Today's telecommunications/datacom industry is coping with its own technical challenges in processing capabilities through the continuous juggle of requirements between greater electrical performance and reduced package sizes. Automatic eutectic attachment techniques, which are desirable for high-performance and high-capacity die attach applications, are constantly in high demand.
The eutectic die attach technique is nothing new. This die-bonding technique has been widely explored and employed, historically on manual die-bonding equipment. What is new, however, is its elevated place in an industry environment where current and future needs—high-volume and high-placement accuracy (±5 microns)—clearly call for flexible automated equipment. In particular, demands for high-volume and high-placement accuracy require automation that is swift, precise and repeatable.
To meet the telecommunications and datacom industries' demands for volume and placement accuracies, flexible automatic work cells are required that support different carriers, methods and automatic materials presentations. The eutectic process, while time-sensitive, is capable of handling the technical requirements involved in recent packaging developments. Meanwhile, packaging demands continue to evolve, requiring higher and higher eutectic placement accuracies. Automated work cell capabilities pose significant technical challenges.
Understanding the Eutectic Process
When different metals are combined into alloys, a range of melting temperatures is created with varying proportions of each metal used, such as Au/Si @ 363°C Au/Sn @ 280°C. 
Material phases—liquid, solid, plastic—and raster structure are usually depicted in "phase diagrams." These diagrams show a distinct mass ratio at which the solid state evolves into liquid state without passing the plastic state. This ratio determines the lowest possible melting temperature for the alloy, which is of great importance to the eutectic soldering process. During eutectic die attach, the substrate is heated to a temperature just below the solder's eutectic temperature. During the bond cycle, an incremental thermal energy is supplied to the solder layer to promote the solder melting process. Liquefied solder then penetrates both bonding surfaces. An intermetallic bond develops, which is also known as wetting.
In many cases during soldering, a cover gas is employed, which usually contains a passive component (such as 90 percent nitrogen) to prevent metal oxides from forming, and an active component (such as 10 percent hydrogen) to break away existing metal oxides. The solder, usually precipitated to the die foundation (backside metallization) or to the substrate surface, can also be supplied as a preform—solder pieces cut to a certain percentage of the die size. While preforms are a cheap solution to apply solder, their use does require an additional pick-and-place process step.
Eutectic die attach offers a number of advantages over similar techniques, of which good thermal solder conductivity and immediate fixing after bonding are most significant.
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Download "Automated Eutectic Die Attach: Ideal for Telecom and Datacom".
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Zeger Bok
Senior Applications Engineer
Palomar Technologies