I use a small amount of Pratley quickset steel to attach those, so long as the surfaces are clean, smooth and lightly sanded just before use with some 400 grit waterpaper, then wiped down with alcohol, the bond will be very strong. You only need a small amount, about a match head of the 2 components, mixed with a toothpick on some plastic scrap, and then applied in the middle of the Ic and then press the heatsink down firmly and leave for a half hour to gel.