The temperature-dependent elastoplastic behaviors of gold wire were taken into account in the nonlinear?nite element model.Both the solid and equivalent shell wire models were used;exhibiting very close agreement,indicating that equivalent model was adequate.The simulation results were veri?ed against the experimental results.The standard triangle loop(STD2-Mode)and trapezoidal loop(LOW2-Mode)were simulated.The?nite element model closely predicted the pro?le of the tested loops.This model was used for an extensive looping factor parameter study.No obvious looping parameter e ects were observed for the STD2Mode.The loop shapes depend primarily on the wire properties,bonding pad distance and tier height. Both the looping and material parameters have signi?cant e ects on the loop shape for the LOW2Mode. Optimizing the reverse angles could reduce the stress on the wire.

?2002Elsevier B.V.All rights reserved.

Keywords:Wirebond;Gold wire;Heat a ected zone;Loop pro?le

1.Introduction

In recent years,as IC packaging development has moved towards higher power,smaller size, thinner dimensions,denser circuits and higher reliability,wire bonding is still the most commonly used bonding interconnection technique in?rst level microelectronic packages[1].In this intercon-nection method,bonding wires carry power and signals between the active semiconductor circuits ?Corresponding author.Tel.:+886-5-2720411;fax:+886-5-2720589.

E-mail address:imedsl@https://www.360docs.net/doc/a63702509.html,.tw(D.S.Liu).

0168-874X/$-see front matter?2002Elsevier B.V.All rights reserved.

doi:10.1016/S0168-874X(02)00226-3

264 D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286

Fig.1.Procedure for wire bonding[1].

and the lead frame or substrate metallization.Gold wire is usually used because of its easy handling and strong bond with the bond pad metal.Shown in Fig.1are the steps involved in thermosonic bonding.The steps are as follows[1]:(1)Gold wire is threaded through the capillary and elec-tric ame-o (EFO)is used to form a ball on the end of the wire.(2)The capillary descends and presses the gold ball onto an aluminum terminal set on the surface of a semiconductor chip.

(3)Ultrasonic bursts of energy are applied with the capillary,creating a weld using atomic in-terdi usion between the gold ball and bonding pad.(4)The capillary ascends vertically to play out su cient wire to form a loop as it moves toward the second bond site.(5)The capillary de-scends to make the second bond(crescent bond).(6)The wire clamp is closed and the capillary moves vertically to break the wire at the heel of the second bond.Once a bonding cycle is com-pleted,a precisely shaped wire connection called a wire loop is created as the capillary descends to a target position for the second bond.Fig.2shows a typical wire loop pro?le after bonding. The critical issues in today’s?ne pitch wire bonding are designing a prescribed trajectory for the capillary so that each wire can meet the packaging con?guration constraints as well as keep the loop height low to avoid wire sweep.Currently,the packaging industry relies heavily on tests to adjust the bonding parameters to meet the design requirement.This trial-and-error process is time consuming and cost ine ective.Therefore,it is important to fully understand the dynamic character-istics of the wire looping process to develop better design tools to quickly obtain an optimum loop pro?le.

In previous studies,scant research has focused on loop pro?le analysis.Shu et al.[2–4]eval-uated the main e ects and bonding parameter interactions on loop pro?le characterization using a statistical method—“response surface methodology(RSM)”.Tay et al.[5]and Srikanth et al.[6] employed the?nite element code ABAQUS to simulate the wire bonding process and obtain a wire-bond pro?le.Lo et al.[7–9]proposed a novel linkage-spring model to analyze the wire bonding loop.

D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286265

Fig.2.Typical thermosonic wirebond loop pro?le.

Besides the capillary trajectory,the wire material properties and EFO play important roles in determining the wire bonding pro?le.Ohno et al.[10]proposed a hardness distribution along the bonding wire corresponding to a V-shaped distribution.Qin et al.[11,12]and Huang et al.[13] performed numerical simulations and experimental works on wire ball formation.Qin et al.found that increasing the discharge current and decreasing the fuse time could decrease the heat a ected zone(HAZ)length.Tay et al.[5]simulated the transient temperature distribution along the wire using the?nite di erence method.

