By Ron Iscoff, Editor
The incursion of two very different technologies-optoelectronics and flip chips, also known as direct chip attach (DCA)-into the IC assembly industry is bringing profound changes.
These changes are evident in many areas, including materials, substrates and processes. Ultimately, however, we may witness the most significant advances in die attach and DCA/wire bonding equipment. First, let's examine wire bonding. Industry seers were playing taps for wire bonding a decade ago, or more. The news was broadcast repeatedly from some quarters that wire bonders had reached their practical limits for speed, pad pitch and bonding accuracy. Breaching the Practical Limits Those practical limits have been breached many times. Although flip chips have become the darling of the IC assembly industry, wire-bonded ICs will continue to grow. Gil Olachea, president of Kulicke & Soffa's Flip Chip Division, contends that wire bonding still accounts for about 98 percent of the ICs assembled today. In five years, that number might drop to 95 percent, Olachea added, speaking at a K&S media luncheon during SEMICON West in July. The world's two largest suppliers of wire bonding tools are Kulicke & Soffa, based in Willow Grove, Pa., and Shinkawa Ltd., headquartered in Tokyo, Japan. Jack Belani, vice president of marketing and sales for K&S-and an industry veteran of decades-says wire bonding will continue as the industry's dominant interconnect technology for many more years. Wire bonding, says Belani, has this edge due to cost advantages and an established infrastructure. "Suppliers will continue developing processes to meet aggressive pad pitch and yield targets while reducing the cost per I/O." Belani notes that the speed of pad pitch improvement using wire bonding "has actually outpaced IC makers' roadmaps. However, going from 45 µm to the targeted 35 µm level will require additional breakthroughs and a higher level of effort. "Simply increasing wire bonder performance won't allow IC makers to reach the next plateau," Belani says. "Integrated assembly processes, developed with suppliers, will become the norm. The convergence of wafer, package and board-level assembly will ultimately be needed, as feature sizes decrease below 70 nm." Wire Diameter Reductions Needed Achieving 35-micron pad pitch will require bonding wire diameter reductions from 25 µm to 15 µm, he maintains. New bonding wire alloys are being developed, says Belani, to improve performance-for example, lower loops for BGAs and less sweep during molding for fine-pitch packages. New molding compounds will also be needed to help eliminate wire sweep-a source of shorts-as finer-wire diameters are employed. At competitor Shinkawa USA in Mesa, Ariz., General Manager Doug Day says next-generation devices will continue to trend toward tighter pitches, with a 35 µm line pitch expected within a year.
Tighter pitches will require an unheard of level of accuracy, says Day, "with a bond placement precision of ±2.0 µm @ 3 sigma-accuracies once in the domain of front-end equipment." Adding to the complexity of bond placement are increasing challenges of wire looping, ball size control (for bumps made with wire bonds), ultrasonic transducer design and fine-wire feeding, Day says. He reports that meeting the demands for wire bonding tools able to handle increasingly finer pitches will challenge bonder suppliers in areas "that were previously Band-Aided, such as machine vibration, optics, thermally induced mechanical drift and ultra-high image acquisition and processing." Incremental and Evolutionary Changes While we will continue to look for incremental and evolutionary changes in wire bonding gear, the most radical changes are likely to be in the die attach and flip-chip equipment areas. Traditional die attach gear-formerly a fairly pedestrian staple of the IC assembly industry-has been joined by the entry of new (and spin-off) companies such as Palomar Technologies (formerly Hughes) and other companies that have have designed die-attach systems from the ground up.
Additionally, the playing field appears to be growing wider every day, as companies such as Universal Instruments (the parent of Alphasem AG) expand their product offerings into highly integrated, highly automated products that offer pick-and-place with flip-chip attach. Die attach equipment, says Shinkawa's Day, "must cope with a broad range of device assembly requirements: multi-stacked chips, very thin die, face-up/flip-chip attach and increased placement accuracy."
Functional and Flexible Tools The key challenge for die attach equipment providers, adds Day, is to offer gear that "combines high functionality with broad flexibility." Dave Halk, general manager for Datacon North America, Trevose, Pa., agrees that advanced packages will require flexible die attach equipment "with high bond force and high temperature capabilities, while maintaining precise die placement."
Looking specifically at flip-chip assembly, it's clear, says Walter Gisler, ESEC (USA) business manager, that next-generation flip-chip die attach will require higher placement accuracy than today's common solder-based processes-which range from 12-18 µm @ 3 sigma. Gisler reports that the trend will be towards 5 µm @ 3 sigma, driven by smaller bump pitch/diameters. To keep a lid on the cost of production, next-generation flip-chip bonders will offer a throughput rate equal to or higher than today's fastest units, he believes. Little Dissent There is amazingly little dissent by tool makers on what is needed in next-generation die attach. Dan Crowley, sales director for MRSI, a Newport Corp. company in N. Billerica, Mass., agrees with many colleagues that equipment combining high speed with the required accuracy and flexibility will win the day. At Universal Instruments, Binghamton, N.Y., Richard Boulanger, vice presidentÐ advanced semiconductor assembly, observes that smaller flip-chip sizes affect traditional specs as well as fluxing and imaging requirements. "The challenge," Boulanger says, "is to create equipment that will be able to handle these ongoing changes for several years." Conclusion New applications-particularly those in opto, datacom, mobile and telecom products-are driving the assembly equipment industry, as is wafer-level packaging. Tomorrow's gear-whether it's for wire bonding or die attach-will be more highly integrated on the line than ever before and will perform tasks at speeds once thought impossible. |
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