Reliability and Lifing Methodologies for Microelectronic Systems
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Mat. Res. Soc. Symp. Proc. Vol. 515 © 1998 Materials Research Society
Perhaps the most important consequence of this fact is the nature of the conditions typical of engineering components. Service conditions are rarely "pure," i.e., they are rarely "just" mechanical, or just thermal, or just cycle dependent, or just time dependent. Real components such as tires, engine or machine parts, or microelectronic devices usually serve under combinations of mechanical, thermal, and environmental applied conditions. Hence, the fundamental question to be addressed is, "how can we predict the life of engineering components under the influence of combinations of applied conditions?" However, this basic problem in composite systems is fundamentally different in many respects from that in homogeneous materials, especially for polymeric and elastomeric composites. [Reifsnider, 1991] Self-similar single-crack propagation is rarely the life-controlling process in composite systems, and even when it is, it rarely constitutes a significant part of the life of the component. More often, combinations of local events, such as microdefect initiation, physical and chemical aging, debonding, and delamination accumulate and interact to create a collective "fatal condition" that causes component failure. This combination of events is often driven by a complex loading condition and unusual material properties. The use of organic materials in microelectrical packaging is driven by the need for increased function density, electrical performance, and heat management capability. Organic materials serve many purposes in Tape Ball Grid Arrays (TBGAs), for example, where they function as thermal (and other) adhesive layers, conductors, insulators, encapsulants, and, most important, as the basis for the flex circuit, the flexible polyimide tape with copper circuitry on both sides. Using thermocompression bonding or flip-chip solder balls, a chip is bonded to one side of the tape, and the other side is bonded to the card, usually with an distribution of solder balls as well. Conductive films may also be used to bond a metal cover plate to the top of the chip-stiffener assembly to manage heat flow. These assemblies must operate under severe cyclic conditions that typically include oxidative atmospheres at up to 150 0C (or up to 220 0C for solder reflow) for thousands of cycles and of the order of 40,000 hours. Thermal mismatches of the order of 14 ppmftC (for silicon to copper bonding) drive the generation of severe thermal stresses that can cause debonding and (especially) delamination of the chip from the thermal cover plate or from the flex circuit. The stress states in such systems are, by definition, complex. Figure 1 shows the conditions one can expect on a material element in the flex circuit in TBGA. The different thermal characteristics of the materials surrounding the flip chip create in-plane and out-of-plane stresses and deformations, that may have sharp discontinuities and elevations at the edges and boundaries of the subcomponents.
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