Low-Moisture Polymer for Die Bonding Fights Delamination

• Dense-Pac Microsystems Inc. (Garden Grove, Calif.) will supply 256Mb memory modules to DeskStation Technology (Lenexa, Kan.) for its Alpha-based workstations and to Lucent Technologies (Murray Hill, N.J.) for it Speech Processing Systems II voice recognition system. Dense-Pac has also successfully tested its 256Mb SDRAM for use in Digital Equipment Corp.'s (DEC, Springfield, Mass.) Alpha Powered workstations.

• General Scanning Inc. relocated its Laser Systems Division product development, manufacturing and support headquarters from Somerville, Mass., to a newly refurbished 78,000 ft² facility in Wilmington, Mass.

• Advanced Micro Devices (AMD, Sunnyvale, Calif.) will offer flash memory devices in fine-pitch, ball-grid array (FBGA) packages to provide a small form-factor package that is independent of die shrinks.

• ChipScale Inc. (San Jose, Calif.) licensed Intarsia Corp.'s (San Jose, Calif.) Micro SMT wafer level packaging technology. Intarsia will initially utilize the proprietary technology to package thin film integrated passive components.
• Analog Devices (Norwood, Mass.) made its AD14160 Quad-SHARC DSP multiprocessor available in a 452-lead ceramic ball-grid array (CBGA) package. Kyocera America (San Diego, Calif.) provided its dimple BGA packaging for the device. The processor performs 480 Mflops and takes up 1.85 in² of space.

• Integrated Packaging Assembly Corp. (IPAC, San Jose, Calif.) sold its facilities on Old Oakland Road and will lease the 82,290 ft² building and related property for an initial 10-year term.

• Abpac has moved into its new facility at 3201 East Harbour Drive, Phoenix, Ariz.

• Electroglas Inc. (Santa Clara, Calif.) acquired Techne Corp. (Albany, Ore.), a supplier of back-end wafer auditing systems.

Engineers at Johnson Matthey Electronics (San Diego, Calif.) have developed a modified cycloolefin thermoset (MCOT) adhesive material for die attach applications that exhibits low moisture absorption. Delamination, or "popcorning," occurs when moisture absorbed by polymer packaging materials are vaporized during high-temperature (220-240°C) solder reflow. They presented the results of tests of the material at the International Electronics Manufacturing Technology Symposium in October in Austin, Texas.

1. MCOT's high-elasticity modulus at high temperatures provides wirebond reliability comparable to epoxies.

2. The thermoset reaction rate for MCOT is maximum near 98°C.

The die attach adhesive consists of thermal and electrical conducting fillers dispersed in liquid prepolymers, which polymerize upon heating. Weight loss is very low, since there are no outgassed products from the reaction. This type of polymer can have a wide range of physical properties from a rigid, high glass transition temperature to an elastomeric material. This adjustability depends on the type of olefin content, and the polymer can be optimized to fit specific applications. The modulus of the material is generally <1 GPa, qualifying it as a low-stress material suitable for large chips.

A common problem for low-stress die attach materials is poor wirebondability. The low cross-link density typical in such materials leads to a significant decrease in modulus when the temperature is raised above the glass transition temperature. When wirebonding is done at temperatures above 200°C, the die tends to float, leaving a poor base for ultrasonic coupling. The problem can be particularly bad for die measuring <5 mm to a side.

The MCOT material has a higher cross-link density than most low-stress materials. The modulus of the material does not drop off as much at temperatures above its glass transition temperature. In fact, tests showed that its modulus is comparable to that of a rigid epoxy at elevated temperatures (Fig. 1). Their tests also showed that both MCOT and a rigid epoxy yielded satisfactory wirebonds, while a typical low-stress material did not.

A differential scanning calorimetry (DSC) trace indicated that the maximum cure rate for MCOT occurs at a temperature of 98°C (Fig. 2). The trace also shows a capability for it to cure at significantly lower temperatures, giving an advantage in some applications.

The MCOT polymer is made primarily of nonpolar olefins, so its moisture absorption is significantly lower than that of typical epoxies. According to their findings, it absorbed <0.05% during an 85/85 test (85°C/85% RH/168 hrs) and 0.1% in a HAST test (150°C/85% RH/168 hrs). A moisture content <0.1%, they observed, should make a package free of delaminations after solder reflow.

The 1997 Roadmap puts forth goals for finding organic substrate and underfill materials with reduced moisture absorption. Though the MCOT material is used here for die attach, it can be viewed as one step toward those goals.