In all of the above studies,the wire properties used to perform the analysis,such as the elas-tic modulus and stress–strain curve,were given by the wire vendors at room temperature and high temperature(250?C).No studies involving the integration of more realistic stress–strain re-lations,especially in the HAZ adjacent to the ball-bond,were attempted due to lack of exper-imental data.Furthermore,little work has been reported on utilizing the three-dimensional?nite element method to simulate the wire looping process and study the bonding loop parameter e ect and residual stress distribution in the wire loop.Therefore,in this research,the looping parameter e ect was examined through computer simulations using a more realistic constitutive model of the gold wire.A newly designed tensile test method was developed to investigate the material proper-ties in the HAZ.The empirical constitutive model was proposed to predict the thermo-mechanical properties of gold wire.A dynamic?nite element code LS-DYNA version940[14]was used to perform the elasto-plastic large deformation contact analyses with the three-dimensional?nite el-ement https://www.360docs.net/doc/a63702509.html,paring the loop pro?les obtained from the computer simulation with the ac-tual wire bond specimens validated the accuracy of the simulation results.The agreement between the experimental and?nite element results was quite good.This computational model can be ap-plied to enhance the package reliability as well as speed up the proper wire bonder parameter settings.

266 D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286

2.Material property of gold bonding wire

2.1.Constitutive relation of gold wire

The thermo-mechanical properties of gold wire are the most important factors in achieving a good computational model for simulating the looping process.A micro-force tensile test method for bonding wire was designed by the authors[15]according to ASTM F219-79testing standard[16]. An elastoplastic constitutive model was proposed to describe the behavior of gold bonding wire

[15].The wire used in this study is Tanaka Inc.25 m FA type gold wire.The FA gold wire is

a middle-strength type for middle loop height.There are four dopant elements in the1ppm range, such as Ag,Cu,Fe,Mg.The major dopant element is Ca in6ppm[17].The constitutive relation for FA25 m wire is written as follows:

=E0(1?e? )+E1 (1?T?);(1) E0(T)=228:9908?0:0778T?0:00015269T2;T unit:?C;(2) (T)=606:0641?0:7032T+0:001215T2;T unit:?C(3) E1=360;(4)

T?=

T?T room

T melt?T room

;T room=25?C and T melt=1070?C;(5)

where E0, ,E1is the material constants,T?,T room,and T melt are the homologous temperature,room temperature and melting temperature,respectively.The?rst term in Eq.(1)expresses the elastic and transition portion of the stress–strain curve.The second term in Eq.(1)denotes the stress–strain relationship in a fully plastic range.

A comparison of the predictions from Eq.(1)is presented in Fig.3.The results calculated from the constitutive model agree well with the experimental data.

2.2.Tensile test of HAZ

Heat energy from the ball formation process is conducted along the wire where the a ected region is recrystallized.The type of dopant added controls the recrystallization temperature of the material,which determines the length of this range called the HAZ.Up to now,the HAZ mechanical properties have never been tested.A new testing method was developed to investigate the tensile strength in the HAZ with various electronic torch input currents.Experiments were performed on an MTS-Tytron micro-force tester equipped with a100gf load cell at room temperature.The HAZ tensile test specimens were from actual wirebonding samples.The testing processes are listed below: (1)set the electronic torch current and1st,2nd bond point on the wirebonder,and then?nish the wirebonding loop.(2)Trim o the wire loop as close as possible to the second bond site.(3)Fix the HAZ specimen onto the microforce tester and start the tensile test.The HAZ specimen and test setup con?guration is illustrated in Fig.4.The material investigated in this study was20 m FA type gold wire.The HAZ specimens were bonded using a SHINKWA wire bonder at28,32,36mA

D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

267

Strain(%)

S t r e s s (M P a )

https://www.360docs.net/doc/a63702509.html,parison of experimental data with model prediction for FA type gold

wire.

