覆晶錫鉛銲錫以熱時效處理後之電遷移失效時間與微結構研究

Abstract

高接腳密度、縮減封裝體積等優勢，讓覆晶銲錫在電子產品走向輕、薄、短、小的趨勢中，成為高階元件的主流封裝型式。伴隨積體電路高電流、小尺寸的設計變化，覆晶銲錫接點內的電遷移現象成為元件可靠度的影響關鍵。在覆晶銲錫電遷移測試中，發現孔洞通常生成於導線與銲錫凸塊界面處。研究發現其係在電子流由導線進入銲錫時，因電子流流通面積改變，造成電流集中效應的影響。因此研究如何讓銲錫球可以減低電流集中，進而抵抗電遷移效應是非常重要的課題。 銲錫球中之Under Bump Metallization(UBM)層通常是Ni或是Cu，並會和Sn產生介金屬化合物(Intermetallic compound,簡稱IMC)，這一層介金屬化合物，不但是銲接上去作為接合用的介面，更由於它是高電阻的物質，我們可以藉由此性質降低電流集中效應，減少電遷移之影響，增加銲錫球的使用壽命。 本研究即利用熱時效(Pre-aging)的方式，使用溫度170℃(約為銲錫熔點絕對溫度的百分之九十)及不同的時效時間，是否對IMC層產生變化，以此來觀察熱時效時間與電阻上升(孔洞生成)時間之關係，並探討其原因。

Flip-chip technology has become a mainstream trend in advanced electronic packaging because of its capability of higher I/O density and smaller package size. With higher current and smaller size trends, electromigration in flip-chip solder has become an critical of reliability concern. The effect of pre-aging on electromigration is investigated in this study using flip-chip SnPb solder joints. The solder joints were pre-aged at 170°C for 25 h, 50 h, and 100 h, and then they were subjected to electromigration tests of 1.0 A at 150°C. It was found that the average failure time increased when the joints were pre-aged for no pre-aged, 25 h, 50 h and 100 h. It is proposed that the major contributor to the prolonged failure time may be the densification of the nickel and copper under-bump metallization (UBM) and the solder due to the aging treatment. The pre-aging treatment at 170°C may stabilize the microstructure of the solder. The vacancies in the solder might be annihilated during the heat treatment, causing a slower diffusion rate. In addition, the IMC structure became thicker after the pre-aging process. Thus, the thicker IMC structure may lead to smaller current crowding effect and slower consumption rates of the nickel and copper layers, resulting in the enhancement of electromigration resistance.