Recently, as the demand for flexible electronic devices has been increasing, flexible substrates based on polymer materials have also been attracting attention as electronic package materials. Since flexible substrates have low glass transition temperatures (Tg), reliabilities of bonding joints such as flexible substrates and electronic chips/packages can be degraded due to thermal deformations of flexible substrates caused by heat caused because of the process temperature when a conventional reflow process is applied to flexible substrates. Consequently, to address these problems, some researches on laser bonding processes for low-temperature and high-speed bonding processes are performed. In the case of the laser bonding process, we can perform local heating with a very short process time. Therefore, the thermal deformations of flexible substrates can be reduced and improve reliabilities of bonding joints on flexible substrates. In this research, we studied the characteristics of bonding joints between polyethylene terephthalate (PET) substrates with various surface treatments and Si chips with Sn-58Bi solder balls using a laser bonding process. The pad surface treatment of PET substrates were organic solderability preservation (OSP), electroless nickel impression gold (ENIG), and electroless nickel electroless palladium impression gold (ENEPIG), respectively. After the laser boning process, joint properties such as formations of intermetallic compounds (IMCs) with various surface treatments were analyzed.