| 摘要: | 摘 要 在覆晶式(Flip Chip)晶片接合封裝技術中，一個銲錫凸塊往往包含兩個不同的金屬化墊層(Under Bump Metallization)結構。例如，最初在IBM的C4(Controlled Collapse Chip Connections)結構中，晶片端的UBM為Au/Cu/Cr，而在基板端的UBM為Au/Ni。但大部分研究至目前為止，都集中在單邊銲錫/UBM界面的反應。當兩種完全不同的UBM結構，同時與銲錫發生界面反應時，其如何的交互影響將會非常重要且有趣。 在本論文的第一部份，即是探討Ni-Sn3.5Ag-Cu三明治結構的界面反應。結果發現在30秒的反應時間下，有一連續Cu-Sn介金屬生成於Ni端，進一步分析其生成機制。首先，在一分鐘的反應時間下，快速的Cu-Sn介金屬生成於Ni端，是由於Cu原子經由Cu端的Cu-Sn介金屬擴散所致。而在一分鐘之後，Ni端的Cu-Sn介金屬生成速率減緩則是與Ni原子擴散進入Cu-Sn介金屬有關，此一Cu-Sn介金屬生成於Ni端防止了Ni墊層與Sn的直接反應。我們發現到Cu原子自Cu端經由液態Sn擴散至Ni端的驅動力是由於一Cu的溶解度梯度所造成。因Ni端Cu-Sn介金屬與液態Sn界面的些微Ni原子使得Cu在Sn中的溶解度降低。由實驗結果計算Cu原子至Ni端的流通量並假設其於液態Sn中的擴散依循Fick’s first Law，求得此一Cu原子的擴散係數為10-5cm2/s。 Spalling的現象在覆晶封裝中一直是可靠度上的問題。在第二部份裡，我們設計一Ni thin film/Sn3.5Ag/Cu的三明治結構。於迴銲的反應過程中，因Cu原子自Cu端擴散進入液態Sn，並反應生成Cu-Sn介金屬沉積於Ni薄膜上。此一介金屬的反應阻障層有效的減緩Ni薄膜的消耗，延遲或避免了Spalling現象的發生。在較長的反應時間下，Ni原子緩慢擴散進入Cu-Sn介金屬而相轉變成Cu-Ni-Sn介金屬，最終產生一Cu-Ni-Sn/Cr的穩定界面，避免了Spalling現象的發生。另外，在進一步的研究中發現，不同Cu含量的Sn(Cu)銲料與Ni薄膜反應，其些微Cu含量的變化對於Spalling現象的產生有著顯著的影響。當Cu含量增加至1.0 wt.%，有一Cu-Sn介金屬化合物生成於Ni薄膜上，且反應時間至20分鐘尚無Spalling的現象發生。此一防止機制為Cu原子來自液態Sn中的流通量填補了界面半圓狀介金屬間的擴散通量。先前的研究已報導Cu原子擴散的驅動力是由於些微的Ni原子溶解進入液態Sn中，使得Ni薄膜端的Cu溶解度降低。而此Cu原子的流通量經由計算，其流通量與界面半圓狀介金屬間的擴散通量相當，此一結果確定了Spalling延遲發生的機制。特別的是，當Cu含量超過1.0 wt.%，快速且大量的Cu-Sn介金屬生成於Ni薄膜上，使得Ni原子毫無機會經由Cu-Sn介金屬擴散進入液態Sn中，造成Cu溶解度梯度的消失。取而代之的是半圓狀介金屬間的擴散通量為主導，進而加速了Spalling現象的發生。 在第三部份的研究中，我們將不同Cu含量的Sn(Cu)銲料與Ni(P)作反應，並將所得結果近一步與純Sn銲料/Ni(P)反應互相比較。結晶化的Ni3P生成將大幅降低銲錫接點的強度。在銲料中添加Cu可有效防止Ni3P的生成，Ni3P的緩慢生成是由於Cu-Sn介金屬生成在Ni(P)進而減緩了Ni自Ni(P)擴散進入銲料。隨著銲料中Cu含量的增加，我們發現明顯減緩了Ni3P的生成。 最後一部份我們將研究不同Cu含量的Sn(Cu)銲料與Au箔及預鍍Ni薄膜之Au箔的界面反應。當Au與以Sn為主的銲料進行界面反應，Au會快速的溶解進入液態Sn中，並在銲錫及界面生成大量的Au-Sn介金屬，而於銲點中產生一脆性接點。但當我們將預鍍Ni薄膜之Au箔與Cu含量超過1.8 wt.%以上之Sn(Cu)銲料進行反應，發現有一層狀的(Cu,Ni)6Sn5介金屬於界面生成，此一介金屬有效的阻絕了Au與Sn間的反應。另一方面，在Au箔與不同Cu含量之Sn(Cu)銲料反應中，Au的消耗因Cu含量的不同而有所變化，我們發現Cu含量在0.7-1.8 wt%其Au的消耗速率最小。 ABSTRACT In the first part of this thesis, the interaction between Cu-Sn and Ni-Sn interfacial reactions in a soldering system has been studied by using a Ni-Sn3.5Ag-Cu sandwich structure. A layer of Cu-Sn intermetallic compound was observed at the interface of the Ni foil after 30 seconds reflowing. Two stages of the Cu-Sn compound growth on the Ni side were observed: (1) in the first minute of reflow, the fast Cu-Sn compound formation was rate limited by Cu diffusivity in the Cu-Sn compound layer of the opposite Cu side. (2) after one minute of reflow, the Cu-Sn compound growth was very sluggish and depended on the Ni diffusion in the Cu-Sn compound of the Ni side. Very little Ni can be detected in the Cu side. This implies that Cu diffused and dissolved in the molten Sn3.5Ag solder much faster than Ni. When the dissolved Cu arrived at the interface of the Ni foil, a Cu-Sn compound layer formed on the Ni interface to prevent the Ni foil from reacting with solder. The driving force of the dissolved Cu atoms toward the Ni side attributed to the Cu solubility difference across the molten solder was established due to the reduction of the Cu solubility near the Ni interface. The reduction of Cu solubility was caused by the presence of dissolved Ni near the Ni interface. Knowing the experimental value of the Cu flux toward the Ni side and assuming the diffusion of Cu atoms in the molten solder following Fick’s first Law, the diffusivity of Cu is found to be 10-5 cm2/s. Spalling phenomenon on UBM (Under Bump Metallization) is one of the current urgent reliability