Copper Systems

The Cu-Au system is vulnerable to void formation at high temperatures, and requires bonding surface cleanliness. In the Cu-Au system, void formation may be a partic­ular issue due to intermetallic growth. In addition, an intermetallic CuAl2 phase can form which is brittle and can cause failure of the bond. These systems are also vulnerable to corrosion in the presence of chlorine and moisture.

12.6 Electrical Connection Processes

Both power and signal connections must be made from the MEMS chip to out­side electronics. The main types are cable connectors, board connectors, chip connectors and probe testers. Cable connectors make a (normally demountable) connection between two electrical cables. They are referred to as in-line connec­tors, if the connection is made by insertion of a male part into a female part, in a direction parallel to the cable axis. The cable may have many conductors. If these lie parallel in a plane, the connector is often known as a ribbon connector. In a ribbon connector, the connecting elements (often known as tongues) are normally arranged on a substrate. The tongues may then deflect in plane (i.e., parallel to the substrate plane) or out-of-plane (perpendicular to the substrate). Thus, it is possible to have an in-line connector with in-plane deflection of the connector tongues [32]. A typical device package is composed of a flat lead frame with numerous leads at the interior die position. The MEMS device is positioned and aligned with the interior opening of the lead frame, after which it is electrically connected to the interior of the lead frame by small wires.

12.7 Encapsulation

Encapsulants come in two physical forms, solids and liquids, but with similar com­positions. Solid types are epoxy molding compounds (EMC) and are blends of solid epoxy resin, hardener, flame retardant, filler, and several additives. EMC preforms or "hockey pucks" are used in transfer-molding machines. Similar in composi­tion, liquid encapsulants are formed from liquid forms of resin and hardeners. In this form, the material may be dispensed and applied directly instead of molded onto the chip and interconnect. Polyimides, polyamide-imides, silicones, acrylics, polyurethanes, fluoropolymers, parylenes and epoxies are typical materials that could be used for encapsulation or conformal coatings. Except for parylenes, which are vapor deposited, the coatings are usually applied in liquid form and are cured using infrared or ultraviolet radiation. This section will discuss both encapsulants and conformal coatings.

12.7.1 Polyurethane

Polyurethanes are available as one or two component resins for conformal coat­ings. Electronic components are often protected from environmental influence and mechanical shock by enclosing them in polyurethane. Typically, polyurethanes are selected for excellent abrasion resistances, good electrical properties, excellent adhesion and impact strength, and low temperature flexibility. The disadvantage of polyurethanes is the limited upper service temperature (typically 250°F (121 °C)). In production, the manufacturer would purchase a two part urethane (resin and cata­lyst) that would be mixed and sprayed or poured onto the circuit. In most cases, the final circuit board assembly would be unrepairable after the urethane has cured.

Date: 2015-02-28; view: 1040