Effect of aligned nanofiber in nanofiber solder anisotropic conductive films (ACFs) on the solder ball movement for flex-on-flex (FOF) assembly

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive materials for decades due to its high resolution, light weight, thin structure, and low power consumption. However, the high demands for miniaturization caused interconnection issues such as short circuits when it comes to fine pitch applications. By introducing nanofiber into the ACF system, nanofiber not only suppresses the solder ball movement during the bonding processes but also forms an insulating layer around the solder ball preventing short circuit between neighboring electrodes to occur. Even though the benefits of nanofiber were proven, the effect of the nanofiber orientation on the solder ball movement suppression ability has not been clarified. In this study, nanofiber/solder ACFs using nanofibers that are aligned in various directions were investigated with respect to solder ball capture rate, electrical/mechanical properties, and reliability. Nanofiber/solder ACFs consists of adhesive resin, solder ball, and nanofiber material. The solder ball incorporated nanofibers were first obtained through electro-spinning technique. In order to align the nanofibers, drum type receiver was used where the receiver was capable of rotating up to 2500 rpm during the electro-spinning process. This allows the nanofiber to deposit in an aligned fashion. After the solder ball incorporated aligned nanofiber layer was obtained, the solder incorporated nanofiber layer was then laminated between two non-conductive films (NCFs) in order to produce the finalized aligned nanofiber/solder ACFs. Here Sn58Bi (58%Sn and 42%Bi) solder ball was used whichhave a low melting temperature of 138 °C. For the bonding method, vertical ultrasonic bonding method was used to make fresh solder joint on metal electrodes. Solder ball normally contains an oxide layer which prevents stable joint to form. Vertical ultrasonic vibration allows us to break the oxide layer during the bonding process forming - stable metallurgical solder joint. The solder movement analysis, electrical/mechanical properties, and reliability measurements were done to confirm the aligned nanofibereffect. As a result, aligning the solder incorporated nanofibers using the electrospinning technique was successfully performed. Since the role of the nanofiber is to suppress the solder movement during the resin flow, it was shown that the solder ball capture rate increases as the aligned nanofibers are parallel to the resin flow direction. Also, by introducing nanofiber and Sn58Bi solder ball, excellent electrical properties as well as reliability was observed.