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Microspring Characterization and Flip Chip Assembly Reliability
Keywords: spring interconnect, multichip module, wafer level packaging and testing
Spring-based electrical interconnects provide new MCM capability for IC packaging by enabling both test and packaging. We describe new springs which achieve low resistance (<100 mohm) and high compliance (>30 micron) in dense 2d arrays (180x180micron pitch). In addition to reliability data, we present details of the spring design, fabrication and characterization. Fabrication is carried out on wafer scale and consists of etch pit formation followed by springs processing based on stress engineered metal. Each pit on two matching chips will later house a microball which selfaligns the chips and facilitates the assembly. The spring metal (MoCr alloy) is sputter deposited and patterned into springs (100 micron long, 30 micron wide). A large stress gradient (GPa/micron) is achieved by varying the sputter pressure during deposition. After the release layer etch, the spring stress relaxes and the tips rise. The springs are plated with gold to increase electrical conductivity. Each test chip (14x16mm) has 2844 contacts, with 2 springs/contact. The spring height is ~45 microns and measurements show the height variation is less than 3 microns across a 4-inch wafer. Release yields >99.9% have been demonstrated (~105 springs/wafer), and 100% yield has been achieved for more than half of the 20 die/wafer. Single spring resistance measurements are performed with four-wire test as a function of compression. Plateau of <100 mohms is repeatedly achieved when contacting a gold pad. FEM simulations determine the resistance contributions from the spring body, tip-pad interface and pad spreading resistance. Force measurements for a single spring show 10 mg force at 20 micron compression. Measurements consistently show >30 micron compliance without damage to the springs. Spring chips were assembled onto pad chips, aided by microballs/pits, and then reliability tested (thermocycle, humidity, high current). Inspection of springs tips and pads show wear patterns stay within tolerances.
E. M. Chow, Research Scientist
Palo Alto Research Center (PARC)
Palo Alto, CA

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