- 學位論文
高導電銀鈀合金線及其在IC與LED封裝之應用
High Conductive Ag-Pd alloy Bonding Wires and their Applications in IC and LED Packages
摘要
基於高導電性及低成本需求，研究Ag-Pd二元銀鈀合金線以取代先前開發完成之Ag-8Au-3Pd三元銀金鈀合金線，一典型的Ag-4Pd合金線具有3.5 μΩ cm的電阻率，與傳統3N 純金線(3.2 μΩ cm)及鍍鈀銅線(1.8 μΩ cm)相當接近；為了進一步改善此Ag-Pd合金線的性能，利用先前在Ag-8Au-3Pd合金線所開發之創新多重退火製程，使其晶粒結構含有大量退火孿晶。 此創新多重退火製程所得到的含大量退火孿晶Ag-Pd二元銀合金線類似於過去的Ag-8Au-3Pd合金線均展現優異的高溫晶粒結構熱穩定性，有別於傳統金線在高溫所呈現晶粒快速成長現象，此特殊Ag-Pd銀合金線的大量退火孿晶亦造就其在通電流試驗時相較於傳統金線呈現較佳斷裂荷重與延伸率，而鍍鈀銅線在通1.23x105 A/cm2電流試驗1小時後已經嚴重氧化，且機械性質大幅劣化。本研究針對通電流1.23x105 A/cm2對於各種Ag-Pd銀合金線的晶粒成長、機械性質及退火孿晶比例進行有系統的研究，同時與Ag-8Au-3Pd、純金線及鍍鈀銅線比較。 關於銀合金線的電遷移機理，實驗發現各種Ag-Pd銀合金線及其他對比銲線(Ag-8Au-3Pd、Au、Pd-coated Cu)通不同電流密度試驗後，其平均失效時間(mean-time-to-failure)依序為：Ag＞Ag-0.5Pd＞Ag-3Pd＞Ag-4Pd＞Ag-8Au-3Pd~Au＞Pd-coated Cu，顯然這些線材的電遷移耐久性與其導電性及通電流造成的焦耳效應密切關連。動力學的分析可歸納出一個與電流密度及活化能相關的銲線通電流的破損模式: MTF-1 = A/πr2．J2．exp(-Q/kT)。 此外，本研究亦利用水滴試驗進行Ag-Pd合金線材進行離子遷移評估，研究發現銀線離子遷移只有在含水條件才會發生，添加1.5%~4%鈀 可以有效降低離子遷移，若再加入8%金於Ag-Pd合金線材更能強化抗離子遷移的能力，另外實驗也發現添加Ni與Pt於銀合金線中對其抗離子遷移並沒有顯著的效果。進一步將Ag-Pd合金線材封膠並置於水中以及空氣下通電流1000小時，觀察並沒有任何銀鬚產生，藉由本研究結果建議：添加Au於Ag-Pd合金線材或適當封膠製程均可以有效抑制銀合金線的離子遷移。 銀合金線材在3.5% NaCl水溶液的電化學腐蝕實驗顯示：隨著Pd 元素的增加，有較低的腐蝕電位及較高的孔蝕電位與較大的鈍態區，其抗腐蝕電流密度的順序為Ag-6Pd＜Ag-4.5Pd＜Ag-4Pd＜Ag-3Pd＜Ag-2Pd＜Ag-0.5Pd＜Ag，此研究證實添加Pd可以有效提昇Ag-Pd合金線的抗腐蝕性。 最後就產品應用，Ag-Pd合金線材產品不僅在一般IC與LED產品取得驗證，對於高階IC封裝的疊球打線及高階LED封裝的BSOS與BBOS打線亦同樣適用，此外含大量退火孿晶Ag-Pd合金線亦保證可應用於一些特殊打線技術，例如：超低線弧打線、短線過頂打線、長線伸打線、多重打線及反轉打線等；此外，銀合金線材以銲球凸塊(stud bump) 形式應用於覆晶封裝、3D IC 封裝及晶圓級封裝時，亦較傳統金銲球凸塊具有許多競爭優勢。
並列摘要
Binary Ag-Pd alloy wires have been produced as an alternative to a previously developed ternary Ag-8Au-3Pd alloy wire to meet requirements for high electrical conductivity and low cost. A typical Ag-4Pd bonding wire reveals an electrical resistivity of 3.5 μΩ cm, close to the values of traditional 3N Au wire (3.2 μΩ cm) and Pd coated Cu wire (1.8 μΩ cm). To further improve the performance of such Ag-Pd alloy wires, a large amount of annealing twins were introduced in the grain structure of these Ag-Pd alloy wires through an innovative concept of sequential drawing and multiple annealing processes developed in the previous Ag-8Au-3Pd bonding wire. Such annealing twinned Ag-Pd alloy wires are similar to the previous Ag-8Au-3Pd wire to possess high stability of grain structure during electrical stressing, in contrast to the rapid grain growth in the traditional pure Au wire. The enrichment of annealing twins in these Ag-Pd alloy wires also results in higher breaking load and elongation in comparison to those of pure Au wire after current stressing. In this case, Pd-coated Cu wire oxidized severely after current stressing with 1.23x105 A/cm2 for only 1 hr, which caused a drastic degradation of its mechanical properties. The effects of current stressing on the grain growth, mechanical properties and annealing twinned grain percentage of various Ag-Pd alloy wires in comparison to Ag-15Au-3Pd, Au and Pd-coated Cu bonding