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研究生:高慧茹
研究生(外文):Hui-Ju Kao
論文名稱:錫表面處理層之銅含量對錫鬚生長及介面反應之影響
論文名稱(外文):Effect of Cu additives on Sn whisker formation and interfacial reaction of Sn(Cu) finishes
指導教授:劉正毓
指導教授(外文):Cheng-Yi Liu
學位類別:碩士
校院名稱:國立中央大學
系所名稱:化學工程與材料工程研究所
學門:工程學門
學類:化學工程學類
論文種類:學術論文
論文出版年:2005
畢業學年度:93
語文別:英文
論文頁數:64
中文關鍵詞:錫鬚無鉛銲料無電鍍鎳磷墊層錫銅合金
外文關鍵詞:Sn whiskerPb-free solderNi(P)Sn(Cu) alloy
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針對被動元件而言,高可靠度銲點必須符合下列的條件 (1) 在兩端點的表面金屬鍍層要與銲料有良好濕潤性(2) 需要有適當的介面反應發生 (3) 避免錫鬚生長。所以本研究的重點在溼潤性,錫鬚生長機制,以及在介面反應上。在溼潤性測試方面,發現隨著廻銲時間延長及增加基板上Sn表面處理層的厚度,皆可提高濕潤性。
在Sn(Cu)表面處理之錫鬚生長有下列探討 (1) 表面處理層厚度效應:以Sn 及Sn0.7Cu 來說,錫鬚密度對表面處理層厚度呈ㄧ線性關係。Sn 及Sn0.7Cu 的安全厚度(即擁有最少的錫鬚數量),分別是10 µm及20 µm。 (2) 合金效應:發現隨著銅含量增加,有緩和錫鬚生長的效果。總結添加銅含量於錫表面處理層會降低錫鬚生長的驅動力原因為銅金屬會對金屬阻障層的溶解與熱應力有很大的影響。而銅含量會自生一銅錫化合物阻障層,可有效阻止金屬墊層的擴散及互相反應。同時,錫銅化合物析出在表面處理層是造成錫銅合金之熱膨脹係數(CTE)較小的主要原因。熱膨脹係數小即表示較低表面處理層中的熱應力。
無電鍍鎳磷層結構被廣泛應用在覆晶(Flip-Chip)或球矩陣列(BGA)封裝結構的多層金屬結構中。無電鍍鎳磷層與含銅之無鉛銲料發生介面反應且會對於銲點強度造成影響。對Sn 來說,Ni3P 結晶層及Ni-Sn 化合物會快速生成於介面上。由固態反應得知,Ni-Sn 化合物生長為一擴散控制程序,其生長活化能為42 KJ/mol。而對Sn0.7Cu而言,在初期固態反應中其表面呈現針狀形態隨著時間加常會轉變成層狀的(Ni, Cu)3Sn4 及(Cu, Ni)6Sn5之混合相。在Sn3.0Cu 系統中,Ni(P)表面會快速生成一層較厚的Cu-Sn 化合物,阻止了Ni-Sn 化合物的生
成而產生一非連續的Cu-Sn/Ni(P)界面。由銲點剪力測試發現在固態反應中,Sn0.7Cu 界面所生成的混合相有較佳的機械強度。相對的,Sn3.0Cu 中非連續的Cu-Sn/Ni(P)介面造成較差的銲點強度。因此添加適量的銅含量(0.7 wt %)於錫銲料中會增加介面銲點強度。
A high reliability solder joints for passive devices must meet the following conditions: (1) Good wettability on the surface of the metal plating of terminations (2) Appropriate interfacial reaction (3) No Sn whisker formation. So, the focuses on this study are wettability, Sn whisker formation mechanism, and interfacial reaction. Our results indicate that we can increase wettability by prolonging the reflowing time and increasing the thickness of Sn finish layer on the substrate.
Sn whiskers formation on Sn(Cu) finishes has been studied. (1) Thickness effect: Sn whisker density for pure Sn and Sn0.7Cu finishes has a linear relationship with the finish thickness. The safety thickness, i.e., with small Sn whisker number, for Sn and Sn0.7Cu finishes are about 10 and 20 µm, respectively. (2) Alloying effect: Sn whiskers formation was found to be retarded by increasing Cu content in Sn(Cu) finishes. We conclude that the Cu additives could reduce the two major driving forces of the Sn whisker formation, i.e., metal under-layer dissolution and thermal stress. The Cu additives formed a self-formed Cu-Sn compound barrier layer, which effectively prevented the reaction and dissolution with the metal under-layer. On the other hands, the Cu additives precipitated out as Cu-Sn compound in the Sn(Cu) finish layer, which is believe to be the reason for smaller CTE values of Sn(Cu) alloys. The smaller CTE values results in a lower thermal stress level in the Sn(Cu) finishes.
Electroless Ni(P) substrate was extensively used as the multi-layer metallization pad for Flip-Chip and ball-grid array (BGA) solder bumps. The correlation exists between the interfacial reaction and mechanical strengths of Sn(Cu)/Ni(P) solder bumps. For pure Sn, Ni3P layer and Ni-Sn compound formed more rapidly than Sn0.7Cu and Sn3.0Cu. Upon Sn/Ni(P) solid-state aging, a diffusion controlled process was observed and the activation energy is 42 KJ/mol. For Sn0.7Cu, the morphology of the interfacial Ni-Sn compound is needle-like at the initial aging, and then transformed to the mixture of Ni-Sn and Cu-Sn compounds which is layer-like shape afterwards. For Sn3.0Cu case, the Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu bumps is relatively weak and exhibits poor resistance against the aging process. So the Cu additives in Sn(Cu) solder (0.7 wt%) can increase the interfacial strength of solder bumps in the solid-state reaction.
Chinese Abstract.............................................................Ⅰ
Abstract.....................................................................Ⅲ
List of Figures..............................................................Ⅶ
List of Tables..............................................................ⅩI

