Challenges of Ultimate Ultra-Fine Pitch Process with Gold Wire & Copper Wire in QFN Packages

Transcription

1 Challenges of Ultimate Ultra-Fine Pitch Process with Gold Wire & Copper Wire in QFN Packages C.E.Tan, J.Y.Liong, Jeramie Dimatira, Jason Tan* & Lee Wee Kok** ON Semiconductor Lot 122, Senawang Industrial Estate, Seremban, Negeri Sembilan, Malaysia. *SPT Asia Pte Ltd 970 Tao Payoh North, #07-25/26, Singapore **ASM Technology Singapore Pte Ltd 2 Yishun Avenue 7, Singapore weekok.lee@asmpt.com Abstract Since several years ago, many semiconductors companies have invested a lot of efforts and resources in the wire bonding conversion from gold (Au) wire to copper (Cu) wire in order to achieve the most competitive cost. Meanwhile, the fast growing mobile application and industry continues to leap forwards with miniaturization especially in the most popular QFN (Quad Flat No-lead) packages. From multiple analysis, it looks like breakthrough competitive edge will depend on the success of running wire bonding process of both wire types (Au & Cu) with the ultimate ultra-fine pitch (UUFP) process on the flimsy QFN leadframes. This new process can aim for targeted pitch size of 20um which is the smallest possible in the industry, and most likely it will become the ultimate pitch size for now and future. There are many challenges in the development of such process with major considerations and evaluations focusing on various designs, hardware, machine capability, process constraints, process control and amplification of respective Au and Cu wire process issues. The UUFP process is firstly started with design considerations on both direct and indirect material. From the discussions with major bonding wire suppliers, current feasible wire size of Au wire is 0.4 mils and Cu wire size is 0.5 mils. With both wires selected, next design consideration is the capillary design. The capillary tip needs to be the smallest possible together with the tightest tolerance of every major dimension. These requirements resulted in extremely tedious fabrication process which requires new molding tool and process enhancement. Furthermore, such small capillary can also encounter robustness issue due to the thin capillary wall and dimension tolerances. Upon receiving the wire and capillary, the subsequent challenge is the machine capability. There are two important considerations, resolution of process parameters (ultrasonic power and force) and bonding placement accuracy. In the previous development of 30/35 um pitch, whereby it was found that bond force requirement already hitting only several grams. It is expecting that the UUFP process will require even more stringent process window. It is expected few 0.1 gram force may result in significant change in the wire bonding responses. Despite that, the conventional process issues will get worse, which include resonance effect of QFN wire bonding, Au wire challenges (IMC growth, reliability risk) and Cu wire challenges (lifted ball, crater, short-tail). Therefore, the process window development can be extremely difficult. In order to obtain better success rate, comprehensive six sigma method is highly recommended. Upon completion of process window development, a series of robustness tests should be performed. There are some unique robustness tests which can disclose any potential issue or risk. Upon completion of bonding success, the bonded units will be loaded into reliability test for standard tests and extended tests. Sometimes, the entire development process may need to be repeated from the beginning design phase if there is any stubborn quality issue. With the utilization of comprehensive methodology, the new UUFP process is applicable to full production mode in near future. This process is believed to be the ultimate process for wire bonding pitch, for both Au & Cu wire types. 1. Wire Bonding Opportunities In general, the wire bonding pitch reduction is intended for 2 main purposes: (a) Increment of device functionality through more inputs / outputs with smaller Bond Pad Pitch (BPP). (b) Decrement of wire cost (wire size). Fig. 1 : Ultra fine pitch process. The ultra fine pitch wire bonding process development was performed consistently with gold wire until the gold price increased significantly. The high gold price forced the entire semiconductors industry switched to alternate bonding

