Underfill adhesives

After attaching the BGA packages to a PWB and reflowing the solder, an underfill adhesive similar to that used for flip chips may be dispensed and allowed to fill the spaces between the solder balls, thus reinforcing the connections and relieving stress in the entire structure. 

When the shear stress of a liquid is directly proportional to the strain rate, as in Fig. 2.4a, the liquid is said to exhibit ideal viscous flow or Newtonian behavior. Most unfilled and capillary underfill adhesives are Newtonian fluids. Materials whose viscosity decreases with increasing shear rate are said to display non-Newtonian behavior or shear thinning (Fig. 2.4b). Non-Newtonian fluids are also referred to as pseudoplastic or thixotropic. For these materials, the shear rate increases faster than the shear stress. Most fllled adhesives that can be screen printed or automatically dispensed for surface-mounting components are thixotropic and non-Newtonian. A second deviation from Newtonian behavior is shear thickening in which viscosity increases with increasing shear rate. This type of non-Newtonian behavior, however, rarely occurs with polymers. ... 

The size of the die is a further consideration in the time it takes for the underfill adhesive to traverse the distance and completely fill the gap. A plot of the square of the distance as a function of time produces a straight line as expected from Eq. (2.9) (Fig. 2.7). For a given die size, increased viscosity of the underfill material also increases the time to fill the gap. For high production rates, the fill time must be as short as possible. With increasing sizes of IC chips, this has presented... 

Underfill adhesive (rapid cure) 30-60% Bisphenol F epoxy reaction product, 15% imidazole catalyst, 30-60% silica filler Loctite 3563 (Henkel)... 

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

Figure 4.12 Dispense patterns for capillary-flow underfill adhesives Top 1 shape Middle L shape Bottom modified XP shape.

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. 

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 5.3 Flow Properties of some underfill adhesives... 

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

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). 

Large amounts of absorbed moisture in the die-attach or underfill adhesive. 

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

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. ... 

Specifications for underfill adhesives. Table 6.9 summarizes requirements for underfill adhesives from the proposed JEDEC J-STD-030. ... 

Viscosity is the internal friction that results from intermolecular forces of attraction and interactions between fillers and resins in adhesives, a measure of resistance to flow. Most adhesives are non-Newtonian fluids that exhibit shear-thinning behavior, or decrease in viscosity with increasing shear rate. Exceptions to this general rule are the capillary-flow underfill adhesives that tend to be Newtonian in fluid behavior. 

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. ... 

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