The EMPF is often called upon to select the wire bonding process that most adequately fits an application for chip interconnection. The selection process for wire bonded chips should including some of the latest advances in the art.
Tip 1. Determine the speed with which the wires must be applied to the chips to meet production requirements.
Figure 1-1 details the ball bonding wire bond process. The ball bonding process exhibits circular symmetry in which the wire is fed through the cylindrical bonding capillary. After making the first, or ball, bond, the capillary bonding tool is free to move in any direction from the first bond. This allows each successive bond wire to “fan out” as it is placed between the chip and substrate, at any angle without rotating either the chip or the wire bonding head, since the wire is fed through the hole in the center of the capillary tool.
Figure 1-2 details the wedge bonding wire bond process. The wedge bonding process requires that the flat wedge bonding tool, after forming the first wedge bond, must traverse straight from the first to the second bond, without changing angle, so that the wire stays under the foot of the wedge bonding tool as the wire pays out. For this reason, either the tool or the bonding table holding the chip must rotate in order to fan out each successive wire from the chip to the lead frame or circuit.
Tip 2. Determine if gold ball bonding, gold wedge bonding, aluminum wedge bonding, copper ball bonding, gold or aluminum wedge bonding, is needed for the production bonding job for some reason other than bonding speed. In some cases, the current carrying, Radio Frequency (RF) performance, Power Consumption, or bonding pitch reqirements might dictate the type of bonding to be used, regardless of the production speed.
For instance, heavy aluminum wire (up to 0.020 in. diameter) is commonly used for very high power (a 0.020 in. dia. Aluminum wire can conduct 25 amperes of direct current without fusing. The slower speed of the wedge bonding process has to be traded off in that case.
RF effects such as inductance and capacitance at high frequency, often dictate that wedge bonding be used because of the rectangular cross-section of the gold ribbon used instead of round wire. Ribbon cannot be ball bonded because of its shape, and wedge bonding is the only alternative for ribbon.
Power Consumption has been mitigated for advanced digital microprocessor chips by metallizing the chips with copper rather than the aluminum traces and bond pads on the silicon chips. These can only be wire bonded using copper rather than gold or aluminum wire. Special copper ball bonding equipment, in which the copper wire “flame-off” that produces the ball shape must be done under a reducing atmosphere for the copper wire. Gold wire poses no such issue with oxidation. With copper metallization on the chip and copper ball bonding, power consumption of the microprocessor can be reduced by up to 50%. Very close bonding pitches, below 75 microns, are often accommodated by using the wedge bonding method, in spite of its slower speed, since the width of the wedge bond tends to be less than the flattened ball of the ball bond for a given diameter wire.
Tip 3. What is the loop height and bond length requirement for the application? The newer stacked packages and multichip packages, as commonly used in cellular telephones today are beginning to infiltrate the communications and military markets. These applications often use wire bonding to interconnect the successive tiers of stacked chips. These stacked chip applications often have severe limits on the height of the wire loop. Wedge bonding has a very low loop height capability. However, to take advantage of the higher production speed of ball bonding, packagers often reverse the bonding sequence, putting the ball bond on the substrate or lead frame (traditionally the second bond, or stitch bond, in ball bonding) and the second bond (stitch or wedge bond) on the chip pad. This results in the higher loop height of the ball bond being formed on the substrate and the lower loop height of the stitch bond on the chip, and retains the high production speed of ball bonding. This technique is known as “reverse bonding.”
There have been recent breakthroughs in forward ball bonding of low loop height and long length bonding. This has been done using new types of wire that retain strength in the long bond lengths required in the newer stacked packages, and have enough ductility in the wire to enable low loop height formation without cracking the wire(1).
Tip 4. Choose the type of bonding needed for the application.
With production speed in mind, trade off the performance requirements of the wire/ribbon type and size, wire or ribbon material, bond pitch, and power needs of the application. Don’t rule out the newer techniques of reverse ball bonding for stacked chips, wedge/ribbon bonding for high power, copper wire for low power consumption, and wedge bonding for close pitch applications. This will result in the optimum choice for wire bonding.
References: 1. “Very Long, Ultra-Low Loop Testing for New Bonding Wire Development,” D.R.MCalpito, Ivy W. Qin, Emily Pasamaanero, Ei Phyu Phyu Theint and Tok Chee Wei, Kulike and Soffa Industries, Semicon Singapore