University Tenaga Nasional, Kajang, Selangor, Malaysia 2

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1 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 8 Characterization of Granular Type Capillary to Wire Bond Yield for Ultra-Fine Pitch Large Quad Flat Package of Semiconductor Packaging 1 Ibrahim Ahmad, 2 Nurul Hidayah Mohamad Nor, 2 Huda Abdullah, 3 Fuaida Harun and 3 Azman Jalar 1 Department of Electronics and Communication, College of Engineering, University Tenaga Nasional, Kajang, Selangor, Malaysia 2 Department of Electrical, Electronis & System, Faculty of Engineering and Built Environment, 3 Institute f Microengineering and Nanoelectronics, University Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia Abstract-- The objectives of this study are three-fold; firstly to improve the bond ability of high reliability gold wire with 99% purity of gold on fine pitch lead frame, secondly to improve second bond quality and reliability using optimal bonding tools and material selection and thirdly, to improve machine stability and efficiency during production mode. All the efforts will be done on the granular type capillary and comparing them with the conventional polished surface capillary. The bonding tool morphology on wire bond quality and production yield using 20 µm ultra fine pitch wire bonded on Argentums plating lead frame were characterized. Quantitative wedge pull test with statistical analysis and internal physical inspection using Scanning Electron Microscopy were carried out for both samples at 0 h after production, 96 and 192 h after high temperature storage and after 500 and 1000 cycle of thermal cycle test. Cross section analysis was also performed to study the alloy formation between wire and lead plating. Results show that significant improvement of wedge strength using granular type capillary as compared to conventional polished capillary with p-value of 0.000, and for samples at 0 and 96 h of high temperature storage and 1000 times of thermal cycle test. The second bond strength of granular type capillary still exceeds that of conventional polished capillary by 0.18 and 0.20 g force, as a result, a reduction of 62% on the capillary cost. In conclusion, granular type capillary offers a better bonding quality for ultra fine pitch package on lead frame packages as well as improvements on machine stoppages occurrence. This capillary also offers an effective and practical improvement in bonding and process robustness, which can be realized not only in development stage, but also in real production arena. tight bond pad pitch of integrated IC has led to an increase demand for fine pitch packages [6-8]. In wire bonding process, reliability plays an important role in the overall device assembly cost. Reliability refers to the probability that a wire bonder machine perform its required functions satisfactorily under specific conditions and within a certain period of time [9] As device density and input output requirements increase, the size and pitch of bond pads, ball bonds and lead finger subsequently decreases [3,10,11]. The transition from conventional bond pad pitch to fine pitch volume production has increased the level of difficulties in the wire bonding process. The thinner and finer lead finger characteristics in fine pitch package makes it more difficult form a stronger and reliable second bond. Frequent bonding interruptions during wire bonding process of ultra fine pitch package due to bonding failures can lead to lower yields, higher machine downtime leading to overall inefficiency of factory operations planning and product quality [9] Common wire bonding failures are premature tail bond break after second bond formation, known as short tail error, unsuccessful Free Air Ball (FAB) formation due to insufficient tail, Non Stick second bond On Lead (NSOL) and Non Stick bond On Pad (NSOP[12,13]. Premature tail bond termination is commonly due to limited tail bond coverage area produced by the of fine pitch capillaries. The tail bond acts as a temporary anchor point during wedge bond formation. Figure 1 shows a cross-section of capillary-wire-lead frame process combinations that occurs during the second bond formation. The highlighted area under the capillary s chamfer where the tail bond is formed and bonded onto the lead is known as tail bond area. Index Term-- Wire bond, second bond, ultra fine pitch, short tail, yield loss. I. INTRODUCTION Wire bonding is an important technology used for making electrical connections for microelectronics packaging. Wire bond carries power and signals between Integrated Circuit (IC) and lead frame [1-3]. It is the most likely used technology in the microelectronics packaging industry [3-5] The continuous miniaturization of packaging footprint having a (Source: [4] Fig. 1: Schematic diagram of tail bond formation

