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Account of a Cleaning Systems Changeover
December 31, 1969 |Estimated reading time: 4 minutes
A self-contained aqueous cleaning and drying unit, supplemented with a wastewater recycling system, replaces an isopropyl alcohol vapor-degreasing system with Positive results.
By Bill Boyd and Shean Dalton
Changes in cleaning methods at Raytheon's Electronic and Missile Systems products facility at Air Force Plant 44 in Tucson, Ariz. have resulted in improved environmental performance, more effective cleaning and substantial production cost reductions. The plant, a producer of printed circuit assemblies for several military weapons programs, has credited the process conversion to eliminating substantial volatile organic compound (VOC) discharge while reducing the cost of cleaning-related consumables by $18,000 per year.
Some missile programs require that circuit assemblies be coated with vacuum-deposited Parylene for environmental and dielectric protection. This coating requires substrate cleanliness for effective adhesion, and the Six Sigma processes were implemented to find a cleaning approach that would be superior to IPA cleaning.
The aqueous cleaning and drying system* provided improved cleaning performance in less floor space than an IPA degreasing unit previously used.Courtesy ELECTROVERT division of Cookson Electronics Equipment.
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A team was assembled to characterize cleaning requirements for the circuit assemblies and study alternatives. The stated vision of the team was to investigate and qualify an alternative cleaning process that would be greener, faster, cheaper and safer.
The team consulted with Phoenix Marketing, a capital equipment specialist for surface mount technology and semiconductor manufacturing, based in Tempe. It recommended installation of an aqueous cleaning system,* an automated multi-function batch cleaning and drying unit.
A Design of Experiments methodology for the cleaning process evaluation began with samples. Boards were cleaned in the IPA vapor degreaser and a 75 percent remainder deionized water rinse in an aqueous cleaner. Contaminants to be removed included fingerprints, oils, and organic and rosin fluxes. Test samples and sample hardware were processed normally.
Team engineers assessed cleaning process effectiveness using an ionic cleanliness test system. This static test methodology** detects resistivity changes in a heated test bath that indicates the presence and volume of manufacturing residues.
Visual inspection reportedly showed no flux contamination on either HF1189 flux or rosin flux samples that had been cleaned in the aqueous process (Table). However, HF1189 flux samples cleaned in the IPA process were visibly contaminated while one-fourth of the rosin flux samples cleaned exhibited visible trace contamination. The team found the aqueous cleaning performance of the tested system to be a clear improvement over the existing IPA process and deionized water rinsing. There was also savings over other solvent cleaning alternatives.
Next, PCB samples cleaned using the processes described were Parylene-coated in a vacuum deposition system. (Parylene is applied to circuit assemblies at room temperature in a gas-vapor-deposition process with no catalysts or solvents.) The coated devices were then subjected to life-cycle testing, followed by evaluation for Parylene-adhesion properties. This involved visual inspection per ANSI-J-STD-001B and ASTM D3359 for adhesion with ratings based on the amount of Parylene removed. The table also shows the results of the ASTM test, where 1 = worst adhesion and 5 = best adhesion performance.
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The 20-year life-cycle test of aqueous-cleaned and Parylene-coated test samples consisted of surface scratching to simulate coating damage. This was followed by 476 time/temperature cycles in the range of 100° to 0°C with a dwell time of 10 min and a ramp rate of 10°C/min. Evaluation per the ANSI and ASTM standards showed no Parylene peeling, lifting, bubbling or apparent damage on units cleaned in the aqueous process.
An environmental-stress screen test of two similarly cleaned and coated qualification units (varying from -45° to 85°C with a one-hour soak and 30 min transition time) also showed no evidence of peeling, blistering or coating degradation for aqueous-cleaned, Parylene-coated boards.
Based on these favorable test results, the vapor degreaser unit and IPA solvent were replaced with the aqueous cleaning system. The new system removes contaminants by maximizing flooding in planar sections. It uses impact force from moving high-pressure nozzles to remove contaminants from tight spaces and in small, narrow apertures, such as those under ball grid array (BGA) and flip chip packages.
The manufacturer modified the aqueous cleaner to suit special requirements for circuit assemblies. System changes included an enlarged front opening, adaptations to circuit assembly fixtures and alterations to the moving high-pressure water jet mechanism.
A typical in-line aqueous-cleaning process requires 4 to 5 gpm of heated, purified water — or approximately 2,400 gallons per eight-hour shift. The cost of water, energy and sewer assessments at this consumption level would be substantial, while trace contaminants in the waste stream are unacceptable for direct disposal. The aqueous cleaning system's closed-loop wastewater treatment unit recycles the entire waste stream to recover 80 to 90 percent of thermal energy, and to capture and separate heavy metals and organic materials for disposal.
The new process uses an associated rinse-water-recycling unit with filters and resin bed tanks to separate sludge and cleaning by-products from the process water, which is returned to the cleaning system for reuse. Thus, wastewater disposal has been eliminated while isolated residues are disposed of following authorized processes. Lastly, since absolute drying effectiveness is important for boards cleaned in preparation for Parylene coating, the aqueous cleaning system incorporates a patented air-knife dryer that removes trace moisture from all surfaces.
This approach has cut VOC emissions at Raytheon's facility by 10 tons annually. The new process also delivers a 10 percent faster cycle time and reduces manufacturing floor-space requirements by 300 sq ft. Since 1995, the company has cut VOC emissions from solvents, alcohols and thinners at the Tucson facility by approximately 80 percent.
*AquaJet
**SCS Omegameter 600SMD
Bill Boyd, western regional district sales manager, may be contacted at Cookson Electronics Equipment, 3211 NW Ivy Lane, Camas, WA 98607; (360) 954-5508: Fax: (360) 9654-5509. Shean Dalton, product marketing manager, may be contacted at Cookson Electronics Equipment, Highway 5 South, Camdenton, MO 65020; (573) 346-3341 ext. 170; Fax: (573) 356-5554; Web site: www.cooksonelectronics.com.