A Primer on Bonding Wire Parameters
A fine wire manufacturer offers some fundamentals for those involved with lead bonding wire.
Mike Greenelsh
California Fine Wire
Grover Beach, CA
Ongoing demands for higher IC utilization with minimal board real estate are producing high densities of connections between chips and packages. As a result, lead wire bonding, used for the formation of interconnects in ICs, has become perhaps the most sophisticated process of all the IC assembly operations. This situation has made the proper selection of bonding wire materials more critical.
Making Judgment Calls
Over the years the specification of bonding wire has been a somewhat moving target. While IC manufacturers and the American Society for Testing and Materials (ASTM) have adopted standard specification and test method guidelines, in truth, many IC designers must make judgment calls about factors affecting wire bonds, such as burnout rate, metal fatigue and current-carrying capacities. These judgments are required mainly because wire bonding parameters are based on what are believed to be typical samples. However, the complexities involved in bonding interconnects, not to mention uncertainties surrounding their applications, may be well outside the realm of typical. There are also a number of issues surrounding the fabrication, shelf life and durability of bonding wire - issues referenced here from a manufacturer's perspective.
Popular Bonding Wire Elements
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 and breaking load. Both gold and aluminum are strong and ductile and have similar resistance in most environments.
Gold
Gold is used because it is normally inert, well suited to the ball bonding process and demonstrates excellent loop formation and cycle performance. However, in a high heat situation, gold presents problems because it tends to absorb the radiated energy, making it unstable, which is especially a problem in outer space. Gold wire can be stabilized with several different dopants including beryllium, calcium and other proprietary dopants. Gold wires for ball bonding are normally supplied in the annealed condition to prevent unwanted break-off partial annealing during ball formation. The proven reliability and flexibility of gold wire bonding have made it the most widely used technology in the IC industry.
Aluminum
Small-diameter aluminum wire is commonly used for ultrasonic wedge bonding. Aluminum alloys also provide the advantage of relative fatigue resistance. In practice, the lightweight silicon-aluminum alloy has proven quite reliable for ICs in billions of devices. Since aluminum is too soft to draw for small-wire dimensions, an alloying metal (normally silicon) must be added to meet necessary breaking load and elongation parameters. Unfortunately, silicon and aluminum do not combine readily and, when heated, silicon alloy particles can cause stress risers, resulting in cracking of the wire. Therefore, the small-diameter aluminum-silicon wire must be heat-treated (partially annealed) in such a way as to cause the silicon to disperse evenly before it is drawn. In larger diameters, the metal is heat-treated to stabilize the silicon before the wire is drawn, then heat-treated again in the final draw to obtain the desired elongation and break point characteristics. (Note that magnesium-doped aluminum wires have advantages including better fatigue resistance than silicon-doped wire, but the silicon-doped aluminum has become the standard.)
Other Materials
In addition to gold and aluminum, 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, therefore, circuit speeds. The choice of copper for interconnections requires that structures be encapsulated with a barrier layer (usually a thin-film layer) to achieve the required adhesion and protect against diffusion of copper atoms into silicon devices, which will degrade performance. Palladium-doped gold wire is used for ball bonds on IC chips for flip-chip applications and ball-in-the-corner interconnects. Platinum wire is sometimes specified for high temperature semiconductor devices. Silver is considered the best conductor of all materials for speed performance.
Concerns about Shelf Life
Some companies adopt a discretionary policy of discarding bonding wire after three or six months. The principle here is that they would rather discard perfectly good wire than risk a change in metallurgical properties that could affect the yield of a given machine setup. Several years ago, tests determined that hard, as-drawn wire began to weaken within six weeks of manufacture. Specifically, the breaking load decreased between five and 15 percent in that period, then decreased more slowly for the balance of the two-year period of the test. However, stress-relieved and annealed wires (both gold and aluminum) stayed within their breaking load specifications for the entire two-year test period. Test results for elongation were somewhat more ambiguous. Annealed or stress-relieved wire may be used for up to two years, although elongation may vary slightly. However, the wire must be stored at near room temperature without exposure to sunlight or drafts.
Metallurgical Fatigue
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. As known from everyday experience, recurring bending can break a wire even though this stress is much lower than that required to fracture it in a single bend or pull.
