銅底材狀態與Cu6Sn5之生成對錫鬚生成之影響

Effects of Cu Substrate States and Cu6Sn5 Formation on Tin Whisker Growth

程凱琪(Chen, Kai Chi)

指導教授 ： 王朝弘

國立中正大學/工學院/化學工程學系/碩士(2012年)

摘要

電子封裝中錫層表面處理經常使用於導線架與銲墊之上，然而錫鬚生成的問題可能會造成元件失效。本研究在探討錫鬚生成的應力來源與相關影響因子，包含不同微結構之電鍍亮與霧錫層、表面機械粗化、電鍍銅與銀底層、基板熱處理與存放溫度，於長時間儲存下觀察，瞭解基板殘存應力、電鍍底層殘存應力、Cu6Sn5介金屬相對於錫鬚生成的影響，並利用硬度計以機械外力形成壓痕，探討錫擠出物生成的應力來源及長程質量傳遞之機制。
在壓延銅基板實驗中，較粗刮痕試片在長時間儲存下，錫層會產生大尺寸隆起物，其頂端處會再延伸出不規則狀錫鬚，隨粗化刮痕越細，錫鬚數量與長度隨之增加，由於細小刮痕易成為應力集中點，導致應力於特定區域釋放而生成較長錫鬚。經退火消除銅基板殘存應力後，大幅減少錫鬚生成數量與長度，證實壓延銅基板之殘存應力為錫鬚生成的主要應力來源。同時以電鍍銅底層進行相似實驗，電鍍銅之殘存應力亦為錫鬚生成所需應力來源之一。
在壓延鎳基板實驗中，未觀察到錫鬚生成，當電鍍銅底層於鎳基板上時，即觀察到大量錫鬚生成，其數量與長度與壓延銅基板實驗結果相似，並在界面處觀察到Cu6Sn5生成。壓延鎳基板經退火後，同樣會大幅減少錫鬚生成數量與長度，證實基板應力為錫鬚生成主要應力來源，但基板應力能夠影響錫層的必需條件是界面處需有Cu6Sn5介金屬相存在。推測基板應力需藉由Cu6Sn5介金屬相傳遞至錫層內部，亦或是Cu6Sn5的生成造成基板應力不平均釋放，導致應力集中釋放而使錫鬚生成。
以硬度計施加外力破壞錫層而形成壓痕實驗中，於短時間內會在壓痕處生成大量錫擠出物。特別的是，當壓痕處外圍之錫層被挖除時，錫擠出物隨之減少，發現錫來源是來自以壓痕中心外200μm處錫層所供應，利用長程質量傳遞造成錫擠出物成長。
亮、霧錫比較方面，霧錫鬚數量遠大於亮錫鬚，但長度遠小於亮錫鬚，由於霧錫層晶界數量少且連續，易使應力由連續晶界快速釋放，所以錫鬚數量較多，因應力快速釋放，使應力沒有過於累積，所以霧錫鬚長度遠小於亮錫鬚。溫度影響方面，室溫(25~27oC)儲存，錫鬚長且多，50oC儲存，大幅減少錫鬚數量與長度，主要是提升溫度有助於消除內部壓應力。由穿透式電子顯微鏡分析錫鬚為單晶結構，其中有許多缺陷存在。由聚焦離子束分析，錫層晶粒結構變化之處只發生在錫鬚生成之處，以外區域的錫晶粒結構沒有變化。
另外施加2.9×104A/cm2之電流於室溫(25~27oC)與50oC進行試驗，發現霧錫試片幾乎沒有錫鬚生成，亮錫試片相對較多，並且錫鬚朝著電子流方向成長。為更了解電遷移影響，在錫層線路上製做不同長度線段於100oC施加104A/cm2之電流，線段100μm以上錫層因電遷移效應造成陰極端產生大量孔洞，陽極端產生錫擠出物，線段50μm以下則無受到破壞，此時背向力會抵消電遷移作用，所以結構不會受到破壞。

