Turning lead into gold. For alchemists, that was a legendary road to riches. For electronics assemblers, it's a path to avoiding the pitfalls of lead-free flip-chip assembly. Converting tin-lead solder bumps into gold stud bumps can improve the performance and reliability of flip chips, without the problems associated with lead-free solder. Moreover, it can save money in the long run.
Gold stud bumps are bonded to dies with a variation of the ball bonding technique for attaching fine gold wire from a silicon chip to a circuit board or package leads. The process begins when gold wire 1 mil in diameter is pushed out from the capillary tip. An electrical spark melts the end of the wire, and the surface tension of the melted gold causes the drop to acquire a spherical shape, called a free-air ball.
When the capillary is positioned over a pad on the die, it descends. The free-air ball is pulled into the capillary's inner chamfer, centered and squashed against the pad. While force and ultrasonic vibration are transmitted through the capillary, heat is applied to the entire assembly, and the ball bonds to the die. The capillary then rises slightly off the pad, shears off the wire, and moves to the next bonding position to start the process anew.
Stud bumps are 2 to 4 mils in diameter and provide a permanent connection through the aluminum oxide on the die pad to the underlying metal. Stud bumping can be performed with standard wire bonding equipment. A high-speed machine can place 10 to 15 bumps per second with an accuracy of ±2 to 5 microns.
As a result of the shearing process, the bump may have a tail, which can be problematic in some applications, says Jerry Jordan, product marketing manager for Palomar Technologies Inc. (Vista, CA). However, coining the bumps will flatten the tails and ensure that the height of all the bumps across the die does not vary by more than 5 microns. This prevents open circuits and distributes the pressing force evenly across the die during flip-chip assembly. Besides coining, tails can be prevented with special capillaries. These tools form the stud bump into shapes that are tailored specifically to various flip-chip assembly methods.
Gold stud bumps can be applied to single dies or entire wafers. Two or three stud bumps can be stacked one atop the other to raise the height of the die above the board. The extra height helps relieve mechanical stress and facilitates flip-chip assembly with conductive adhesive.
Although the stud bumping process was originally developed with gold wire, some electronics assemblers are experimenting with wire made from other metals, such as copper and platinum. "Gold is the perfect conductor," says Phil Couts, sales manager for the Factory Automation Div. of TDK Corp. of America (Mount Prospect, IL). "The problem with copper is that it oxidizes, and that leads to concern about how strong that bond will be."
Going for Gold
Today, the bumps on most flip chips are made from tin-lead solder, which is applied to the wafer through screen-printing, evaporation or electroplating. Flux is applied to the bumps prior to attaching the die to the substrate. Once the die is in place, the assembly is reflowed at approximately 230 C.
Compared with solder bumping, gold stud bumping has several advantages. For one, gold stud bumping has fewer processing steps and is therefore inherently less expensive. The die does not need an under-bump metallization layer, and the flip chip does not have to be fluxed before assembly or cleaned afterward. In many cases, the flip chip also does not require an underfill material.
Another advantage of gold stud bumping is that it's a lead-free process. Gold is also a better conductor than solder, and gold stud bumps are more compatible with alternative die materials, such as lithium-niobate, gallium-arsenide and indium-phosphide, than solder is.
Gold stud bumping won't entirely replace soldering as a method for flip-chip assembly, says Couts. Soldering is still the technology of choice for assembling dies with more than 1,000 I/O.
However, that could change as wafer fabrication evolves from 130-nanometer process technology to 65-nanometer process technology, Couts says. In high-volume production, soldering is limited to bump pitches of 200 to 250 microns. In contrast, gold stud bumps can be produced at a pitch of 30 microns, even in high-volume production.
These advantages make gold stud bumps the technology of choice for many microelectronic devices, such as cell phones and handheld computers. "The main market for gold stud bumping is mobile devices, where the form factor-the height and the area of the final package-is the most important design consideration," says Couts.
Gold stud bumps are ideal for assembling surface acoustic wave filters for cell phones and microwave circuits for military radar systems. That's because these components cannot tolerate flux residues, and neither encapsulants nor underfill material can be applied after assembly.
