Zymet has introduced a new reworkable underfill encapsulant, CN-1736, designed to underfill 0.4-mm pitch Package-on-Package (POP) assemblies. It has low viscosity and a lower CTE than its predecessors. Plus it has greater flux compatibility, making it suitable for use with a broader range of solderpastes and tacky fluxes.
With finer pitch devices, flow of an underfill is slower. To retain superior flow speed for 0.4-mm pitch POP’s, CN-1736 has a viscosity of 650 cps, compared to 900 cps for its predecessor. Also, finer pitch solder joints are more fragile and could be more susceptible to underfill-induced thermal cycle failure. Consequently, CN-1736 has been designed with a CTE of 55 ppm/°C , compared to 64-75 ppm/°C for its predecessors.
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.
Zymet is a manufacturer of microelectronic and electronic adhesives and encapsulants. Its products include die attach adhesives, substrate adhesives, UV curable glob top and cavity-fill encapsulants, and underfill encapsulants.