Supercritical  CO 2 Fluid for Chip Resistor Cleaning

Abstract

Cleaning of electronic components has become more difficult in recent years because of the need to eliminate the use of ozone-depleting chlorofluorocarbons. Toxicological studies of hydrochlorofluorocarbon (HCFC), the planned freon replacements, have raised questions about the feasibility of their widespread use for cleaning applications. 1 Unfortunately, the remaining options are few. Among these, vacuum baking works only for a limited number of contaminant/substrate systems, and aqueous solvents require a drying step, leading to a waste-disposal problem. Although water also has a high surface tension in comparison to freons, 2 surfactants added to water to improve its wettability may leave unwanted residues. Finally, various chlorinated solvents used for analytical extraction or cleaning are under scrutiny for health reasons and may generate corrosive hydrochloric acid if overheated during processing.As an alternative, supercritical fluids, compounds existing above their critical temperature and pressure, possess liquid-like solvent and gas-like diffusibility and viscosity properties that enable rapid penetration into crevices and boundary layers and allow for complete removal of organic and inorganic contaminants contained therein. [3][4][5] Moreover, by cycling pressure between supercritical and subcritical values, particles can be effectively dislodged during the expansion phase of the pulsation. 6 Specifically, supercritical CO 2 has the advantage of versatility over other liquid solvents with a minimum of waste. For instance, the solvent properties of CO 2 can be adjusted by manipulating their density via external temperature and pressure application. Small amounts of polar cosolvents can also be added to enhance the solubility of polar materials. 7,8 In addition, the low surface tension of supercritical CO 2 makes it an attractive candidate for cleaning porous materials 9 and possibly removing particles from electronic components. For example, chip resistors are important and commonly used electronic components in PC boards for surface mount technology (SMT). Most of the chip resistors on the market today have wraparound terminations with a barrier of 3-5 m nickel and 6-8 m Sn/Pb solder electroplated by a barrel-plating process. 10 After the Ni and Sn/Pb plating, it is necessary to rinse the chip to take out Ni and Sn/Pb residuals to ensure the quality of solderability during termination soldering application. 11,12 Thus, the termination purity level is the key controlling factor. Applying deionized (DI) water to rinse the electroplated terminations is a conventional method for barrel-plating processes. Such a process generates a mass consumption of DI water and a need for an expensive recycling system for waste treatment. As a result, advanced rinsing methods to improve current disadvantages are expected. We have therefore selected the removal of Ni and Sn/Pb residuals from the terminations of chip resistors as a test for this novel supercritical CO 2 cleaning technique.The main focus of our current work...

Atomic and Molecular Data and Their Applications: An Overview of the 4th ICAMDATA Program

“…It is supposed, judging by the potential curves, that C 60 molecules are stabilized with the assistance of ethane molecules if ethane molecules are trapped between C 60 molecules. It is, in fact, well known that supercritical fluids' molecules can enter even tiny gaps, and therefore critical fluids are used for washing and cleaning semiconductors [22,23]. As we mentioned, it is known that nanoparticles or nanostructures, which are composed of C 60 molecules, can be produced by injecting C 60 -dissolved organic solvents into supercritical fluids such as scCO 2 , scC 2 H 4 and scC 2 H 6 , in which case supercritical fluids' molecules are trapped in the crystals [30][31][32].…”

“…Reactions are encouraged [14][15][16][17], chemicals are extracted [18][19][20][21], semiconductors are washed and cleaned [22,23] and nanomaterials can be produced [24,25] efficiently utilizing supercritical fluids such as supercritical carbon dioxide (scCO 2 ), which has both gaslike and liquid-like characteristics.…”

Low temperature synthesis of fibres composed of carbon–nickel nanoparticles in super-critical carbon dioxide

“…Reactions are encouraged [5,6], chemicals are extracted [7,8], semiconductors are cleaned [9,10] and nanomaterials and nanostructures are produced [11,12] efficiently utilizing super-critical fluids such as super-critical carbon dioxide (scCO 2 ), xenon (scXe) and ethane (scC 2 H 6 ), which have both gas-and liquid-like characteristics. Incident light cannot penetrate fluids under near-critical conditions because of the formation of large molecular clusters, known as critical opalescence, and as a result, the physical properties such as specific heat at constant pressure and isothermal compressibility diverge as the fluids approach their critical points [13].…”

Phase Behavior and Modeling of CO2/Methanol/Tetramethylammonium Bicarbonate and CO2/Methanol/Tetramethylammonium Bicarbonate/Water Mixtures at High Pressures

“…Supercritical CO 2 is currently being considered as an environmentally benign approach to photoresist and etch residue removal. In particular, CO 2 has been applied to precision cleaning (for metal surfaces) − and to wafer cleaning − and drying. − Because of its easily attainable critical temperature and pressure, nonflammability, ease of availability, and low cost, CO 2 presents a very attractive green solution for photoresist and residue removal. However, since SCF CO 2 has little solvating power for photoresist or inorganic materials, it requires the addition of modifiers or additives. − Recently we demonstrated that tetramethylammonium hydroxide (TMAH) is an efficient base additive in the cosolvent due its ability to attack the plasma-processed photoresist crust. − Because of the polar nature of TMAH, its solubility in CO 2 is expected to be small.…”