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Process Cleaning Magazine
© 2012
AMT-The Association For Manufacturing Technology
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PC FEATURE
Two-compartment cabinet oven
Class 100 cleanroom truck oven
600ºF electrically heated truck oven
Electrically-heated tunnel oven
Whether removing flux, curing potting compound, drying or coating, the applications for heat processing equipment in the manufacture and assembly of printed circuit boards (PCBs) are many and varied. Freestanding, inline and conveyor integrated equipment abounds in the EOEM, and a number of examples are shown here. The Grieve Corporation (Round Lake, Illinois) is a manufacturer of heat processing equipment for the electronics industry.
In a cleanroom environment, such ovens as Grieve’s Class 100 cabinet style are often used for delicate heat cleaning of circuits, drying and setting coating films onto PCBs. Heat cleaning is used to remove flux particles and other contaminants from the boards to prevent circuit disruption or signal malfunction.
The maximum operating temperature of this unit is 260ºF, or approximately 127ºC, in a shelved workspace measuring 36" wide x 36" deep x 39" high. 20kW are installed in Incoloy-sheathed tubular heating elements while a 1,000-CFM, 1-1/2 HP recirculating blower delivers a horizontal airflow to the workload. In such ovens, the in-cycle temperature variation within the entire oven workspace might be less than a degree, owing to the 4" thick insulation, Type 304, 2B finish stainless steel interior with backwelded seam construction and heavy-duty, explosion-venting door hardware (mandated on equipment where flammable solvents are processed). In properly designed and code-compliant equipment, many other devices are required on board such ovens. The unit also carries a 30" x 24" x 6" thick stainless steel high-temperature HEPA recirculating air filter, plus a digital indicating programming temperature controller and SCR power controller.
The two-compartment cabinet oven processes independent batch runs of PCBs at a customer of Grieve. Separate heat chambers are located at the rear of the unit, one for each compartment. Air is recirculated in a full horizontal pattern through each of the compartments, independently operated by separate digital indicating temperature controllers, mounted onto a single control cabinet for operator efficiency. For further control, each compartment is equipped with additional safety devices, including a manual reset excess temperature controller with separate heating element control contactors. Also, a recirculating blower airflow safety switch turns off the heat in the event of a blower malfunction, thereby preventing damage to the workload.
Each compartment contains six rollout shelves made of stainless steel, supported by ball slides. A primary shutdown timer is adjustable to ten hours and shuts down the oven heat only. A secondary timer shuts down the recirculating blower when the process cycle is completed. A pilot light on the oven control panel signals when the oven cycle is complete.
The oven shown here has dual compartments, each measuring 24" wide x 18" deep x 24" high. Alloy heating elements of 12kW are installed in each compartment to supply operating temperatures to 500ºF.
The Class 100 cleanroom truck oven has a maximum operating temperature of 500ºF and workspace dimensions of 36"wide x 40"deep x 66" high. The Incoloy-sheathed tubular elements to heat the chamber have 30kW installed while a 2,400-CFM, 3-HP recirculating blower delivers a horizontal airflow to the workload. Often, whether truck loaded or shelved, an oven chamber must be properly ducted and the airflow pattern must be sufficient to deliver constant heated air to the entire workpiece, only its top side or bottom, or other combinations. Good oven companies will take all load, workpiece configuration and spatial packing requirements into consideration when suggesting the right piece of equipment for cleaning, drying, coating, curing, setting or other heat processing tasks. This oven further features 4" insulated walls, an aluminized steel exterior, stainless steel interior with continuously backwelded seams, white epoxy paint, #4 brushed stainless steel door cover and control panel face, 325-CFM powered forced exhauster, two 24" x 30" x 12" thick stainless steel high-temp HEPA recirculating air filters and a 4" insulated floor with removable truck wheel guide tracks.
For safety and precise process control, this oven further features a digital indicating temperature controller, manual reset excess temperature controller with separate control contactors, recirculating blower airflow safety switch and 10" diameter circular chart temperature event recorder.
Grieve’s 500ºF electrically heated cabinet oven with vertical airflow was designed to fully integrate with the customer’s conveyor system in an electronics assembly plant. Penetration of the sidewalls was made for pass-through of the conveyor, but the oven was designed in such a way as to minimize the air volume loss and related energy costs. With a workspace of 44" wide x 24" deep x 24" high, this oven has 10kW installed in Incoloy-sheathed tubular elements to heat the chamber while a 1,000-CFM, 1-HP recirculating blower provides a vertical downward airflow to the workload. This apparently obvious airflow pattern to save energy (heat does rise and would be blown out the sidewalls if upward in flow) requires a special ducting to capture and recirculate the maximum volume of air with minimal heat loss, thus conserving the energy while still achieving the desired results in the heat processing cycle.
The 600ºF electrically heated truck oven is well suited for more stringent cleaning requirements. Nichrome (special alloy for this temperature range and resistance to oxidation) wire heating elements have 24kW installed while a 1500 CFM, 1-1/2 HP recirculating blower provides horizontal airflow to the higher-capacity batch workspace measuring 36" x 36" x 60" high. Unique features mandated by the customer’s facility and production layout included a top-mounted heat chamber. This oven has 5" insulated walls for heat containment and uniformity, a Type 304, 2B stainless steel interior, stainless steel exterior with #4 brushed finish and motorized dampers on the intake and exhaust for accelerated cooling, working in conjunction with a 325-CFM powered forced exhauster. As this oven is used for handling flammable solvents, all the necessary hardware and auxiliary devices required by NFPA, IRI and FM are on board.
An electrically-heated tunnel oven, this unit is also used for circuit board processing and runs at similar operating temperatures to 500ºF. However, a much higher heat input of 60kW is needed to supply consistent operating temperatures to this workspace, which measures 24" wide x 120" long x 15" high. This tunnel oven is used in conjunction with the customer’s conveyorized production line to produce finished boards. Special supports were engineered into the body of the oven to integrate with the customer’s conveyor line—a frequent occurrence in the industry. Special care must be taken whenever such a production scenario is needed, as the integration of different equipment heights, transfer lines, conveyors and ancillary devices can result in excessive heat loss and energy waste if not properly designed.
In this tunnel oven, for example, vertical lift doors at each end of the workspace are built into the oven body while access doors to the workspace and oven heat chambers are likewise onboard. All have special insulation to contain the process heat and related energy costs. Additional features of this oven include a digital indicating temperature controller, manual reset excess temperature controller with separate contactors, blower airflow safety switch and circular chart recorder with remote data transmission capability. Many versions of this oven type now carry wireless data and alarm signal sending capability, resulting in significant savings of hard wiring.