2 wire material. Most companies selected the Cu wire. The Cu wire is truly cost effective, but the bonding process provides many challenges. Therefore, the ultra fine pitch process development was put on hold during the learning of Cu wire bonding development and implementation in production. Over time, mobile applications continue to make tremendous progress especially in introducing new products. These new products require more miniaturization to address the market requirements. The miniaturization requirements drive wire bonding to go for further development of ultra fine pitch process. With the development startup again, the popular question is about the ultimate & final pitch size. For company who can utilize that final pitch size, it will have the competitive strength in offering the best wire bonded products in both functionality and cost. This pitch process is defined as Ultimate Ultra Fine Pitch (UUFP) process. Fig. 3 : Selection of wire material for UUFP development. 2. Key Wire Bonding Challenges The wire bonding challenges of UUFP development is depending to all the common factors in the bonding process. In order to achieve excellent performance, all the identified factors need to be characterized and optimized by utilizing Six Sigma methodology. The development efforts are expected to be very comprehensive, not only changing wire, capillary & bonding parameters. The following figure shows some of highly possible factors that need to be considered in the development of ultra fine pitch process. Camera Wire Fig. 2 : Development towards UUFP process. While continuing the ultra fine pitch trend with Au wire, the other selected wire material is Pd coated Cu (PdCu) wire. Both bare Cu wire and Cu alloy (doped) wire may not be feasible due to their inferior 2 nd bond performance may not survive small stitch bonding. For Ag (silver) alloy wire, it is still struggling on wire purity concern, balancing between reliability performance and electrical conductivity. Therefore, only Au wire and PdCu wire should be considered in the UUFP process development. The development of Au wire is expected to be slightly more advanced in comparing to the PdCu wire due to its longer history in the past development. However, the Au wire process is considered as near term goal for PdCu wire ultra fine pitch process development. Track Cover Motorized System EFO Torch Track Capillary Top Plate Transducer Cu Kit Die / Pad Leadframe Window Clamp = Possible variations that can affect wire bonding responses Fig. 4 : Possible factors in affecting UUFP development. 2. Material Challenges The first factor of UUFP process development is getting the proper wire material. By looking into the wire manufacturing process, there are 2 major factors getting the highest priority, wire properties and drawing die set. Despite that, there are 3 process steps in wire manufacturing that require characterization. These 3 process steps include wire drawing (for getting the wire size), annealing (for getting the wire properties) and winding (for getting wire in the final spools).

3 Final Wire Size Furthermore, entire capillary dimensions will require shrinking to the minimum level, reaching the manufacturing constraints. In order to reach the extremely close bond pad pitch, capillary tip diameter needs to be as small as possible. Wire Properties - No breakage during Draw Down Process - Dopant selection - New annealing process - Properties uniformity & stability - Pd coating consistency Design of Drawing Die Set - Special techniques - Die reduction steps - New series of die - Drawing speed - Tension control - Friction effect - Lubricant Fig. 5 : Important considerations in wire manufacturing process. 70um BPP, T=90um 50um BPP, T=63um What is the ultimate size? 35um BPP, T=45um 25µm wire By looking into the previous fine pitch development activities, UUFP process will require new wire size. The feasible wire size is 0.4 mils for Au wire and 0.5 mils for PdCu wire. Though Au wire size may be smaller, it is expected to be relatively easier in comparing to PdCu wire. The PdCu wire requires good oxidation protection, verification of pad metal displacement, higher risk of lifted bond, potential damage underneath bond pad and inconsistent 2 nd bond performance (bonding, termination and reliability). 25um BPP, T=30um Fig. 8 : Capillary tip diameter shrinks with the BPP. In parallel with the tip shrinkage, all the other capillary dimensions need to be reduced. Some of important dimensions include chamfer diameter, chamfer angle, hole diameter, face angle, outer radius,,etc. The following figure illustrates the comparison between an ultra fine pitch capillary and a normal capillary. The UUFP process is expected to go for even more stringent reduction in dimensions. Fig. 6 : Bond ball size & wire size changes in fine pitch process development. ~8um Wall thickness ~25um wall thickness The second factor is designing and making the right bonding tool (capillary) to bond the new wire types. In the past, there were several guidelines established in order to enable the fine pitch process development. For example, ultra fine pitch capillary would require increment of wire material mass inside the chamfer area. Tip size =32um (25um BPP) Tip size =90um (70um BPP) Superimpose Capillary tip as big as hole size for 1mil wire capillary Fig. 9 : Comparison of ultra fine pitch & normal capillary. The ultra fine pitch capillary also requires tightening of dimension tolerances. The following example is the previous study performed on ultra fine pitch process in comparison to a normal pitch process. The tolerance control of ultra fine pitch capillary was about 75% tighter than normal capillary. By just doing some simple interpolations, maximum tolerance of UUFP capillary must not exceed 1 micron. CD - More contained wire material inside Chamfer Angle Typical 90 Deg Chamfer Angle UFP 50 / 60 Deg - Less squeezed out Fig. 7 : Capillary chamfer angle design consideration in ultra fine pitch process.

