Assessment of Die Attach Film for Thin Die and SiP Applications
The drive for package miniaturization and die thickness reduction has created new challenges for conventional die attach paste materials. Die attach film (DAF) is seen as a potential candidate especially in applications requiring die thickness =100 mum. However, processing of DAF materials also presents certain difficulties especially in dicing of DAF-laminated wafers. This paper outlines the challenges and process optimization carried out for 5 different DAF materials. Process feasibility study and dicing optimization were conducted on 100 mum thick wafers. Subsequently the DAF materials were assembled into a ball grid array (BGA) package and subjected to reliability testing (Moisture Sensitivity Level 3, Unbiased Highly Accelerated Temperature and Humidity Stress Test and Temperature Cycling). Results showed that minimum chipping and whisker formation can be achieved using a 2-step dicing process with optimized cutting depths and feed rate. It was also found that optimum adhesion of the DAF material to both the dicing tape and die backside is necessary to prevent "flying die" during dicing and die pick-up problem during die attachment. Positive results for 2 DAF materials were obtained from reliability performance assessment. These findings show that there is a robust DAF material and process for thin die and System-in-Package (SiP) applications.
New proposed adhesive tape application mechanism for stacking die applications
One approach to develop a high-density electronic package for memory modules is stacking the dice in the packaging and assembly processes. The development of the stacked die has been speeded up significantly in recent years by the maturity of the wafer thinning technology. Conventionally, if the top die and bottom dice are the same size, a dummy silicon die is used as a spacer to separate the dice and make room for wire bonding. Methods including using a much thicker epoxy or epoxy with spacer balls would be the alternative techniques. A new technique has been developed recently. A thick adhesive tape of size a bit smaller than the die is used as spacer as well as adhesive for the bonding of the bottom and top dice. This new process, known as adhesive tape application process, is introduced for stacking the dice. In the process, a predetermined tape size is cut, picked and placed on the top surface of the bottom die or substrate. The advantages of this technique over the dispensing process are (i) there is no die tilt and (ii) there are no epoxy bleed out issues. However, there may be new issues, such as delamination failure that occurs at the bonding interface. In this study, we find that void formation due to trapped air in the bonding interface is introduced by the tape application process while a collet with flat surface is used to place the tape on the bottom die or substrate. The uneven contact points between tape and bonding surface trap air bubbles in the bonding interface. We invent a new tape application tool that can basically eliminate trapped air bubbles during the tape bonding process. A convex compliant collet is used to pickup and place the tape on the bonding surface: The line contact between tape and bonding surface is formed in the middle of the tape. The air is not trapped in the bonding interface as the collet moves further down to deform the collet and squeeze the tape out sideways. Finite element simulation has been carried out to determine the optimized configuration in terms of force requirements to flatten the convex collet, strain/stress distribution of the compliant collet and the tape itself. Selection criteria of this compliant material for the tape application process are also discussed. Experiments have been carried out to verify the void-free condition is achieved using this new convex compliant collet.
Ozonation of polycyclic aromatic hydrocarbons in oil/water-emulsions: mass transfer and reaction kinetics.
The ozonation of highly condensed polycyclic aromatic hydrocarbons (PAH) was studied in oil/water-emulsions, which are comparable to poorly water-soluble PAH in industrial wastewaters and at contaminated sites. As there was a lack of knowledge about the ozonation in oil/water-emulsions, first the ozone mass transfer was studied and optimized from the gas to the water phase and from the water to the oil phase. The ratio of mass transfer and oxidation reaction was determined by the Hatta-number and revealed a slow, quasi homogeneous reaction of ozone with PAH inside the oil droplets. Because the ozone gas concentration had no influence under the optimized conditions, the selective PAH-ozonation could be described microkinetically by a direct ozone reaction of pseudo-first order regarding PAH-concentrations. The determined PAH mean reaction rate constants of 1.02 min(-1) in oil/water-emulsions are in the upper range as found for PAH dissolved in water. These results give a new insight into the ozonation in the three-phase systems and into the treatment of highly condensed, hardly biodegradable PAH.