metrology LEDs
comparison, with automated imaging, analysis software reports the positions of defective die via the user’s factory automation system. This allows sub-standard devices to be discarded after dicing.
If individual devices are very small, it may be prudent to program the software to remove the eight die that are adjacent to the defective one. This makes sense when the defect is large relative to the die area, and may consequently degrade nearby die.
An example of the capability of our tool is provided in Figure 1, which shows, in very high resolution, an acoustic image of a small portion of one LED wafer. Pale grey regions indicate an absence of defects, while red features represent regions of high- amplitude reflections. These areas consist of numerous tiny gap-type defects, such as delaminations, located between various layers built up on the substrate.
Often LED die are small in area, and when the wafer is diced the delaminations can cause the die to fall apart. Even if these delaminations are very small, the chip can separate when scribing is used to cut up the wafers. Defective or potentially defective die must be uncovered and removed from the production process to improve the efficiency of the manufacturing process. To do this using automated analysis of images, an overlay map is employed to precisely locate each device and key each defect to a specific die for subsequent automatic removal.
...and LED assemblies
Common features of a completed LED assembly include one or more LED chips, die attach material for bonding the LED to a substrate, and attachment material for bonding the substrate to a heat sink (see Figure 2).
The substrate on which the LED rests may be a PC board; problems with FR4 boards – a very common, flame-retardant backbone for rigid printed circuit
Figure
3.Acoustic imaging performed on the topside of an array of LEDs can reveal cracks,such as that highlighted by an arrow,in the sapphire
chips.This array had no lens,and the depths of interest were the LEDs and their attachment to the substrate
boards – are that they have relatively high thermal resistance and transmit ultrasound poorly.
Engineers design the assembly so that it removes heat from the LED at a fast enough rate to prevent overheating – if an LED operates above its rated temperature, it will have a shorter life span. The key consideration is the total thermal resistance of the materials above and below the heat-generating LED chip.
The biggest problem – and the target of acoustic imaging – is gap-type defects. They are frequently found in: an attachment material layer or die attach layer; or between attachment material or die attach and the adjacent die, substrate or heat sink. Even extremely thin gaps can block heat transmission, leading to a hike in junction temperature and a plummeting life span for the LED.
Process engineers have the option to use our tool to inspect the topside of an LED assembly or the bottom side that features the heat sink. Scanning the latter can expose voids and other gaps in the
Often LED die are small in area,and when the wafer is diced the delaminations can cause the die to fall apart. Even if these delaminations are very small, the chip can separate when scribing is used to cut up the wafers. Defective or potentially defective die must be uncovered and removed from the production process to improve the efficiency of the manufacturing process
October 2012
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