在封裝製程中，擴散阻擋層能有效減緩銲錫劣化，提升接點壽命。在目前，主要以無電鍍鎳最為常見，因為在潤濕性、擴散阻擋能力以及成本上，鎳都有相當不錯的表現。然而在無電鍍鎳製程中有磷參雜，這會導致Ni3P的生成。Ni3P會導致接點脆化、劣化，使之失效。因此有人提出以鈷來作為取代，因為鈷具有鈷UBM錫間反應速率慢、高接點強度、良好的接點可靠度以及可接受的潤濕性。
另一方面，錫-3.5銀合金銲錫在隨著含鉛銲錫的禁用之後，在特性上除了熔點接近傳統錫鉛銲錫，也具有更佳的彈性係數，更小的熱膨脹係數，逐漸成為銲錫的主要選項之一。且相較於純錫，銀在銲錫中的參雜，能夠減緩介金屬化合物生成速率以及降低反應活化能。
本實驗討論無鉛錫-3.5銀銲錫介面，在150℃、170℃及190℃下固態時效處理後的變化行為。根據實驗結果，系統生成的介金屬化合物主要為CoSn3，但也有Ag3Sn以及介穩態的CoSn4的析出。在截面大多觀察到垂直表面生長的CoSn3，在銲錫與介金屬化合物接面上的CoSn3大多呈現破裂的片狀結構，Ag3Sn則有顆粒狀以及塊狀兩種形貌，CoSn4較多生成為塊狀析出物。反應時間在5-600小時之間所生的介金屬化合物厚度與時間成正相關。利用聚焦離子束在試片截面上進行標記，藉由標記移動情況，可知在鈷錫系統中，主要的擴散元素是錫，其速率明顯大於鈷。根據介金屬化合物生成厚度與反應時間關係做圖，發現在反應初期，介金屬化合物厚度未達臨界厚度，反應行為是介面控制反應，隨著反應時間增長、介金屬化合物厚度增加超過臨界厚度之後，則轉換為擴散控制反應。實驗結果顯示，在190℃固態時效處理下，介金屬化合物生成達到一定厚度之後，反應會轉換由擴散控制反應主導。但在介面控制反應與擴散控制反應之間，存有一個轉換區，該區域同時符合符合介面控制反應與擴散控制反應的厚度變化條件。透過觀察轉換區的標記位移情況來計算鈷、錫在190℃的狀況下在介金屬化合物中的擴散速率，結果分別為：1.60*10-13與2.11*10-12。透過時間與厚度的關係圖與阿瑞尼士方程式計算可得，在實驗條件下鈷錫系統介面反應控制區域生成介金屬化合物的反應活化能為122.47kJ/mole。
在另一方面，透過推球測試來分析固態時效處理後對於接點性質的影響。相較於未經過熱處理的試片，接點的剪切強度下降，可能是因為銲錫中的銀析出形成Ag3Sn以及錫與鈷反應生成介金屬化合物，導致銲錫組成改變而造成機械性質改變。斷裂面觀察上，僅有在150℃長時間固態時效處理的部分接點在推球測試後出現同時具有脆性與延性的混和性斷裂，其餘斷裂面接點均為延性斷裂。推測在長時間固態時效處理時孔洞得以粗大化，導致該處形成應力集中，形成脆性斷裂。接點的斷裂面則與透過蝕刻液移除銲錫之後所觀察到的介金屬化合物相同，該處主要成份為CoSn4。

Under bump metallization(UBM) is an important issue in packaging industry, because it directly effects the reliability and the life time of solder joint. To overcome the failure of solder joints by Ni3P in the Ni-P UBM, the Co-bases UBM becomes the substitute of the Ni-P UBM, because of the well diffusion-barrier capability, better reliability and acceptable solderability.
On the other hand, the Sn-3.5Ag solder is the one of the replacement for lead-solder. Because the Sn-3.5Ag solder has the better elastic modulus and the lower coefficient of thermal expansion. The addition of Ag in solder can decrease the growth rate of intermetallic compounds (IMCs). For these reasons, the Sn-3.5Ag solder becomes the prefer options in packaging industry.
In this study, we systematically investigated the solid-state interfacial reaction between the Sn-3.5Ag and the Co substrate through different aging time at temperature of 150℃, 170℃ and 190℃, respectively. Based on the results from XRD and EDX, CoSn3 , Ag3Sn and CoSn4 IMCs are formed in this system. The CoSn3 IMC layer grows thicker with longer reaction time and higher temperature. The kinetic analysis shows that the formation of CoSn3 is mainly control by the interfacial reaction in the beginning. When the thickness of CoSn3 over the critical region, the mechanism changed to diffusion control. The activation energy of CoSn3 is calculated to be 122.47kJ/mole. The diffusivity of Co and Sn in the IMC at 190℃ is 1.60*10-13 cm2/s and 2.11*10-12 cm2/s, respectively.
The result of ball push test shows that the shear strength of solder joint decreased after solid-state aging. Most of the solder joints are the ductile fracture. Only 3 of the long-time solid-state aging solder joints showed both ductile fracture and brittle fracture. The coarsening of holes is the main reason to form the brittle fracture.

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