Recent advances in anisotropic conductive adhesives (ACAs) materials and processing technology
This paper deals with three recent advances in the area of ACA materials and processing technologies, (1) wafer-level ACA, (2) ultrasonic ACA bonding, and (3) Solder ACAs. Patented idea of Wafer level package (WLP) ACA/NCA was introduced earlier as one of the promising ACA packaging technologies due to its advantages such as fewer processing steps, lower cost, and enhanced device performance compared to conventional single chip ACA packaging. WLP ACA can be widely used for flat panel display (FPD), general semiconductor packaging, and 3D-TSV applications because of cost and performance advantages. In addition, another patented novel anisotropic conductive film (ACF) bonding process using vertical ultrasonic vibration was also introduced in flex-on-board (FOB) and Touch Screen Panel(TSP) applications. The ACF temperature was precisely controlled by adjusting both U/S power and bonding pressure. The significant meaning of this technique are (1) significantly reduced ACF bonding time typically 1/3 ∼ 1/5 of conventional T/C bonding times in FOB applications, (2) less thermal damages especially for TSP assembly, and (3) compact and energy saving machine. Lastly, newly developed solder ACFs combined with Ultrasonic processing can increase twice the current handling capability of conventional Ni ACFs and show excellent reliability performance in FOB applications because of metallurgical bonding at the joint rather than physical contact of conventional ACAs.
Wafer-applied underfill: flip-chip assembly and reliability
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.
Effects of substrate design on underfill voiding using the low cost, high throughput flip chip assembly process
Voiding is a concern in low cost, high throughput, or "no-flow" flip chip assembly. This process involves chip placement directly on the pad site with pre-dispensed no-flow underfill on it. The forced motion causes a convex flow front to pass over pad and mask-opening features, promoting void capture. This paper determines the effects of substrate design on underfill voiding using the no-flow process. A full-factorial design experiment analyzes several empirically determined factors that can affect void capture in no-flow processing, including pad height, solder mask opening height, pad/solder mask opening separation, pad pitch, chip placement speed, and underfill viscosity. Test substrates were designed and manufactured at Georgia Tech's Packaging Research Center to ensure process control. The design consisted of 6 factors with a mix of levels for each. These included four levels of copper pad heights, two solder mask opening heights, three pad/solder mask separation distances between copper pad and solder mask opening edges, three feature (pad/mask openings) pitches, two chip placement speeds and four pad/mask geometries separated into pad site quadrants. The experiments involve placement of a transparent glass chip on the pad site through a predispensed no-flow underfill. Subsequent flow of the underfill is carefully recorded and resultant voids are logged and analyzed. The response variable is defined as the number of voids created in the process, and is further analyzed for the location and any visible modes of void formation. Thus, improved substrate designs can be derived.