2 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 9 done on the granular type capillary and comparing them with the conventional polished surface capillary. During this movement, the capillary and wire clamp combination rises through the stationary wire to a pre-set height before the latter is clamped and pulled back from the lead frame, prior to breaking the tail bond. If the tail bond is prematurely broken during the second bond formation, the resulting wire tail length is either short or missing. [12, 13]. Missing tail happens when the wire was blown out of the capillary by the machine air tensioning. Figure 2 shows tail bond length after wedge bond formation. Figure 2a shows good tail bond length for next step of the bonding process, while Figure 2b shows bad or too short wire tail formation. When the wire tail is too short or missing, it can cause an electro flame off misfires during the free air ball formation. Should either situations occur, the wire bonder will stop and issues an electro flame off open failure, short-tail or tail liftoff error. These stoppages require operator assistance that counted as machine downtime. There are also instances when, during electro flame off misfires, the machine continues into the bonding cycle but due to insufficient formation of the free air ball, lifted ball, ball short-to-metal, smashed ball or defective ball bond rejects can occur thus reduces the overall wire bond yield and reliability [12,13] Traditionally, semiconductor capillaries and wires were individually developed because of their inherently different material and fabrication requirement [13] However, the performance of each component must complement each other during wire bonding process in order to obtain highest bond ability and reliability [15,16] Although various continuous individual improvements efforts were made to wires and capillaries to address the ever-changing IC interconnection requirements, its makes better sense to integrate both components during product stage development to obtain better results. From the capillary design point of view, possible solutions to the premature tail bond termination are optimizations done to the tip diameter, face angle, outer radius and surface finishing of the capillary. However, there are still limitations to the optimization of these components. The tip diameter is limited by the bond pad pitch of the devices while reduction in face angle even though able to improve the second bond formation but it can cause lower readings during the pull process. As for smaller outer radius, while it can help to improve the second bond quality, it can also cause heel crack [14,15]. Out of all of these components, the capillary tip surface is the major part that comes into direct contact with the wire and affects the shape of the second bond [16] The objectives of this study are three-fold; firstly to improve the bond ability of high reliability gold wire with 99% purity of gold on fine pitch lead frame, secondly to improve second bond quality and reliability using optimal bonding tools and material selection and thirdly, to improve machine stability and efficiency during production mode. All the efforts will be II. MATERIALS AND METHODS Second bond formation on Argentums lead frame is far more challenging than bonding on gold plated substrates. Several factors that can affect the second bond quality are, the lead finger clamping where poor calibration or design causes bouncing leads, plating inconsistency and surface roughness, Burrs beneath the lead fingers that cause instability and plating oxidation and contamination. Due to the many possible factors that can affect the quality of second bond on leaded packages, it is crucial to minimize all the potentials by calibrating the wire bonder machine prior to starting the following wire bonding process. Table I Combination of the bonding parameter for sample A, B, C, D and E Leg # Ultra sonic power Bond force For the wire bonding process, two types of samples were assembled using two different types of capillary tip surface, namely the granular type capillary versus the conventional polished capillary. All the other dimensions of both capillaries under investigations were kept the same. Sample A was assembled using granular type capillary, while sample B was assembled using conventional polished capillary. The granular characteristics of capillary used for sample A is meant to improve and enhance the bondability of the second bond especially when bonding on an unstable lead frame along with the 20 um, 99% purity wire. Both samples were bonded on 60 µm bond pad pitch of ultra fine pitch of Low profile Quad Flat package. The second bonding parameters were optimized through a detail work by applying design of experiments on the bonding ultrasonic power and force in order to obtain optimum tail bond area for both capillary types. As the work is concentrating on the second bond process, the first bond parameter was kept constant during the evaluations. Design of Experiment on second bond parameter: A 2 2 Full Factorial experiment was designed with a center point for this purpose. A complete matrix strip of 2 7 units per leg was bonded with the two types of capillaries. The combination of the bonding parameter for all samples is as shown in Table 1. The tail bond area was inspected using high power microscope and compared to reference data. Bonding parameter selected must produce at least 50% of tail bond area with consistent formation through all package side in order to obtain stable bonding [9].

