Impact of the final finish on the solder joint reliability and IMC formation after thermal storage

The need for higher temperature stability of solder joints is steadily increasing which is among others driven by the growing demands in the automotive industry. In the recent years an increasing number of studies focused on the understanding of the aging of the solder joints under high temperature and long-term storage conditions and its impact on the solder joint reliability. Still, not much information is available on the impact of the final finish under harsh temperature storage. The intention of this paper is to evaluate the impact of different types of final finishes on the solder joint which was exposed to long term thermal aging. The comparison covers common final finishes such as Nickel/Gold (Ni/Au) including the variation between high-P nickel and mid-P nickel, Electroless Nickel/Palladium/Gold (Ni/Pd/Au), organic solder preservatives (OSP) and immersion tin (I-Sn), as well as new finishes such as Palladium/Gold (Pd/Au). All tests were conducted on Ball grid array (BGA) test coupons with 250 µm solder resist openings, soldered with SAC 305 solder balls of 300 µm diameter. After the solder ball application, the samples were heat treated for 1000 h at 175°C ad 2000 h at 150°C. To study the intermetallic phase and the solder joint connection, high speed shear tests have been performed and the solder joint was investigated by means of scanning electron microscopy. The paper describes the differences in the solder joint formation depending on the final finish type and identifies those final finishes bringing in the best properties to support a good and reliable solder joint formation under those challenging aging conditions. To judge the solder joint quality the IMC is characterized by structure and thickness and high-speed shear tests are performed as indicator for the reliability and brittleness of the solder connection. A major difference is thereby reflected between those final finishes where the IMC is formed directly between copper and the solder like in OSP, I-Sn or Pd/Au and the nickel containing finishes like Ni/Au and Ni/Pd/Au. Here the impact of the additional Pd-layer on the reliability and brittleness of the solder joint is studied and compared with earlier investigations under different aging conditions.