添加鈰之錫鉍銲錫球格陣列構裝接點界面反應研究

Abstract

錫鉍(Sn-58Bi)銲錫合金由於與錫鉛(Sn-37Pb)合金類似，僅有一個共晶反應且無介金屬相存在，在機械性質方面錫鉍銲錫合金比傳統錫鉛合金具有更高的抗拉強度、剪力強度以及較低的潛變速率，重要的是錫鉍銲錫合金熔點僅139℃，在軟銲過程中可使用較低的迴焊溫度，故可避免塑膠球格陣列構裝基板老化並減少對第一層構裝的熱衝擊效應，另外在所有無鉛銲錫中，錫鉍銲錫合金的價格頗具有競爭性。 在許多研究當中證實添加稀土元素可以增加銲錫合金潤濕性、提升機械性質以及增加抗潛變性等，然本實驗室首度發現添加稀土元素所產生的稀土介金屬化合物會引發錫鬚異常生長的負面效果，故本研究中嘗試在錫鉍銲錫合金中添加不同含量的稀土元素Ce，觀察其對原材料的影響。另外將Sn58Bi-XCe的銲錫合金與化學浸鍍錫及無電鍍鎳/化學浸鍍金二種基板迴銲接合並施以時效處理，觀察其界面反應以及利用推球方式測量強度。最後再觀察Sn58Bi-0.5Ce銲錫合金之稀土介金屬化合物的錫鬚生長情形。 實驗結果顯示適當的添加量能增加機械強度並提升錫鉍銲錫合金所欠缺的延性，但過量的添加則會大幅降低構裝接點強度。在銲錫合金與二種BT基板的界面反應中，可以觀察到添加稀土元素Ce能夠抑制界面介金屬層的成長，減緩有害的界面介金屬層Cu3Sn生成。最後在錫鬚生長情形的觀察實驗中，Sn58Bi-0.5Ce銲錫合金所生成的稀土介金屬化合物並不會引發有害的細長錫鬚生長。

Sn-58Bi solder alloys are similar to Sn-37Pb alloys as they have only one eutectic reaction with no existence of intermetallic phase. From the mechanical perspective, compared with traditional Sn-37Pb alloys, Sn-58Bi solder alloys are with higher tensile strength, shear stress and lower creep rate. Moreover, the most important is, the melting point of Sn-58Bi solder alloys is only 139 ℃, therefore, by using lower reflow temperature in the process of soldering, we can avoid the aging of plastic ball grid array substrate and also reduce the thermal shock on the 1st level of package. In addition, the pricing of Sn-58Bi solder alloys is quite competitive among all Pb-free solders. It has been confirmed in many studies that adding rare earth element into solder alloys can improve the wettability, enhance the mechanical properties and also strengthen the resistance to creep, etc. However, our laboratory was the first to discover that the “Rare Earth intermetallic compounds” resulting from adding rare earth element will lead to the negative effect of abnormal growth of tin whiskers. Therefore, in this study we attempt to add different content of rare earth Ce into Sn-58Bi solder alloys to observe the impact on raw materials. In addition, we reflow Sn58Bi-XCe solder alloys with Immersion Tin substrate and with Electroless Nickel / Immersion Gold substrate then proceed with aging process to observe the interfacial reaction and also to measure the strength by using the ball shear test. Finally, we observe the growth of tin whisker resulting from the Rare Earth intermetallic compounds of Sn58Bi-0.5Ce solder alloys. The results of experiments indicate that adding appropriately can improve the mechanical strength and enhance the ductility of Sn-58Bi solder alloys. However, adding too much will significantly reduce the strength of solder joint. From the interface reaction of solder alloys with two kinds of BT substrate, it’s observed that adding rare earth element can inhibit the growth of interface intermetallic layer, especially slowing down the growth of harmful interface intermetallic layer Cu3Sn. Finally, we observed from experiments on the growth of tin whisker, the rare earth intermetallic compounds generated from Sn58Bi-0.5Ce solder alloys does not result in the growth of harmful slender tin whisker.