3 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 10 accelerated test for standard JEDEC High Temperature Storage life test (HTS) (JESD-22A-103B at 150 C for 1000 h) [3&20]. Subsequently, tail pull testing was then used to characterize the bonding quality qualitatively after the accelerated test and predict how bonds will performance during aging. At the same time, in order to predict wire bond quality under ever-changing pressure and working ambient, temperature cycle test at -65 C/+150 C condition per the JESD22-A104-B JEDEC specification at 500 cycles up to 1000 cycles test were performed for both capillary types followed by a wedge pull test[18]. Cross section test was also done for both samples in order to observe any potential alloy formation between the wire and the lead surface using the SEM. Wire bond performance after production: At 0 h, wire bond formations for both capillaries were inspected using high-resolution microscope with 500 magnifications. Internal Physical Inspection (IPI) using Scanning Electron Microscopy (SEM) was performed to look for any abnormalities such as peeling or inconsistent first and second bond formation. Ball shear, ball pull and wedge pull test using wire pull tester were done for quantitative assessment. In order to achieve a robust process, only failure modes of broken wire at stitch, centre or ball neck are accepted. Table 2 and 3 show the failure mode classification for wedge or ball pull test and ball shear test. Table II Failure mode classification for wedge or ball pull test [17-19] Test type Wedge pull Ball pull Code failure description accept/reject accept/reject 0 Operating error Broken at centre Accept Accept 2 Broken at ball neck Accept Accept 3 Broken at stitch Accept Accept 4 Lifted ball Reject Reject 5 Lifted stitch Reject Reject 6 Cratering Reject Reject 7 Peeling aluminum Accept if no crack Accept if no crack Table III Failure mode classification for ball shear test [17-19] Code Failure description Accept/reject 0 Operating error - 1 Lifted ball from Al Reject 2 Gold residue not 100% Accept 3 Peeling aluminum Accept if no crack 4 Cratering Reject 5 Ball sheared Accept Performance of granular capillary in production: Granular type capillaries effectiveness for increasing wire bond yield will be proven in actual production during wire bonding assembly operation in comparison to the polished type capillary. Machine downtimes such as no tail, short tail and EFO open error for both capillaries were counted during this production process. Capillary touchdowns monitored and second bond quality was observed for every 100,000 touchdown to maintain good bond quality. To ensure testing procedures were closely duplicated as the actual production environment, test conditions such as product line, wire bonder and bonding temperature were set to be the same as that of the production s settings with all assist counters reset to zero. Wire assists data including the number of NSOP, NSOL, EFO and short tails that occurred during each interval were recorded Diagnostic data was recorded for each capillary type and wire bond were tested using wedge pull test and optical test using High power microscope for every 100,000 bonds to maintain consistent wedge strength and stitch shape and intensity. Average yield was calculated and compared between devices using granular versus the polished type capillary to obtain yield improvement. Capillary touchdowns to be able to make the cost comparison effectiveness for both capillaries were also recorded. In order to compare the wedge strength performance between granular and polished type capillary, 30 data were III. RESULTS AND DISCUSSION collected and data were compared accordingly. Internal Physical Inspection after wire bonding: As wire Reliability test of wire bonding: To characterize the bond bond formation plays an important role in determining quality to enable the prediction of wire bond performance bonding quality, internal physical inspection was done to under high temperature, pressure and different ambient observe bonding formation using