Bryan Smith and Siamak Akhlaghi, PhD
Gold-tin (AuSn) solder is becoming the preferred answer to many modern packaging challenges. An innovative solder deposition method promises to improve AuSn solder performance while reducing costs and complexity.
The challenges driving the move to AuSn solder are common to today’s packaging: higher power, higher operating temperatures, higher connection density, and better geometric accuracy. High power electronic and optical devices require better heat transfer to the substrate, but smaller, high-density flip chip bumps make heat transfer more difficult. High-power LEDs operate at temperatures beyond that of eutectic tin-lead, requiring better electrical and thermal conductivity. Precision optical alignments can be distorted by high-temperature ‘creep’ of lead-tin solders. MEMS devices are intolerant of fluxes and their residues. Biomedical devices require corrosion resistance along with good electrical, thermal, and mechanical properties. And looming over all electronic packaging is the black cloud of lead-free requirements.
Eutectic AuSn solder has long been recognized as offering superior high temperature performance, excellent electrical and thermal conductivity, high mechanical strength, and fluxless soldering. The drawback is that maintaining the desired eutectic composition requires extreme accuracy and close control. The simplified phase diagram (Figure 1) shows that the desired 280 ° C AuSn eutectic point has steep walls. As a recent survey article points out, a 1% shift in the AuSn solder composition to the Au rich area can raise the melting point of the solder 30 ° C, making the solder unusable. 
Figure 1. AuSn phase diagram, showing the eutectic point at 280 ° C, 20 weight % Sn. (Courtesy Advanced Packaging)
The same survey article describes the traditional 20th century approaches to controlling AuSn solder composition, and details their complexity and limitations. However, it omits the newest, 21st century method of composition control. Alloy electroplating of AuSn promises better composition control, lower mechanical stress, and finer dimensional capability, with lower processing complexity, higher throughput, and low capital cost.
Conventional electroplating of AuSn solder moves the substrates repeatedly between two separate Au and Sn plating baths, to deposit successive layers of Au and Sn. The gold plating baths may contain hazardous materials such as cyanide, and may also require special additives to control stress in the deposited layers. The baths may attack conventional photoresist, limiting both geometric accuracy and deposit thickness.
Moreover, depositions are controlled by process variables such as time, temperature, and agitation, to obtain Au and Sn layers in the proper ratio so that post-processing by annealing or by reflow will yield the eutectic proportions of the AuSn and Au5Sn alloys. In addition to the complexity and cost of alternating between two baths, there is risk of cross-contamination between the baths, and of exposure of the Sn layers to oxidation during processing.
Unlike conventional electroplating, co-deposited alloy electroplating of AuSn solder directly deposits the desired gold-tin alloys, AuSn and Au5Sn, in the proper proportions, rather than depositing their Au and Sn precursors. The alloys are deposited in a single step, in a single, composite plating bath. The ratio tin to gold, and therefore the composition of the deposit, may be continuously varied during the deposition over the range from 10 to 40 weight-percent Sn, by controlling the plating current density.
This automatic composition control makes it easy to deposit multilayers of solder. The hard AuSn alloy adds strength. The softer Au5Sn alloy bears most of the plastic deformation applied to the solder.
Au5Sn co-deposition also permits better stress matching, and provides oxidation protection. If the plating seed layer is gold, the first deposited layer is Au5Sn, for lattice congruity, to minimize stress at the interface without chemical additives. AuSn and Au5Sn layers then alternate to the desired thickness, which may be as much as 60 microns. The final layer is again Au5Sn, to minimize surface roughness, and to protect against tin oxidation in the exposed layer. Figure 2 shows a cross-section of alternated AuSn and Au5Sn layers.
Figure 2. Multiple layers of electroplated AuSn and Au5Sn.
Click for larger 93k image. (Micralyne photos.)
Alloy co-deposition has many advantages over the traditional method of separately plating successive layers of tin and gold from separate plating baths. It avoids the added complexity and cost of maintaining and controlling two plating baths, and the attendant risks of cross-contaminating the baths, or of oxidizing the Sn between baths.
The slightly-acidic chloride based single bath solution is compatible with common alkaline-developable photoresists. Since the single bath does not attack the resists, resolution is not compromised when plating is extended to obtain thicker deposits. The single, one-step bath minimizes exposure of the tin to oxidation during processing.
The resulting deposit is low-stress, dense, and pinhole free. It is comparable to thin film metal depositions, both in quality and in dimensional tolerances, but without the high capital and operating costs of thin film deposition. Automated current density control of composition over the 10% to 40% Sn range gives the flexibility to make precise composition adjustments. This allows modifying the composition to compensate, for example, for a thick-gold substrate layer.
While the photoresist compatibility allows precise patterning and thick deposits, the process is equally capable of blanket-coating an entire unpatterned wafer. It needs no post-annealing to blend two dissimilar layers. The computer control of composition during deposition makes for simple, repeatable operation.
The single bath system is a cost saver compared to conventional plating, since eliminating the two-bath tango increases production throughput. It has far lower capital costs than evaporative or sputtering systems, while offering similar deposited solder characteristics, and far greater processing flexibility.
Alloy co-electroplating offers technical, manufacturing, and financial advantages for depositing AuSn solder. As the demand for high-performance solder continues to grow, alloy co-plating could soon become the preferred deposition method.
 Robert S. Forman & Gerard Minogue, “The Basics of Wafer-Level AuSn Soldering,” Chip Scale Review, Vol. 8 #7, pp 55 – 59, October 2004.
FOR MORE INFORMATION
Micralyne offers a patented process for electroplating AuSn solder alloy, at compositions from 10 to 40 Sn (wt%), in patterned or unpatterned shapes onto metallized substrates at virtually any thickness.
Micralyne, Inc. 1911 – 94 Street
Edmonton, Alberta, Canada T6N 1E6