覆晶(Flip-Chip)銲接結構中，銲料合金通常連結晶片與基板的鎳與銅基凸塊底層金屬。其中，銲料合金與凸塊底層金屬材料的選擇是影響銲接點微結構、相生成與可靠度的重要議題。在此研究中，錫基無鉛銲料接點界面反應、結構變化與衝擊可靠度將分成六個主題來深入探討。
(1) 在Ni/Sn-Ag-Cu/Cu覆晶結構中，探討鎳添加對微結構變化與銲接界面反應。銲接結構迴銲後於150 oC下時效熱處理，Ni與Cu元素在銅與鎳凸塊底層金屬間交互作用，Ni往銅端擴散，Cu往鎳端遷移，影響兩端的界面反應，兩種不同鎳含量的(Cu,Ni)6Sn5介金屬化合物生成於鎳端界面。相對地，銅端凸塊底層金屬之界面只有一層厚的低鎳含量(Cu,Ni)6Sn5與一層薄(Cu,Ni)3Sn生成。當微量鎳添加於銲料合金中，其接點微結構與相生成因此改變。熱處理對於Ni與Cu元素的重新分布關聯著接合界面(Cu,Ni)6Sn5介金屬化合物的生成機制。
(2) 高低鎳含量(Cu,Ni)6Sn5介金屬化合物的生成對Sn-Ag-Cu/Ni銲點衝擊可靠度是關鍵影響因素。藉由高速剪應力衝擊測試，破壞形貌分析顯示高與低鎳含量(Cu,Ni)6Sn5的雙相界面促使裂痕易成核於介金屬化合物內。裂痕的延伸與介金屬化合物的韌性有一定程度的關係。壓痕測試結果指出：高鎳含量(Cu,Ni)6Sn5相對於低鎳含量(Cu,Ni)6Sn5有較低的韌性，使裂痕易延著高鎳含量(Cu,Ni)6Sn5延伸。
(3) 比起solder/Ni接合界面，solder/Cu接合界面有較厚的介金屬化合物伴隨許多孔洞的生成。發展新的凸塊底層金屬以取代舊有的銅凸塊底層金屬係值得考量的選項。以銅鋅合金作為新穎凸塊底層金屬材料並探討其與純錫間之界面反應。經實驗後發現銲料與銅鋅合金接點處銅錫介金屬化合物的成長與孔洞明顯被抑制。藉由穿透式電子顯微鏡與場發射式電子微探儀觀察及分析發現CuZn與Cu-Zn-Sn兩種富鋅相生成，其相生成機制可由熱力學與動力學之觀點來探討。
(4) 在迴銲的過程，Cu與Zn從銅鋅凸塊底層金屬溶入熔融銲料中，進而影響銲料組成與微觀結構。探討銲料與銅鋅凸塊底層金屬間液態反應係重要課題。與solder/Cu銲接點相比，solder/Cu-Zn銲接點有較粗大的共晶結構與較小的Cu6Sn5析出於銲料內，接合界面上相生成也因Zn元素的溶入有所影響。研究證實，在solder/Cu-Zn銲接點中「迴銲時間」與「鋅於凸塊底層金屬內濃度」控制著接點微結構變化與相的演進。
(5) 界面介金屬化合物的性質通常影響銲點可靠度。 Cu6Sn5是錫基銲料與銅凸塊底層金屬主要的界面化合物，其晶體結構隨溫度不同而改變。經250 oC高溫迴銲，Cu6Sn5呈現六方晶結構(η相)。於150 oC熱處理後，Cu6Sn5轉變以單斜晶結構(η’相)存在。在銅鋅凸塊底層金屬的銲接點內，Zn溶入Cu6Sn5內生成Cu6(Sn,Zn)5介金屬化合物。利用X光繞射儀鑑定與熱叉分析儀分析，斷定Cu6(Sn,Zn)5相為一個穩定的六方晶結構，且不隨溫度產生相變化。熱力學計算也證明鋅的溶入穩定Cu6(Sn,Zn)5的生成。
(6) 評估銅鋅凸塊底層金屬應用於Ni/solder/Cu-Zn覆晶結構的可行性。比較Ni/solder/Cu銲接點，Ni/solder/Cu-Zn有較薄的介金屬化合物，界面上的介金屬化合物隨熱處理時間增加緩慢成長，界面上也無任何孔洞生成。元素於鎳端與銅鋅端間的交互作用行為也因為鋅溶入銲料內而被抑制。Ni/solder/Cu-Zn銲接結構優異的熱穩定性主要歸因於鋅元素的分布阻擋了其他元素間的劇烈反應。
綜上所述，新穎的銅鋅凸塊底層金屬顯現出許多接合上的優勢，包括：(一)減少銅錫介金屬化合物生成、(二)抑制界面孔洞產生、(三)生成穩定的Cu6(Sn,Zn)5介金屬化合物、(四)阻止元素的交互作用於Ni/solder/Cu-Zn中。由此研究得知銅鋅合金是具有潛力且可用於先進封裝製程的凸塊底層金屬材料。

In the flip-chip (FC) solder joint, solder alloys usually connect with Ni and Cu based under bump metallurgies (UBMs) at chip-side and substrate-side, respectively. The material selection for solder alloys and UBM material is a critical issue to affect the microstructure, phase formation, and the reliability of the solder joints. In this study, the interfacial reaction, microstructure variation, and impact reliability of Sn-based Pb-free solder joints with Ni, Cu, and novel Cu-Zn UBMs are discussed and categorized into six topics.