4 - 0 + (g) QFN wire bonding constraints, with solutions that require scrubbing mechanism rather than ultrasonic energy [2] In the previous 30um BPP process, bonded balls are already located very close to each other. Moving towards UUFP, another round of tightening will happen, estimated by 30%. Fig. 10 : Tolerance comparison of ultra fine pitch & normal capillary. With all the capillary requirements established, next challenges occur in the actual manufacturing process. An entire new mold will be established in order to deliver the new designed capillary blanks for UUFP process. In the manufacturing process, the capillary tip is expected to break easily due to the extremely thin capillary wall. Furthermore, super tight tolerances that may require up to sub-micron control level. That requires new customized capillary finishing processes. When manufacturing process is becoming more stable, additional efforts will be required to achieve more competitive cost by enhancing all the process steps again. 4. Process Development The wire bonding process window need to be established with all the variations in wire, capillary and bond pad. Wire Variations Pad Variations Capillary Variations Process Variations Fig. 11 : Variations in material and process. The predicted challenges in the process window development include the followings: (a) Capillary tip breakage due to extremely thinner capillary wall. (b) Capillary hole clogging issue. (c) FAB (Free Air Ball) consistency. (d) Non-sticking on first bond. For PdCu wire, additional risk is pad metal displacement & damage underneath bond pad [1]. (e) Low stitch pull strength. (f) Tail cutting consistency. - MBD : 23 um - MBH : 6 um - Ball Shear : 5 g - Neck Pull : 3 g - Stitch Pull : 2 g Fig. 12 : Previous works done on 30um BPP process. In order to prepare the UUFP process development, some of the machine features may need to be considered for enhancement. (a) Bonding accuracy - Optics resolution - Pattern recognition accuracy - Placement mechanical accuracy (b) Free Air Ball control - Low sparking EFO current - Disturbance by air turbulence (c) Bond Force - Impact force during touch down - Force accuracy - Force calibration jig & hardware - Disturbance by cooling air, cable & vibration (d) Ultrasonic Energy (USG) - Ultra fine/low USG accuracy & stability - Micro transducer vibration stability (e) Wire Clamp - Clamp response time - Clamping force & mechanism (f) Looping - Ultra low looping straightness - Ultra low loop height (g) Machine Operation - Difficulty in wire threading - Bond sticking detection consistency & stability. With the machine readiness achieve satisfactory level, process window characterization and optimization can be started. All the evaluations will require comprehensive study from fundamental material study until execution of Design of Experiment (DOE) or Response Surface Methodology

5 (RSM). Upon getting a workable window, further verifications need to be performed to understand the corner matrix performance and robustness level. For example, PdCu wire may encounter severe displacement issue, and it need to be enhanced before sending for reliability tests. Only having all good wire bonding responses, then reliability test performance will be meaningful. References 1. C.E.TAN, Copper Wire Bonding Process in Leaded Packages with Zero Loss in Quality, Capacity, Scrap & Machine Efficiency, Proc 13 th Electronic Packaging Technology Conference, Singapore, Dec, C.E.TAN, Y.J.PAN & Geale Fonseka, QFN Wire Bond Dilemma Stress Neck & Stress Heel, Proc 9th Electronic Packaging Technology Conference, Singapore, Dec, Extreme Splash Zero Pad Remnant Fig. 13 : Severe bond pad displacement by PdCu wire. 6. Conclusion The ultimate ultra fine pitch process development with both Au & PdCu wire will be recognized as a new milestone for many coming years. It addressed the needs to go for larger I/O and same time also addressing the potential cost saving. More importantly, this development is aligned perfectly with the popular miniaturization activities performed in the aggressive mobile applications. The process development started by creating close partnership with all the major material suppliers, especially those who produce bonding wire, capillary and wire bonding machines. Each supplier must challenge their design and manufacturing process in order to produce the world smallest bonding wire and capillary. Most of the material needs to comply to the tightest manufacturing tolerance, mostly are less than 1 um. Together with proper material development, wire bonder capability also requires relevant upgrade. Upon getting the proper material & machine, then only process development can be initiated. In the process window development, all the possible factors and variations will require thorough considerations. Once achieving good responses, quality verifications with corner matrix and robustness tests will need to be performed accordingly. In the preparation of ultimate ultra fine pitch process, it is expected to encounter many challenges. By utilizing strong teamwork and attention to details, that will ensure high success rate in the UUFP process development. Acknowledgments The authors would like to thank all the key team members from ON Semiconductor, SPT, Tanaka and ASM for their hard work and valuable inputs throughout the execution of ultimate ultra-fine pitch process development.