A growing requirement to include optical devices such as lasers and high-brightness LEDs in microelectronic assemblies presents new challenges to the manufacturer.
Maintaining position and alignment of optical components to other circuit elements for the lifetime of the assembly may be crucial for maximum performance and high reliability.
Accurate initial placement and bonding of sensitive circuit elements depends on equipment capability and control. Maintaining lifetime accuracy depends on assembly materials.
Solder attachment of components remains the most used assembly method, with solder-bumped flip chip in high density assemblies. Gold-tin (AuSn) solder preforms have long been used for fluxless hermetic lid sealing, die attach and heat sink attach.
Now AuSn solder has become the material of choice for flip chip mounting of precision optical die. Gold-tin has many advantages for this application:
Gold-Tin Solder Advantages
Fluxless. Unlike most common solders, AuSn does not require a chemical flux to remove oxides and prepare the surface. Eliminating flux and flux cleanup shortens the assembly process, while avoiding contamination of the optical surfaces with flux residues.
Hardness. AuSn forms a very hard solder joint, with no creep, relaxation, or deformation in the joint, so that alignment remains unchanged over time.
Good Wetting. AuSn solder readily wets bond pads for strong, uniform, void-free joints, a deficiency with some newer lead-free substitutes.
Corrosion resistant. AuSn solder joints are highly resistant to corrosion, without the additional protection required by some substitutes.
Excellent thermal properties. The high thermal conductivity of gold/tin rapidly carries heat away without creating excessive stresses, a major concern in high-density packaging.
High electrical conductivity. The high electrical conductivity of gold provides low-resistance connections important for high power devices.
Long-term stability. Intermetallic growth rates are low when deposited over nickel, palladium, or platinum.
Lead-free proven. AuSn device mounting was used for decades before the political lead-free solder mandate created a flood of less known lead-free solders.
Active Atmosphere. Eliminating flux also eliminates flux removal of surface oxides. Instead, AuSn assembly avoids oxidation by proceeding in a controlled, active atmosphere.
Narrow Process Window. As discussed below, obtaining the desired characteristics of AuSn requires careful process control in bumping and assembly.
Material Cost. The worldwide financial crisis has recently driven the price of gold to new highs.
Figure 1, a simplified AuSn phase diagram, shows a eutectic point at 278 °C, with a composition of 80 weight-percent gold and 20 weight-percent tin. Figure 1 also shows that the melting temperature rises rapidly with the increasing gold percentage beyond this eutectic ratio.
For example, a 1% increase of gold in the composition increases the melting temperature by 30 °C. Accurate composition control is the essence of Au80-Sn20 bumping.
Figure 1. Gold-Tin Phase Diagram.
One proven bump-forming approach is to electroplate a thick gold layer capped with a thin tin layer in the correct proportion. The bump is reflowed to obtain the eutectic composition.
Other bumping methods include sequential evaporation of alternating gold and tin layers in the correct ratio, placing and reflowing preformed solder spheres of the proper composition, stenciling or jetting AuSn solder paste, or alloy electroplating with a single composition-controlled gold-tin alloy solution.
Optoelectronic Assembly Process
AuSn optoelectronic assembly requires highly accurate alignment and placing of the device, as well as close control of bonding temperature, pressure, and time. A program-controlled single assembly machine is the best way to automate the process while maintaining control.
The following video demonstrates single-machine high-accuracy assembly of bar lasers in microelectronic assemblies.
This 4 minute video below demonstrates aligning, placing, and soldering a bar laser device, with placement accuracy to less than 1 micrometer. (Courtesy of Finetech, Inc.)
Gold-tin eutectic solder has many established advantages over other solders for accurate, stable mounting of optoelectronic devices, but the process requires close control. An automated machine can place and solder these devices to better than 1 micron accuracy.
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