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Wedge Bonding New Wire Alloys for Medical Electronics
Keywords: Wedge Bonding, Wire Alloy, Medical Electronics
Wire bonding is the dominant interconnection method for electronic packaging, currently over 90% of all interconnections are wire bonded. The flexibility, reliability and low cost of wire bonded interconnections is unsurpassed. Even with the high cost of gold, the average wire bond uses less than $0.002 gold. However, even that small cost becomes large (more than $10 billion) when many trillions of wires are considered. This has led to the rapid conversion of gold ball bonding to copper wire. Currently approximately 15% of the entire market has converted and as early production problems have been resolved and more products have been successful the trend has increased. Initial conversions were less expensive, lower reliability devices but more advanced products are right behind. There are, however, devices that will not be converted and many medical electronic packages are among them. The high value, demanding reliability and low volumes of medical devices make gold wire costs a less critical factor. Medical devices require high reliability and must perform flawlessly in difficult environments. New wedge bonding quality tools, not available on ball bonders, are capable of monitoring the bond in real time, a patented sensor, built into the ultrasonic transducer, monitors the amplitude of the tool movement, and senses friction at the bond interface. Bond deformation, ultrasonic current and frequency are also monitored in real time. Algorithms that use all of these real time signals to derive a Quality factor for each bond are available for wedge bonding. User defined limits on deformation, current and Quality factor are used to flag any bonds that don’t meet the strict limits required for high-reliability medical devices assuring high quality products and high yields. Interfacing this quality data to a real-time engineering factory control system allows storage of bond quality data for each bond produced. Wedge bonding has traditionally used either Al or Au wire for interconnection but recently new silver alloy wires have become available. Earlier, pure silver wires had reliability problems when bonding to Al bond pads, during 85C/ 85%RH testing they were reported to lose 90% of their bond strength in less than 10 hours. The new silver alloy wires, because of their chemical composition, are able to survive accelerated testing and meet reliability requirements. New wedge bonders have many new capabilities: • They are much faster than previous generation machines, and now are capable of bonding more than 6 wires/second. • New constant loop height and constant loop length algorithms achieve optimum control of electrical properties (impedance, capacitance). • Low loop capability is superior because there is no ball adding to the bond height and the low take off angle of the wire naturally tends toward lower height. New looping motion algorithms provide the lowest loops achievable. • In 8 die stacks wedge bonds achieved a 32% decrease in cross section and a 20% decrease in total stack height compared to standard ball bonded devices. This provides an inexpensive alternative to TSVs in 3-D packaging. • Bond placement accuracy has improved. New machines are capable of 1µm bond placement repeatability @3. Wedge improvements (V groove and oval hole tools) in conjunction with bonder advances have significantly improved bond placement enabling them to achieve the finest pitch available for any wire bonding process. Wedge bonding, because of its smaller bond size, room temperature bonding, lowest loop capabilities and low cost is the best interconnect method for these new devices. The discussion will focus on the use of fine pitch wedge bonding for medical electronic interconnections.
Lee Levine, DMTS
Hesse Mechatronics
Fremont, CA

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