Challenges & solutions in the die attach process for micro thin die

Abstract

The drive for package thickness thinning and package size reduction has created new challenges for current wafer fabrication and assembly technology which lias led to the smaller die and thinner wafer thickness trend. Thus, the conventional wafer thickness of 8 mils decreases to 4 mils is more challenging for the new innovation package of Micro Leadless Land Grid Array (μLLGA) with only 0.4 mm package thickness. The uniqueness of μLLGA package construction is that it has no die paddle for die attach and the die backside is exposed to customer view, thus no indentation or sign of ejector pin is allowed visible. The level of difficult also increased by the inclusion of 14×14 mil die size and challenge in pick up the micro thin die without having a concern of die back cracking, and dies rotation. Die back cracking is one of the major quality issues for die attach with major contributor due to improper ejector pin selection (made up of tungsten carbide). The gross die back crack can be detected during in process monitoring but the micro crack die back will be difficult to detect and it usually will only be detected through customer application or reliability stress test. Another aspect this paper will cover is the conventional epoxy dispensing no longer viable for thin die attach as critical control is required in order not to have epoxy overflow with 4 mils die thickness die. In replacing this, a new approach known as epoxy screen print on wafer back is introduced to enable a more robust die attach process. This paper explains in details the challenges and solutions of die attach process for the μthin die into this unique μLLGA package. The challenges include: (i). Micro die size of 14 × 14 mils (near to minimum machine capability of 10 mils in die size) (ii). Thin die of 4 mils thickness with epoxy screen printon wafer back, (iii). μLLGA package construction with expose die back (with no die paddle). Design of Experiments (DOE) were carried out to enablea an optimize die attach process which consists key deliverablea as below: (i). Selection of appropriate ejector needle tip size in order not to induce epoxy on die back from cracking or indentation mark during die pick up. (ii). The optimization of die bond critical input parameter in order to achieve optimized process with no issue on die rotation, die unpick and die crack.