Tutorial #59
George Riley
January 2006

Introduction

With the lead-free deadline only months away, more solder users are worrying about the production hazards of switching to higher temperature lead-free alloys. Their concerns rise both from the limited reliability history of those alloys, and the risks that temperature sensitive components, including organic circuit boards, may be damaged by the high temperature exposure. Meanwhile gold bumping and indium bumping, proven lower temperature alternatives to lead-free alloys, continue to improve in cost and performance.

Gold bumping includes high quality, fine pitch electroplated gold bumps and lower cost gold stud bumps. Gold stud bump flip chip is winning new applications based on declining cost and improved performance. The new generation of stud bumping equipment deposits bumps up to three times faster than earlier equipment, with speeds sometimes exceeding 30 bumps per second. Stud bumps may have pitches down to 50μm, at bumping temperatures below 150°C. Stud bumping normally requires no special metallization, fluxing, or cleaning.

Since stud bumping is a serial process, higher bumping speed lowers the cost per bump, making stud bumping cost competitive with batch technologies over a wider range. For example, the model presented in Figure 1 shows that stud bumping cost may lower than plating cost below 300,000 bumps per wafer.

Figure 1. Cost model for gold stud bumping.
(Courtesy Advanced Packaging)

Bumping Costs Down

Gold stud bump flip chips may be assembled with conductive or non-conductive adhesives, or by thermosonic assembly, at temperatures of 120°C to 150°C, well below even eutectic lead-tin solder.  Gold stud bumping with conductive adhesive assembly has become competitive with indium for assembly of high bump count detector arrays. Conductive adhesive assembly may have lower electrical and thermal conductivity and lower mechanical strength than solder. However, recent research shows that adding nanoparticle fillers to adhesives and underfills improves conductivity and greater mechanical strength.

With temperature-limited components, such as pyroelectic films (60C maximum), conductive adhesives and underfills may be specially formulated for lower curing temperatures, even down to room temperature.  These are typically two-part adhesives that are mixed immediately before use, and require 16 to 24 hour cures at room temperature.

In addition to protecting temperature sensitive components, room temperature assembly of large or fragile components avoids the mechanical stresses between dissimilar materials that may be locked in at higher curing temperatures. Room-temperature underfills avoid chip cracking and intermittent open connections.

Thermosonic assembly of gold stud bumps or of plated gold bumps as an alternative to adhesive assembly provides a complete gold-to-gold system, with high thermal and electrical conductivities and good mechanical strength. The assembly process is similar to that in thermosonic wire bonding, except that all of the connections on the chip are made simultaneously. Assembly temperatures are typically 100°C to 150°C.

Thermosonic bonding has long been used for high volume production of low bump count assemblies, such as SAW filters. Recent equipment and process developments have increased the practical thermosonic bump-count limit to about 100 stud bumps when assembly onto rigid substrates and to more than 1,400 plated gold bumps assembled onto flex substrates. The plated gold bumps, demonstrated to 25μm, are suitable for high-density display driver connections.

Thermosonic gold stud bump assembly has recently been introduced for packaging high power LEDs. The benefits include better thermal and electrical conductivity than for tin-lead solder bumps, with lower assembly temperatures. This allows retaining proven organic packaging, which would not withstand lead-free solder temperatures.

Indium bumping has been common for many years in bonding large detector arrays to multiplexers, lasers to drive electronics, and making similar chip to chip assemblies. Indium also forms a room temperature hermetic package seal. Indium’s cryogenic stability, thermal and electrical conductivity, and self-welding at room temperature are well suited to these high-value applications. Indium is evaporated or plated onto both the IC and substrate bond pads, at pitches down to 15μm. It will cold weld when brought into contact at moderate pressures, or reflow at temperatures below 100°C.

Summary

Gold bumps (plated or stud bumps) with adhesive or thermosonic assembly, and indium bumps with cold welding or reflow are gaining popularity as lower temperature, proven alternatives to new lead-free solder alloys. Moving to lower temperatures instead of to higher melting point ternary alloys can avoid costly redesign, equipment replacement, and reliability considerations. Taking the low route on temperature may let you go lead free without fears or tears.