Thermosonic bonding of flip chips has advanced greatly over the years. Originally limited to ceramic substrates, thermosonic assemblies are now made on a variety of organic substrates. Originally limited to small die with low bump counts, improved equipment now allows larger die with higher bump counts.
The thermosonic bonding advantages described below and the disadvantages of lead-free solder are stimulating the growth of this low temperature gold-to-gold connection.
The thermosonic assembly process is similar in many ways to its ancestor, gold wire bonding. In the first step of gold ball wire bonding, the bonder places a gold bump on a bond pad. For thermosonic bumps, instead of extending the wire to a second pad, the bonder breaks the wire above the first bump.
As shown in Figure 1, this generally leaves a little tail of wire behind. The bumps may be coined by uniform compression or mechanically sheared to eliminate the wire tail, create a larger contact area, and coplanarize all of the bump heights on the die.
As with wire bonding, plasma cleaning the bumps and substrate pads immediately before assembly can improve bond strength and underfill flow. After cleaning, the substrate is placed in an aligner-bonder.
Bumps on the die are aligned with the substrate pads, brought into contact, and bonded by sonic energy while under heat and pressure. The 4 minute VIDEO shows the bonding process in detail. 
A major advantage of thermosonic flip chip bonding is the low temperature and low force required compared with other flip chip assembly methods. Figure 3 shows the temperature – pressure regime for common bonding methods. 
As shown in Figure 3, thermosonic bonding temperatures are lower than other common bonnding methods. Thermosonic assembly generally is below 200 deg-C, and may be below 100 deg-C. Thermosonic bonding forces of 50 to 100 grams per bump also are lower than thermocompression forces. 
The thermosonic bonding cycle is short, with high-volume automated bonders capable of bonding 3,600 units per hour. The shorter cycle and fewer process steps contribute to high throughput and reduced costs.
The gold-to-gold connection offers lower (5 milliohms) and more stable resistance than conducting particle connections.
The compliance and strength of gold stud bumps makes it possible to eliminate underfill on smaller die. This allows mounting die that cannot tolerate underfill, such as MEMS pressure sensors with mechanically moving parts. 
The original thermosonic limitation to small die with few bumps has been extended to larger die with 50 to 100 bumps.  Assembling large die requires the excellent coplanarity and uniform distribution of bonding force of improved assembly equipment. Fine pitch bumping with over 1,000 bumps has been demonstrated. 
Similarly, substrate materials have been extended from ceramic to flex and to a variety of rigid organic substrates.  Again, improved equipment has been key to that extension. In summary, advances in equipment and technology have stimulated the growth of low-temperature thermosonic flip chip bonding in many new applications.
Exlpanation to Figure 3: thermo-compression gold to gold (TC AuAu); thermo-compression gold-tin (TC AuSn); thermo-compression soldering; reflow soldering, and thermo-sonic. 
For More Information
[ 1] L. K. Cheah, Y. M. Tan, J. Wei and C. K. Wong Proceedings HDI 2001, April, 2001, pp.165-175.
 Gerard Kums, J. van Delft, H. de Vries, “Thermosonic Bonding of High Pin Count Flip-Chips on Flexible Substrates” IMAPS Annual Symposium on Microelectronics, 2005.
 George Riley, “Stud Bump Flip Chip Assembly of MEMS Motion Sensors,” IMAPS New England Symposium, May 8, 2001.
 Ian Hardy, “Thermosonic Gold Stud Bump Attach for Large ICs on Laminate Substrates.” IMAPS Annual Symposium on Microelectronics, 2008.
 Nick Stolper, “A Comparative Study: Thermosonic Bonding of Flip Chips with Gold Stud Bumps for a Variety of Organic Substrates,” IMAPS Annual Symposium on Microelectronics, 2008.
Editor: George Riley