(1) Microstructural variation and interfacial reaction in Ni/Sn-Ag-Cu/Ni assemblies with and without Ni doping
Ni and Cu elements cross-interacted between the Cu and Ni UBMs during thermal aging and affected the interfacial reactions at both Cu and Ni sides. Dual-phased (Cu,Ni)6Sn5 intermetallic compounds (IMCs), which has high and low Ni concentration, formed at the solder/Ni interface. In contrast, thicker low-Ni (Cu,Ni)6Sn5 and thin (Cu,Ni)3Sn layers formed at the Cu side. The Ni doping into solder varied the microstructure of solder alloys and the formation of interfacial IMCs. The re-distribution of Ni and Cu was correlated to the formation mechanism of interfacial (Cu,Ni)6Sn5.
(2) Effect of dual-phased (Cu,Ni)6Sn5 IMCs on the impact reliability of
Sn-Ag-Cu/Ni solder joints
By employing the high-speed shear impact test, the impact fracture morphology reveals that the interface of high-Ni (H)/low-Ni (L) (Cu,Ni)6Sn5 facilitates the crack nucleation within the IMC. It is believed that the crack propagation depends on the fracture toughness of interfacial IMCs. The indentation data shows that bulk H-(Cu,Ni)6Sn5 exhibits distinctly lower fracture toughness than bulk L-(Cu,Ni)6Sn5. In correlating the impact fracture behavior and mechanical properties of two kinds of (Cu,Ni)6Sn5, cracks tend to propagate through H-(Cu,Ni)6Sn5 due to the relatively low fracture toughness of H-(Cu,Ni)6Sn5.
(3) Development of a novel Cu-Zn UBM for Pb-free solder joints
To suppress the thicker IMCs and voids at the solder/Cu interface, the Cu-Zn alloy was designed for a novel UBM material. The interfacial reactions of Sn/Cu-xZn (x = 0, 15 and 30 at.%) solder joints were investigated. Interestingly, the growth of Cu-Sn IMCs was significantly reduced and no void was found in the Sn/Cu-Zn solder joints after thermal aging. Transmission electron microscopy (TEM) images and the field emission electron probe microanalyzer (FE-EPMA) analysis show that there are two types of Zn-rich phases, i.e. CuZn and Cu-Zn-Sn phases, to form in Sn/Cu-Zn joints. The formation mechanisms of IMCs were probed and proposed with regard to the thermodynamics and kinetics.
(4) Liquid-state reaction of Sn-Ag-Cu solders and the novel Cu-Zn UBM
During the reflow process, Cu and Zn atoms would dissolve from the Cu-Zn UBM into the molten solders, leading to the variation of the composition in the solders. Then, the composition variation further altered the microstructure of the solders. In comparison with the Sn-Ag-Cu/Cu, it was found that the coarser eutectic region and smaller Cu6Sn5 IMCs inside the solder matrix of Sn-Ag-Cu/Cu-Zn. In addition, the interfacial reaction was also affected by Zn dissolution. In this study, it was demonstrated that the microstructural variation and the phase evolution in the solder joints were controlled by the reflow time and the Zn concentration in the Cu-Zn UBM.
(5) Characterization of the Cu6(Sn,Zn)5 intermetallic compound
Cu6Sn5 is a dominant IMC at the Sn-based solder/Cu joint interface. The crystal structure of Cu6Sn5 varies with temperature. After reflow at 250 oC, the interfacial Cu6Sn5 revealed hexagonal structure (η-Cu6Sn5). During aging at 150 oC, hexagonal η-Cu6Sn5 would transform into monoclinic η’-Cu6Sn5. According to literature, the phase transformation between η’ and η would induce crack easily propagating through the Cu6Sn5 at the solder joint interface. In the novel solder/Cu-Zn joints, and Zn would dissolve into Cu6Sn5 to form the Cu6(Sn,Zn)5 IMC at the interface. X-ray diffraction and differential scanning calorimetry analyses show that doping small amounts of Zn into Cu6(Sn,Zn)5 can stabilize the hexagonal structure during the thermal aging process. Thermodynamic calculation also demonstrates that Zn can stabilize the hexagonal Cu6(Sn,Zn)5.
(6) Application of Cu-Zn UBM on the Ni/Sn-Ag-Cu/Cu-Zn assemblies
The feasibility of novel Cu-Zn UBM applied for the Ni/solder/Cu-Zn assemblies was also evaluated. In comparison with the Ni/Sn-Ag-Cu/Cu solder joint, Ni/Sn-Ag-Cu/Cu-Zn solder joints revealed thinner Cu6Sn5-based IMCs at both Ni/Sn-Ag-Cu and Sn-Ag-Cu/Cu-Zn interfaces after aging. (Cu,Ni)6(Sn,Zn)5/(Cu,Ni)6Sn5 dual-phase formed at the Ni side while (Cu,Ni)6(Sn,Zn)5 single-phase at the Cu-Zn side. The interfacial IMCs grew very slowly, and no void formed in these Zn-contained solder joints during thermal aging. Additionally, the dissolved Zn in the solder alloy reduced the elemental cross-interaction between the Ni and Cu-Zn substrates. The noticeable thermal stability of Ni/Sn-Ag-Cu/Cu-Zn solder joints is attributed to the Zn re-distribution retarding the reaction of Ni, Cu and Sn. Phase formation and IMCs suppression mechanisms in Ni/Sn-Ag-Cu/Cu-Zn solder joints were probed and discussed.
In summary, novel Cu-Zn UBM shows lots of advantages for soldering, including: (I) reduction of Cu-Sn IMCs, (II) suppression of voids at the interface, (III) formation of the a hexagonal Cu6(Sn,Zn)5, and (IV) retardation of the elemental cross-interaction in the Ni/solder/Cu-Zn assemblies. The Cu-Zn alloys could be a potential UBM material for the advanced electronic packaging.

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