Comparative Analyses between Bare Cu Wire and Palladium Coated Cu Wire Performance in IC Packaging Assembly

Transcription

1 Comparative Analyses between Bare Cu Wire and Palladium Coated Cu Wire Performance in IC Packaging Assembly Dr. Jerome Palaganas NANOTECH Solutions, Inc. ABSTRACT Cu wirebonding has been the interconnect technology of choice that has been rapidly replacing Au wirebonding in the semiconductor industry today. Primarily, the direction has been influenced and driven by lower cost and better reliability performance of Cu bonding wire vs. that of Au bonding wire. However, users or companies are faced with two options when converting to Cu wire solution - either to go to bare Cu wire or to adopt PCC, also widely known today as palladium coated copper wire. This paper provides up-to-date summary of comparisons between the Cu wires, namely bare Cu and PCC wire, starting at cost going through workmanship, then through reliability as well as providing other important information commonly overlook during Cu wirebonding production in IC semiconductor assembly. Explanation on interactions of Cu and PCC wire to key interfaces namely on the first bond and second bond are likewise discussed in the paper. Insight on industry trend has also been included, providing a macro view of for the Cu bonding wire history and rationale of migration. Lastly, author s recommendations are provided at the later part of the paper for those who consider converting from Au wire to Cu wire for the first time. In addition, the paper also touches on brief introduction of the alternative and latest interconnect technologies for wirebonding to provide users idea of what the future holds for wirebonding. INTRODUCTION In the middle of the decade of 2000, the push for wirebonding in the semiconductor industry to convert from Au bonding wire to Cu bonding wire was primary driven by finding an interconnect option as an answer to increasing price of gold in the market. However, due to the infancy phase of Cu bonding wire in the industry, and the lack of full support then from wirebond OEMs, the proliferation of Cu bonding wire has seen resistance and hesitation from different companies for the adoption of this interconnect technology. It was not until the late part of 2000s and early part of 2010s, when the price of Au was sky rocketing in the market, demand for Cu bonding wire adoption has become the key priority of many semiconductor companies. However, this move was not without some level of calculated risk involved to these Cu bonding wire industry adopters. Due to the recent introduction of Cu bonding wire, starting 2005, only a few number of players emerged in the market. Thus, selection option by the companies who embraced this new interconnect technology was limited to a few Cu bonding wire suppliers, who were then existing and known suppliers in the Au bonding wire market. The main thrust of early players had been more towards providing workable bonding wire that can be readily be used in manufacturing On the other hand, as the Cu bonding wire becomes more widely accepted in the semiconductor industry, with more and more

2 companies, IDMs and OSATs alike adopting, newer and better Cu bonding wire players have entered the bonding wire space. Some of these players have long and extensive experience and technology over Cu wire manufacturing which provided competitive advantage over Au bonding wire suppliers, who are now also migrating to Cu bonding wire supply and manufacturing. More so, armed with the technology these new players brought along the technology knowledge and patent for Cu bonding wire manufacturing. Thus, soon enough the emergence of insulated or plated Cu bonding wire has been introduced in the market. The introduction came as an innovation to improve the shelf life as well as the reliability of bonding of Cu wire to different device applications. True enough, the Pd plated Cu wire or PCC wire has become the interconnect of choice for those who have done extensive R&D and reliability study, as well as for those late adopters who have accessed to better and newer information on interconnect technology. This paper provides comparative analyses between the two existing Cu bonding wire the bare Cu wire and the PCC wire. The information contained herein provides the reader as well as the industry salient points needed to make a decision when migrating from Au wire to Cu bonding wire, and selecting the wire type between the two technology options. METHODOLOGY Using internal data generated during codevelopment works with customers combined with the literature review with respect to the advancement of Cu wirebonding, the author summarizes the: 1) material, 2) workmanship and 2) reliability performance for both bare Cu and PCC wire in this paper. In order to avoid variability in the results to be presented, most of the studies done had used same device, on both die and leadframe perspective, during the conduct of the evaluation studies completed. Thus, results presented, provide comparative analyses of all the data that shall be presented in succeeding pages of this paper. In this case, TAYA Cu bonding wire has been used in the evaluation studies particularly on the PCC wire. DISCUSSION Cu bonding wire has been seen and proven to provide better electrical performance and thermal conductance as compared with Au bonding wire. These properties of Cu has made it as a very attractive replacement for Au bonding wire as interconnect for wirebonding in the semiconductor industry. In addition, material cost savings derived from conversion from Au wire to Cu wire is relatively significant with larger wire diameter, and higher no. of I/O application in a package. In fact, an 85% estimate material savings can be achieved assuming 1Km length Au bonding wire is replaced by 1Km length Cu bonding wire. On the Cu wire processing side, the only difference between bare Cu and PCC wire is the addition of the Pd plating process. However, critical requirement to achieve a good Cu wire, regardless if bare Cu or PCC wire is the quality of raw materials used. The higher the quality or purity, the better would be quality of the Cu wires produced. For PCC wire, it is a factor that Pd plating over Cu wire must be uniformed and controlled. The quality of the Pd plating affects directly the hardness of the wire as well as the workmanship during wirebonding process. Controlled and uniform plated Pd Cu wires produced consistent

