附录1WIRE BONDING: PRESENT AND FUTURE TRENDSABSTRACTGold wire bonding has been a standard operation inplastic IC package assembly for several decades. However, as the packages grow larger, thinner, and denser, the capabilitiesof equipment, and of the wire itself, are being taken to thelimit. Recently, specialty wires have been introduced with different dopants and processing procedures giving higherstrength and stiffness. These improved properties can offerbetter wire wash protection as well as capability for longspans and low loops. Disadvantages come from higher cost and potential reliability issues from the high dopant levels.Advances in the wire bonders allow better machinemotion control with precise placement of the wires. Improved control software can produce low loops and long straight spans by simultaneously regulating motion along all three axes, from ball placement to the wedge bond on the lead tip. Some manufacturers even believe that all improvements can be achieved through the equipment alone, without having toresort to the specialty wires.The combination of better wires and better machines should be able to produce the type of wire bonds desired in thin fine pitch plastic packages. Nevertheless, the physical limits of ball bonding may eventually be reached, if the current trend of finer and finer pitch continues. In this case, other types of interconnection will start looking attractive, such as TAB, flip-chip, or the development of high speed, flexible wedge bonding.1.0 CURRENT STATUSThe gold (Au) bonding wire generally used in the industry for automated thermosonic bonding is primarily beryllium (Be) doped, at about 3 to 8 parts-per-million (ppm). In this range, beryllium gives the wire increased strength by precipitation hardening in the matrix of the intermetallics formed from the two elements. It also allows the gold to be more easily drawn, as very pure gold is too soft for shaping into a fine wire (1,2). Stronger and harder wires have been introduced into the market recently in response to the needs for fine pitch and longer spans. These wires are also formulated to produce lower loops for thinoutline packages (3).While adequate for most plastic packaging bonding needs, the previous generation of bonders do not have the ability to bond accurately for the next generation of high density packages. These machines tend to have analog motion control, which does not lend to precise travel and placement of the wires. Also, being analog, each machine is unique with respect to its set-up. Parameters optimized on one bonder cannot easily be transferred to another.2.0 TRENDS IN GOLD BONDING WIRE2.1 MATERIAL REFINEMENTSTable 1 compares the properties of some typical commercially available wires. Generally all the “new” wireshave improved properties, such as greater breaking load,lower loop height, and increased pull and shear strengths.Dopants increase the strength of Au wires. Forinstance, Be increases grain refinement and strength by introducing intermetallic precipitates, since it is a small atom compared to gold and is electronically issimilar. Even in minute amounts, the atoms will distort the lattice, making Be an effective hardening agent. This is currently the primary dopant used in the industry. However, increasing the amount to above 12 ppm will cause Be to come out of solution, and induce intergranular fracture during bonding (1).Other metals are also used to maximize mechanical properties. Silver (Ag), palladium (Pd), copper (Cu), and platinum (Pt) are added for solid solution strengthening and chemical stability. Three of the four elements cited are in the same group in the periodic table as gold. In addition, just aswith gold, all four elements are face centered cubic (FCC)with similar lattice constants and electronic structure.Strengthening is by substitution of lattice atoms with dopant atoms to provide the chemical stability (4). Other metals, like calcium and lanthanum, can also be used for strengthening by precipitation hardening, in much the same manner as Be. In addition to the dopants, the production process also influences the wire properties. Careful rolling, annealing, and drawing improve the wire properties by insuring grain uniformity and even dispersion of the precipitates (1).2.2 LOWLOOPING CHARACERISTICSAnother purpose for these refinements is to increase strength and hardness and to reduce the length of the heat affected zone (HAZ). The HAZ is the location where the loop is formed, and should ideally be between 25 pm to 100 pm from the ball bond. Through recrystallization and grain growth, the HAZ can easily be deformed. Larger grains are generally weaker from a lack of dislocations due to work hardening. Opant precipitates tie up the grain boundaries during heating and prevent grain growth. One reason for the high dopant level is to insure that an adequate amount of precipitates exists in the atomic lattice. Thus,the quality of t