無鉛錫球的孔洞對覆晶球柵陣列構裝元件之可靠度影響研究

Effects of voids in the unleaded solder balls on the reliability of FCBGA components

李俊蒼(Chun-Tsang Li)

指導教授 ： 陳永樹

元智大學/工程學院/機械工程學系/碩士(2008年)

摘要

近年由歐盟所主導之綠色電子產品法規的推動，所有銷往歐盟之電子產品自2006年起不得使用含重金屬鉛之銲錫。相關無鉛銲錫材料因應而生，最常用之典型銲錫材料如錫銀銅合金已廣泛應用錫球接點之材質。但由於材料之特性，無鉛銲錫在電子元件製程過程中容易在錫球內產生空洞(Void)，此瑕疵往往造成以極小的應力就可能引其起銲點的破壞，對無鉛銲錫元件之可靠度影響至鉅。因此，本研究針對循環剪力測試下，孔洞對於覆晶球柵陣列構裝元件壽命之影響，分別從理論分析、可靠度實驗、有限元素分析與田口參數方法等方面著手探究。

理論分析方面是根據錫球受剪力之情形。發現孔洞在錫球裡確實會顯著提高所受之剪應力值，其剪應力與孔洞大小幾乎成正比。並且假設孔洞位置在錫球的上、中、下三處，發現孔洞位置在錫球上方之處的剪應力效應最大。而可靠度測試即針對應用無鉛銲錫材料之FCBGA元件，進行一系列機械循環剪力測試，觀察孔洞是否會造成錫球之易於受到破壞。失效分析發現，當FCBGA元件疲勞壽命偏低時，其失效模式大都為裂縫擴展路徑穿越孔洞。此外，當失效模式為發生在無孔洞之錫球本體時，其特徵壽命為裂縫擴展路徑穿越孔洞之這類錫球失效模式特徵壽命之1.6倍 ~ 4.2倍。由此判斷空洞隱藏在錫球內部顯著影響構裝元件之壽命。

可靠度測試雖證實錫球中含有孔洞會影響元件之壽命，但在理論分析時，發現不單單是孔洞大小，甚至孔洞位置亦會影響剪應力效應。因此，運用有限元素分析再配合田口參數方法，在資料解析過程中發現主要影響覆晶球柵陣列構裝元件壽命為孔洞尺寸約佔電子元件壽命54%。藉由以上各種研究之方法，以釐清孔洞尺寸及位置對FCBGA壽命影響多寡，此為相關文獻中所闕如。至信此研究有助於了解孔洞結構的特性及其相關影響。

關鍵字

無鉛銲錫孔洞；循環剪力測試；FCBGA；有限元素分析；疲勞壽命；田口參數方法

並列摘要

In recent years, as a result of the proclamation for green electronics products conducted by European Union (EU), all the electronics products exported to EU are prohibited to use leaded solder alloy effective from 2006. Consequently, unleaded solder materials are developed rapidly to meet this demand. Among them, the most commonly used solder material like Sn-Ag-Cu alloy is widely adopted as the solder ball joint material. However, due to its material characteristic, unleaded sold alloy is liable to generate voids during manufacturing processes. In slight of stress, this flaw often causes damage to solder joints, which dramatically influences the reliability of unleaded solder components. Thus, this research is aimed to investigate the influence of voids on FCBGA components’ life under cyclic shear stress test, from the theoretical analysis, reliability test, finite element analysis, and Taguchi method respectively.

Based on status of solder balls under shear stress, theoretical analysis shows that voids in solder balls can significantly increases the shear stress it takes. The shear stress is almost directly proportional to void size. Among the voids assumed to exist at the top, middle, and bottom locations of solder ball, it is found that shear stress effect is greatest when the void is at the top location of solder ball. Reliability test is carried out on the FCBGA components with lead-free solder materials through a series of mechanical cyclic shear test. It is intended to observe if voids are the major cause most likely to damage solder balls. Failure analysis reveals when FCBGA components have a relatively low fatigue life, their failure modes are mostly voids that exist on the crack propagation route. Besides, when failure happens on solder balls without voids, their characteristic life is 1.6 to 4.2 times longer than those with voids on the crack propagation route. It concludes that voids stay inside solder balls could greatly affect components’ life.

Though reliability test confirms that solder balls contain voids inside would influence the components’ life, it is found that not only the size, but also the location of void are all related to the shear stress effect in the theoretical analysis. Thus, with finite element method and Taguchi method, it is found through the data analysis process that 54% of components’ life is mainly influenced by void size. The research distinguishes degrees of influence between void size and location on FCBGA components’ life which has never been mentioned in literatures. The corresponding results are believed to have made a remarkable contribution to the understanding of void characteristics and its effect on the reliability of unleaded solder.

並列關鍵字

cyclic shear stress test； fatigue life； FCBGA； finite element method； Taguchi method； unleaded solder； void

參考文獻

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