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Effect of Capillary Trace on Dynamic Loop Profile Evolution in Thermosonic Wire Bonding Fuliang Wang, Member, IEEE, Yun Chen, and Lei Han
Abstract— Thermosonic wire bonding remains the most commonly used interconnection technology in microelectronic packaging, and looping is an important aspect in modern wire bonders. To identify the loop formation mechanism, the effect of capillary trace on the standard wire looping process was studied. Dynamic looping processes with different capillary trace parameters and reverse motions of 4, 8, and 16 mil were recorded by a high-speed camera. The capillary trace and wire profile evolution were obtained from the looping videos using a digital image processing program, and the relationship between capillary trace and loop profiles was analyzed. A finite-element model was established to study the strain distribution on wire during looping. Experimental and simulation results show that the wire profile of the standard loop is mainly affected by capillary position and is not sensitive to capillary velocity. The upward capillary trace mainly affects the loop configuration, including the number, position, and deformation of kinks and the loop length. The downward capillary trace affects the stress states, loop height, kink deformation, and loop profiles. A kink is the wire with the largest local curvature, and it is a plastically deformed wire segment with little elastic core. The kink has two functions: 1) shaping the loop and 2) isolating the pulling force on the first bond and neck caused by capillary movement. This paper can be of great help in loop profile optimization in the industry and in academic research of loop dynamics. Index Terms— Apillary trace, high-speed camera, kink forming, loop profile, reverse motion, thermosonic wire bonding.
I. I NTRODUCTION
T
HERMOSONIC wire bonding is widely used in microelectronic packaging because it meets most interconnection needs from the most common dual in-line package to the latest 3-D packaging [1]. Keen competition in the microelectronic industry drives wire bonding toward higher reliability, fine pitch, and higher looping performance [2], [3], which makes loop profile control increasingly important in modern wire bonders.
Manuscript received January 2, 2012; revised June 24, 2012; accepted June 25, 2012. Date of publication July 27, 2012; date of current version August 31, 2012. This work was supported by the China Department of Science and Technology Program 973 under Contract 2009CB724203 and Contract 2011CB013104, the Natural Science Foundation of China under Contract 51175520, the National S&T Major Project under Contract 2009ZX02038, and the Program for New Century Excellent Talents in University NCET-11-0523. Recommended by Associate Editor J. J. Pan upon evaluation of reviewers’ comments. The authors are with the State Key Laboratory of High Performance Complex Manufacturing, Changsha 410083, China, and also with the School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2012.2206593
Looping is a complex dynamic process related to the wire material and capillary trace [4], and many studies have been conducted on this subject. In relation to the material, Ohno et al. [5] and Shah et al. [6] observed a V-shaped hardness distribution in the heat-affected zone (HAZ) of a gold wire, and the loop height is directly proportional to the length of HAZ. Saraswati et al. found that the elastic modulus and break load of gold wire significantly influence loop height, length and spring-back behavior [7]. Seuntjens et al. developed a 4N gold wire to address ultrafine pitch package requirements and discovered that the loop profile is dramatically altered by the material properties of the wire [8] Kung et al. developed a microwire sweep machine to investigate the sweep resistance of a wire loop. They found that loop stiffness depends on kink numbers, position, loop spans and bond heights [9], [10]. However, the kink formation process has not been fully understood. With regard to the capillary trace aspect, Shu experimentally studied the effect of the capillary trace parameter on the loop profile using the statistical method—response surface methodology—and empirical models were developed to predict the loop profiles [11], [12]. Abdullah et al. studied the effect of looping parameters on pull strength using the statistical method and observed that the low loop weakened the neck strength [13]. In addition to the above statistical and static studies, simulation studies on dynamic looping processes were also conducted. Lo et al. developed a link-spring model and discovered that the HAZ length and capillary trace have significant influence on the final profile [14]–[16]. Liu et al. measured the construction relationship of a gold wire and developed a finite-element (FE) model to study the effect of capillary trace on the looping process [17], [18] Tay et al. used an FE model to investigate the effect of wire tensioning and bonding force on the neck profile [19], [20] However, the model-predicted looping process failed to agree with the experimental details because of too many simplifications and assumptions. Experimental studies on the dynamic looping process are very few because the loop time is only 50–70 ms, and the wire diameter is only 20–30 μm. Thus, it was found to be difficult to observe the tiny plastic deformation of the wire during the looping process, except with the use of a high-speed camera. In this paper, the standard loop-formation process with different reverse motion parameters of the capillary trace is experimentally studied using a high-speed camera. By analyzing the looping process videos with a digital image program, the capillary traces and wire loop profiles were
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WANG et al.: EFFECT OF CAPILLARY TRACE ON DYNAMIC LOOP PROFILE EVOLUTION
obtained, the relationship between the capillary trace and the loop profile was studied, and the effect of the reverse motion parameters on the loop profile evolution process was discussed. This paper can be beneficial in loop profile optimization in the industry and in looping simulation verification in academic research.