3 yield and quality performance during wirebonding and assembly. Also, it has been found out that overall wire hardness of the Bare Cu or PCC wire translates to the FAB hardness outcome and performance. Bare Cu or PCC wire with lower hardness measure produces softer FAB than those with higher wire hardness measure with wirebond parameters set at a constant. Looking at workmanship, using 20 um bonding wire diameter on device A with an Al bond pad structure from Customer A, 1 st and 2 nd bond performance of Au wire, Cu wire and PCC wire are presented in Figures 1, 2 and 3 respectively. All bonds from the three different wires show good 1 st and 2 nd bond appearance relative to the bonding pad and leadframe leadfinger structures. Specific to the 2 nd bond, there is not much tool or capillary mark indentation which can be seen for the stitch bond created using the PCC wire. This is apparent as PCC wire used in the wire evaluation used lower bond force and bond power comparable bare Cu wire used. However, this setting of second bond parameter does not translate to lower wire pull strength which later on can be seen on the subsequent page. Figure 2: Bare Cu wire 1 st and 2 nd bond Figure 3: PCC wire 1 st and 2 nd bond Likewise, crater test results for all the wires, as presented in Figures 4, 5 and 6 for Au wire, Cu wire and PCC wire, respectively. However, it can be noted that to test the true performance of the Cu wire and PCC wire, a circuit-under-pad or CUP device has been pre-selected. It has been an early notion before that PCC wire is hard, and difficulty to bond CUP bond pad structure was an earlier challenge. However, based on evidence presented below, using CUP device, PCC wire is able to pass crater test even with this kind of structure underneath the bonding pad. Figure 1: Au wire 1 st and 2 nd bond

4 shear strength, wire loop height, bonded ball height and bonded ball size. Cpk values for all five workmanship dimensions are used to compare the overall wire performance for each wire. For workmanship dimensions having Cpk >5, these are standardized or transformed to maximum of 5 Cpk value. Figure 4: Au wire crater test result Figure 5: Bare Cu wire crater test result Figure 6: PCC wire crater test result In Fig. 7, comparison for the three wires - Au wire, Cu wire, and PCC wire workmanship is presented. For Au wire and Cu wire, suppliers A and B were used. Whereas, PCC wire used in the comparative study is from TAYA. Apart from visual workmanship, the spider web chart presented in Figure 7, summarizes key mechanical workmanship performance of the three wire used namely, Au wire, Cu wire and PCC wire in terms of wire pull strength, ball Figure 7: Au vs. Cu vs. PCC Workmanship Matrix Cu wire performance, represented by brown color, shows better workmanship performance compared to Au wire for wire pull, ball size and ball height. And slightly better workmanship performance on loop height than Au wire. Whereas, PCC wire performance, on the other hand, represented by blue color, surpasses that of Cu wire on loop height and wire pull. And, on comparative performance with Cu wire for ball size and ball height. The good performance of PCC wire in this case is attributed to the softer wire property of TAYA bonding wire, thus, allowing consistent loop formation during wire bonding due to its highly workable wire property. This outcome has provided new evidence that PCC wire can no longer be considered as a hard wire, a belief earlier formed during the introductory phase of the PCC wire in the semiconductor market. Specific to 2 nd bond, the higher performance of wire pull values between the Cu and Ag interface on leadframe for PCC wire is attributed