CHAPTER 1. Introduction.......................................................1
CHAPTER 2. Literature review..................................................5
2.1 Introduction of Sn whisker................................................5
2.2 Calculation of the diameter of Sn whisker................................10
2.3 Theory of Sn whisker formation...........................................10
2.4 The failure models of Sn whisker formation...............................11
2.5 Driving forces of Sn whisker formation...................................12
2.6 Mitigation methods.......................................................18
CHAPTER 3. Experimental procedures...........................................20
3.1 Wettability of Sn on Ni metal finish.....................................20
3.2 Sn whisker formation on Ni metal finish..................................22
3.2.1 Metal finish substrate preparation.....................................22
3.2.2 Sn(Cu) finish layers preparation.......................................24
3.3 Interfacial reaction between Sn(Cu)/Ni(P) solder bumps...................26
CHAPTER 4. Results and Discussions ..........................................28
4.1 Wettability..............................................................28
4.2 Sn whisker formation.....................................................33
4.2.1 Morphology of Sn whiskers..............................................33
4.2.2 Thickness effect.......................................................38
4.2.3 Driving forces.........................................................40
4.2.3.1 Interfacial reaction effect..........................................40
4.2.3.2 Thermal stress in the finish layer...................................43
4.2.4 Alloying effect .......................................................48
4.3 Correlation between interfacial reactions and mechanical strengths of
Sn(Cu)/Ni(P) solder bumps....................................................49
4.3.1 Sn/Ni(P) solid-state interfacial reaction..............................49
4.3.2 Sn0.7Cu/Ni(P) solid-state interfacial reaction.........................52
4.3.3 Sn3.0Cu/Ni(P) solid-state interfacial reaction.........................54
4.3.4 Effect of the aging process on the strength of the solder joint........55
CHAPTER 5. Conclusions.......................................................58
References...................................................................61