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(a)
LED Lighting
II. L OOPING E XPERIMENT A. Experiment Equipment and Materials An experimental setup was established to record the wire looping process, as shown in Fig. 1(a). It consisted of a KNS 8028 bonder, a Photron FastCAM SAl.1 high-speed camera, and 100-W high-intensity light-emitting diode (LED) lighting. The KNS 8028 bonder was used to bond the 4Ntype 25-μm-diameter gold wire with a standard bottleneck capillary (made by PECO Inc., Korea, with part number B1014-51-20-10) on aluminum pads in the silicon chip. A high-speed camera with a 5400-frame-per-second speed and LED lighting was used to monitor the deformation of a small gold wire during the fast looping process. Fig. 1(b) is a photo of the gold wire taken by the high-speed camera. The circles show the wire profile recognized by the image processing program. B. Loop Parameters In this paper, the standard loop is investigated. The loop formed with the capillary trace, as shown in Fig. 2, is called a standard loop. The capillary trace of the standard loop was changed by varying the reverse motion parameters in the experiments from 4 to 8 mil and then to 16 mil. In the following sections, these experiments are referred to as Cases 1–3. The major bonding parameters are listed in Table I. During the experiments, room temperature and humidity were carefully controlled to avoid altering the gold wire properties. After the experiment, the capillary traces and loop profiles of Cases 1–3 were extracted from the high-speed camera videos, using a digital image processing program to study the relationship between them.
Wire Bonder
High speed camera
(b)
Capillary Gold wire
Fig. 1. (a) Experimental setup for looping process study. (b) Wire photo from high-speed camera.
III. A NALYSIS OF W IRE P ROFILES E VOLUTION D URING L OOPING P ROCESS A. Relationship Between Capillary Trace and Loop Profile
Fig. 2.
The capillary traces of the three cases are shown in Fig. 2. Points O and F represent the position of the first and second bonds, respectively. The looping process is divided into upward and downward stages according to the traces, as shown in Table II. The capillary traces are similar in the three cases, and only the lengths of reverse motion AB and loop flat (LF) CD are different, indicating that the reverse motion parameter only changes the length of the capillary trace on the upward stage, not on the downward stage. The final loop profiles from the different capillary traces are shown in Fig. 3. The wire in Case 1 is the lowest and the shortest. The highest point B1 is located in the middle of the loop, about 150 μm from the substrate. The right-tilted wire
above the first bond makes the loop appear to be pulled by two bond points. The wire in Case 2 has the highest point B2 approximately 240 μm from the substrate and near the first bond. The wire directed straight upward above the first bond makes this loop appear to have almost no residual stress. The wire in Case 3 is the highest and the longest. The highest point B3 is approximately 400 μm from the substrate and near the first bond. The left-tilted wire above the first bond makes the loop appear to be pushed by two bond points. Figs. 2 and 3 show that the reverse motion parameter only changes the length of the capillary trace on the upward stage
Capillary traces with different reverse motion parameters.
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TABLE I B ONDING PARAMETERS Parameters
Value
Loop type Loop span (first and second bond distance) Kink height Reverse motion Reverse angle Bonding temperature Ultrasonic current of first and second bonds Bonding time of first and second bonds Bonding force of first bond Bonding force of second bond
Standard loop 1382 μm 5 mil 4 mil, 8 mil, 16 mil 90° 160 °C 120 mA 15 ms 25 g 35 g
TABLE II L OOPING P ROCESS S TAGES A CCORDING TO C APILLARY T RACE Stage
Fig. 3.
Name
Upward stage
OA AB BC CD
kink height reverse motion loop-up LF
Downward stage
Arc DE EF
loop-down search height
Fig. 4. Loop profile evolution during reverse motion stage. (a) Case 1. (b) Case 2. (c) Case 3.
