Flip-chip applications underfill

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. 

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. 

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. 

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. 

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. 

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. 

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. ... 

Loctite 3594/ Loctite Fluxing (no-flow) underfill NA Device passivations, OSP - copper, copper-nickel-gold Automated pattern dispense Flip-chip-on-board using SMT reflow profiles. Suitable for high-fi-equency applications ( - 2.9 at 100 kHz). 

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)... 

Amicon E1330LV/ Emerson Cuming Unfilled, no-flow fluxing underfill (reworkable) Not applicable Nickel-gold and OSP finishes Syringe, jet dispense, or stencil print Flip-chip, CSP, EGA modest post-reflow cure (offline) required. Reworkable, 300 second reflow cycles 225°C peak. Variable fi-equency microwave curable. 

Guidelines for Selection and Application of Underfill Materials for Flip-Chip and Other Micropackages, JEDEC J-STD-030 (Dec. 2000)... 

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... 

Although a reduction in feature sizes referred to as shrink is one of the ways to accommodate an increased transistor count, it is often not sufficient to provide the required I/O counts, necessitating an increase in die size as well. However, as the strain that a solder joint experiences is directly proportional to the distance from the neutral point (DNP, i.e., the temperature invariant point), this can have a significant impact on the reliability of solder joints located in the outer rows of area-array, flip chip devices. An increased die size may result in a maximum DNP that poses a potential reliability concern for some applications, requiring that additional precautionary steps be taken, among them enhanced cooling capability, chip underfill, and increased inherent fatigue resistance of the solder. 

There are a number of methods available to avoid the tin pest reaction, although some are not practical. Clearly, the tin pest reaction will not occur if the temperature is not reduced below the transformation temperature. Unfortunately, the transformation temperature is not so low that it can be avoided either during product use or shipping therefore temperature control is not an acceptable solution. The imposition of a compressive stress is known to inhibit the transformation. This may be useful in some applications, for example, the underfill used in conjunction with flip chip solder joints (C4) exerts a compressive stress on the joints as the volume increases during the tin pest reaction. The effect of the compressive stress may be sufficient to eliminate or retard the tin pest transformation. Unfortunately, this mechanism would not apply to solder joints not utilizing an underfill material such as ball grid array (BGA) and other surface-mounted components [65]. 

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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