Gage Length (300 Fig.4.Con?guration of HAZ specimen and test setup.

electronic torch current.Tensile tests were performed at room temperature with a loading rate of 0.3mm/min.The gage length was 0.3mm.Fig.5compares the results for force versus displacement curve obtained over various electronic torch currents with the FA gold wire tensile test results at room temperature.No signi?cant di erent among the stress–strain curves is observed with various torch currents.The breaking load for the specimen with HAZ decreases about 25percent from 7.0to 5.2gf in comparison to the FA wire tensile test results at room temperature.The breaking position was located at 40–80 m from the neck portion.Furthermore,the decrease in breaking strain indicates

268 D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

Displacement(mm)

F o r c e (g f )

Fig.5.Force vs displacement curve of 20 m FA gold wire and HAZ at room temperature.

that the mechanical properties of the material in the HAZ zone are more brittle than the original gold wire.

3.Wirebond looping simulation model 3.1.Finite element model

The con?guration of the typical wirebond model used in this study is shown in Fig.6.The model consists of a die,a pad,a lead frame,a ?rst ball bond,a capillary,a clamper and the gold wire.The wire diameter is 25 m.The capillary inner diameter is 38 m.The total wire length is 1400 m.The distance between ?rst bond and second bond is 1100 m.The chip height is 370 m.

In our study,a three-dimensional solid wire model was developed to simulate the wire bond loop formation https://www.360docs.net/doc/a63702509.html,paring the loop pro?les obtained from the computer simulation with the actual wire bond specimens validated the accuracy of the simulation results.A three-dimensional equivalent shell wire model was also developed in this study.The purpose of this model was to reduce the computation time so that the looping parameter e ects can be inves-tigated quickly through computer simulations.The shell element was a 4-node type,while the solid wire model was an 8-node brick element type.The model geometries,material properties,and boundary conditions used in the equivalent shell wire and solid wire models were the same except for the element type used in the wire model.The speci?c shell thickness was 6:29 m assigned to the shell wire maintain the weight of the shell wire equal to the real bonding wire.

D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286269

Fig.6.The con?guration of wire bond model.

3.2.Material properties

The material properties of the gold wire are crucial in producing an accurate simulation.Two phenomena must be carefully examined.First,because of the large amount of heat energy transferred into the wire from the?rst bond formation process,the temperature distribution along the wire is not uniform.Second,the minimum mechanical strength and its position in the HAZ is important as a governing factor for the loop pro?le.Tay et al.[5]found that for a wire bond process with a duration of about0.15s,the temperature distribution will remain stable along the wire after?rst bond form (time?0:04s)was approximately a linear distribution that varied from200?C to room temperature. Therefore,in our study,the temperature distribution along the wire length was separated into four ranges.A constant temperature is given in each range,as shown in Fig.7,to approximate the linear decrease in each range.Once the temperature is de?ned in each range,the resultant temperature can be brought into the constitutive model proposed in Eqs.(1)–(5)to obtain the correlative stress–strain curve.Furthermore,according to the micro-force test results presented in the previous section,the minimum tensile strength at the HAZ should decrease by25%and be located40–80 m from the neck portion.Therefore,the HAZ was re-divided into?ve ranges and used the gold wire stress–strain curve at200?C as the reference strength.The minimum tensile strength assigned to range II declined by25%,and the strength in the other ranges was assumed to increase linearly back to the reference strength,as shown in Fig.8.A piecewise-linear-isotropic-plasticity material law in LS-DYNA3D was assigned to gold wire.The stress-strain curves in each part of the gold wire can be described by dividing the curve to several line segments and successively inputting the tangent modulus for each segment and e ective plastic strain value at each linking point.

