Capillary flow underfills

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

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. 

In addition to capillary-flow underfilling, a new approach to underfilling has been developed whereby the underfill material is applied before attaching the flip-chip or BGA device. Specially formulated adhesives, known as no-flow underfills (NFU) or... 

Figure 5.8 Capillary-flow underfill process steps...

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. 

Alternate approaches that eUminate the capillary-flow underfill process currently used by electronics assemblers, involve pushing the problem back to the wafer stage. One process, wafer-level underfill (WLU) (Figure 5.12), consists of applying the underfill material to a bumped wafer and then B-staging the underfill. The wafer is then diced and the die are positioned on corresponding connection pads on an interconnect... 

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. 

Amicon E 1216/ Emerson Cuming Snap-cure, nonanhydride chemistry, capillary-flow underfill >5 nul Ceramic, plastic, laminate Automated dispensing (21-gauge needle) FHp-Chip, CSP, and BGA... 

Amicon E 1172/ Emerson Cuming Fast-flow, nonanhydride capillary-flow underfill >1 mil Laminates Automated dispensing (21-gauge needle) CSP, BGA, and Flip-chip on board... 

A property related to viscosity is the thixotropic index, ameasure of the dependence ofviscosify on shear-rate. The thixotropic index (TI) is the ratio of viscosities measured at speeds that have aratio of 1 10 rpm, for example, at 2 rpm and 20 rpm. For non-Newtonian materials used in automated dispensing, shear thinning is apparent from TI values of 2-6. In general, capillary-flow underfills have thixotropic indices of 1, or close to 1. 

Figure 23 shows photomicrographs of failed CSPs after drop tests for lead-free and tin-lead solder alloys. Without underfill, lead-free solder and tin-lead eutectic solder drop results were comparable. With capillary flow underfill, the... 

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