電子封裝Ag-4Pd銀合金銲線之退火處理及電遷移材料特性與晶粒結構研究

Abstract

電子封裝製程技術的進步，促使消費性電子產品達成短小與輕薄化的要求，然而便攜性帶來的可靠度影響實為一不容忽視之問題，近年來電子產業開始使用銀合金線做為金線替代材，然而高頻通訊與電動車模組的高電流密度常態使用，加劇導線發生廣為人知的電遷移現象；有鑑於此，本研究第一部份即針對銀合金銲線探討電遷移現象與機制，基於此項研究經驗，發展針對銀合金帶材與線材的電子背向散射繞射分析技術，並透過此技術從另一面向究明銀合金帶材與線材的退火與電遷移過程的材料特性與晶粒結構。 為求電遷移實驗線材實際溫度準確性，本研究第一部分之電遷移實驗採用本實驗室自行研發之耐高溫夾具，對線徑17.6 μm的四種不同鈀含量之銀鈀合金線，於高溫環境下進行電遷移，結果顯示添加量2~6 wt.% Pd之銀鈀合金線，電遷移活化能為0.36~0.46 eV，為表面擴散路徑主導。為徹底了解Ag-4Pd銀合金線線材結構成因，本研究第二部分進行銀合金帶材輥軋退火研究，並以創新的電子背向散射繞射技術對Ag-4Pd銀合金線內部晶格結構做一全面的了解；結果顯示，Ag-4Pd銀合金帶材經退火後晶粒大小與孿晶晶界佔比和退火溫度高低較相關，而與退火時間長短較無關聯。晶粒大小成長與孿晶密度佔比都與退火溫度呈現線性趨勢增加，其晶粒成長活化能為0.47 eV，與第一部分Ag-4Pd線材活化能接近，並且，多道次輥軋能有效增加Ag-4Pd帶材內部孿晶晶界佔比，並由KAM圖得知銀合金退火前後殘留應力釋放與晶粒成型相關性。 本研究第三部分針對17.6 μm的Ag-4Pd銀合金線，以電子背向散射繞射技術對銀合金線的電遷移過程特性做分析；本實驗室基於帶材經驗，發展出一套特殊的線材EBSD試片製備技術，成功突破銀合金線材EBSD試片製備瓶頸，為世界首創關於線材電遷移過程特性的EBSD研究。透過電子背向散射繞射技術分析線材結構，可以得知線材中心部分長條晶結構的晶格取向為<100>，線材外圍部分則是無序、富含退火孿晶的細晶粒，並透過與KAM圖與帶材結果對照可知，中心長條晶成因實為冷加工抽線殘留織造(texture)，外圍則是經線材退火製程而再結晶成長之晶粒。後續設計室溫(25 oC)與高溫(150 oC)線材電遷移實驗，取各時間段線材進行EBSD分析，結果顯示線材通電後，內部晶粒快速成長，原始線材晶界電遷移後絕大多數快速地轉為Σ3或是Σ9的孿晶晶界與垂直電子流方向的竹節狀晶界，並在總體孿晶晶界密度達到約80 %後停止，此結果與孿晶晶界的低自由能特性有關，並受溫度影響在高溫環境加速進行。透過EBSD分析各時間段下線材結構與晶格取向，可推論銀合金電遷移破壞機制分為三階段：1)初期晶界快速轉化為孿晶與竹節狀晶界；2)中期線材內部結構穩定後轉為表面電遷移效應；3)線材表面形貌改變，線徑減縮區域使電流密度加大而燒斷。

The flourish improvements of electronic packaging technology allows the developing trend of electronic devices to be light-weight, gaunt and neat. However, reliability issues such as electromigration caused by relatively high current density usage inside high-frequency components and power modules also arises. Hence, in order to establish a comprehensive understanding of Ag-alloy wire property and failure mechanism, our research focus on the complete study of electromigration phenomenon and the wire structure itself. Firstly, the failure mechanism of Ag-Pd bonding wire has been investigated by stressing with 1.23 x 105 A/cm2 current density at 150 ~ 300 oC. The results showed that the activation energy of Ag-Pd bonding wires to be 0.36 ~ 0.46 eV as the doping content of Pd arises from 2 to 6 wt.%. On the other hand, grain structures and surface morphologies of Ag-4Pd bonding wires were investigated after electromigration, indicating surface diffusion to be the dominant mechanism during electromigration for Ag-4Pd bonding wire. Secondly, in order to clarify the characteristic and grain structure of Ag-4Pd Ribbon, an innovative Electron Back Scatter Diffraction analysis for Ag-4Pd ribbon was proposed. By applying EBSD technique, characterization of twin boundary and average grain size were investigated during the annealing treatment of Ag-4Pd ribbon. The result showed that twin boundary densities increased originally from 3.7 % up to 77 % as the annealing temperature arose, and the average grain size were also grew from 0.28 μm to 8.46 μm with elevating annealing temperature. The grain growth activation energy was calculated to be 0.47 eV which is quite near to the activation energy of electromigration. Our study also found that after long time treatment there was not significant difference among twin boundary densities(60~70%) and grain structure(1.20 μm) of Ag-4Pd ribbon itself, whereas staged rolling and annealing can increase the twin boundary densities up to 70% above. Last, the mechanism and grain structure of electromigration of Ag-4Pd was investigated. In present study, the grain structure of Ag-4Pd bonding wire was characterized by EBSD analysis, and the microstructure of grain evaluation during electromigration were recorded. Regions in the centre of Ag-4Pd bonding wire was characterized to be <100> orientation, and they are reckoned as the inheritance of the cold-draw texture during wire manufacturing procedure. Twin boundary densities arose from 25.5 % to about 80 % after electromigration. The evolution of the increasing twin boundary densities during electromigration can be attributed by relatively lower grain boundary energy of CSL boundary and the resistance against electromigration. Hence, the failure mechanism of Ag-4Pd bonding wire can be classified into three stages: 1)Initial Stage: grain boundary migration; 2)Secondary stage: Termination of grain boundary migration, surface diffusion dominates; 3)Failure stage: failure occurs due to the local shrinkage of wire diameter.