Recently extensive researches on the lead-free solder and fluxless soldering have been conducted. Since high soldering temperature of the lead-free solder has detrimental effects on the packages, development of the low temperature soldering process is needed. Thermosonic bonding using the longitudinal ultrasonic is investigated in this study for its application to low temperature fluxless flip chip bonding with the lead-free solder(Sn-3%Ag-0.5%Cu). Effects of various bump configurations such as solder bump and metal pad, metal bump and solder bump, and crossed strip solder bumps on bond quality are studied through numerical analysis and experiments.
Numerical analysis using the finite element method(FEM) showed that larger strain thus larger viscoelastic or viscoplastic heat generation occurs at the bond interface. Heat generation by viscoplastic heating was larger than viscoelastic heating because most of deformation energy is transformed into heat under viscoplastic behavior while only small portion is under viscoelastic behavior. Solder temperature was calculated to increase by 40-135oC in a few seconds of bonding time by viscoelastic heating or viscoplastic heating.
Experiments using various bump configurations have been conducted with the preheating temperature lower than the melting point of the lead-free solder. The solder temperature increased over melting temperature locally in the package, and the measured solder temperatures with bonding time showed reasonable agreements with the calculated results. While longer bonding time resulted in higher joint shear strength, excessive bonding time deteriorated bond quality. Higher pressure reduced bonding time needed for the maximum strength. Joint shear strength under the optimal bonding condition satisfied the specification required in the industry.
Since the multiple flux-free solder joints can be formed with short bonding time and low preheating temperature, thermosonic soldering method is applicable to the flip-chip bonding of lead-free solders with low thermal effects on the package.
최근 무연 솔더와 무플럭스 솔더링에 관한 연구가 활발하게 진행되고 있다. 무연 솔더의 경우 용융 온도가 높아 패키지에 높은 열영향을 미치므로 저온 솔더링 공정을 개발하는 것이 필요하다. 본 연구에서는 종방향 초음파를 사용하는 열초음파 접합을 무연 솔더 의 플립칩 본딩에 적용하여 저온 무플럭스 플립칩 접합을 수행하였다.
수치해석 결과 점탄성 발열 또는 점소성 발열에 의해 수초안에 솔더의 온도가 약 40-135℃ 정도 상승하는 것으로 계산되었다. 실험 결과 솔더의 온도가 패키지 내에서 국부적으로 상승하여 솔더가 용융하였으며 수치해석 결과와 비교적 일치하였다. 최적 접합 조건에서의 접합 강도는 산업에서 요구하는 조건을 만족시켰다.
열초음파 솔더링을 이용하여 다수의 무플럭스 솔더 접합부를 고속으로 형성할 수 있으므로 무연 솔더의 플립칩 본딩에 적용 가능하다.