Microprocessors become warm due to their high operating current and device manufacturers are usually somewhat cautious about using fans or radiators to cool these devices. However, thermal cycling in ICs constantly flexes bonded wires and can produce failures. In addition, during thermal cycles any undispersed silicon in aluminum-silicon wire may enlarge and serve as a stress riser, making the wire prone to crack and fail. Loop height can affect thermal cycling as much or more than the properties of the wire itself. Such problems must be considered when designing ICs that will encounter appreciable temperature variations.
There is no doubt many IC designers find themselves having to use limited data to make estimates concerning wire fatigue life. According to the ASTM, the practical solution for limiting wire fatigue in open-cavity IC packages is to increase the loop-height-to-bond-length ratio, which limits the amount of flexing in wire bonds.
Wire Burnout
Wire burnout is related to metallurgical fatigue, but results from certain factors causing the current-carrying capacity of wire to be exceeded and the wire to be fused. Most of these factors are metallurgical, including resistivity, thermal conductivity, temperature coefficient of resistance and melting point. Another major contributor is the length of the wire - the longer the wire, the lower the current required for burnout. Even plastic-encapsulated devices, which compose the majority of ICs, are subject to burnout when the encapsulating material that is melted becomes thermally insulating and quickly burns out. Recognizing the difficulty in predicting the maximum current a wire can carry and the likely incidence of wire burnout, many designers simply overspecify wire diameter or use multiple wires. In fact, wire burnout is not a common problem in the field.
Elongation and Breaking Load Parameters
There are trade-offs between elongation and breaking load, two fundamental metallurgical characteristics of bonding wire that influence wire specification. Elongation pertains directly to the elasticity of a particular wire in a certain state of hardness depending on the requirements of an application or how much a wire can stretch under various stress-strain conditions before plastic deformation (permanent stretching) occurs. Breaking load is the amount of elongation a wire can sustain before the breaking point.
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 strength. This goal can be accomplished to some extent by slightly annealing (stress relieved) or fully annealing the wire. The elongation of a user's specified wire has been successfully doubled without sacrificing break strength by routinely employing these annealing processes.
Wire Quality Factors
Quality is acutely critical for the manufacture of bonding wire. Of course, most wire producers do some things differently during various manufacturing processes, but there are standards and practices that should be in place throughout the industry.
Base metals must meet ultra-pure standards; for example, gold must be 4/9 minimum. These minimum standards usually are exceeded to assure that component manufacturers will have a quality product. The higher the quality of melting stock, the fewer contaminants are present such as oxides that could cause fatigue or wire drawing problems. In addition, dopants are added (for example, beryllium with gold) so that everything possible is done to ensure a good melt. For example, with aluminum a specialized crucible with a nitrogen cover is used to protect the melt. In addition, dopants, especially silicon, must be mixed properly to prevent problems with drawing the wire or wire fatigue.
If the wire is slightly out of round, it may 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.
Most fine wire takes approximately 50 draws through a wire machine to reduce it to the final finish size. When considering that the beginning stock may be 0.250" in diameter and end up as 0.00125", one can appreciate the size of the reduction and number of dies involved.
Heat-treating (annealing) is an important process in making bonding wire. The raw materials must be heat-treated as soon as they are received. In the case of aluminum-silicon alloy, the material is heat-treated to ensure that the silicon is dispersed properly. After the wire is drawn down to finish size, it is heat-treated again to stabilize the alloy. During this final heat-treating process the wire receives a degree of external protection in the form of a light patina of oxidation, which helps to prevent rapid oxidation. The wire's tensile strength is determined by performing yield strength, gram breaking load and elongation tests.
Finally, some wire damage may occur at the user level due to handling. This damage seems to be a result of mishandling of the little spools by accidentally dinging them with fingernails as they are removed from packages. Complete handling instructions must be included with every spool of wire to advise bonding machine operators how to handle the spools with great care.
Mike Greenelsh has been involved in the wire manufacturing industry for 35 years. Currently, he is president of California Fine Wire, a manufacturer of specialty wire and a leading supplier of bonding wire to the microelectronics market. He can be reached at (805) 489-5144. Additional information is available at the California Fine Wire Web site: www.calfinewire.com.