Fillets, underfill

The volume of underfill must be controlled to assure complete filling beneath the die and some filleting around the die. A complex combination of factors (gap height, contact angle, viscosity, wettability) affects the amount of underfill required for complete coverage. The volume of underfill (V) is calculated based on the volume of free space under the die (Vc), the volume of the solder bumps (Vb), and the fillet volume (Vf) or V=Vc — Vb +... 

Table 2.2 Contact angle and fillet shape factors for underfills ...

The simple I dispense pattern is widely used for square devices. Multiple applications, however, may be needed followed by a fillet or seal at the other three sides using a fine gauge needle. A limitation of this approach is the time required to underfill large die. For die smaller than 3-mm square, a single pass usually suffices. Yet another option for small die is to apply a single dot at one edge. ... 

Figure 2.9. Shape factors for underfill fillets. (Source Electronics Engineer, 1998.)...

Ablefill UF 8822/ Ablestik Labs. Moisture-resistant cyanate ester (silica filled) N/A Plastic, ceramic, FR-4, polyimide, and Rigid BT Dispense (22-25 gauge needle), capillary underfill Underfill for lead-free interconnects die sizes over 10-mm square (self filleting)... 

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