Challenge and process optimization of Thermal Compression bonding with Non Conductive Paste

TCNCP (Thermal Compression bonding with Non Conductive Paste) was developed for fine pitch interconnection and advanced silicon node (20 nm or below) in Flip chip technology. With pre-applied NCP dispensing process, small bump joint (<;40 um) is well protected during bonding process. TCNCP provides an excellent solution for fine pitch and small interconnection. A FCCSP with fine pitch (40um) Cu pillar bump was designed in this study. 14×14 mm² package was assembled with 11×11mm² die, over 3,000 bumps are connected by daisy chain between die and substrate. Flux clean was applied prior to TCNCP to remove OSP layer on top of the Cu trace. The study also investigated NCP Q (queue) time at 70^oC and 80^oC of stage temperature. To ensure a robust solder joint, solder gap (distance between Cu pillar and Cu trace) was investigated and was well controlled. Bump crack is observed at higher solder gap after reliability test (such as HTST), Sn-Cu IMC formation and Sn consumption are the failure mechanisms. Experiments show that convinced solder joint shape is found at peripheral bump, while obvious NCP entrapment is noticed at core bump. The detailed mechanism is discussed in the paper. In addition to joint quality, 2-step bonding also demonstrates 17.3% UPH enhancement as compare with normal TCNP.