覆晶技術(FCT)搭配球腳格狀陣列(BGA)的連接方式已經被廣泛的使用在現今的微電子封裝技術中。其中，凸塊底層金屬(UBM)的材料選擇成為最重要的議題之一。鎳基的凸塊底層金屬除了可與銲錫潤濕外亦可作為阻擋銅金屬擴散的擴散阻絕層。因此，受到業界或學界的注目。本研究利用顯微分析與詳細的定量分析來探討銲錫與鎳/銅凸塊底層金屬間的元素分佈情形與相關的相變化行為。
在迴銲的過程中發現：銲接反應會加速銅原子的擴散與介金屬化合物(IMC)的生成。銅原子擴散的通量約為1015~1016 atoms/cm2s。在一次迴銲後，銅原子穿過鎳層並溶於Ni3Sn4中而形成(Ni1-x, Cux)3Sn4。在不同鎳厚度的試片中，所量測到(Ni1-x, Cux)3Sn4的組成均相當均勻。在三次以上的迴銲後，銅原子更進一步的穿過(Ni1-x, Cux)3Sn4。當銅在錫銅鎳合金中的濃度超過0.6 wt.%時，另一個扇貝狀的介金屬化合物(Cu1-y, Niy)6Sn5開始生成。(Cu1-y, Niy)6Sn5生成的量與鎳層的厚度和迴銲的次數有關。其中，所量測到的y值約維持在0.4左右。而x值卻從0.02變化到0.35。這些介面生成物的元素分佈情形可利用錫銅鎳三元相圖加以解釋。此外，(Cu1-y, Niy)6Sn5生成的相關機制也是探討的重點之一。
在經過的適當蝕刻與試片處理後，介金屬化合物Ni3Sn4和Cu6Sn5的型態可以明顯的顯現。實驗發現：此二種介金屬化合物具有截然不同的型態。另外，在一次迴銲後，在錫銀系統中生成的(Ni, Cu)3Sn4較錫鉛系統中的具有較大的晶粒，導致銅原子擴散的通道減少。因此，在錫銀系統中，僅偵測到一層較厚的(Ni, Cu)3Sn4，並沒有發現(Cu, Ni)6Sn5。
凸塊強度與銲錫凸塊邊緣的元素分佈有著密不可分的關係。因此，在銲錫凸塊邊緣的介金屬化合物生長情形也是研究的方向之一。迴銲的過程中，在鎳/銅凸塊底層金屬的側邊僅發現(Cu1-y ,Niy)6Sn5的生成。然而，在經過高溫熱儲藏(150C，1000小時)後，除了(Cu1-y, Niy)6Sn5外，還偵測到另一個介金屬化合物(Cu1-z, Niz)3Sn。(Cu1-z, Niz)3Sn的生成是由於銲錫凸塊邊緣的錫被耗盡所致。另一方面，由於(Cu1-y, Niy)6Sn5生長快速，因此甚至可以在銲錫與凸塊底層金屬的介面中偵測到它的存在。