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6001400

Distance from neck of ball bond ( )

T e m p e r a t u r e d i s t r i b u t i o

n

10001200200m μFig.7.Temperature distribution along wire.

II

S t r e s s

(a)

(b)

200m

m

m m

m o C

Fig.8.(a,b)Wire property on heat a ected zone (HAZ).

All other parts of the FEM model,including the capillary,die,lead frame and wire clamper were much sti er than gold wire,so they are modeled as a rigid material with no temperature variation.Table 1summarizes the material properties used in this model.

D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286271 Table1

Material properties used in the model

Part Density(g=cm3) E(GPa) y(MPa) u(MPa) Capillary 3.960.22370

Gold wire

200?C19.320.2864.5152214

190?C19.320.2864.7163216

180?C19.320.2864.8171218

160?C19.320.2865180220 HAZ I19.320.2860118182 HAZ II19.320.2850108169 HAZ III19.320.2854118182 HAZ IV19.320.2861128208 HAZ V19.320.2864140214

Fig.9.Equivalent shell wire tensile test model.

3.3.Material laws veri?cation and mesh density experiments

To verify the material law,a simple wire tensile test?nite element model with4-node shell elements was developed as shown in Fig.9.The wire length is400 m.The wire diameter is 25 m.The shell element size is10 m×5 m(length by width),and the thickness is6:29 m.The ?nite element model simulated the gold wire tensile test with one end?xed and the other moving at?xed strain rate.Fig.10shows the comparison between the simulation stress–strain relations with input material stress–strain curves.They agreed very well.This indicates that the material law developed in previous section is adequate.It also shows that reasonable simulation results should be obtained using an element size smaller than10 m×5 m.Therefore,an element dimension of 10 m×5 m(length by width)was selected for modeling the gold wire.The three-dimensional ?nite element model is shown in Fig.11.A total of16,131nodes and13,536solid elements were used to model the solid gold wire model.However,only2729nodes and2720shell elements were needed for the equivalent shell wire model.

272 D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

Strain (%)

S t r e s s (M P a )

Fig.10.Simulation result of tensile test.

Fig.11.FEM mesh of wire bond model.

D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286273

3.4.Loads and constraints

The?nite element model simulated the capillary movement along a speci?c trajectory.To keep the capillary moving on the x–z plane and vertical to the x–y plane,the nodes on the capillary,the rotation degree of freedom and the translation degree of freedom were restrained in the y direction. The nodes on the wire were free to translate and rotate in any direction,except for the?xed-end nodes on the ball bonds.The?xed boundary condition was given by restraining all of the degrees of freedom for the nodes on the lead frame,die,pad and ball bond.

The contact-interface was de?ned between the wire and the capillary to simulate the contact, however,the friction was negligible.In an actual bonding processes once the capillary ascends vertically to play out su cient wire to form the wire loop between the bonds,the wire clamps are closed at the highest point,causing the wire to move along the same trajectory as the capillary.To simulate this constraint condition,the contact interfaces were set at wire clamps and the end of the wire.Once the capillary ran to the top position of the movement path and started going downward to the second bond,the elements on the wire end between the wire clamps were free to move in the vertical direction between the clamps although their horizontal displacements were made the same as that of the capillary.

The HAZ and the turning range on the loop pro?le carry large plastic deformation;however,most of the other loop portions are deformed only in elastic range.According to the tensile testing results in Fig.3,the stress–stain relation of the gold wire did not very signi?cantly with temperature in the elastic range.Therefore the thermal contractions as the wire cools back to room temperature were not considered in this model.