issues for the Pb-free solder implementation in flip chip technology. In the second part, we report that spalling of Ni thin UBM can be prevented during the soldering reaction, if a Cu reservoir is introduced into the structure of C4 (Controlled Collapse Chip Connections) solder joints. Once molten Sn3.5Ag solder was saturated with Cu atoms, Cu precipitated out as a layer of Cu-Sn compound on Ni thin UBM. Cu-Sn compound layer served as a reaction barrier to retard the consumption of Ni thin UBM. So, spalling is retarded. After prolonged reflowing, Ni thin UBM was converted to ternary Cu-Sn-Ni compounds. Unlike interfaces of Ni-Sn compound/Cr, interface of Cu-Sn-Ni compound/Cr is very stable and no spalling was found. Furthermore, the effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni thin film and Sn(Cu) alloys has investigated. We have found that the variation of Cu content has a strong influence on the spalling of the Ni thin film. With small Cu additives in the Sn, spalling was deferred to longer reflowing time. When the Cu content increased to about 1.0wt.%, a layer of Cu-Sn compound formed on the Ni thin film and no spalling was observed after 20 minutes reflowing. The possible mechanism of spalling deferring is proposed. A Cu flux from the solder to the interface compensated the ripening flux of the semi-spherical compound grains, therefore, spalling was retarded. The driving force of the Cu flux was attributed to the reduction of Cu solubility due to the presence of Ni at the interface of the Ni thin film. The Cu flux from solder to the interface is calculated to be in the same order with the ripening flux of Cu6Sn5 compound grains, which confirms the proposed mechanism of spalling deferring. For the Sn(Cu) alloys having Cu content over 1.0wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial compound layer was free of Ni. There was no Cu flux to compensate the ripening flux, therefore, the ripening flux dominated and spalling occurred after short reflowing time. In the third part, We have studied interfacial reactions between Sn(Cu) alloys and Ni(P) substrates. Comparing to Sn/Ni(P), the formation of Ni3P layers in Sn(Cu)/Ni(P) reaction couples were very limited. The sluggish growth of Ni3P layer attributed to a layer of Cu-Sn compound layer formed on the Ni(P) substrate, which effectively isolated the Ni(P) substrate from reacting with solder. The eventual formation of Ni3P compound layer depended on the Ni diffusion in the Cu-Sn compound. Also, we found that the higher Cu-content Sn(Cu) alloys had less Ni3P formation. We have investigated interfacial reactions between Sn(Cu) alloys with Au and Ni-coated Au foils in the fourth part. As Au foils reacted with Sn-based alloys, Au foil quickly dissolved into the molten solder and form large amount of Au-Sn compound in the solder and compound layer at the interface. As the Au coated with a thin Ni layer reacted with the Cu content over 1.8 wt.%, a layer of ternary (Cu,Ni)6Sn5 compound was observed at the interface between Sn(Cu) alloys and Au foils. This ternary compound layer effectively isolated the Au foil from reacting with the molten solder. Also, as Au foil reacted with Sn(Cu) alloys. The Au consumption rate depended on the Cu content. In the middle Cu content region, we found that it has the least Au consumption rate. |
