Flip-chip applications process

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)... 

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. 

Some adhesive materials and processes are used across many apphcations. For example, adhesives are used to attach bare die, components, and substrates in assembling commercial, consumer and aerospace electronic products. Adhesives are also widely used for surface mounting components onto interconnect substrates that serve numerous functions for both low-end consumer products and for high rehability applications. Underfill adhesives are used to provide stress relief and ruggedize the solder interconnects for almost all flip-chip and area-array devices, regardless of their function as integrated circuits. 

Smart cards are among the fastest growing applications for electrically conductive adhesives. Smart cards are pocket-size plastic cards with thin embedded ICs that store, process and transfer data much in the manner of a mini-computer. Smart labels are also being developed where the substrate is paper instead of plastic. Flip-chip transponder chips and flexible antenna are incorporated producing, for example, low-cost radiofrequency identification (RFID) units. 

The use of underfill adhesives has resulted in the development of the draft version of J-STD-030, Guideline for Selection and Application of Underfill Material for Flip Chip and Other Micropackages. The guideline covers critical material properties for underfill materials to assure compatibility in underfill applications for reliable electronic assemblies as well as selected process-related qualification tests such as thermal cycling. Table 6.9 summarizes selected materials requirements for underfill adhesives from the proposed JEDEC J-STD-030. ... 

Being monolithic in design, this diamond actuator is substrate independent and can be integrated to any microwave substrate depending on the application. For the first set of results, these actuators were flip-chip bonded to an alumina substrate using a Cu/Sn solid liquid interdiffusion (SOLID) process [43]. Figure 12.15 shows the integration process of the diamond actuator to the host substrate. 

The development of anisotropic adhesives was initiated by the LCD industry but an extensive survey of the patent literature indicates other general trends [102]. The resins and fillers intended for use as anisotropic adhesives must satisfy new requirements compared to those previously defined for isotropic materials. In particular, the geometrical factors influence the formulation and the processing technique in relation to the final application LCD, surface mounting or flip chip bonding. 

In soldering and when an ICA is used, the electrical connection is first established by the solder or the conductive adhesive before the bond is mechanically stabilized by application of an underfill. Using ACA or NCA makes it possible to combine electrical connection and mechanical stabilization in one process step. Electrically conductive connections made with nonconductive adhesives are based on mutual contact between the joining partners. A constant contacting force therefore has to be applied until the adhesive has cured. Contacting flip chips with adhesive... 

As such, many applications of the capillary flow is widely developing in some modem processes, such as underfilling of flip chip, flow in microfluidic chip or biochip, and a variety of other fields. The capillary phenomena involved with flow behavior, driving principle and the flowing control is becoming quite important[l][2]. 

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