Cracked die is a serious failure mode in Light Emitting Diode (LED) industry-affecting the LED quality and long-term reliability performance. In this paper, an investigation has been carried out to find out a relation between die bonding force and the occurrence of die crack at Germanium (Ge) substrate due to die attach (DA) ejector pin indentation. Based on the analysis, the results show that cracks start to occur at 60 gram-force (gF) bond force and above. The crack length at the die substrate increases with respect to the bond force. These indented dies were further analyzed by using Scanning Electron Microscope (SEM). The results show plastic deformation, slip traces and material pileup at the vicinity of ejector pin crater. Some samples were sectioned using Focus Ion Beam (FIB) and it was found the crack depth does not exceed beyond 20.5µm and it follows the (111) plane. These findings, concludes that cracks start to appear at 60gF and they are confined to surface level even indented at extreme load (140gF). These cracks are far away from the active region of LED.
Abstract:Cracked die is a serious failure mode in the Light Emitting Diode (LED) industry -affecting LED quality and long-term reliability performance. In this paper an investigation has been carried out to find the correlation between severe cracked germanium (Ge) substrate of an aluminum indium gallium phosphate (AlInGaP) LED and its electrooptical performance after the Temperature Cycle (TC) test. The LED dice were indented at several bond forces using a die bonder. The indented dice were analysed using a Scanning Electron Microscope (SEM). The result showed that severe cracks were observed at 180 gF onward. As the force of indentation increases, crack formation also becomes more severe thus resulting in the chipping of the substrate. The cracked dies were packaged and the TC test was performed. The results did not show any electro-optical failure or degradation, even after a 1000 cycle TC test. Several mechanically cross-sectioned cracked die LEDs, were analysed using SEM and found that no crack reached the active layer. This shows that severely cracked Ge substrate are able to withstand a −40 ∘ C/+100 ∘ C TC test up to 1000 cycles and LED optical performance is not affected. A small leakage current was observed in all of the cracked die LEDs in comparison to the reference unit. However, this value is smaller than the product specification and is of no concern.
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