ZYMET: Reworkable Underfill (9/22/09)
This company has introduced a new reworkable underfill encapsulant, CN-1728, designed to underfill package-on-package (POP) assemblies. Underfilled POPs have greater difficulty passing thermal cycle tests than underfilled BGAs. Compared to earlier generation underfills, CN-1728 has a lower coefficient of thermal expansion and higher Tg, and better compatibility with flux residues, both of which contribute to its superior thermal cycle performance.
CN-1728 is a fast-flowing capillary underfill with a viscosity of 900 cps, and it can be cured in as little as one minute at 150°C. These properties make the underfill particularly well suited for high-volume, high-speed in-line processing.
Rework is accomplished by use of elevated temperature, 170°C to 180°C, to remove the underfill fillet. Then, the BGA is lifted from the board after heating it to reflow temperature. Underfill residue is easily scraped off, again at 170°C to 180°C.
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