Flip chips with underfill

Flip-Chip with Underfill. Flip-chip with underfill thermal cycling results from two sets of experiments, (Ref 25-27) and are plotted in Fig. 7 with the failure times given as mean cycles to failure for Sn-Pb and lead-free assemblies ... 

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... 

Figure 5.22 shows a specimen process flow for flip-chip gluing with ICA. Conductive adhesive is stenciled on to the substrates. The devices are then placed on the chip, and the conductive adhesive is thermally cured. As in flip-chip soldering, underfill Is applied as mechanical protection and cured. 

Another process for bonding a flip chip with metal bumps consists of screen printing an ICA on a chip carrier, aligning and placing the chip, curing the ICA to form bonds, and underfilling. By... 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

DeBarros T, Katze D. Achieving SMT compatible flip-chip assembly with no-flow underfills. Proc. NEPCON West Conf Anaheim, CA 2000. 

Figure 6.3 Reliability of flip-chip devices with and without underfill.

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

Underfill for flip-chip-attached devices. With the availability and increased use of flip-chip devices, a need has arisen to reinforce the connections and dissipate stresses resulting from mismatches in expansion coefficients among the solder, the substrate, and the device. This need has been met by underfilling the space between the solder connections with specially formulated underfill adhesives (Fig. 1.5). Underfill adhesives are... 

FluxFill 2000/ Loctite No-flow, fluxing underfill (volcano reflow) Not applicable CSP (ceramic), FCOB (silicon with passivation) and OSP -copper, copper-nickel-gold, and PWBs Dispense, 18-gauge needle 10 psi Fluxing CSP or flip-chip on board (volcano cycle)... 

DeBarros, T., and Katze, D., Achieving SMT Compatible Flip-Chip Assembly with No-Flow Underfills, Proc. NEPCON West Conf., Anaheim, CA (2000)... 

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