(A) study on the electrical performances and reliability of flex-on-board interconnection using solder anisotropic conductive films솔더 ACFs를 사용한 FOB 접속의 전기적 특성과 신뢰성에 대한 연구

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Amid the current trend for wearable electronics assembly, flex-on-board (FOB) assembly is attracting great attention because of its important role in replacing conventional physical contacts-based socket type interconnections. In the case of conventional socket type interconnections, there are three main drawbacks, which include: physical contact, large package size, and low packaging density. For reducing the package thickness from 4mm to 0.1mm, and fine pitch capability to less than 100um, FOB application is an obvious choice in the replacement of conventional socket type interconnectors. Anisotropic conductive films (ACFs) are the interconnection material used in FOB assembly. ACFs generally consist of thermos-setting polymer resins and micron-sized conductive particles, with the cruel requirement in fine pitch applications of providing electrical paths only in the z-axis, and insulation property by polymer resins in the x-y plane. Physical contacts between conductive particles and metal electrodes are the main electrical paths for micron-sized Au/Ni coated polymer ball joints and metal ball joints, but they will easily fail with the thermal expansion of polymer resin in high-power applications. Therefore, micron-sized solder particles are added to polymer resin matrixes to replace micron-sized Au/Ni coated polymer ball joints and metal ball joints to form metallurgical joints for lower joint resistance, higher power-handling capability, and better reliability. Recently, two bonding methods have normally been used to remove the solder oxide layer and get better solder wettability. One method uses ultrasonic bonding, and the other method adds flux materials in adhesives using thermo-compression bonding. However, polymer resins show a low modulus at high temperature when the pressure is automatically released at the end of the thermo-compression bonding, and adhesive rebound may crack solder joints after FOB assembly. For highly reliable solder ACFs FOB assembly, and consequent improved electrical performance, it is necessary to investigate and eliminate solder ACF joint cracks. The effects of thermo-compression bonding temperatures and pressures, as well as ACF resin properties on the solder ACF joint cracks, are discussed in Chapter 2. It was found that SnBi58 solder wettability was so poor below 200$\circ C$, and there were no solder ACF joint cracks at the optimized 200$\circ C$. However, solder ACF joints cracked above 225$\circ C$, and could not be eliminated by using higher bonding pressures. On the other hand, even at the optimized 200$\circ C$ bonding temperature, solder joint cracks were obtained when using lower modulus ACF resin. Finally, adhesive rebounds were tested by a thermal mechanical analyzer (TMA) as a function of temperatures and pressures. Then, solder ACF joint cracks were due to adhesive rebound when bonding pressure was released at the end of thermo-compression bonding. In addition, this rebound can be reduced using a higher resin modulus. The effects of delaying bonding tool lift time and adding silica fillers, which were carried out to increase the ACF resin modulus and remove solder cracks, are detailed in Chapter 3. For maintaining the bonding pressure on the FOB in the cooling process until room temperature, a conventional thermo-compression bonding could not be used, and a novel ultrasonic bonding was used instead. Even though cracks could disappear when maintaining bonding pressures until cooling to polymer Tg, over 30 seconds was too long for assembly. Therefore, 7nm silica fillers were added in the adhesives to reduce adhesive rebound using the conventional thermos-compression bonding. It was found that solder cracks can be eliminated by adding 10wt% silica fillers. AC and DC electrical performances of optimized solder ACF joints are investigated in Chapter 4. In AC conditions, a network analyzer in a 100MHz?20GH high-frequency test was compared between an Au/Ni coated polymer ball and solder ball joints. The polymer ball was 0.1 dB lower than solder joints above 17 GHz. In the DC condition, the power-handling capability was compared between an Au/Ni coated polymer ball, Ni ball, SnBi58, and Sn-3Ag-0.5Cu (SAC305) solder ball ACF joints. The Sn-3Ag-0.5Cu ball showed two times better power-handling capability than the polymer ball due to a higher mechanical property under its melting points. The failure mechanism of solder ACF joints on an Au/Ni surface finish in a pressure cooker test (121$\circ C$ 100% humidity 2atm) is demonstrated in Chapter 5, and the best ACF resin candidate and solder materials were optimized in terms of hydroscopic swelling and solder property for higher FOB reliability. Cationic epoxy was selected due to having the lowest hydro-swelling, and SAC305 solder was chosen due to better mechanical property in the pressure cooker test. As a result, there is no resistance change after five days of the pressure cooker test using cationic epoxy-based SAC305 solder ACF joints after the optimization of the FOB interconnection.
Advisors
Kyung-Wook Paikresearcher백경욱researcher
Description
한국과학기술원 :신소재공학과,
Publisher
한국과학기술원
Issue Date
2017
Identifier
325007
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 신소재공학과, 2017.8,[vi, 83 p. :]

Keywords

solder ACF▼athermo-compression bonding▼aelectrical performance; reliability▼aflex-on-board; 솔더 ACFs▼a핫 용접▼a전기 성능▼a신뢰성▼aFOB 접속

URI
http://hdl.handle.net/10203/241946
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=718958&flag=dissertation
Appears in Collection
MS-Theses_Ph.D.(박사논문)
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