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July, 2017
The Benefits and Risks of Copper Pillar Bumped
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Flip Chips: Part 2 Continued from page 59
copper pillar. Several studies show that stresses and/or strain energies in the joint are decreased with lower copper pillar height, while others seem to show the opposite. These seemingly conflicting find-
ings, together with several investiga- tions of how various design parame- ters (polyimide thickness and opening size, solder thickness, die thickness, underfill properties, pillar shape, etc.) impact stresses and strain energies within the joint, emphasize the point that broad, general statements about the performance of copper pillar joints must be made cautiously. Clearly, details of the joint and package design, together with material properties, greatly impact the development of stresses and strains within the joint during package assembly and thermal cycling. Care must be taken to analyze
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any given design and materials set to understand reliability performance. Finally, in many cases, modeling pre- dictions have not been validated experimentally, either at the package level or for packages mounted to print- ed circuit boards, emphasizing the need for more experimental studies.
Effects of Underfill Regardless of the interconnect
technology, underfill properties will dominate thermal cycling perform- ance. If the underfill material needs to change to accommodate the copper pil-
lar package assembly process — from those commonly in use today to NCP, NCF or molded underfill — then this change may have a far greater effect on thermal fatigue lifetimes than the interconnect parameters. For example, consider the
impact of temperature on the coeffi- cient of thermal expansion (CTE) and storage modulus of a widely used low Tg underfill material. Near the glass transition temperature, which is within the range of temperatures under which thermal cycling may take place, the CTE changes more rapidly than the modulus. This large increase in expansion
rate with negligible change in the mod- ulus can result in significant tensile stresses in the solder interconnect. It is known that tensile stresses are far more damaging than shear stress to the lifetime of solder joints under tem- perature cycling. Experimental testing has demon-
strated a 100-fold reduction in time-to- failure for comparable stress ampli- tudes in tension as opposed to shear. It has also been demonstrated that the mean stress plays a critical role in the solder response. In one experiment, two solder joints were subjected to stress-controlled fatigue at the same 42 MPa amplitude. One had a mean stress of 0 MPa (fully-reversed), and the other had a mean stress of 7 MPa. The fully-reversed sample had life- times that were 30 to 300 times longer
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