Final loop profiles from different capillary traces.
and has little effect on the downward stage; however, the slight change in the capillary trace greatly alters the height, profile, and stress state of the wire loop. To investigate these phenomena, the wire profiles during each capillary trace stage were obtained, and the analysis is presented in the next sections. B. Wire Profiles Evolution During Looping Process 1) Wire Profiles During Reverse Motion Stage: The wire profiles of the three cases during the reverse motion stage are shown in Fig. 4. The original point is the first bond, and the wire end is the capillary tip. At the start of the reverse motion stage, all wires were directed straight upward. When the capillary moved toward the left, the length and profiles of the loop gradually changed, and the profiles were completely different at the end of the reverse motion stage. Wire profiles may be affected by the capillary horizontal movement velocity and/or position. To address this issue, the loops in the three cases were compared, as shown in Figs. 5 and 6. Fig. 4 shows that the reverse motion stage of all three cases starts at about frame 104 and ends at frame 140, and the reverse motion time is about 6.67 ms, but the distances are 4, 8,
Fig. 5. Loop profiles at different capillary positions. Capillary at (a) −27 μm, (b) −80 μm, (c) −138 μm, (d) −315 μm, and (e) −420 μm.
and 16 mil for Cases 1–3, respectively. Therefore, the capillary velocity is different when it passes by the same position in different cases. However, the wire profiles are the same for the same capillary position, as shown in Fig. 5(a)–(c). Therefore, it seems that the standard loop profile was mainly affected by the capillary position, instead of by capillary velocity. The final wire profiles of the three cases at the end of the reverse motion stage are shown in Fig. 6. One kink was present in Case 1, two kinks in Case 2, and three kinks in Case 3. The positions of kink I were the same for the three cases. However, kink II deformation in Case 2 was more severe than that in Case 3. Therefore, the reverse motion parameter first changed the reverse motion length of the capillary trace; affected the number, position, and deformation of the kinks; and induced different wire profiles during the reverse motion stage.
WANG et al.: EFFECT OF CAPILLARY TRACE ON DYNAMIC LOOP PROFILE EVOLUTION
Fig. 7. Fig. 6.
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Spring-back of gold wire.
Wire profiles at the end of the reverse motion stage.
2) Wire Profiles During Loopup and LF Stages: A “springback” phenomenon of the gold wire was observed at the beginning of the loop-up stage when the capillary started to move vertically up, as shown in Fig. 7. Spring-back is the elastic recovery of kinks, and each kink has a tiny recovery, suggesting that a kink is a plastically deformed wire segment with little elastic core. The spring-back of the three kinks cannot be easily identified because the distance is very small. Except for the spring-back, the wire profiles had no noticeable change during the loop-up stage. During the LF stage, the horizontally moving capillary resulted in the spinning of the wire loop, as shown in Fig. 8. For Case 1, the loop was spun around kink I. For Cases 2 and 3, the loop appeared to spin around kink II, which increased the deformation of kink II. Therefore, the reverse motion parameter changed the loop length, deformation of kinks, and loop profiles by changing the LF distance of the capillary trace. 3) Wire Profiles During Loop-down Stage: During the loopdown stage, the capillary moved along arc DE (Fig. 2). As the clamp was closed, the wire length did not increase. The capillary movement resulted in wire bending under the capillary tip, continuous wire spinning, and kink deformation, which dramatically changed the loop profiles. The loop profile evolution of Case 1 is shown in Fig. 9. The capillary movement bent the wire near the capillary. The bent wire caused the entire loop to tilt toward the second bond because the loop length was very short, and the wire spin center gradually turned from kink I to the first bond. The wire segment around kink I recovered from an S-shaped curve to straight upward and then to a C-shaped curve because of the pulling force, which made the loop appear to suffer from tensile stress. This pull can also cause neck damage. Therefore, a short reverse motion parameter results in short loop length and may cause tensile stress on the wire and even neck damage during the loop-down stage. The loop profile evolution of Case 2 is shown in Fig. 10. The capillary movement mainly affected the deformation of kink I. The spin center appeared to be always on kink I, and kink I only recovered to a slight C-shaped curve. The bent wire near the capillary caused a slight tilt of the loop above the first bond. Compared with Case 1, kink II and the long loop were deduced to be the effect of capillary movement in the first bond and neck. Therefore, tensile stress and neck damage during capillary loop-down can be reduced by appropriately increasing the reverse motion parameter. The loop profile evolution of Case 3 is shown in Fig. 11. The capillary movement mainly affected the deformation of
(a)
(b)
(c)
Fig. 8. Wire profiles at start and end of LF stage. (a) Case 1. (b) Case 2. (c) Case 3.
