In-Ag Solid-Liquid Interdiffusion Bonding for LED Die Bonding

近年來由於環保意識高漲，綠能產業的發展備受重視，其中發光二極體(LED)具有節省能源、使用壽命長、可靠度高、較無環境汙染等優點，被視為未來最有潛力的照明技術。然而LED受高溫影響甚鉅，當接面溫度過高時，其發光效率與使用壽命會大幅衰減，因此有效的熱管理是目前LED發展中最重要的議題之一。 本論文針對高功率LED封裝中的固晶製程進行研究，目前LED主要的固晶方式有三種，分別是銀膠、無鉛錫膏與金錫共晶接合，但都各有其優缺點，沒有任何一種固晶方式可完全取代其他方式。本研究則是利用「銦-銀固液交互擴散接合技術」進行低溫固晶接合，而多層結構的製備是選用設備簡易、成本較低的電鍍與無電鍍製程，在接合後進行推力強度測試與接合熱阻分析，另外並觀察、分析破斷面與接合橫截面的形貌和組成。 研究結果顯示，在多層結構設計中增加銀鍍層的厚度(15µm→45µm)或降低接合製程溫度(200℃→180℃)，皆可減少材料間熱膨脹係數差異所引入的熱應力之影響，進而提升接合強度，且已達到美國軍用規範MIL-STD 883G的標準，但是與上述三種目前常見的LED固晶方式相較下仍較差。熱阻分析的部分，本研究設計的接合製程所引入的熱阻較上述三種目前常見的LED固晶方式都來得低，有較優異的熱傳導性質。另外在接合橫截面中整體接合狀況良好，幾乎無孔洞、缺陷的存在，主要由Ag2In介金屬化合物及金屬Ag所構成。 綜合比較下，銦-銀固液交互擴散接合的製程溫度低，可以保護LED晶片，且整體接合的熱傳導性質相當優異，有助於LED晶片的散熱，但在接合強度方面表現較差，仍需改善以提升接合可靠度。As the awakening of environment-friendly conscious, green-industries are attracting much attention in today’s society. We regard light emitting diode (LED) as the most potential lighting technique in the future due to its advantages: energy conservation, long life, high reliability and low pollution. However, LED is affected by temperature significantly. Both the light output efficiency and life will be cut down dramatically with increasing junction temperature. Therefore, precise thermal management is one of the most important issues of LED development. This research focuses on the die bonding process in high-power LED packaging. There are three major methods for LED die bonding now: silver paste, lead-free solder and Au-Sn eutectic bonding. Each of them has pros and cons; no one can replace the others. In this study, we use “In-Ag solid-liquid interdiffusion bonding (SLID) technique” for LED die bonding because it allows LED chip to bond at low temperature; for the multilayer structure preparation, we use electroplating and electroless plating process because it costs less and uses simple equipments. After bonding, we execute the die shear test and the thermal resistance measurement. Moreover, we observe and analyze the micrograph and composition of the broken interface and the joint cross-section. The results show that, both increasing the thickness of Ag deposits and lowering the bonding temperature could reduce the thermal stress induced by coefficient of thermal expansion (CTE) mismatch between materials and enhance the bonding strength. Although the bonding strength is worse than the three commercial techniques for LED die bonding, still, it already surpassed the MIL-STD 883G. On the other hand, the thermal resistance of In-Ag SLID is lower than the other three common LED die bonding methods. It indicates that In-Ag SLID is a technique of great thermal conductivity. Besides, the joint is nearly void-free, and mainly composed of Ag2In IMC and Ag. To sum up, In-Ag SLID is a technique of low bonding temperature which enables LED chip to remain complete during the process. The thermal conductivity of the joint is outstanding, and it can help the heat dissipation of LED chip. But the bonding strength has to be further improved in order to enhance the joint reliability