Underfill adhesives applications

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. 

Automated dispensing is used for the application of die-attachment, surface-mount, and underfill adhesives. 

In the application of capillary-flow underfills, there are so many material, process, and equipment variables that empirical methods are widely used to establish the optimum processing conditions. The viscosity and flow properties of underfill adhesives are among the most important variables in rapidly filling different gap sizes and devices. A list of some commercially available underfill adhesives and their flow properties, as related to filler size and gap height, is given in Table 5.3. 

Table 5.5 Examples of capillaiy-flow and no-flow underfill adhesives and applications... 

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. ... 

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured according to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low-Shear Rates by Brookfield Viscosity The viscosity of typical electrically conductive die-attach adhesives is measured with a Brookfield HBT viscometer with Spindle TB and Speed 5. For higher-viscosity conductive adhesives and for underfill adhesives, a Brookfield RVT or RVF viscometer is used with Spindles 6 or 7 at speeds of 4—10.4 rpm. Another Brookfield viscometer, the Cone-and-plate viscometer with a CP-51 spindle is used for low-to-intermediate viscosity adhesives. Finally, the Brookfield HAT and HBT instruments are used for the high-viscosity (1-2 million cP at 1 rpm) adhesives typically used in SMT applications. ... 

Table 5.5. Examples of Capillary-flow and No-flow Underfill Adhesives and Applications... 

MAJOR PRODUCT APPLICATIONS rubber pads, liquid encapsulants, underfills, printed circuit boards, adhesives, greases, hquid coatings, aerosol sprays... 

Avatrel Dielectric Polymers (low-k dielectric applications such as interlayer dielectrics, passivation layers, die attach adhesives, chip encapsulants (both molding and adhesive) and underfill materials). 

Once cured, the properties of NFUs are similar to those of capillary-flow underfills. One major difference is that the no-flow materials are generally unfilled and, as a result, their expansion coefficients are higher than those of their filled counterparts. However, their lower moduli more than compensate for the mismatches in expansion coefficients. NFU adhesives have shorter shelf lives than capillary-flow types because of the incorporation of the fluxing agent into the adhesive formulation. Table 5.4 is a compilation of underfills and their properties while Table 5.5 lists examples of capillary flow and no-flow underfills and their applications. 

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. 

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... 

In no-clean flux applications, excess flux residue is of particular concern because it has been shown to impede the adhesion of underfill materials that facilitate improved solder joint reliability. 

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