granular over the polished condition, both samples were then tested with two reliability type capillary. This is to ensure the absence of peelings for tests namely, High Temperature Storage (HTS) and Thermal second bond and homogeneous IMC formation of first bond. Cycle Test (TCT). HTS tests were performed at 175 C for 96 The results of IPI using SEM and high power microscope and 192 h, with pull test failure mode of unmolded devices. with 500 magnifications at first bond shows symmetrical ball High temperature storage is an accelerated aging at high bond formed with the intermetallic coverage exceeding more temperature used to promote gold-aluminium intermetallic than 50% of the surface. This is shown in Figure 4b. growth for the first bond. HTS test at 175 C used is the However, only small IMC was observed at time 0 h due to the

4 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No: % wires characteristics which has a small amount of efficiently transferred from the ultrasonic generator of the wire palladium that helps to slow down the formation of bonder through the capillary and right to the leads as compared intermetallic between the gold wire and the aluminium from to the polished type capillary where the insufficient gripping the bond pad surface [21]. On the other hand, IPI at second can create some loss of ultrasonic energy thus causing an bond shows large tail bond area formed with small torn area. insufficient transfer of energy which directly impacting the No peelings and consistent tail bond formation observed. quality of the second bond. Figure 3 shows good second bond quality. (a) (a) (b) (b) Fig. 4. (a) Cross section of first bond with clear intermetallic layer with granular type capillary; (b) Consistent inter metallic formation at the gold bond surface with granular type capillary Fig. 3. (c) (a) First bond formation using granular type capillary; (b) Second bond formation using granular type capillary; (c) Second bond formation using polished type capillary Figure 3 shows first and second bond formation at bond pad and leads using granular capillary, while Figure 4 shows cross section of first bond. The granular tip surface effect on stitch, as observed in Figure 3b, has helped to enhance the gripping between the capillary tip and the wire. This is based on research done by [22] on the mechanical theory of adhesion. Modification of the topography of a surface may alter the way that the stress is distributed when the joint is loaded. This can increase the energy dissipation,, which occurs during fracture and provide an important increase in adhesion [22]. Therefore, the ultrasonic energy generated was Wedge pull test for wedge bond process optimization: Wedge pull tests serve as a useful tool in achieving a robust process during wire bond process [17]. As weak tails lead to premature separation of the wire from the second bond, the tail bond strength becomes critical in bonding process especially during the finer wire application (Wittenwiller et al., 2005). The tail bond should be strong enough to allow consistent wire tail formation. To do this, every sample was pulled and the strength was then recorded with a minimum guidance of 2.3 g [23]. Wedge pull results for both granular and polished type capillary are shown in Figure 5. Three units used per sample with 30 wires were pulled during the test. Results is showing higher wedge pull strength obtained using granular type capillary as compared to the polished type capillary. This mechanical interlocking between the gold wire and the lead frame [22] When good interlocking exist, relative displacement between the capillary s tip and the wire reduces thus directly improve the efficiency of energy transfer between wire and lead surface [22]. Both samples exceeds minimum spec, with granular type capillary wedge strength is 0.7 g force higher compare to polished capillary as shown in Figure 5. Statistical Two T-test of 95% confidence (using α = 0.05), shows that wedge pull strength using granular capillary exceeds that of polished capillary between and g force.