Fig. 9.
Loop profile evolution of Case 1 during loop-down stage.
kinks I and II. The wire spin increased the deformation of kink II and decreased that of kink I because the spin center moved from kink I to kink II. After a slight recovery, kink II still retained an S-shaped curve. As the loop length is long enough, the wire above the first bond tilted against the second bond, apparently caused by the compressive force in the loop. Compared with Cases 1 and 2, the long loop in kink III avoided the effect of the capillary movement in the first bond and neck. Therefore, although the capillary traces during the loopdown stage are the same, the wire spin center is different because the loop configurations (i.e., loop length, kink number,
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Fig. 10.
Loop profile evolution of Case 2 during loop-down stage.
Fig. 11.
Loop profile evolution of Case 3 during loop-down stage.
and deformation) are different, which resulted in different recoveries of kink I, loop tilt above the first bond, stress state on the loop, and effect on the neck and first bond. The kinks avoided the capillary movement effect in the loop profile by isolating and absorbing the pulling force. 4) Wire Profiles During Loopend Stage: During the loopend stage, the capillary continued to move down; the wire under the capillary was squeezed and bonded on the pad, further affecting the loop shape, as shown in Fig. 12. For Case 1, the wire part that first made contact with the substrate at frame-315 instant was approximately 100 μm away from the capillary tip. The downward movement of the capillary made the contact point move toward the capillary tip and loop to kick up. The formation of the second bond squeezed the wire under the capillary tip and pushed the loop toward the first bond, which released part of the tensile stress in the loop, resulting in the loop-down stage and transforming the loop from a concave to convex curve. A new kink formed at the middle of the loop at frame-380 instant and became the highest point. The maximum kick-up distance was approximately 50 μm in the new kink position. The loop profile was obviously changed by the kick up and formation of the new kink, as shown in Fig. 12(a).
Fig. 12. Loop profile evolution at loop-end stage. (a) Case 1. (b) Case 2. (c) Case 3.
Fig. 13.
Wire looping model.
For Case 2, the downward movement of the capillary and formation of a second bond also squeezed the wire under the capillary, pushing the loop rightward and kicking up the loop, which caused the wire profile to transform from concave to convex. However, no new kink was formed, and the loop height was only slightly increased. The maximum amplitude of the kick-up was approximately 25 μm, as shown in Fig. 12(b). For Case 3, the squeezed wire under the capillary caused a slight kick-up and a 16-μm-height increase of the loop but had no significant effect on the loop profiles. The recovery of kink II avoided the effect of the pushing force in the neck and first bond, as shown in Fig. 12(c). Therefore, although all loops suffered pushing force caused by the squeezed wire, the different stress states and loop configurations resulted in different kick-up distance, loop-height increase, and loop-profile change. From the aforementioned looping process analysis, we can conclude
WANG et al.: EFFECT OF CAPILLARY TRACE ON DYNAMIC LOOP PROFILE EVOLUTION
Fig. 14.
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Capillary geometry. TABLE III G EOMETRIC PARAMETERS OF C APILLARY IN M ODEL Parameters
Value
Inner hole size (d1 ) Inside chamfer diameter (d2 ) Tip diameter (d3 ) Back hole size (d4 ) Outer diameter (d5 ) Tool length (d6 ) Cornet radius (R1 ) Cornet radius (R2 ) Cornet radius (R3 ) Cornet radius (R4 ) Inside chamfer angle (α)
35 μm 50 μm 130 μm 78 μm 200 μm 2000 μm 5 μm 10 μm 3 μm 3 μm 90°
(a)
(b)
Fig. 16. Comparisons of wire profiles when capillary moved (a) to point B, (b) from points C–F, and (c) to point F.
(c)
Fig. 17.
Strain distribution on wire for capillary at point B.