5 to the low presence of oxygen in the said interface [1]. Overall, PCC wire workmanship performance is seen better than Cu and Au wires for all dimensions examined except for ball shear where in Au wire still has higher Cpk performance than PCC wire. This performance of Au wire on ball shear, with better performance than PCC and Cu wires, is attributed to the softer property of Au wire allowing it to produce better bonding adhesion with the bond pad after the 1 st bond formation at wirebond. In terms of reliability, recent studies show comparing bare Cu and PCC wire performance in reliability in terms of uhast and PCT, results so far from studies conducted favors that of PCC wire [2]. For uhast, using 135 o C/85%RH, Cu wire translated product life is 100 hr as compared to 1300 hr for PCC wire with both wires are used in combination of green molding compound [3]. In addition, under PCT 400hrs, Cu wire when combined to green molding compound shows cracking or separation at the Al-Cu IMC. Whereas, under the same condition, of PCT 400hrs, PCC wire combined with green molding compound does not exhibit any cracking at the IMC interface of the PCC wire and Al bonding pad [2]. Apart from workmanship and reliability performance, another factors needed to be considered in the Cu wirebonding are the assembly yield and process window when using bare Cu or PCC wires. Due to the presence of Pd plating, unlike the bare Cu wire, PCC wire extends the open air shelf life of the wire to a 15 day nominal period. However, there are cases which the open shelf life is set up to 30 day still achieving the same yield and quality performance. CONCLUSION / RECOMMENDATION In summary, it is presented that current level of bare Cu and PCC wire, with the introduction of innovation in both process and material in the Cu bonding wire manufacturing, are able to provide good visual and mechanical performance when doing wirebonding on IC device in lieu of Au bonding wire. However, comparing the two wires, PCC wire reliability performance has been found to be superior to that of bare Cu wire. In addition, working with PCC wire in production, based on our experience, using advanced PCC wire type like TAYA PCC bonding wire, is seen already to be almost comparable to that of Au bonding wire. However, just a caveat to engineers doing work on PCC bonding wire, not all PCC wires have the same level of Pd plating technology and overall wire hardness, thus, a careful selection is needed when adapting to the PCC bonding wire. With the correct wire selection, full benefits of adopting PCC wire can be achieved. In addition, there are new interconnect material options emerging in the market other than bare Cu and PCC wires. These are the Ag wire, Pd-doped wire, and Au+Pd plated Cu wire. With the experience gained in Cu wirebonding, the selection criteria on overall workmanship, quality, reliability and price can be applied during the selection process. However, the author believes that PCC would be the mainstay for Cu wirebonding for a while until any of identified next generation bonding wires would be able to provide similar extensive data with better quality and reliability performance results. Likewise, the wires have to prove itself capable to the different packaging applications in the semiconductor industry.

6 REFERENCES 1. Qin, I., Clauberg, H., Cathcart, R., Acoff, V., Cylak, B., and Huynh, C., Wirebonding of Cu and Pd Coated Cu wire: Bondability, Reliability, and IMC Formation, Electronics Components and Technology Conference (2011), pp Uno, T., Bond reliability under humid environment for coated copper wire and bare copper wire, Microelectronics Reliability, (2011) Vol. 51, pp ABOUT THE AUTHOR Dr. Jerome Palaganas is currently the Business Development and Operations Head of Nanotech Solutions, the materials and automations division under SA Technologies, Inc. Dr. Palaganas has more than 19 years of engineering and management experience in the semiconductor and electronics industry, including process, development, R&D, NPI and program management responsibility at Amkor Technology Philippines working at various sites and with different packaging technologies. He also played a key role as a start-up wirebond technical development and process engineer for Amkor s MQFP and TQFP packages as well as done pioneering and successful works in Cu wirebond development and qualification for CABGA in 2009 among others. Apart from the industry experience, Dr. Palaganas taught at the De La Salle Graduate School of Business for more than 8 years. He received his Doctor of Business Administration and Master of Business Administration degrees from De La Salle University finishing both With High Distinction Honors. And, he earned his Bachelor of Science in Electronics and Communications Engineering from Mapua Institute of Technology graduating at the Top Ten list of his class.