4.Veri?cation of?nite element model

To verify the accuracy of the computational model,simulations were carried out with two types of bonding loop modes according to the actual wire bonding experiments:(a)STD2-Mode with a stan-dard triangle loop pro?le.(b)LOW2-Mode with a trapezoidal loop pro?le.The samples were bonded using a Shinkawa wire bonder.The pro?le for each sample was measured using a scanning electron microscope(SEM).Fig.12shows the capillary trajectory and loop pro?le of the STD2-Mode.The loop height indicates the distance between point G and point H.The capillary moves up to the middle of the HAZ for the inverse angle and an elliptical trace is chosen in the downward stage. The loop forming stages as shown in Fig.13.The capillary trajectory of the LOW2-Mode with dual reverse points can be applied to form a trapezoidal wire loop.As a result,the loop can be formed with the height of the wire loop kept low and contact prevented between the wire and the die.Fig.14shows the capillary trajectory and loop pro?le of the LOW2-Mode.The loop height indicates the distance between points H and I.The upper side length indicates the distance between points I and J.The wire loop forming stages are shown in Fig.15.

4.1.Solid and equilibrium shell wire model comparisons

Figs.16and17show the maximum Von-Mises stress from the LOW2-Mode simulations for the solid wire and equivalent shell wire models.As shown in the?gures,the agreement between the solid and shell simulations is very good except near the second bond point.This di erence

274 D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

Reverse Length

Fig.12.Capillary trajectory and loop pro?le of STD2-Mode loop.

Fig.13.(a–e)Various stages of STD2-Mode loop formation.

D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

275

2nd Reverse Length

Fig.14.Capillary trajectory and loop pro?le of LOW2-Mode loop.

Fig.15.(a–f)Various stages of LOW2-Mode loop formation.

276 D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286

Fig.16.Simulation result of LOW2-Mode loop with shell wire model.

Fig.17.Simulation result of LOW2-Mode loop with solid wire model.

is due mainly to the solid elements not supporting the bending loads;therefore the wire near the capillary tip would form a large bend radius during the downward stage.This leads the pro?le near the second bond to contact the lead frame.The solid wire model and equivalent shell wire model stress distributions are very similar.This implies that the equivalent shell wire model is

D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286277

X-axial

(10E-03mm)

-400

-200

https://www.360docs.net/doc/a63702509.html,parison between simulated and actual wirebond pro?le for STD2-Mode loop.

X-axial (10E-03mm)

Z -a x i a l (10E -03

m m )

-400

-200

200

https://www.360docs.net/doc/a63702509.html,parison between simulated and actual wirebond pro?le for LOW2-Mode loop.

adequate for simulating the wire loop formation.This is a very good ?nding because the compu-tation time was decreased from 28h to 4h using the equivalent shell wire model.To reduce the computation time,the equivalent shell wire model was used to investigate the other parameters in this study.

4.2.Model veri?cation

The actual STD2and LOW2Mode pro?les were compared with the pro?les obtained from the three-dimensional wire computer simulations in Figs.18and 19.The loop obtained from the

278 D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286 Table2

Comparison of simulated loop height with actual specimens

Loop mode Loop height( m)Discrepancy(%)

Experiment Simulation

STD2Mode110.39108.58 1.6

LOW2Mode100.93105.07 4.1

Fig.20.Von-Mises Stress distribution for STD2-Mode loop.

?nite element analysis closely simulated the experimental bonding loop.Table2compares the height of the actual wire bond pro?le with the height of the simulated wire bond pro?le.It can be seen that the discrepancy in the loop height was less than5%.The agreement between the simulation and experimental results demonstrate the reliability of the?nite element model predictions.

Figs.20and21show the Von-Mises stress distribution for each bonding mode.The maximum Von-Mises stress on both loops is about220MPa.The HAZ strain energy for the STD2-Mode is2:025N m and1:9526N m for the LOW2-Mode.Because of the strain energy discrepancy in the HAZ,the wire loop strength for the reverse trapezoidal loop would be stronger than that for a standard-triangle loop.This result agrees with the study by Hu[18]that indicated that neck-ing can be removed and pull strength increased when a reverse loop is introduced during the bonding process.The HAZ strain energy may be a good index to correlate the wire bond pull strength.

D.S.Liu et al./Finite Elements in Analysis and Design40(2004)263–286279

Fig.21.Von-Mises Stress distribution for LOW2-Mode loop.

