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Choosing the Right Bonding Wire for ICs
By Mike Greenelsh, President, California Fine Wire, Grover Beach, CA Our love of small electronics with big features is creating ongoing demands for miniaturized integrated circuits (ICs) with ever more inputs and outputs. This results in very crowded connections between tiny ICs and their packages. As a consequence, the lead wire bonding — used to form those connections — has become perhaps the most sophisticated process of all IC assembly operations. This situation makes the proper selection of bonding wire materials even more critical. While there are standard specification and test guidelines, many IC designers must make judgment calls about factors other than basic material selection, such as metal fatigue, and elongation and breaking load parameters. There are also issues concerning wire quality, which can be referenced from a manufacturer's perspective. Most Common Metals Generally speaking, the elements most commonly used to make bonding wire are gold and aluminum. Bonding wire is usually specified because of its strength, based on the metallurgical characteristics of “elongation” (how much a wire can stretch before becoming permanently stretched) and “breaking load” (how much elongation a wire can take before breaking). Both gold and aluminum are strong. Both are ductile. Both also have similar resistance in most environments. Gold is used because it is normally inert, well suited to the “ball” bonding process, and well suited for providing better loop formation and cycle performance. The proven reliability and flexibility of gold wire bonding have made it the most widely used technology in the IC industry. However, in a high heat situation, gold presents problems because it tends to absorb the radiated energy, making it unstable, which is a particular problem in outer space. Aluminum alloys can be better conductors than gold, and provide the advantage of relative fatigue resistance. In practice, the lightweight silicon-aluminum alloy has proven quite reliable for the ICs in billions of devices. However, small-diameter aluminum-silicon wire must be heat treated (partially annealed) in such as way to cause the silicon to disperse evenly before it is drawn. In larger diameters, California Fine Wire heat treats the metal to stabilize the silicon before the wire is drawn, then heat treats it again in the final draw to get the elongation and break point we're seeking. Many IC manufacturers today are opting for copper, palladium-alloy, platinum or silver bonding wire because of the potential for substantial gains in conductivity and, thereby, circuit speeds. In bonding wires, the problem of metal fatigue is usually an installation question, rather than a field problem. It is the result of repetitive stress, such as the repeated bending of a wire. According to the ASTM (American Society for Testing & Materials), the practical solution to limiting wire fatigue in open-cavity IC packages is to increase the loop-height/bond-length ratio, which limits the amount of flexing in wire bonds. There are instances when an IC manufacturer may want to exceed the normal elongation of a certain type of wire without encountering plastic deformation or compromising breaking load. This can be accomplished to some extent by slightly annealing (stress relieving) or fully annealing the wire. At California Fine Wire we have successfully doubled the elongation of a customer's specified wire without sacrificing break strength, because we routinely employ these processes. Standards and Practices Quality is acutely critical for the manufacture of bonding wire. Of course, most wire producers do some things differently throughout the various manufacturing processes, but there are standards and practices that should be in place throughout the industry. Metal Purity. Base metals must meet ultra-pure standards; for example, gold has to be 4/9 minimum. At CFW we usually exceed the minimums to assure that component manufacturers will have a quality product. The higher the quality of melting stock, the fewer the contaminants such as oxides that could cause fatigue or wire drawing problems. After dopants are added — such as beryllium with gold — we do everything possible to ensure a good melt. For example, with aluminum, we use a specialized crucible with a nitrogen cover to protect the melt. We also make sure that dopants, especially silicon, are mixed properly to prevent problems with drawing the wire or with wire fatigue. Shape. If the wire were slightly out of round, it could not pass through the bonding machine capillary and could slip or not run smoothly. Here again it is important not to have stress risers in silicon-based alloys, or the silicon “chunks” may cause capillary blockage or compromise bond integrity. Thickness. Most fine wire takes about 50 draws through a wire machine to reduce it to the final finish size. When you consider that we may start with 0.250-inch stock and finish with 0.00125, you can appreciate the size of the reduction and number of dies involved. Annealing. Heat treating (annealing) is an important process in making bonding wire. We heat treat the raw materials as soon as we receive them. With aluminum-silicon alloy, we heat treat to make sure the silicon is dispersed properly. We heat treat again after we've drawn the wire down to finish size, in order to stabilize the alloy. During this final heat treating process the wire gets a degree of external protection in the form of a light patina of oxidation, which helps to prevent rapid oxidation. Tensile strength. Tensile tests are crucial to determining a product's tensile strength, yield strength, gram breaking load (GBL) and elongation. Handling. There is some wire damage done at the customer level due to handling. This seems to be a result of mishandling the little spools by accidentally dinging them with fingernails as they are removed from packages. At CFW, we include complete handling instructions with every product to advise bonding machine operators to handle the spools with great care. For more information, contact: California Fine Wire, P.O. Box 446, Grover Beach, CA 93433-0199. Tel: 805-489-5144; fax: 805-489-5352. Web: http://www.calfinewire.com |
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