Feature Article - Silver Loaded Glass Die Attach - By Christopher Henderson

Developed by American scientists in the mid-1980s, silver-glass adhesives are a potential substitute for gold-silicon eutectic, as they have high thermal conductivity and a coefficient of thermal expansion (TCE) close to that of silicon. Silver-glass has been evaluated for the manufacture of power hybrids and for attaching active devices with gold or silver back metallization to substrates plated with nickel or silver-plated copper. A number of military and space customers now use silver-loaded glasses for die attach since the TCE reduces the stress in the package.

A typical silver-glass paste contains 80-85% by weight of silver flake and lead borate glass dispersed in an epoxy-resin-based system. The paste is dispensed into the package cavity, the die placed on the paste, and the package inserted in a furnace. The heat removes the organic vehicle and then centers the residual solder and glass to bond the die strongly to the ceramic.

The material should form a smooth fillet free of voids and cracks. To achieve this, the conventional process includes a pre-drying step of 2-10 hours at 60-80°C, depending on die size. Sufficient solvent needs to be eliminated to avoid the formation of voids, and the void density depends on the drying temperature. However, too high a drying temperature can reduce shear strength because there is no intimate contact between die and glass. Optimized conditions should yield voids less than 1%. Several researchers have determined this level of voiding experimentally using radiographic techniques.

The figure below illustrates the sequence of events that happen to silver glass paste during firing. Solvent evaporation (50-200°C) is followed by polymer resin burn-out (300°C). The glass starts to soften at 340°C, which causes silver-glass sintering to accelerate and further wet the surfaces of die and substrate. Reaction bonding of silver-glass with silicon and alumina also starts at about 340°C, and the reaction is completed when the temperature has been maintained at 410-430°C for approximately 10 minutes.

The shear strength of the material increases with sintering temperature towards an asymptotic value, probably due to the densification of silver particles. Therefore, one must take into account degradation mechanisms on the die. For example, too high of a temperature can cause aluminum to silicon ohmic contacts to degrade.

SEM observation shows that the attachment to the die is not continuous, that there is a high concentration of lead at the interface between the silver-glass and the substrate, and that adhesion seems to be the result of diffusion of silver into the gold layer. Researchers like Dequidt explained the absence of gold in the silicon and gold-silicon eutectic by the nickel or titanium under-layer preventing migration of gold into the silicon. Similarly, bonds between the silver glass and the plated package are probably interdiffusion mechanisms, whereas bonds between alumina and silver glass are chemical and similar to conventional thick film.

In investigating the process, thermogravimetric analysis (TGA) is used to determine the temperature at which solvents are eliminated from the binder and differential thermal analysis (DFA) is used to characterize the glass and show its recrystallization.

Advancements in silver-glass die attach materials have mainly been focused on:

• Stress relief for large area dice.
• A single pass firing process with no pre-drying.
• Precise rheology for high speed dispensing.
• Lower firing temperature.

As with resin attach, the TCE of the bonding material is not a significant factor in die stress. This assumes that the thickness and modulus of the die bonding material is small compared to the thickness and moduli of die and substrate. However, cracks due to thermally induced stresses arising from a TCE mismatch between silicon and ceramic have been reported by Nguyen for die larger than 10mm on a side. Furthermore, the bond line thickness has to exceed a minimum value for acceptable performance.

The stress in the bonding material is related to the sintering behavior of the silver/lead borate glass mixture. Control of the sintering rate is typically achieved by small quantities (0.1-1%) of proprietary additives which increase the surface roughness and slow down regrouping of the particles. This helps to eliminate cracks without adversely impacting on adhesion and void formation.

Despite the claim that silver-filled glass offers lower cost, faster processing, high yield, better thermal performance, and a lower die stress than gold-silicon eutectic, it has not replaced eutectic or other solders for high power chips. This seems to be because companies are reluctant to change. However, the material is widely used in the fabrication of Ceramic Dual Inline Packages because it resists the high temperature of the furnace sealing involved.