List of Figures

Fig. 2-1 (a) Microstructure of the pure Sn (b) Hillock formation of the pure Sn after 40 h current stressing at the ambient temperature....................6
Fig. 2-2 Whisker growth of pure Sn after (a) 20 h, (b) 40 h, (c) 60 h, (d)80 h of current stressing..........................................................7
Fig. 2-3 The morphology of Sn whiskers (a) Nodules (b) Kinks and striations (c) Solid and striations (d) Filament and straight (e) Pyramidal shape (f) Irregular shaped whisker tip..................................................9
Fig. 2-4 The failure model of Sn whisker formation...........................12
Fig. 2-5 (a) The whisker is indicated by an arrow. Many grain boundary precipitates can be seen and they have been identified as Cu6Sn5 for the eutectic SnCu finish. (b) Very few grain boundary precipitates of Cu6Sn5 can be observed for pure Sn finish...............................................14
Fig. 3-1 The typical metal layer cross-sectional structure in terminals of the array R-Chip.........................................................................21
Fig. 3-2 The set-up structure of steam aging. And the red solid rectangle is cooling cycle system.......................................................................22
Fig. 3-3 The cross-sectional structure of MLCC...............................23
Fig. 3-4 Sn(Cu)/Ni/Ag metallization was electroplated on square BaTiO3 substrates sequentially (studied samples)....................................24
Fig. 3-5 The structure of the pad on the substrate is Au/Ni(P)/Cu. Their thickness was 600 Å and 5 µm, respectively. The concentration of P in Ni(P) is about 10 at. %............................................................27
Fig. 4-1 The tin (10 µm) layer was coated on Ni/brass substrate. The wetting behavior is better with increasing the reflowing time........................28
Fig. 4-2 The tin (5 µm) layer was coated on Ni/brass substrate. The wetting behavior is better with increasing the reflowing time........................29
Fig. 4-3 The tin (3 µm) layer was coated on Ni/brass substrate. The wetting behavior is better with increasing the reflowing time........................29
Fig. 4-4 The relationship between the contact angle and the reflowing time at different thickness of Sn plating............................................30
Fig. 4-5 The net-force describes the wettability behavior....................31
Fig. 4-6 (a) Without steam aging (b) 16 hours steam aging. All samples reflowed for 1 minute on the Sn/Ni/brass substrate...........................32
Fig. 4-7 SEM top view images of Sn whiskers on Sn(Cu) finishes. Both “Filament-like” and “Nodule-like” Sn whiskers coexist on the Sn finish. For Sn0.7Cu finish, Sn whiskers show “Nodule-like” shape. Chunky “Nodule-like” Sn whiskers are dominant on higher Cu content finish..................34
Fig. 4.8 During annealing, grains proceeded recovery, recrystallization, grain growth stages.......................................................................36
Fig. 4-9 The growth of new Sn grains would create a high pressure and eventually break the surface oxide to extrude out as Sn whiskers. At constant properties of the surface oxide (thickness, strength, and phase), the diameter of Sn whisker should be controlled...........................................37
Fig. 4-10 The relationship between the thickness of finish layer and the Sn whisker density for different Sn(Cu) alloys..................................40

Fig. 4-11 The SEM cross-sectional pictures of Sn(Cu) finish layers reacting with Ni under-layer .......................................................43
Fig. 4-12 Upon heating, the sample would bend, and the average length of finish layer is the sum of the length of substrate L0+(L0�炒S�炒GT), and ∆d due to bending curvature.........................................................46
Fig. 4-13 The order of Sn whisker density is Sn > Sn0.7Cu > Sn1.8Cu > Sn3.0Cu at 2~3 µm of the finish layer...............................................49
Fig. 4-14 The SEM cross-sectional images show the interfacial reaction between Sn and Ni(P) substrate for the different aging time at 150 ℃...............50
Fig. 4-15 The plot shows that the relationship exists between the Ni3Sn4 compound thickness and the square root of the aging time for pure Sn under three different aging temperature............................................51
Fig. 4-16 Arrhenius plot for the formation of Ni3Sn4 at the pure Sn/Ni(P) interface....................................................................51
Fig. 4-17 The SEM cross-sectional images show the interfacial reaction between Sn0.7Cu and Ni(P) substrate for the different aging time at 150 ℃...........................................................................53
Fig. 4-18 The SEM cross-sectional images show the interfacial reaction between Sn0.7Cu and Ni(P) substrate for the different aging time at 200 ℃...53
Fig. 4-19 The SEM cross-sectional images show the interfacial reaction between Sn3.0Cu and Ni(P) substrate for the different aging time at 150 ℃...........55
Fig. 4-20 Shear strength of Sn(Cu) solder bumps versus the aging time at 200 ℃...........................................................................57
Fig. 4-21 Cross-sectional SEM image of a half-fractured Sn3.0Cu solder bump.........................................................................57

List of Tables

Table 2-1 The diameter of Sn whisker at the different experimental conditions....................................................................8
Table 4-1 The value of CTE for different bulk solders at the range of 40 ~ 80 ℃...........................................................................47
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