Fluxless Solder Flip Chip bonding using Non-Conductive Paste

A. Yamauchi, T. Mori
Toray Engineering Co., Ltd

Our paper describes underfill-less and fluxless bonding processes by thermal compression, and presents data. This fluxless flip chip bonding technique for solder bumping application allows underfilling and thermal compression processes to be done simultaneously.

Currently, flip chip bonding methods using the solder bump with thermal compression process require flux to prevent oxidation problems. These problems can be separated into primary and secondary oxidation.

Primary oxidation is generated on the surface of the solder bump when it is plated on a chip electrode. This oxidation layer is typically generated on a ball-bump after reflow. By applying oxide plasma during the reflow process, the undesirable oxide layer is removed with a reducing process (effect). Our new process creates ideal solder bumping with almost no oxidation.

The Non-Conductive Paste (NCP) method we describe is an effective measure to prevent secondary oxidation when applying thermal compression. In our suggested NCP method, the NCP that remains as underfill can be applied prior to mounting chips. Secondary oxidation can be prevented by applying thermal compression of bumps in the NCP. However, to make sure pertinent wettability during soldering in NCP does not occur, the NCP is required to decrease its viscosity during the melting of the solder phase while its thermal setting starting time should be delayed after gaining wettability.

The flux-free soldering bump method can be realised by optimising the above processes and the result is that underfilling and solder flip chip bonding can be conducted simultaneously by thermal compression.