Analyses of propagation behavior of crack at interface between die-attach and Cu base and cracks´ effects on reliability of high brightness light-emitting diode (LED)

Reliability problems heavily hinder light-emitting diode (LED), especially high brightness LED, from developing rapidly and putting into mass production. One of the most important reasons is that it has so many interfaces which are always the weakest places. LED chip´s working temperature is about 90°C and the high mismatch of coefficient of expansion of different materials can inevitably cause failures, such as delamination at the interfaces. What´s more, due to inappropriate manufacturing processes, defects, like cracks and delaminations, are often generated in LED devices before they reach customers, which would propagate at a fast pace and bury a potential threat causing the device to fail to work. In this paper, delaminations between die attach and copper heat sink are analyzed by finite element analysis (FEA). Different sizes are considered to investigate how the crack propagates and the results show that the crack can easily propagate at first, but it would be a little more difficult and finally the crack is so long that there is little chance to propagate, if the crack growth is stable, or if the strain energy release rate gets smaller. Furthermore, two kinds of die adhesives, lead free solder and sintered silver paste, are selected and analyzed under the same conditions and it is found that they follow a similar propagation behavior, but cracks in sintered silver paste has much lower strain energy release rate. Effect of interfacial delamination on thermal performance is also investigated and it is found that delaminations have a great impact on LED´s thermal resistance.