5 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 12 (a) Fig. 5. Wedge pull strength at 0 h for polish and granular capillary (b) Fig. 7. Alloy thickness of second bond after two conditions: (a) 500 and (b) 1000 times of thermal cycle test Fig. 6. Wedge pulls results at time 0 and after reliability tests Statistical analysis after reliability tests: Table 4 shows the estimated difference, confidence intervals difference and p- value for both samples bonded by granular versus the polished type capillary. Significant improvements of wedge pull strength as shown in Figure 6 were obtained for sample at 0 h, after 96 h HTS and 1000 times TCT since all p-values are less than 0.05 significance level [24]. It is also 95% confidence that wedge strength of granular capillary is higher than the polished capillary between and g force at 0 h, and g force after 96 h HTS and and g force after 1000 times TCT. However, no significant improvements of wedge strength were obtained after 192 h HTS and 500 times TC. These results showed that, second bond quality of granular type capillary is either higher or equal to that of the polished type capillary even after the reliability test. Meanwhile, the SEM image of cross-sectioned sample bonded using granular capillary at 500 and 1000 times TCT on the leads is as shown in Figure 7a and b. It can be found that homogeneous interface compound formation between gold (Au wire) and silver plating (Ag) with 0.80 and 0.70 µm average IMC thicknesses. No sign of crack and abnormalities were found after TCT test. Thus, Fig. 8. Stoppages rate between Polished and granular type capillary showing that good second bond quality was produced using the granular type capillary. The improvement of wedge strength between and g force proven that the effectiveness of granular tip surface capillary in enhancing the bond ability of the gold wire on lead frame. Effect of granular capillary to machine stoppages rate: The effects of granular capillary were further investigated on machine stoppages rate. The machine stoppages that were calculated using wire bonding process including Non-Stick On Lead (NSOL), Non-Stick On Pad (NSOP), Electro Flame Off (EFO) errors and short tails. Figure 8 shows, lower stoppages rate of wire bonder using granular tip surface capillary as compared to polished capillary. Stoppages reduced from 28 stoppages per 1000 units bonds using polished capillary to only 7 stoppages per 1000 units bonds resulting 75% improvement.

6 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 13 Fig. 9. Average yield for polished versus granular type capillary Table IV Results for two-sample t-test Term Difference 95% CI p-value 0 h 0.7 (0.542, 0.858) HTS 96 h 0.82 (0.599, 1.041) HTS 192 h 0.2 (-0.014, 0.414) TC 500 times 0.18 (-0.010, 0.370) TC 1000 times 0.19 (0.007, 0.373) (a) (b) Fig. 10. Yield breakdown for (a) polished capillary versus (b) granular type capillary Reduction in machine stoppages rate helps to improve machine uptime and overall equipment efficiency and as a result, minimizes the production cost [9]. In addition, these few stoppages from the usage of granular type have further increases the Mean Time Between Assist (MTBA) for the wire bond machine. The mean time between assist for the same equipment used has increased tremendously from 18 min for the polished type capillary to 76 min for the granular type capillary, which is 304% of improvement. Effect of granular type capillary to overall production yield: Average yield for all samples using granular type and polished capillary was calculated in order to study the effect of their surface morphologies to wire bond machine stability and yield loss. In order to minimize other negative factors contributed by machine s technical failures, wire bonder was firstly calibrated before prior to the yield loss calculation. Figure 9 shows average yield for both granular type and polished tip capillaries for 18 devices used in the comparison work. Average yield for polished capillary is 96.9%, while the yield for granular type capillary is 99.5%. This 2.6% improvement of average yield is because the enhancement on the bond ability when using the granular type capillary, which indirectly has tremendous effect to the machine stability [9]. Larger tail bond area, smaller torn area and with a full moon tool mark as observed through physical inspection at 0 h and after reliability tests represents good second bond quality [13]. In Figure 10, it is shown that the yield breakdown in ppm for polished and granular type capillary respectively. The total of NSOL and NSOP impoved by 34.5 and 79.3% using the granular capillary respectively. In this study, the NSOP are directly affect from short tail error. This is resulting from nonsensitive machine charecteristics of wire bonder machine.

