Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive materials for various applications such as interconnection material for flat panel displays (flex to PCB, chip-on-chip, and chip-on-glass) and flip chip semiconductor packaging applications 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. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection method such as Flex-on-Flex (FOF) are growing more and more, where the bending characteristics are important. In this thesis, nanofiber is introduced into the ACF system. Nanofiber suppresses the solder ball movement during the bonding processes which improves the conductive particle capture rate. This eventually improves the interconnection electrical property as well as reliability properties. In addition, nanofiber forms an insulating layer around the solder ball preventing short circuit between neighboring electrodes to occur when it comes to fine pitch assemblies. These nanofiber are fabricated through a well-known technique called electrospinning, where high voltage is applied on the needle and charged polymers are fabricated on the ground receiver. Here we mix conductive particles (Sn58Bi solder ball) to incorporate nanofibers with the conductive particles. Even though the benefits of nanofiber were proven in the past, the effect of the nanofiber orientation on the solder ball movement suppression ability and bending properties has not been clarified.
In the introduction section, first electronic interconnection trend will be introduced by giving mobile phone as an example. Then we will explain why we introduced Sn58Bi solder incorporated nanofiber anisotropic conductive films (ACFs). In the past Ni incorporated ACFs were commonly used for flip chip assembly application. However, Ni particle has a high melting temperature and when it comes to bonding, Ni ball only forms a physical contact with the electrode. However, by applying solder ball, stable metallurgical joint forms which eventually improves the reliability of the interconnection joint. In addition, by incorporating nanofiber into the ACF system, nanofiber suppresses the solder movement which increases the solder ball capture rate. This will improve the electrical properties as well as reliability. Next, the advantages of aligned nanofiber will be explained. When performing electrospinning, there are few ways to align the nanofibers. Here we fabricated aligned nanofibers using a drum type receiver. When aligning the nanofibers parallel to the main resin flow direction, the conductive particle movement suppression capability of the nanofibers improve as well and the bending characteristics of nanofiber/solder ACFs. Lastly, the importance of flex-on-flex (FOF) assembly will be introduced. Here, we will explain what disadvantages socket type interconnectors have and because of what demands flex on flex interconnection have advantages over socket type interconnectors for wearable device applications.
In detail, chapter 2 focusses on fabrication of nanofiber/Sn58Bi solder ACFs and its interconnection properties. As mentioned above, interconnection problems such as short circuit tends to occur when it comes to fine pitch interconnections. Solder ball incorporated polyvinylidenefluoride (PVDF) nanofiber made by electrospinning technique was added into the ACF system to suppress solder ball movements during the flex-on-flex (FOF) bonding process, and forming an insulating layer around the solder ball preventing short circuit between the neighboring electrodes. Also, to improve the thermal mismatch of the flexible substrate which can lead to electrode misalignment during the bonding process, the bonding temperature was set to $200^\circ C$. In order to do that Sn58Bi solder ball which has a melting point of $138^\circ C$ was used instead of SAC305 solder ball which has a high melting temperature of $217^\circ C$. In addition, to form excellent metallurgical solder joints while removing the solder oxide around the solder ball surface, vertical ultrasonic (U/S) bonding method was used. When performing FOF assembly using PVDF nanofiber/Sn58Bi solder ACF and vertical ultrasonic bonding, it was shown that PVDF nanofiber/Sn58Bi solder ACFs showed 34% higher solder capture rate on an electrode compared to conventional Ni ACFs and conventional Sn58Bi solder ACFs. Additionally, PVDF nanofiber/Sn58Bi solder ACFs showed 100 % insulation between neighboring electrodes where conventional Ni ACFs and conventional Sn58Bi solder ACFs showed 75 % and 87.5 % insulation. Other electrical properties such as contact resistance and current handling capability as well as reliability test of PVDF nanofiber/Sn58Bi solder ACFs showed improved results compared to conventional Ni ACFs, which proves the formation of stable solder joint of PVDF nanofiber/Sn58Bi solder ACFs.
In chapter 3, 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. The solder ball incorporated nanofibers were first obtained through electrospinning 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 electrospinning process. This allows the nanofibers 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. The solder movement analysis, electrical/mechanical properties, and reliability measurements were done to confirm the aligned nanofiber effect. It was shown that parallel nanofiber/Sn58Bi solder ACFs showed 7% higher solder capture rate on an electrode compared to random nanofiber/Sn58Bi solder ACFs. Additionally, parallel nanofiber/Sn58Bi solder ACFs as well as random nanofiber/Sn58Bi solder ACFs showed 100 % insulation between neighboring electrodes where perpendicular nanofiber/Sn58Bi solder ACFs showed an insulation rate of 95 %. Other electrical properties such as contact resistance and current handling capability as well as reliability test of parallel nanofiber/Sn58Bi solder ACFs showed improved results compared to the other nanofiber/Sn58Bi solder ACFs which proves the excellent properties of parallel nanofiber/Sn58Bi solder ACFs.
In chapter 4, the bending characteristics of flex-on-flex assembly using nanofiber/Sn58Bi solder ACFs were investigated. The bending was performed by using 3-point bending method. The interconnection properties and reliability of nanofiber incorporated ACFs were introduced above where the nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability which not only prevents short circuit to occur but also improves the joint reliability. Depending on the solder capture rate which leads to different solder joint area can affect the bending characteristics of the flex-on-flex assembly. This is because solder joints tend to prevent any sort of delamination during the bending process. Since same amount of stress is applied on the bonded area, the one with a larger joint area will have higher sustainability. It was shown that parallel nanofiber/Sn58Bi solder ACFs and random nanofiber/Sn58Bi solder ACFs a stable contact resistance even after 100,000 cycle bending where the bending radius was 0.33 mm radius. However, perpendicular nanofiber/Sn58Bi solder ACFs showed an open failure after 90,000 cycles due to the low solder ball capture rate in other words smaller solder joint area.
Lastly, chapter 5 introduces large scale nanofiber/solder ACFs based on the lab scale. The spinning apparatus consists of 5 needle setting all which contains a rotation system. This prevents any solder ball precipitation to occur during the spinning process. Also, inflatable pump rate controller is applied in order to supply the polymer solution continuously. Once the 10 m long solder incorporated nanofibers were fabricated, this was then laminated between two NCFs right after the film was coated. When comparing the interconnection characteristics as well as reliability properties with lab scale nanofiber/solder ACFs, it was shown that the results were similar proving the possibility of nanofiber/solder ACF mass production.