wires were systematically investigated. Concerning with the mechanism of electromigration for Ag-alloy bonding wires, Ag-Pd alloy wires were stressed with various current densities. For comparison, Ag-8Au-3Pd, Au and Pd-coated Cu bonding wires were also included. The results indicated that the sequence of mean-time-to-failure against current stressing for various Ag-Pd alloy wires in comparison to Ag-8Au-3Pd, Au and Pd-coated Cu wires is shown as follows: Ag＞Ag-0.5Pd＞Ag-3Pd＞Ag-4Pd＞Ag-8Au-3Pd∼Au＞Pd-coated Cu. It is evidenced that the durability to electromigration for these bonding wires is strongly dependent on their electrical resistivity, and wire temperature increases during current stressing due to the Joule effect. With kinetics analysis, a failure mode has been concluded in correlation to the current density and activation energy as: MTF-1 = A/πr2．J2．exp(-Q/kT). In addition, electrolytic migration in Ag-Pd alloy wires was also evaluated with water drop tests. The results indicated that water must be present between the wire couple for Ag-ion migration to occur. The addition of 1.5 to 4% Pd decreased the ion migration rate obviously. Further alloying with about 8% Au into the Ag-Pd wires enhanced this effect. It seems that the addition of Ni and Pt in Ag-Pd alloy wires does not influence Ag-ion migration. Ag-Pd alloy wires sealed with silicone gel and stressed in air and in distilled water revealed no dendrites after more than 1,000 hr. It is suggested that the electrolytic migration in Ag-Pd alloy bonding wires can be alleviated by Au addition and completely inhibited by a suitable encapsulation process with an appropriate molding compound. The corrosion behaviors of various Ag-Pd alloy wires in 3.5 % NaCl solution were also studied with the electrochemical tests. The results indicated that the corrosion potential, the pitting potential and passive range of various Ag-Pd alloy wires increases with the increase of Pd contents. The sequence of corrosion current density is as: Ag-6Pd＜Ag-4.5Pd＜Ag-4Pd＜Ag-3Pd＜Ag-2Pd＜Ag-0.5Pd＜Ag. It is obvious that the addition of Pd element can effectively enhance the corrosion resistance of Ag-Pd alloy wires. Finally, in terms of applications, the Ag-Pd alloy wires have been verified not only in general IC and LED products but also in advanced packaging processes such as the stacking ball bond, bonding ball on stitch (BBOS), and bonding stitch on ball (BSOB) in IC and LED packages. In addition, the annealing twinned grain structure in these Ag-Pd alloy wires ensures their application in many special wire bonding methods, such as ultra-low loop wire bonding, short wire overhand, long wire span, multi-tier loop, and reverse bond. The Ag-alloy wires also possess advantages over traditional Au stud bumps that make them applicable to stud bumping in flip chip assembly, 3D-IC package and wafer level package.