Reliability flip-chip devices

Figure 6.3 Reliability of flip-chip devices with and without underfill.

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

Although a reduction in feature sizes referred to as shrink is one of the ways to accommodate an increased transistor count, it is often not sufficient to provide the required I/O counts, necessitating an increase in die size as well. However, as the strain that a solder joint experiences is directly proportional to the distance from the neutral point (DNP, i.e., the temperature invariant point), this can have a significant impact on the reliability of solder joints located in the outer rows of area-array, flip chip devices. An increased die size may result in a maximum DNP that poses a potential reliability concern for some applications, requiring that additional precautionary steps be taken, among them enhanced cooling capability, chip underfill, and increased inherent fatigue resistance of the solder. 

TABLE 7 Reliability Tests for Flip-Chip Devices Assembled Utilizing Micron Bump... 

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

Materials for use as anisotropically conductive adhesives must satisfy requirements even more stringent than those defined previously for isotropically conductive adhesives. No specifications, however, have been defined specifically for these materials. When used for flip-chip applications, the adhesive not only serves as a physical and electrical interconnection between the device and the substrate, but also serves as the environmental protection and passivation layer. This fact, combined with high adhesive concentrations, makes the ionic contamination levels of these materials more critical than for isotropic conductive adhesives. In addition, the processing of these materials has a greater influence on joint reliability as the anisotropic electrical properties develop only after heat and pressure are applied to the joint. 

Data describing the reliability of joints assembled with anisotropically conductive adhesives are incomplete. Several papers have been published, but usually the sample size investigated is small, the accelerated stress tests are not standardized, and the results are highly dependent on device type (e.g., flexible circuit to rigid PWB, surface-mounted components, and flip-chip assembles). Further work is required in this area. 

Dressier, M. Reliability Study of Stud Bump Bonding Flip Chip Assemblies on Molded Interconnect Devices. Dissertation, Technische Universitat BerUn, 2010. 

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