銅、鎳元素在迴銲測試中錫銀微凸塊銲錫內的熱遷移研究

Abstract

隨著電子產業的發展，電子產品紛紛追求高效能、體積小的趨勢發展，三維積體電路技術因應而生，而其中微凸塊接點被採用來連接上下層的矽晶片。因為微凸塊的銲錫高度小於20微米（μm），許多新的議題隨之衍生而出，晶片上下兩端交互作用現象為其中之一。 以錫銀銲錫連接上一層銅金屬墊層或者鎳金屬墊層的單邊結構作為本研究的試片，將兩片相同的試片上下對接、對準後，以迴銲（Reflow）接合的方式迴銲3分鐘形成Cu/SnAg/Cu以及Ni/SnAg/Ni的結構。對於Cu/SnAg/Cu的結構，包含迴銲接合的3分鐘，在260 ℃，進行3、8、13、23、43分鐘的迴銲測試後，利用掃描式電子顯微鏡觀察上下兩端銅金屬墊層與錫銀銲錫的介面處。對於Ni/SnAg/Ni的結構，散熱係數為4.5 W/m-K的散熱膏和一片矽晶片則放置在冷端，然後在260 ℃下，再進行5、10、20、40、100分鐘的迴銲測試。最後，利用掃描式電子顯微鏡觀察上下兩端鎳金屬墊層與錫銀銲錫的介面處。 經過43分鐘的迴銲測試後，在Cu/SnAg/Cu結構的兩端介面處上可以發現Cu6Sn5介金屬化合物(Intermetallic Compounds, IMCs)生成的厚度不一樣，冷端的厚度為12.3 μm，而熱端的厚度為3.5 μm。同時，也發現冷端銅墊層的消耗量遠比熱端來的少許多。根據以往的文獻，我們可以推斷，銅的熱遷移現象可能是造成兩端Cu6Sn5 IMCs厚度不一樣的原因。而在結構為Ni/SnAg/Ni的介面上，則是在經過100分鐘的迴銲測試後，發現兩端Ni3Sn4 IMCs的厚度有明顯的差異，分別為2.12 0.06 μm和4.61 0.34 μm，而且熱端鎳墊層的厚度，與熱端銅墊層的情形相同，消耗量比冷端多了一些，因此我們推斷鎳金屬在液態錫銀銲錫中同樣會發生熱遷移現象。 在實驗過程中，沒有外加電流使電遷移現象發生，而應力效應在融熔銲錫中也不明顯，只有溫度梯度促使原子的擴散移動可能是造成兩端介面IMCs厚度不一樣的主因。然而，在液態銲錫中，很難利用儀器量測上下兩端的溫差，所以我們利用有限元素分析法模擬其過程並取得液態銲錫內部上下兩端的溫度梯度。最後，根據熱遷移的通量公式，可以得到Q*值(Heat of Transport)，銅為57.97 kJ/mole，鎳為0.84 kJ/mole。

Microbumps are adapted as the interconnect between chips in 3D-IC packaging. With the interconnect shrinks to micrometer scale, the cross interactions across the two ends of the microbump may become an important issue, since the bump height is below 20μm SnAg solders with Cu metallization and Ni metallization was used in this study. The two sets of samples were aligned and jointed together at 260°C for 3 min to form sandwich structures of Cu/SnAg/Cu and Ni/SnAg/Ni. Included the 3 min of forming the joint, reflow times of 3, 8, 13, 23, 43 min at 260°C were performed for the Cu/SnAg/Cu structure. For Ni/SnAg/Ni structure, additional thermal paste and a Si chip were put on the cold end of the structure to form a larger thermal gradient and then additional 5, 10, 20, 40, 100 min reflow at 260°C were performed. After that, scanning electron microscope (SEM) was employed to observe the thickness of Cu6Sn5 and Ni3Sn4 intermetallic compounds (IMCs) at the SnAg/Cu and SnAg/Ni interface. We observed asymmetrical growth of Cu6Sn5 intermetallic compound (IMC) at the two interfaces of Cu/SnAg/Cu solder joints during reflow at 260˚C hotplate. For Cu/SnAg/Cu, the Cu6Sn5 IMC grew to 12.3 μm on the cold end and 3.5 μm on the hot end after reflow for 43 min. The consumption of Cu on the cold end is less than that on the hot end. We propose that rapid thermomigration of Cu is responsible for the asymmetrical growth of the IMC. As to the Ni/SnAg/Ni microbumps, we also observed the asymmetrical growth of the Ni3Sn4 IMC after reflow for 100 min on hotplate: 4.61μm on the cold end and 2.12 μm on the hot end. Furthermore, the consumption of Ni on the cold end is slightly less than that on the hot end. It inferred that the thermomigration of Ni also occurs in liquid SnAg solder. No electrical currents were applied in the tests, and only the thermal gradient in molten solder joint should be responsible for the asymmetric IMC growth. Thermal gradient across the molten solder was simulated by finite element analysis due to the measurement difficulty. Based on thermomigration flux calculation and the simulated thermal gradient, the heat of transport (Q*) of Cu and Ni was calculated to be57.97 kJ/mole and 0.84 kJ/mole, respectively.