銲錫薄膜之電遷移與錫晶鬚生長之研究

Abstract

於銲點微小化與無鉛表面接著產品廣泛應用的考量下，電遷移效應與晶鬚成長便成為電子構裝中日益重視的可靠度議題。 因此於本論文中，我們將分別利用Blech結構與銅基材之引腳架來對電遷移效應與高溫高濕環境下如何促使晶鬚成長進行研究。 於電遷移研究中，我們求得了共晶錫鉛於攝氏100度、電流密度2 □ 104 A/cm2下之臨界長度(critical length)與臨界值(critical product)分別為10 到15 um與20 到30 A/cm。同時，我們亦求得錫銅介金屬化合物於攝氏225度、電流密度5 x 103 A/cm2下之臨界長度(critical length)與臨界值(critical product)分別為5 到10 um與2.5 到5 A/cm。且於實驗中我們發現共晶錫鉛與錫銅介金屬化合物因back stress 之關係皆有length-dependent電遷移的現象。 於晶鬚成長之研究方面，我們於100度、電流密度5x104 A/cm2下在共晶錫鉛表面發現複合式晶鬚之生成。但因於較小之電流密度2 □ 104 A/cm2下無發現晶鬚之生成，故我們推斷生成複合式晶鬚之臨界電流密度將介於此二者之間。且我們亦利用繞射圖形得知複合式晶鬚中Pb part為[110]， 與 [112]。 在高溫高濕實驗中，我們借由TEM之EDS與繞射圖形證實了，此環境下錫的氧化反應將促使晶鬚之成長，而此氧化反應將通常於晶界處產生。因此，我們發現藉由熱處理減少錫之晶界將有助於延緩因氧化反應生長之晶鬚成長速率。且經過迴銲過程後，因錫微結構改變成等軸結構將因增加晶鬚成長之錫原子與氧化反應之擴散路徑，故將有效延緩晶鬚之成長。

The miniaturization of bumps and tin-based Pb-free electronic packaging has increased the importance of electromigration whisker formation, which significantly affect reliability. Therefore, this investigation investigates electromigration behavior in eutectic PbSn solder and Sn-Cu intermetallic compounds using a Blech structure and Sn finishing on a Cu leadframe. In the investigation of electromigration, the critical length and critical product of eutectic PbSn solder and Sn-Cu intermetallic compounds are measured to determine the resistance to electromigration resistance. The critical length of eutectic PbSn was 10 to 15 □m based on a current density of 2 □ 104 A/cm2 at 100 °C. That of Sn-Cu intermetallic compounds was 5 to 10 □m under stress by a current density of 5 □ 103 A/cm2 at 225□C. The corresponding critical products were 20 to 30 A/cm at 100 °C and 2.5 to 5 A/cm at 225 □C. Also, due to the effect of back stress, length-dependent electromigration behavior was observed in the stripes of in both eutectic PbSn and Sn-Cu intermetallic compounds. In the study of whisker growth, composite whiskers were formed at 100□C when the eutectic PbSn stripes were subjected to a current density of 5 x 104 A/cm2. For the current-induced whisker study, the critical current density for composite whisker formation on eutectic PbSn surface was between 2 x 104 A/cm2 and 5x104 A/cm2. The growth orientations of the Pb components of composite whiskers were [110] and [112]. In whisker formation in the Pb-free finish on a Cu leadframe in a humid environment, oxidation of the Sn film is proposed to be the driving force of whisker growth. Since the reflowing treatment can change the columnar grain structure of the Sn film to the equiaxed grain structure in some of its regions, lengthening the grain boundary diffusion path for whisker growth and oxidation, the reflowing process is an excellent means of mitigating Sn whisker growth in the Sn finish on Cu leadframes. Also, thermal treatment at 220 °C mitigates whisker growth by reducing grain boundary area of the Sn finish.