Flip-chip applications

Francis D. Thinning wafers for flip chip applications. High Density Interconnect Magazine 1999 2(5) 22-25. 

E. K. Yung and I. Tbrlik, Electroplated Solder Joints for Flip-Chip Applications, IEEE Comp., Hybrids., Manuf. TechnoL, 14, No. 3, 549-559 (1991). 

Sun et al. (2006) examined the use of novel silica nanofillers in underfill for flip-chip applications, and showed that pre-cure rheology and post-cure values of Tg are effected by nanosilica surface treatment. 

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. 

Leong, W. H., Developing an Underfill Process for Dense Flip-Chip Applications, Proc. 1996 lEEE/CPMT Inti. Electronics Mfg. Technol. Symp., pp. 10-17 (Oct. 1996)... 

FP4531/ Loctite Fast-flow underfill 1 N/A Automated dispensing (21-gauge needle) Flip-chip applications requiring snap cure. 

Conductive Columns. Nitto Denko Corp. developed an ACF for fine pitch flip chip applications (27). The features of this ACF were (1) connectability between bumpless chips and fine pitch PCB (2) high electrical conductivity (3) repairabil-ity (easy to peal off chips from a printed circuit board at elevated temperatures) (4) high reliability and (5) potential storage at room temperature. There are other notable features too (7) ACF is usable at pitches down to 25 fim, (2) the conductive elements are micrometallic columns as opposed to random-shaped particles, and (3) this adhesive matrix consists of a thermoplastic polymer resin, conductive columns coated with an insulator, and a high Tg polymer, which completely separates the columns from the adhesive (Fig. 6). 

Metal-Bumped Flip Chip Joints. ICAs can also be used to form electrical interconnections with chips that have metal bumps. ICA materials utilize much high filler loading than ACAs to provide electrical conduction isotropically (ie in all directions) throughout the material. In order for these materials to be used for flip chip applications, they must be selectively applied to only those areas that are to be electrically interconnected. Also, the materials are not to spread during placement or curing to avoid creating electrical shorts between circuit features. Screen or stencil printing is most commonly used to precisely deposit the ICA pastes. However, to satisfy the scale and accuracy required for flip chip... 

Organometallic conq)ounds have been explored as tiie latent catalysts for various epoxy resin systems [7,8,9]. Metal acetylacetonates (AcAc s), in particular, are foimd to be effective latent accelerators for epoxy and anhydride cure reactions [10,11,12]. Based on the epoxy/ anhydride/ metal AcAc system, underfill materials have been developed for flip-chip applications [5,6,13]. Metal AcAc s are unique as catalysts for epoxy cure reactions in that they not only provide high cure latency, but also offer a wide range of cure temperatures. 

Ceramic materials provide excellent CTE match between the die and package, which then reduces the risk of first-level solder bump failure in flip-chip applications. Unfortunately, this superb CTE match between the ceramic and the silicon die creates a large CTE mismatch between the ceramic package and the PWB. This CTE mismatch creates severe stress on the second level interconnects (see Fig. 58.31). This CTE mismatch, combined with DNP issues... 

Underfills are a specific class of adhesives designed to protect silicon dies which are soldered active face down onto the PCB. In these flip-chip applications, the imderfiU material flows beneath the die by capillary action. These materials are generally highly loaded with inorganic fillers to reduce the coefficient of thermal expansion. 

Lead-free reflow may be done in air or N2. Typically N2 is not required, and the use of N2 may even increase certain defects (such as tomb-stoiflng) especially for small passive components. In certain situations, N2 may help improve wetting (which in turn may help reduce the amount of voids in the solder joints). For flip chip applications, where flux is used instead of solder paste, N2 becomes necessary to form reliable solder interconnects. An N2 atmosphere with O2 level below 1000 ppm has been found to be effective (Ref 65, 67). 

Estes, R. H., Kulesza, F. W. (1995). Conductive adhesive polymer materials in flip chip applications. In J. H. Lau (Ed.), Flip chip technologies (pp. 223-267). McGraw Hill, New York. 

Van Noort, H. M, Kloos, M. J. H., Schafer, FI. E. A. (1994). Anisotropic conductive adhesives for chip on glass and other flip chip applications. Adhesives in Electronics 94, International Conference on Adhesives Joining Technology Electronics Manufacture, VDI/VDE-IT. 

S. Luo, T. Yamashita, C. P. Wong (2000) Study on the property of underfill based on epoxy cured with acid anhydride for flip chip application, J. Electronics Manufacturing 10, 191. 

Kim JW, Lee YC, Jung SB (2008b) Reliability of conductive adhesives as a Pb-ffee alternative in flip-chip applications. J Electron Mater 37 9... 

For flip-chip applications, the eutectic Bi-Sn solder alloy was electroplated to form solder bumps on various under-bump metallizations (UBMs) [37]. Based on the nature of the interfacial reactions and ball shear strength exhibited when exposed to multiple reflow cycles, Bi-Sn solder bumps were identified as a potential candidate for replacing the Pb-Sn eutectic solder bumps. 

Master, Raj, et al. Ceramic column grid array for flip chip applications. Proc. 45th Electrical Components and Technology Conference IEEE Las Vegas, NV, 1995 925 929 pp. 

III. FLIP-CHIP APPLICATIONS USING ANISOTROPICALLY CONDUCTIVE ADHESIVES... 

TABLE 4 Low-Cost, Heat-Sensitive Chip Carriers Utilized in Polymer Flip-Chip Applications Applications of polymer flip-chip bonding... 

FIG. 17 A schematic depicting flip-chip application utilizing chips with micromachined polymer bumps, (a) Process flow for creating micromachined polymer bumps in the wafer state, (b) Die attachment to a chip carrier. 

Moon, K. Wu, J. Wong, C.P. Study on self-alignment capability of electrically conductive adhesives (ECAs) for flip-chip application. Proceedings of the International Symposium on Advanced Packaging Materials Processes, Properties and Interfaces, Braselton, GA, March 2001 341-346. 

Kang, S.K. Horkans, J. Andiicacos, P. Carruthers, R. Cotte, J., et al. Pb-free solder alloys for flip chip applications. Proc. 49th Electronic Comp, and Technol. Conf San Diego, CA, June 1999 283-288. 

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