This describes the development and evaluation of advanced technologies for the integration of electronic devices within membrane polymers. Specifically, investigators thinned silicon die, electrically connecting them with circuits on flexible (liquid crystal polymer (LCP) and polyimide (PI)) circuits, using gold thermo-compression flip chip bonding, and embedding them within the material. The influence of temperature and flexure on the electrical behavior of active embedded assemblies was evaluated. In addition, the long-term thermal cycle resistance of the passive daisy chain assemblies was determined within the Mil-Std (-55° to +125°C), extreme low #1 (-125° to +85°C), and extreme low #2 (-125° to +125°C) temperature ranges. The results of these evaluations will be discussed, along with the application of this technology for future NASA missions.

Silicon thinned to 50 mum and less is flexible allowing the fabrication of flexible and conformable electronics. Two techniques have been developed to achieve this goal using thinned die: die flip chip bonded onto flexible substrates [polyimide and liquid crystal polymer (LCP)] and die flip chip laminated onto LCP films. A key to achieving each of these techniques is the thinning of die to a thickness of 50 mum or thinner. Conventional grinding and polishing can be used to thin to 50 mum. At 50 mum, the active die becomes flexible and must be handled by temporarily bonding it to a holder die for assembly. Both reflow solder and thermocompression assembly methods are used. In the case of solder assembly, underfill is used to reinforce the solder joints. With thermocompression bonding of the die to an LCP substrate, the LCP adheres to the die surface, eliminating the need for underfill.

Thinned die can be used to realize ultra-thin flexible electronics for applications such as conformal and wearable electronics. Three techniques have been developed to achieve this goal using thinned die: die flip chip bonded onto flexible substrates, die laminated onto LCP films, and die embedded in polyimide. A key to achieving each of these techniques is the thinning of die to a thickness of 50 mum or thinner. Conventional CMP processing can be used to thin to 50 mum. At 50 mum, the active die become flexible and must be handled by temporarily bonding them to a holder die, for further processing. Once bonded face down to the holder die, the active die can be further thinned by DRIE etching the exposed backside. The thinned die can then been packaged in or on the flexible substrate.

Driven by a growing range of applications in the automotive, industrial, military, aerospace, computer, telecommunication, consumer electronics, and medical electronics industries, miniaturization and the use of flex circuits continue to be of prime interest to electronics manufacturers. The assembly of thinned silicon die (25-100 mum) onto flex substrates provides options for ultrathin, flexible electronics for applications ranging from smart cards to space-based radars. For high-density applications, 3-D modules can be fabricated by stacking and laminating preassembled and tested flex layers and then processing vertical interconnections. This paper describes a low cost, highly manufacturable process developed for flip chip assembly of thinned die to poly-imide flex substrates that eliminates the need for special handling tools and techniques. In this paper, solder bumped thinned die are reflow soldered to the patterned flex using a method that maintains the flex substrate flat during die placement and reflow. Reflow is followed by underfill dispense and cure. The underfill dispense process is critical to avoid underfill flowing onto the top of the thin silicon die and will be discussed. Parts assembled using these processes have undergone reliability testing, a high degree of reliability has been found, and those results are presented.

Manufacturers of consumer electronic products are continuously striving to confer greater functionality to smaller, lighter, and less expensive packages, and flip-chip is an important enabling technology for these product trends. Underfill between the die and an organic substrate is necessary to compensate for the coefficient of thermal expansion mismatch. The underfill dispense and cure step is not a typical process for a surface-mount technology (SMT) factory, and demands additional capital equipment, floor space, cycle time, and headcount. An alternate approach to traditional capillary underfill is wafer-applied underfill. The underfill is applied after wafer bumping and sawing, but prior to the picking of the individual die from the saw tape. This paper describes the coating and assembly processes. Liquid-to-liquid thermal cycle shock tests (-55 to +125/spl deg/C) have been performed on test vehicles assembled with the wafer-applied underfill. First failures were at over 1000 cycles. Weibull plots of the data and failure analysis are presented.

The move to replace lead in electronic assemblies continues to gain momentum. The driving factors are potential lead banning legislation, primarily in Europe, recycling laws in Japan and global market pressures for more environmentally friendly products. After more than 10 years of research, a 'drop-in' solder alloy replacement for eutectic Sn-Pb has not been found. While a number of alloys are still being discussed, the current industry trend is to use the eutectic (or near eutectic) Sn-Ag-Cu alloy. The melting point of this alloy (similar to 217 degreesC) is significantly higher (34 degreesC) than eutectic Sn-Pb (183 degreesC). This higher melting temperature will impact the assembly process. In addition, the lead-free alloys have a higher modulus than eutectic Sn-Pb. This may change the stress distribution during thermal cycle testing, affecting reliability and failure modes. This paper examines the assembly process for flip chip die with Sn-Ag-Cu solder bumps and on-going reliability testing.

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246/spl deg/C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235/spl deg/C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40/spl deg/C to +125/spl deg/C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts.

This work focused on investigating a novel recycling technique, swelling method, to recovering PVC coated PET fabrics. This technique used a chemical as a swelling agent to swell the only PVC component in the coated fabric. This process degrades the mechanical properties of PVC component which allows it to be broken into small pieces and detached from the PET fabric substrate by mechanical agitation. The recovered products, PVC particles and PET fabric or yarn agglomeration, were obtained by screening. The choice of swelling agent is very important in this method since it influences the processibility and cost of the recycling procedure. By comparing the physical and chemical parameters of some candidates, methyl ethyl ketone (MEK) was selected as a swelling agent, mixed with water in different amounts to form a swelling bath. In the swelling system, the phase property of MEK/water influenced the solubility parameters of swelling bath, which changed swelling behavior of PVC component in coated fabrics and finally affected the recycling process. However, recovered PVC component lost more than half equivalent of the plasticizer. The other recovered components were very similar to those in virgin PVC component in coated fabric, so that it could be reused as coatings in the manufacture of PVC coated fabrics. Recovered PET fabric scraps had glue and kept their properties unchanged. For the purpose of added value recycling, they were further treated with dimethyl formamide (DMF) to remove the glue and yield pure PET fibers. If other conditions are constant, temperature influences glue-removing process and final PET fiber properties. The orientation of both molecular chains and segments in treated PET fiber decreased with DMF bath temperature, reducing the tensile strength and sonic modulus of PET fibers but increasing the elongation. The experimental results showed that average size of crystalline region was a function of temperature. The research work also covered applications of recycled substances. Recycled PET fabric scraps with glue was used as substrate to reinforce epoxy composites. Glue-free PET staple fibers were used to make needle-punched nonwoven fabrics. This research completed the first part of work in the development of swelling method to recycle of PVC coated PET fabrics. It made a theoretical investigation and yielded some fundamental data for future development. There are still problems to solve, such as separation of PVC and polyester fiber, recovery of washing wastewater and DMF/glue mixtures, designing of corresponding equipment and pilot tests.