Drying and gas or vapor contact with solids – Apparatus – Houses – kilns – and containers
Reexamination Certificate
2002-01-18
2004-07-13
Bennett, Henry (Department: 3743)
Drying and gas or vapor contact with solids
Apparatus
Houses, kilns, and containers
C034S212000, C034S219000
Reexamination Certificate
active
06760981
ABSTRACT:
FIELD OF THE INVENTION
The invention is generally directed to a compact drying chamber for drying articles by convection heating. More specifically, the invention provides a compact drying chamber for drying printed circuit boards and other electronic assemblies by enhancing evaporation. The compact drying chamber of the invention provides high velocity convection-heated air and maximizes dwell time of printed circuit boards within heated air to achieve enhanced rates of evaporation.
BACKGROUND OF THE INVENTION
In the field of manufacturing printed circuit boards and electronic assemblies, prior art systems and apparatuses provide various treatments of printed circuit boards such as reflow or wave soldering, etching, and cleaning that are typically followed by rinsing and drying processes. Printed circuit boards are typically cleansed in water-based cleaning systems and then subsequently dried in various types of dryers and drying systems that employ air. Many prior art designs supply air to printed circuit boards at high velocity (typically >10,000 fpm) to physically remove the bulk of residual water from the surfaces of printed circuit boards and to dry boards by evaporation. Air is typically delivered by a number of discrete air nozzles, air knives or other devices as high velocity heated air streams through which printed circuit boards pass during the drying process.
High velocity air streams striking printed circuit boards blows off the bulk of exposed water residing on the surfaces of boards after cleansing and breaks up such water into small and fine water droplets that are eventually removed by evaporation. Although striking boards with high velocity air readily removes the bulk of residual water on printed circuit boards, the impact breaks water into very small and fine water droplets that are difficult to physically remove from printed circuit boards due to their high surface tension. A layer of air at an intermediate temperature forms between small water droplets and the surrounding air, which produces an insulating or “skin” effect that reduces a rate of heat exchange. Impinging small and fine water droplets with high velocity heated air disrupts this “skin” effect and enables water to be removed by evaporation.
In addition, small and fine water droplets residing in or around the different types of electronic connectors and components connected or surface-mounted to printed circuit boards are often hidden and essentially removed from the path or flow of heated air. Small and fine water droplets also reside or pool within narrow or deep recesses of electronic connectors and components and, thus, are not sufficiently exposed to the heated airflow. Heated air streams simply cannot reach small and fine water droplets embedded within such electronic connectors and components. Such small and fine water droplets will eventually be removed by evaporation as the temperature of printed circuit boards and electronic connectors and components is raised and maintained upon continued exposure to heated air. However, evaporation takes place at the interface of heated air and water. Since such small and fine water droplets have little or no contact with heated airflow in comparison to their volume, removal of water from electronic connectors and components by evaporation alone would take too long.
At practical speeds of conveyance provided by many prior art dryer designs, the time printed circuit boards are exposed to high velocity heated air, or the dwell time, is insufficient or too short to completely dry printed circuit boards by evaporation. Thus, reliance upon evaporation alone to remove small and fine water droplets from surfaces of printed circuit boards and electronic connectors and components to completely dry boards requires an increase in the dwell time. The more time each point along the surfaces of printed circuit boards is exposed to heated air, the faster the rate of heat exchange and the faster the temperature of printed circuit boards and electronic connectors and components is raised to a sufficient degree to enhance evaporation of water. Hence, the longer the dwell time, the more completely printed circuit boards are dried.
Many prior art dryer designs that employ high velocity heated air increase the dwell time of printed circuit boards by increasing the number of air nozzles or air knives in order to increase the streams or jets of high velocity heated air through which printed circuit boards pass during drying. However, an increase in the number of air nozzles or air knives requires one or more large air blowers and a delivery system to effectively pressurize and circulate a sufficient volume of heated air through a large number of nozzles or knives. Use of large air blowers to accommodate the required output typically results in a substantial increase in the size of the dryer and an increase in the dryer's power consumption and exhaust requirements. In addition, high power blowers are often limited to intake temperatures of about 125° C., which requires cool air to be added to the blowers' intake, which increases the exhaust requirements of the dryer. For instance, one prior art design employs a large number of discrete air knifes to increase dwell time that requires a twelve foot drying chamber through which printed circuit boards are conveyed to be completely dried. This dryer design is effective in drying printed circuit boards. However, its disadvantages are its large size and high power consumption. In addition, such large dryer designs operate at high noise levels. Another prior art design increases the dwell time of printed circuit boards by delivering a large volume of heated air on the order of about 3,000 cfm at a high velocity that requires three 15 hp air blowers and does not recirculate air. The disadvantages of this dryer design are its very high power consumption and exhaust requirements. Thus, prior art designs that use a large number of air nozzles or air knives and/or a large volume of heated air to lengthen the dwell time of printed circuit boards result in very large dryers and drying systems with high power consumption and high exhaust requirements. Such dryer designs are also expensive to manufacture and operate.
Therefore, it is desirable to provide a dryer or a drying system with the capability of rapidly heating printed circuit boards and particularly electronic connectors and components connected or mounted thereto to efficiently and completely dry printed circuit boards by an enhanced rate of evaporation without substantially increasing the size of the dryer or drying system, its power consumption, exhaust requirements and manufacturing and operating costs.
SUMMARY OF THE INVENTION
The invention provides a compact drying chamber for drying printed circuit boards and the different types of electronic connectors and components connected or surface-mounted thereto by convection heating after printed circuit boards have been washed or otherwise processed. Embodiments of the compact drying chamber according to the invention dry printed circuit boards and electronic connectors and components by enhanced evaporation that removes residual water otherwise difficult or impossible to physically remove by heated air streams. Embodiments of the compact drying chamber according to the invention overcome the limitations and disadvantages of prior art dryer designs with respect to overall size, power consumption, exhaust requirements and manufacturing and operating costs.
The invention provides a compact drying chamber comprising features that control the parameters found to significantly affect a rate of evaporation and, consequently, affect a rate at which printed circuit boards are completely dried by evaporation. Such parameters include the velocity at which heated air impacts surfaces of printed circuit boards, the dwell time or the time printed circuit boards are exposed to heated air, and the temperature of the heated air provided to printed circuit boards.
Embodiments of the compact drying chamber of the invention enhance a
Bennett Henry
Lowrie Lando & Anastasi, LLP
Ragonese Andrea M.
Speedline Technologies, Inc.
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