7 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 14 Shinkawa for providing bonding tools and wire bonder machine. Fig. 11. NSOP error shows capillary marked When short tail errors occurs, wire bonder will continues and proceed with the normal cycle by firing the EFO. However, due to short tail length, none FAB were formed. Figure 11 confirmed this verification. Internal physical inspection of the bond pad using high power microscope shows only capillary marked, not ball bond marked as per observed on NSOP error causing by bad quality of ball bond. IV. CONCLUSION As conclusion, granular surface morphology capillary therefore improved the bond ability of high reliability gold wire on fine pitch lead frame, improve wire bonder stability and the machine efficiency during wire bonding production mode. The granular type capillary enhances the capillary tip contact with the wire during ultrasonic transmission thus strengthens the metallurgical weld between wire and second bond formation on lead frame surface which in turn improves the overall bond ability. Improvements of second bond quality have also directly affects the process stability and production yield. Major reduction in machine stoppages was achieved with 75% improvement of stoppages rate. An average of 2.6% improvement has resulted in 99.6% production yield using the granular type capillary. Therefore, it can be concluded that the granular type surface capillary will be a better choice of bonding tools for ultra fine pitch on lead frame bonding process as this granular type capillary not only improves the bonding quality and reliability but also improvement in the overall production environment stability with the high overall machine efficiency and total production cost. REFERENCES [1] Zhong, Z. and K.S. Goh, 2007, "A new bonding tool solution to improve stitch bondability", J. Microelect. Eng., vol. 84, pp [2] Levine, L., M. Osborne, H. Clauberg and S. Hilsenbeck, "Improving intermetallic reliability in ultra-fine pitch wire bonding", Proceedings of the SEMICON Singapore, Dec. 2004, Singapore. [3] Gehman, B., Bonding wire microelectronic interconnections, IEEE transactions on components. Hybrids Manuf. Technol.,vol. 3, pp [4] Tok, C.W., I. Langut, A. Menache, D.R.M. Calpito and Y.H. Chew,. "Wire bonding Improvement through optimal bonding tools and materials selection", Proceedings of the 9th Electronics Packaging Technology Conference, (EPTC 2007), Dec. 2007, pp [5] 5 Oian, O., Y. Liu, T. Luk and S. 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Garnier et al., "Tail pull strength of Cu wire on au and Ag-plated bonding leads", Proceeding of the SEMICON Singapore, 2005 [15] Tian, Y. H., I. Lum, S.J. Won, S.H. Park, J.P. Jung, M. Mayer and Y. Zhou, "Experimental study of ultrasonic wedge bonding with copper wire", Proceeding of the 6th IEEE International Conference on Electronic Packaging Technology, IEEE Press, Sept 2005, pp [16] Breach, C., F. Wulff, K. Dittmer, D.R. Calpito and M. Garnier et al.,. "Reliability and failure analysis of Gold Ball bonds in fine pitch and ultra fine pitch application", Proceedings of the SEMICON Singapore, 2004, ACKNOWLEDGEMENT This study was supported by the Malaysian Government and University Kebangsaan Malaysia, under the IRPA grant of PR0075/09-08 and ScienceFund SF SF0495. Special thanks to K and S bonding tools and

8 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:10 No:06 15 [17] MIL-STD-883E, Method bond strength (destructive bond pull test). [18] Edwin Bradley, C.A. Handwerker, J. Bath, R.D.Parker, R.W.Gedney (2007);iNemi Projects Lead to Successful Manufacturing; John Wiley, NJ, USA [19] Electronic Industries Alliance, EIA/JEDEC standard, wire bond shear test method, EIA/JESD22-B116, [20] MIL-STD-883D, Test method condition C: High temperature storage test, (2003). [21] King, N.T., "Solder reactions on nickel, palladium and gold-solder joint technology", Mater. Propert. Reliabil.,2007, vol. 117, pp [22] Packham, D.E., "The mechanical theory of adhesion-a seventy year perspective and its current status", Proceeding of the 1st International Congress on Adhesion Science and Technology: Invited Papers (ed W.J. van Ooij and H.R. Anderson, Jr.), VSP Publishers, Utrecht, 1998, pp: [23] AEC Standard, Qualification test method, test group C-package assembly integrity test, AEC-Q100-REV-G, [24] Mongomery, D.C., Introduction to Statitical Quality Control. 6th edition., Wiley, USA, ISBN , pp Corresponding Author: Ibrahim Ahmad, Department of Electronics and Communication, College of Engineering, University Tenaga Nasional, Kajang, Selangor, Malaysia Tel: Fax: