Gold ball bumps are serving a widening number of flip chip applications. This increased activity has led to a new generation of dedicated gold ball bumping equipment, capable of bumping single die or entire wafers with a variety of specialized bump shapes, to improve performance and yield.
Bumping and Shaping
Gold ball bumps are made by a modification of the process used for many years in wire bonders for gold ball bonding. The first step in that process is to melt ("flame-off") the tip of the gold wire, so that surface tension forms the molten gold into a sphere. The wire bonder capillary tool then attaches that sphere to the device bond pad, using heat, pressure, and ultrasonic energy. For gold ball bumps, the wire must then be severed from the ball. The resulting ball size and shape is determined by the wire composition and diameter, the bonding tool, the bonder settings, and the method used to sever the wire.
Figure 1 (5 Kb) shows the typical shape of a ball bump as it is created by a wire bonder. Note the small length of wire that remains attached to the top of the ball. This "tail," common in early gold bumps, results when the wire is severed by pulling until it breaks at some point above the ball. However, this tail is not suited to some flip-chip assembly processes and applications. A process step known as "coining" must then be used to flatten the tail and create a flat, circular top to the ball, as shown in Figure 2 (6 Kb). Coining refers to an application of force that will "smash" the peak into a smooth, flat surface. The diameter of this "plateau" is an important process characteristic. It determines the portion of the gold ball that will contact the 2nd surface, defining the conductive area.
The introduction of a new generation of equipment developed for and dedicated to gold ball bumping, and the broadened requirements of different gold ball flip chip applications, has lead to more subtle and rewarding methods of shaping the ball bump. New bump formation techniques have been developed that can create a gold bump without the traditional tail. Now, the optimization of any gold flip-chip process also includes optimizing the bump shape.
The bump shown in Figure 3 (6 Kb) differs from the typical bump of Figure 1 in not having the wire tail or pointy peak. This bump was designed specifically for flip chip assembly using either a thermocompression or a thermosonic assembly process. The bumps in Figure 3 show a softer, more blunt peak than the Figure 1 bump. This shape will help direct the compression forces to assist in the formation of an intermetallic bond at the second surface. By focusing the applied energy onto a smaller surface area, the other bonding factors (heat, force and ultrasonic energy) can be reduced. As this bump continues to be compressed during assembly, the surface area in contact with the substrate pad will grow, increasing the conductive area.
A ball bump shape like the one in Figure 4 (5 Kb) may be preferred for isotropic conductive epoxy assembly. This bump has a center stud, with a matte surface finish on the upper surface of the bump. The matte finish was created by the impression of a special capillary onto the gold ball. The capillary is the bump-forming tool that carries the wire down to the surface to be bonded. It is the same tool that presses the gold ball onto the surface. This matte finish provides a better surface for conductive epoxy to adhere to, and to remain in place during the flip and attach assembly steps.
The third specialized shape we will describe is the coined or flat-top shape. This is an attractive shape for non-conductive epoxy assembly using compressive dispersion. The SEM photo shown here as Figure 5 (5 Kb) shows a flat top bump that looks similar to the coined bump shape of Figure 2. The flat top, in this case, is created by mechanically shearing across the top of the bump, to sever the wire and shape the bump immediately after it is formed. The shearing step eliminates the need for a separate coining step.
Conclusion
The new generation of dedicated gold ball bumping equipment allows tailoring the final bump with a variety of gold ball bump shapes. Now, gold ball bump shapes can be matched to the assembly and application needs, to increase yield and performance.
FOR MORE INFORMATION
Full Text , downloadable version of the original article "Gold Stud Bump in Flip-Chip Applications" by Jerry Jordan, Palomar Technologies
Tutorial 3, "Gold Stud Bump Flip Chip"
Tutorial 9, "Thermosonic Flip Chip Assembly"
Tutorial 10, "Flip Chip Assembly of Detector Arrays"
Technology Update, "Stud Bump Flip Chip Assembly of MEMS Motion Sensors"