關鍵字

錫鬚 ； FIB ； 電遷移 ； 退火 ； Cu6Sn5

並列摘要

In the electronic packaging processes, surface treatment of the tin coating layer is frequently used upon leadframes and solder pads; however, the generation of tin whiskers may cause the failure of electronic components. This study investigated the sources of compressive stresses by which tin whiskers was generated and the related determinants, including the electrodeposited bright and matte tin layers of different microstructures, surface mechanical roughening, electrodeposited Cu and Ag underlayer, heat treatment and storage temperature of substrates. With the observation of long storage period, this study figured out the influences which substrate residual stresses, electrodeposited underlayer residual stresses and Cu6Sn5 IMC have on the generation of tin whiskers and applied the micro hardness instrument to form the indentation through external mechanical forces, so as to probe the stress sources generated by tin extrusions and the mechanism of long distance mass transfer.
In the experiment of the rolled Cu substrate, under prolonged storage, the tin layer on the coarse scratch sample produced large-sized bumps, with the irregular-shaped tin whiskers stretching out from the top; moreover, the finer the coarse scratch was, the larger number and the longer the whiskers got. Because the fine scratch is apt to form a stress concentration, it causes stress to release in a specific area and thus produce longer tin whiskers. The annealing treatment eliminates the residual stresses of the Cu substrate, significantly reducing the number and the length of tin whiskers. The experiment proves that the residual stresses of the rolled Cu substrate are the main stress source in the generation of tin whiskers. A similar experiment on electrodeposited Cu underlayer was simultaneously conducted, indicating that the residual stresses of electrodeposited Cu underlayer is also one of the stress source required in the generation of tin whiskers.
In the experiment of the rolled Ni substrate, the generation of tin whiskers was not found; however, when Cu underlayer was electrodeposited on the Ni substrate, the generation of tin whiskers of a large quantity was observed. The generated number and length of tin whiskers are similar to the experimental result of rolled Cu substrate, and Cu6Sn5 IMC at the interfacial surface was observed. After the rolled Ni substrate was annealed, the number and the length of tin whiskers were significantly reduced, showing that substrate stresses are the main stress source in the generation of tin whiskers, but the existence of Cu6Sn5 IMC is a requirement for the substrate stresses to influence the tin layer, meaning that the substrate stresses was transferred to the interior of the tin layer by means of Cu6Sn5 IMC or substrates residual stresses release non uniform caused by Cu6Sn5 IMC.
In the experiment of applying the micro hardness instrument to exert external forces to undermine the tin layer and form the indentation, a large amount of tin extrusion would be generated upon the indentation within a short period of time. Particularly, when the tin layer at the periphery of the indentation was excavated, the tin extrusion is reduced, revealing that the source of the extrusion comes from the tin layer 200μm outside of the indentation center, using long distance mass transfer to cause the growth of tin extrusion.
As for the comparison of the bright and matte tin layers, the number of the matte tin whisker was far bigger than that of the bright tin whisker, but its length was much less than the bright tin whisker’s. Due to the small number and continuity of the grain boundary of matte tin, the stresses tend to be released rapidly from the continual grain boundary so that more tin whiskers are generated, and because of the rapid release of the stresses, they are not over-accumulated; therefore, the length of the matte tin whiskers is much less than the bright tin whisker’s. In terms of the influence of temperature, when the tin layer is stored at room temperature(25~27oC), the tin whiskers are long and abundant; when it is stored at 50oC, the number and length of tin whiskers are considerably reduced, giving the fact that the raise in temperature helps eliminate the interior compressive stresses. According to the analysis of the transmission electron microscopy, the tin whisker is a single crystal structure which has a lot of defects. Besides, the analysis of focused ion beam indicates that the change of tin layer strusture only occurs at the area where tin whiskers are generated, while there is no change in the structure of tin layer outside the area.
In addition, in a test of imposing 2.9 × 104A/cm2 current at both room temperature(25~27oC) and 50oC, there is barely any tin whisker generated upon the matte tin sample, while there are relatively more tin whiskers upon the bright tin sample with the tin whiskers growing in the direction of electrons flow. To better understand the impact of electromigration, line segments of different lengths were made and imposed with 104A/cm2 current at 100oC. Upon the line segments above 100μm, due to the electromigration effect, a large number of holes and cracks were created at the cathode region, while the tin extrusion was generated at the anode region; however, the line segments under 50μm were not damaged because the back stresses would offset the electromigration effect, preventing the structure from being damaged.

並列關鍵字

tin whisker ； FIB ； anneal ； Cu6Sn5

參考文獻

2. B. D. Dunn, “Mechanical and Electrical Characteristics of Tin Whiskers with Special Reference to Spacecraft System”, Ecological Society of America Journal, Vol. 11, pp. 1-17, (1987).

3. N. Jadhav, E. Buchovecky, E. Chason, A. Bower, “Real-time SEM and FIB of Whisker Growth and Surface Modification”, Journal of Electronic Materials, Vol. 62, No. 7, pp. 30-37(8), (2010).

4. K. S. Kim, C. H. Yu, S. W. Han ,K. C. Yang, J. H. Kim, “Investigation of Relation between Intermetallic and Tin Whisker Growths under Ambient Condition”, Microelectronics Reliability, Vol. 48, pp. 111-118, (2008).

5. M. E. Williams, K. W. Moon, W. J. Boettinger, D. Josell, and A. D. Deal, “ Hillock and Whisker Growth on Sn and SnCu Electrodeposits on a Substrate Not Forming Interfacial Intermetallic Compounds”, Journal of Electronic Materials, Vol. 36, No. 3, pp. 214-219, (2007).

6. T. H. Chuang, “Rapid Whisker Growth on The Surface of Sn-3Ag-0.5Cu-1.0Ce Solder Joints”, Scripta Materialia, Vol. 55, pp. 983-986, (2006).