Printable optically transparent adhesive processing for bonding of LED chips to packages

Die bonding is the first step in the packaging of light-emitting diodes and its role in lumen output performance and consistency of the packaged LED emitters has recently been investigated: there is an optimal range of bond-line thickness (BLT) and the fillet coverage has to be minimized. One method which can accurately control the BLT and fillet coverage is to employ the pre-made die attach adhesive (DAA) films, which cost more than the use of conventional DAA pastes. However it is difficult to control BLT as well as fillet by using conventional DAA pastes. In this work, a silicone-based transparent DAA is used for LED chip bonding. DAA layers are formed by using stencil printing, as an alternative method for LED die bonding with an objective of exploring a cost-effective method for LED die bonding with the controlled DAA BLT and fillet coverage. It is demonstrated that with other conditions being the same, a low BLT value of 3.9 µm could be achieved by stencil printing. The lumen output of the packaged white LED emitters using printed DAAs with no fillet coverage, is found to be 3.5% higher than using the conventional pin-transferred DAA pastes with 33% fillet coverage, while the reliability of the packaged white LEDs is found not to be altered. The method of printable DAAs for LED die bonding is thus a cost-effective alternative to the one using pre-made DAA films.