| 摘要: | 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. |