Flip chip technology with BGA interconnection has attracted a great deal of attention in today’s electronics packaging. One of the challenging issues is the material selection for under bump metallization (UBM). The Ni-based UBM acts as a wetting layer of solder and also as a diffusion barrier for Cu metallization. In this study, the elemental distribution and related phase transformation between solders and Ni/Cu UBM were investigated with the aid of microstructure evolution and deliberately quantitative analysis by an electron probe microanalyzer.
The soldering-induced Cu diffusion and intermetallic compound (IMC) formation were revealed. The atomic flux of Cu diffusion during reflow was in the order of 1015~1016 atoms/cm2s. After one reflow, Cu atoms diffused through electroplated Ni and dissolved into the previous formed IMC Ni3Sn4 to produce another layered IMC (Ni1-x, Cux)3Sn4. The composition of (Ni1-x, Cux)3Sn4 IMC was homogeneous in each joint despite of different Ni thickness. After more than three times reflow, Cu atoms further diffused through the boundaries of (Ni1-x, Cux)3Sn4. As the concentration of Cu in Sn-Cu-Ni alloy is greater than 0.6 wt.%, another scalloped IMC (Cu1-y, Niy)6Sn5 would form. The amounts of (Cu1-y, Niy)6Sn5 IMC formation could be associated to the Ni thickness and reflow times. The values of y were evaluated to remain around 0.4, however, the values of x varied from 0.02 to 0.35. The elemental distribution of IMC in the interface of the joint assembly was further correlated to the Ni-Cu-Sn ternary equilibrium. In addition, the mechanism of (Cu1-y, Niy)6Sn5 formation was also probed.
Through appropriate etching and sample preparation, IMCs of Ni3Sn4 and Cu6Sn5 were revealed with distinct morphologies in a solder/Ni-Cu UBM joint assembly. The grain size of (Ni, Cu)3Sn4 IMC formed in the Sn-Ag system after one reflow was larger than that in the Sn-Pb system, thus the pathways for Cu diffusion were reduced. As a result, only thicker (Ni, Cu)3Sn4 IMC was observed, and no (Cu, Ni)6Sn5 IMC could be detected between SnAg solder and Ni.
The elemental redistribution in the edge of the solder bump is crucial for its correlation with the bump strength. Hence, the IMC formation in the edge of solder bump between UBM and eutectic Sn-Pb solder was also discussed. During reflows, only (Cu1-y ,Niy)6Sn5 IMC was observed in the sideway of Ni/Cu UBM. After high temperature storage (HTS) at 150C for 1000 hours, both (Cu1-y, Niy)6Sn5 and (Cu1-z, Niz)3Sn were found in the sideway of Ni/Cu UBM. The (Cu1-z, Niz)3Sn IMC formed since Sn atoms were almost exhausted near the sideway of Ni/Cu UBM. Moreover, (Cu1-y, Niy)6Sn5 IMC was visible even in the interface of solder and UBM after HTS test due to the fast growth rate.

List of Tables........................................................................................... III
Figures caption....................................................................................... IV
Abstract................................................................................................... IX
Chapter I Introduction.......................................................................... 1
1.1 Background................................................................................. 1
1.2 Motivations and Goals in This Study.......................................... 2
Chapter II Literature Review............................................................... 7
2.1 Electronic Package...................................................................... 7
2.2 Flip Chip Technology.................................................................. 8
2.3 Solder Bump................................................................................ 9
2.3.1 Solder Materials................................................................ 9
2.3.2 SnPb Solder....................................................................... 10
2.3.3 Lead-Free Solder............................................................... 12
2.4 Under Bump Metallization.......................................................... 13
2.4.1 Cu-Based UBM................................................................. 14
2.4.2 Ni-Based UBM.................................................................. 15
2.4.2.1 Electroplated Ni....................................................... 15
2.4.2.2 Electroless Ni-P....................................................... 15
2.5 Metallurgical Reactions in solder joints...................................... 16
2.5.1 Metallurgical Reactions between Solders and Cu-Based
UBM................................................................................. 16
2.5.2 Metallurgical Reactions between Solders and Ni-Based
UBM................................................................................. 18
Chapter III Experimental Procedure................................................... 45
3.1 Fabrication of Solder Bump........................................................ 45
3.2 Solder Reflow and Heat Treatment............................................. 45
3.3 Sample Preparation..................................................................... 46
3.4 Characterization and Analysis..................................................... 47
3.4.1 Microstructure Evolution.................................................. 47
3.4.2 Composition Analysis....................................................... 47
Chapter IV Results and Discussion...................................................... 53
4.1 Effects of Ni Thickness and Reflow Times on Interfacial Reactions
between Ni/Cu UBM and Eutectic Sn-Pb Solder.............................. 53
4.2 Phase Equilibrium between Ni/Cu UBM and Eutectic SnPb Flip
Chip Solder Bumps........................................................................... 64
4.3 Soldering-Induced Cu Diffusion and Intermetallic Compound
Formation between Ni/Cu UBM and SnPb Solder........................... 80
4.4 Metallurgical Reaction of Sn-3.5Ag Solder and Sn-37Pb Solder
with Ni/Cu UBM............................................................................... 99
4.5 Interfacial Reactions and Compound Formation in the Edge of
PbSn Flip Chip Solder Bumps on Ni/Cu UBM................................ 110
Chapter V Conclusions.......................................................................... 120
References............................................................................................... 123

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