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Conductive Die Attach

The eutectic bonding process occurs when the substrate is secured just below the eutectic melting point on a heated work stage. The die and preform are placed on the substrate, and a light scrubbing motion is made with modest pressure by the bond head. This scrub generates a rise in the temperature of the bond to the eutectic melting point. The melted material solidifies, and creates the bond. Nitrogen is used as a cover gas in order to prevent oxidation due to the high heat. In some cases, the die may have a eutectic alloy pre-plated on its back omitting the need for a preform. The perform is a thin foil of a solder alloy of two or more dissimilar metals placed in the spacet between the die and substrate. The preform has a melting point that is lower than the melting point of its base materials. Consider a typical preform composed of gold and silicon. The melting point of gold is 1640 °C, and the melting point of silicon is 1414°C. However, when the materials are combined into a

Materials Melting point, °C
Au97-Si3
Au88-Sn12
Au80-Sn20
Pb63-35Sn-1.8Sb

preform, the melting point becomes 363°C. Table 12.8 shows examples of eutec­tic preform materials. The actual bonding temperature is about 20°C higher than the eutectic point. Gold-based eutectics have high flow stresses and offer excellent fatigue and creep resistance. The high flow stress is a result of lack of early onset of plastic flow. This lack of plastic flow can lead to high stresses in the MEMS chip due to the thermal conductive mismatch between the chip and substrate.

12.4.2 Metal-Filled Glasses and Epoxies

A glass bond is formed by adhering the die to the substrate using glass in the form of a paste. In some cases, such as in silver-glass die attach, the pastes contain silver par­ticles that enhance thermal and electrical conductivities. A mix of lead borate-based glass frit with 80 volume% Ag is a typical glass die attach paste.. The glass bonding process is similar to adhesive bonding, although differences in bonds arise from the type of material and process temperatures. Glass bonds are heated to 350-450°C, forming a low viscous liquid. As it cools, the glass hardens to form a bond.

Benefits of using a metal filled epoxy include relative insensitivity to substrate metallization, low void content, and low contamination. In addition, the resulting bonds have good thermal and electrical conductivities and limited stress relax­ation. However, drawbacks such as higher oxidation rates during high temperature processing should be noted.


Date: 2015-02-28; view: 1059


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