5.Looping parameter study

In general,the wirebond loop pro?les are governed primarily by the material and looping param-eters.The material parameters primarily indicate the gold wire strength and the HAZ length,which depends on the EFO.The looping parameters include the capillary movement trajectory(looping mode),wire length,reverse angle,reverse length and bonding speed.Adapted looping parame-ters can reduce the loop height and increase the reliability.The looping parameters on both the LOW2-Mode and STD2-Mode considered here included the reverse angle,reverse length and wire length.

5.1.STD2-mode(standard triangle mode)

Fig.22shows the capillary trajectory of the STD2-Mode with various reverse angles.As the reverse angle increases,the highest capillary moving position descends to?x the wire length.The reverse length is180 m in these cases.Loop pro?les with various reverse angles are shown in Fig.23,from which it may be seen that the loop height increases slightly and bevel edge drops with the increment of reverse angle.Fig.24shows that the reverse length has no obvious e ect on the looping pro?le.From the above,the reverse angle and reverse length have no obvious e ects on the looping pro?le for the STD2-Mode.In other words,the STD-2Mode loop pro?le is dominated mainly by the material parameters and bond pad con?gurations instead of the looping parameters.

5.2.LOW2-mode(reverse trapezoidal mode)

The2nd reverse angle e ects were studied?rst.Fig.25shows the capillary path with a?xed 15?1st reverse angle and di erent2nd reverse angle.The?rst reverse length is110 m,and the

280 D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

X-axial (10E-03

mm)

Fig.22.Capillary trajectory for STD2-Mode loop with di erent reverse

angle.

Z -a x i a l (10E -03m m )

-400

-200

https://www.360docs.net/doc/a63702509.html,parison of STD2-Mode loop with di erent reverse angle.

second reverse length is 520 m in these cases.Two phenomena were observed for di erent loops by increasing the 2nd reverse angle,as shown in Fig.26.First,the loop shape tends to be more trapezoidal with the 2nd reverse angle.This implies that the purpose of the 2nd reverse angle is to form a trapezoidal pro?le.Second,the loop height varies from 140to 110 m with a change in the increment of the 2nd reverse angle from 15?to 50?.The HAZ Von-Mises stress distribution for loops with various 2nd reverse angles is shown in Fig.27.The maximum Von-mises stress

D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286281

X-axial(10E-03mm)

-400

-200

https://www.360docs.net/doc/a63702509.html,parison of STD2-Mode loop with di erent reverse length.

X-axial (10E-03mm)

Fig.25.Capillary trajectory for LOW2-Mode loop with di erent 2nd reverse angle.

on the HAZ decreases with a change in the increment of the 2nd reverse angle from 15?to 30?.The maximum Von-Mises stress on the HAZ increases as the 2nd reverse angle becomes greater than 30?.Furthermore,obvious necking is observed when the 2nd reverse angle is greater than 40?.From the above,the increment of the 2nd reverse angle not only decreases the loop height,but also decreases the loop strength.The adapted 2nd reverse angle is from 15?to 30?.

282 D.S.Liu et al./Finite Elements in Analysis and Design 40(2004)263–286

X-axial(10E-03mm)

Z -a x i a l (10E -03m m )

-400

-200

https://www.360docs.net/doc/a63702509.html,parison

of LOW2-Mode loop with di erent 2nd reverse angle (1st reverse angle =15?).

Fig.27.Von-Mises stress distribution on HAZ for LOW2-Mode loop with various 2nd reverse angle (a)?=15?,(b)?=30?,(c)?=40?and (d)?=50?(1st reverse angle =15?).

The e ects on the 1st reverse angle were investigated next.Fig.28shows comparisons between the loop pro?les with a ?xed 2nd reverse angle and various 1st reverse angles.The ?rst reverse length is 110 m,and the second reverse length is 520 m in these cases.As the 1st reverse angle