Conductive die attach

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured according to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low-Shear Rates by Brookfield Viscosity The viscosity of typical electrically conductive die-attach adhesives is measured with a Brookfield HBT viscometer with Spindle TB and Speed 5. For higher-viscosity conductive adhesives and for underfill adhesives, a Brookfield RVT or RVF viscometer is used with Spindles 6 or 7 at speeds of 4—10.4 rpm. Another Brookfield viscometer, the Cone-and-plate viscometer with a CP-51 spindle is used for low-to-intermediate viscosity adhesives. Finally, the Brookfield HAT and HBT instruments are used for the high-viscosity (1-2 million cP at 1 rpm) adhesives typically used in SMT applications. ... 

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured aeeording to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low Shear Rates by Broolfield ViscosityThe viseosify oftypieal electrically conductive die-attach adhesives is measured... 

US 5,204,399 (American) 1991 Thermally conductive thermoplastic polyimide film die attach adhesives and their preparation National Starch and Chemical Investment Holding Corp. R Edelman Adhesive formulations having excellent die shear strength A polyimide-siloxane was prepared from aromatic amines, bis aminophe-noxybutylsiloxane and aromatic dianhydrides... 

During die attach, an adhesive creates a mechanical interface between the die and substrate in a semiconductor assembly. A typical dispense pattern has intersecting lines with strategically placed dots so that after die placement, the fluid is evenly distributed (Figure 10-12). Most die attach adhesives also provide thermal and/or electrical conductivity between the die and substrate. 

Vitreous materials comprise a third type of attachment material. Glasses may be used to seal lids to packages for hermetic enclosures. Silver-glass pastes have been developed for die attachment. They have high thermal and electrical conductivities and are used extensively for single die attachment in single packages. 

Polyimide adhesives are commercially available as electrically conductive or insulative pastes used for die attachment. They are also available as thermoplastic preforms for chip-on-lead and lead-on-chip attachments. 

DM 3030, Silver/glass die attach paste. Thermally Conductive Adhesives Selection Guide. Diemat, a Namics Co, www.diemat.com 2003. 

After attachment to the substrates, LED chips can be wire bonded or flip-chip bonded. If wire bonded (see section 1.2.1), the entire backside is available for heat dissipation. In flip-chip bonded formats, however, the thermal path is through the bumps only. Therefore, from a thermal dissipation viewpoint, direct die attachment to a heat spreader followed by wire bonding is preferred. Regardless of the interconnect approach, package materials having high thermal conductivities are required. 

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

Electrical-stability testing is essential for conductive adhesives used for electrical connections. Electrical conductivity can degrade at elevated temperatures, on aging with or without power, and on exposiue to humidity and temperature. The specific test method used depends on the application. One test used for die-attach adhesives specified in NASA MSFC-SPEC-592 (now inactive) involves a series of gold-plated Kovar tabs attached with conductive epoxy to metal pads on an interconnect substrate. In the test vehicle, a bias of 5 V and cmrent density of 139 3.9 A7cm (900 A/in ) are applied to a series of wire-connected tabs, and the resistance change is measured after exposure to 150 °C periodically up to 1,000 horns. The maximum allowable resistance change is 5%. 

In this section we discuss three applications of electrically conductive adhesives die attach adhesives, anisotropically conductive adhesives for liquid crystal display (LCD) assembly, and conductive adhesives for surface-mounted assembly of packaged components on printed wiring boards (PWBs). These applications were selected based on overall... 

Conductive adhesives also form sufficiently robust joints that withstand the temperatures and pressures experienced during wirebonding and over molding. The formulation of die attach materials is discussed in Section IV. 

The following list includes several commercial manufacturers of conductive adhesives commonly used for die attach as well as other interconnection applications. Many manufacturers sell both isotropically as well as anisotropically conductive materials. The list is not intended to be inclusive but merely to provide initial guidance. 

---