以錫銅化合物來抑止銲錫與銅多次回銲時Cu6Sn5的成長

To inhibit the growth of Cu6Sn5 during reflow using Cu-Sn compounds as a diffusion barrier

胡致嘉(Chih-Chia Hu)

指導教授 ： 陳智

國立交通大學/工學院/材料科學與工程系所/碩士(2012年)

https://doi.org/10.6842/NCTU.2012.01045

摘要

本論文主要研究如何抑制錫銀銲錫凸塊在多次迴銲時Cu6Sn5介金屬化合物的成長。銅與錫在高溫迴銲時會迅速反應生成介金屬化合物(Intermetallic Compounds, IMCs)，此反應甚至在室溫時仍會持續進行。因為錫銅彼此間容易產生反應，因此在微電子封裝產業中銅常常被當作金屬墊層(Under Bump Metallurgy, UBM)的材料。但在接上銲錫後若銅持續擴散進入銲錫中，將會在銲錫中產生壓應力導致錫鬚晶的生成。故若能在銲錫凸塊接合完成後抑制介金屬化合物繼續反應，則將是一項利多的事情。不少研究都針對此議題提出解決的辦法，諸如改變銲錫成分、添加鎳層在銅金屬墊層上等等，但效果有限。根據前人的文獻，介金屬化合物Cu6Sn5的成長有賴於其彼此間的通道(Channel)，銅原子藉由通道自金屬墊層擴散進入銲錫內反應。換句話說，通道在扇貝形的Cu6Sn5成長上扮演了舉足輕重的角色。
我們發現一個有效能抑止錫銅介金屬化合物成長的方法。藉由先在銅墊層上鍍上一層約2 μm的銲錫，並在260 oC下迴銲 10分鐘，生成層狀的Cu6Sn5或是Cu３Sn，通道將會大幅的減少。本研究中主要使用兩類試片來證明此論點：第一類為實驗組，以2 μm薄銲錫鍍在銅墊層上；第二類則是對照組，用20 μm厚銲錫鍍在銅墊層上。我們將薄銲錫試片先在260 oC下迴銲 10分鐘，減少通道的數量，當該試片再接上厚銲錫時，由於銅原子擴散途徑變少，介金屬化合物的成長明顯也趨緩。
在本研究中，有擴散阻擋層的試片在迴銲10分鐘後介金屬化合物僅成長0.2 μm；而沒有作任何前處理的對照組試片則成長了1.2 μm。顯而易見地，介金屬化合物的成長確實被減緩。

關鍵字

介金屬化合物 ； 銲錫 ； 銅

並列摘要

In this study, we reported an effective approach to inhibit the growth of Cu-Sn intermetallic compounds (IMCs) during multiple reflows of SnAg2.3 solder on Cu.Cu reacts with Sn at a very high rate and they even continue to form Cu-Sn IMCs at room temperatures in flip-chip solder bumps. Because of its excellent wettability, Cu serves as the popular under-bumpmetallurgy (UBM) in flip-chip boundary. After joining, if Cu continues to diffuse into solder, compressive stress may generate in the solder layer and whisker of Sn may grow.　It will be beneficial if the Cu-Sn reaction were inhibited after jointing. Therefore, several approaches have been proposed to inhibit the growth of the IMCs, including altering solder composition, and incorporating Ni into Cu UBM. However, none of them is able to suppress the formation of the IMCs effectively. The method for the inhibiting growth of the IMCs is proposed in this study.It is reported that there are many channels in between the scallop-like Cu6Sn5IMCs. The Cu underneath the IMCs can diffuse through the channels and facilitate the formation of the Cu-Sn IMCs; in other words, the channels between the Cu6Sn5 scallops play critical role in the growth of the Cu-Sn IMCs.
We propose a highly effective approach to inhibit the growth of the Cu-Sn IMCs. By depositing a thin layer of solder on a Cu film and after a reflow metallurgical reaction at 260 oC for 10 minutes, the channels between Cu6Sn5 IMCs can be closed up or a layer-type Cu3Sn IMC can be formed. The system used to demonstrate this approach is SnAg2.3 solder, pure Sn and electroplated Cu. Two schemes of structures were fabricated: the first one is 2-μm-thick solders on Cu UBMs, and the second one is 20-μm-thick solders onCu UBMs. The 2-μm-thick solders samplewas reflowed for 10 minutes to close the channels between Cu6Sn5 IMCs. When the sample was jointed to solder afterwards, this layer-type Cu6Sn5 IMCs becomes a diffusion barrier for Cu dissolution during additional reflowing, and the growth of the IMCs was significantly retarded.
In this study, the increasing thickness of the Cu-Sn IMCs was only 0.2 μm after the reflowat 260°C for 10 minutes. Without the diffusion barrier layer, the increasing thickness of Cu-Sn IMCs is 1.2 μm. It is evidentthat the growth of Cu-Sn IMCs is significantly inhibited in the solder bump.

並列關鍵字

IMC ； solder ； Cu

參考文獻

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