The technology is also used to assemble hearing aids, radio frequency identification (RFID) tags, high-power LEDs, smart cards, memory devices, digital signal processors, microelectromechnical systems, DNA analyzers, X-ray detectors and image sensors in camera phones.
Attaching the Chip
Once the gold bumps have been attached, the die can be flipped and bonded to the substrate. Assemblers have three options for attaching the die: ultrasonic welding, thermocompression welding and adhesive bonding. Which to use depends on the size and design of the die; the shape, number and arrangement of the bumps; and the requirements for cost and throughput.
"Metal-to-metal bonding methods provide a higher level of reliability," argues Couts. "Adhesives have their niches, such as RFID tags. But, with adhesives, you're just putting the substrate electrodes in contact with the die and holding that contact. With welding, you're creating a bond at the atomic level."
Ultrasonic welding is used to bond individual dies, and it works the same as the ultrasonic process for splicing wires or sealing copper tubes, just on a much smaller scale. First, the gold bumps on the die are aligned over the gold-plated pads on the substrate. The latest vision-guided equipment accomplishes this with an accuracy of ±8 microns. The die is then pressed down onto the pads with a force of up to 100 grams per bump. At the same time, horizontal vibrations are applied at an ultrasonic frequency of 58 to 64 kilohertz. This scrubbing action disrupts the oxide layers on the parts, allowing atoms to diffuse from one part to the other and creating a true metallurgical bond.
Heating the substrate speeds up the welding process, and a temperature less than 150 C is sufficient for this purpose. The entire process takes slightly more than 1 second: 800 milliseconds to align and mount the die and 300 milliseconds to weld it to the substrate.
Ultrasonic welding is primarily used to bond small dies measuring 5 millimeters square and having up to 30 bumps. However, with greater pressing force and ultrasonic energy, dies up to 10 millimeters square with 100 bumps can be welded. The pad pitch can be as low as 30 microns, and dies can be as thin as 100 microns.
"For ultrasonic welding, the ideal die would have a symmetrical array of gold stud bumps in a rectangular pattern," says Couts. "That's because we're applying downward force on the die and scrubbing it, and we have to distribute that energy evenly across all the bumps."
Thermocompression welding is similar to ultrasonic welding, except that ultrasonic vibrations are not applied to the die. The process relies solely on force and heat to create the weld. As a result, the substrate is heated to much higher temperatures, 350 to 400 C, than with ultrasonic welding.
"Thermocompression allows you to attach dies with a larger number of bumps than ultrasonic welding can," says Couts. Whereas ultrasonic welding can handle dies with 100 bumps, thermocompression welding can handle dies with several hundred bumps. On the other hand, thin, brittle dies may be unable to tolerate the high temperatures.
For optimal ultrasonic and thermocompression welding, the bumps should not have a tail, but rather a soft, blunt peak. They should look a bit like the haystacks in Monet's famous paintings. This shape will help to direct the compression forces and form an intermetallic bond.
Adhesives are another option for attaching the die to the substrate, and assemblers can choose between conductive and nonconductive epoxies.
When using nonconductive epoxy, the die is placed on the substrate, and the adhesive is dispensed to fill the entire space between the die and the substrate. As it cures, the epoxy shrinks and pulls the die tight against the substrate. To enhance the electrical connection, the stud bumps on the die are usually coined prior to bonding. This maximizes the surface area in contact with the substrate pads.
The most common adhesive for this process is anisotropic conductive epoxy. When compressed, conductive particles inside the epoxy align themselves and create a conductive path between the die bumps and the substrate pads. Conduction between bumps is blocked.
When using conductive epoxy, a dot of adhesive is applied to each die bump or substrate pad before the die is placed. The epoxy can be applied through positive-displacement dispensing, screen printing or transfer printing. After curing, the assembled flip chip can still be underfilled with a nonconductive adhesive.
For optimal assembly with conductive epoxy, the bumps should look a bit like a Hershey's Kiss. They should have a central stud and a matted finish around the top half.