IV. F INITE -E LEMENT M ODEL FOR L OOPING P ROCESS A. Finite-Element Model (d) Fig. 15. Wire profile evolution during looping. Wire profile when capillary moved to (a) point A, (b) middle of AB, (c) point B, and (d) from points C–F.
that a large reverse motion parameter resulted in a long loop with more kinks and compressed stress, which avoided the effects of capillary trace in the first bond and neck but resulted in a high loop. A small reverse motion parameter resulted in a short loop with fewer kinks and tensile stress. In this case, the kinks shaped the loop but did not isolate the neck and first bond from the effects of capillary movement.
To study the loop profile evolution process and strain distribution on wire, a 2-D dynamic FE model was developed using LS-DYNA. The wire looping model consists of a capillary, gold wire, silicon substrate, silver layer, and silver pad, as shown in Fig. 13. The capillary geometry is shown in Fig. 14, and the major parameters are shown in Table III. The gold wire was 2000-μm long and 25 μm in diameter. The gold wire included an FAB and an HAZ formed by an electronic flame off [21]. The FAB size was measured to have a 70-μm diameter and 23-μm height. The HAZ length was assumed to be 200 μm. The thicknesses of the silver pad and silicon substrate were 8 μm and 33 μm, respectively, and widths of both were 100 μm.
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TABLE IV P ROPERTIES OF G OLD W IRE , C APILLARY, PAD , AND S UBSTRATE Part No. Gold wire HAZ I HAZ II HAZ III HAZ IV HAZ V 200 °C 190 °C 180 °C 160 °C Capillary Substrate Silver pad
Fig. 18.
Length (μm) 2000 35 30 35 50 50 400 400 500 500
Density (g/cm3 )
Poisson ratio (v)
E (GPa)
σ y (MPa)
Etan (MPa)
19.32 19.32 19.32 19.32 19.32 19.32 19.32 19.32 19.32
0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28
60 50 54 61 64 64.5 64.7 64.8 65
118 108 118 128 140 152 163 171 180
188.5 130.5 140 141 148 149 150.5 152.5 156
-
3.96 3.96 10.5
0.28 0.3 0.32
220 168.9 73.2
287
413
Strain distribution on wire for capillary at point F.
The Plane162 element with hourglass control was applied to mesh the model and to avoid abnormal distortion for large deformations of the gold wire. The meshes in the FAB and HAZ were refined. The typical size of the refined area was set to 6.25 × 6.25 μm (length by width). A total of 1383 nodes and 1106 elements were used to model the gold wire, and 1898 nodes and 1644 elements were used to model other parts. Considering the general properties of the gold wire at different temperatures [14], [15] and the grain size and Vickers hardness distribution [7], a nine-segment wire property model was used. The silver layer was defined using the bilinear kinematics model. The capillary and silicon substrate are considerably stiffer than the gold wire, and are, therefore, defined as rigid. The properties are listed in Table IV. The boundary conditions and displacement load are defined as follows. The rotation of the capillary is constrained as it only moves along the XY plane. The bottom of the substrate is fixed along all degrees of freedom as the substrate is clamped to the work-hold. A 0.015-g pull force is applied to the top nodes of the wire to simulate air tension force. An automatic 2-D single surface contact is defined between the wire and the capillary inner face to simulate contact and friction. A displacement load (see Case 3 in Fig. 2) was defined for the capillary. Each dot point represents a simulation step, and 269 steps in total were used. B. Simulation Result and Analysis With the above FE model, the looping process was simulated, as shown in Fig. 15. When the capillary moved from A
to B, the gold wire was fed out, bent, and plastically deformed, and several kinks were formed, as shown in Fig. 15(a)–(c). When the capillary moved from C to D, E, and F, the gold wire was rotated and deformed to its final profile, as shown in Fig. 15(d). To validate the accuracy of the FE model, the simulated wire profiles during the forming process (see Fig. 15) and the experimentally obtained profiles (see Figs. 3–12) were compared. Fig. 16(a) shows this comparison for the kink number, kink position, and loop profile during capillary loop-up. Fig. 16(b) shows a comparison for the profile evolution during capillary loop-down. Fig. 16(c) shows a comparison of the final loop profile. No significant difference is observed between the compared profiles, indicating that the simulation results agree well with the experimental results. Fig. 17 shows the effective plastic strain distribution on the wire for the capillary moving to point B (during loop-up). The maximum strain of 0.45 is located at the wire neck, which may be a cause of neck damage during looping. The average strain of 0.27 is exhibited on the kinks. Notably, the center of the kinks has a small strain of
