Power parameters of vacuum coating high-voltage power supplies in decorative coatings and smart-window glass

 

　　The sputtering power is set at 3 kW for decorative films, and at 5 kW for smart-window glass. The cathode voltage runs at 400 V at 3 kW, and at 500 V at 5 kW. The voltage rises with the target erosion, because the magnetic field weakens as the target thickness decreases. The power supply holds the power constant, adjusting the voltage to maintain the setpoint. The voltage drift over a 100 hour run is 5 percent, from 400 V to 420 V, and the drift is within the 10 percent acceptance limit.

 

　　The decorative film process deposits a 20 nm silver layer on stainless steel trim parts. The parts hang on a rotating drum, 500 mm diameter, rotating at 5 rpm. The drum holds 40 parts per run, and the run time is 20 minutes. The film thickness is measured by a quartz crystal monitor, positioned near the cathode. The monitor reading is calibrated by a profilometer on a witness sample, measured after the run. The thickness uniformity across the parts is within 5 percent, measured at five points on each part.

 

　　The smart-window glass process deposits a 500 nm tungsten oxide layer on 1 m by 1 m glass panels. The glass sits on a substrate holder, moving past the cathode at 10 mm per second. The film thickness is measured by the quartz crystal monitor, and the monitor reading is calibrated by a spectrophotometer on a witness sample. The thickness uniformity across the glass is within 3 percent, measured at nine points on each panel.

 

　　The high-voltage cable connects the supply to the cathode, through a 5 m shielded cable. The cable is a 10 kV silicone type, with the outer jacket grounded at both ends. The cable once developed a hairline crack in the jacket, and the leakage current rose from 5 mA to 30 mA. The leakage did not trip the 50 A overcurrent threshold, but the extra 25 mA added noise to the cathode voltage measurement. The cable was replaced at the next scheduled maintenance, and the leakage returned to 5 mA.

 

　　The supply cabinet is 600 mm wide, 800 mm deep, 1800 mm tall, with IP54 protection. The cabinet contains the inverter, transformer, rectifier, divider, and control board. The cabinet cooling fans run on a thermal switch, starting at 35 degrees and stopping at 25 degrees. The fans once failed in summer, and the cabinet temperature reached 55 degrees. The electrolytic capacitors on the DC bus reached 65 degrees, above the 60 degree rating. The fan fault alarm now routes to the central monitoring room, and the fan is replaced within 10 minutes of alarm.

 

　　The control board uses a 16-bit ADC, sampling the output voltage and current at 1 kHz. The ADC reference is a buried-zener type, with a drift of 2 ppm per degree Celsius. The cabinet temperature is held at 25 plus or minus 2 degrees by the fan and heater, and the reference drift over a 24 hour operation stays below 10 ppm. The drift is within the 0.1 percent acceptance limit, and the reference is recalibrated annually against a 0.01 percent standard divider.

 

　　The safety interlock on the chamber door was tested during commissioning. A technician opened the door while the supply was running, and the output dropped to zero within 20 ms. The residual charge on the output capacitor bled through a 10 MΩ resistor within 100 ms. The interlock response time was logged at 18 ms, within the 25 ms design limit. The chamber door carries a high voltage warning label, and entry requires a sign-in log. The non-authorized personnel are not permitted inside the coating bay.

 

　　The maintenance log lists routine tasks. Each shift, the operator records the output voltage, current, and cabinet temperature. Weekly, the technician inspects the cable jacket and the cathode feedthrough. Monthly, the insulation resistance is measured with a 1000 V megohmmeter, and the reading should exceed 1000 MΩ. Quarterly, the divider is calibrated against a 0.1 percent standard. Annually, the ADC reference is recalibrated. The log is signed at each step, and the record is filed in the plant archive.

 

　　The input side of the supply draws 400 V three phase at 15 A. A 30 kVA isolation transformer sits between the plant bus and the supply, to isolate the chamber ground from the plant ground. The isolation transformer has a 2500 V insulation rating between primary and secondary. The input cable is copper, 10 mm², rated for 35 A, routed through a cable tray. The input breaker is a 32 A molded case type, with electronic trip set to 1.05 times rated current long delay. The input power factor is 0.96, measured with a power analyzer during commissioning.

 

　　The high-voltage transformer inside the supply cabinet is oil insulated, with a 10 L oil volume. The oil temperature reaches 42 degrees after 8 hours of continuous operation at 5 kV and 10 A, against an ambient of 25 degrees. The silica gel breather on the transformer tank changes color after 12 months, and the gel is replaced before the color reaches half saturation. The oil breakdown voltage is tested annually, and the last measurement showed 40 kV per 2.5 mm gap, well above the 25 kV replacement threshold. The transformer sound level is 48 dB at 1 m, measured with a sound level meter during commissioning.

 

　　The output divider is a series chain of 100 MΩ high-voltage glass glaze resistors, totaling 1 GΩ. The divider tap at 1:1000 feeds the control board ADC. The divider is calibrated quarterly against a 0.1 percent standard divider. The calibration drift over 12 months was 0.2 percent, equivalent to 10 V at 5 kV. The drift is within the 1 percent acceptance limit, but the divider is scheduled for resistor replacement at the 24 month mark.

 

　　The chamber vacuum system uses a turbomolecular pump, rated at 200 L per second, backed by a rotary vane pump. The chamber base pressure is 5e-3 Pa, measured by a Pirani gauge. The gauge is calibrated annually, and the drift is checked quarterly. The gauge once drifted, reading higher than the actual pressure, and the chamber ran at 2 Pa instead of the required 0.5 Pa. The drift was caught during a quarterly check, and the gauge was recalibrated. The turbomolecular pump cooling water flow is monitored at 2 L per minute, and a low flow alarm routes to the central monitoring room.

 

　　The process gas supply uses argon, 99.999 percent purity, from a gas cylinder manifold. The manifold has two cylinders, with automatic switchover when one cylinder empties. The gas pressure is regulated at 0.1 MPa, and the flow is controlled by a mass flow controller at 20 sccm. The mass flow controller is calibrated annually, and the drift is checked quarterly. The gas cylinder changeover takes 5 minutes, during which the supply is offline.

 

　　The customer acceptance test was run over two days. Day one covered the decorative film process at 3 kW, measuring the film thickness uniformity and the color consistency. Day two covered the smart-window glass process at 5 kW, measuring the film thickness and the optical transmittance. The test report was signed by the customer representative, and the supply entered the 12 month warranty period.

 

　　The plant environment around the supply cabinet is held at 23 plus or minus 3 degrees Celsius, with 45 to 65 percent relative humidity. Winter heating raises the room temperature to 26 degrees, and the cabinet fan runs continuously. Summer cooling failure once pushed the room temperature to 32 degrees, and the cabinet internal temperature reached 50 degrees. The ADC reference drifted 0.3% during that week, equivalent to a 15 V error at 5 kV. The drift was corrected after the cooling unit was repaired, and the supply was recalibrated. The plant floor is epoxy coated, cleaned daily with a vacuum cleaner, and the illumination is 300 lux with LED fixtures at 24 V.

 

　　The coating process data is recorded on the line server, with the run number, the product type, the film thickness, and the optical transmittance. The data is stored for 5 years, and is available for customer audit. The quality department reviews the data weekly, and trends the film thickness and the transmittance. The data system generates a weekly report, showing the process yield and the defect rate. The report is distributed to the plant manager and the customer.

 

　　The coating bay safety audit is conducted annually, by an internal safety team. The audit covers the vacuum system, the high-voltage system, and the gas supply. The audit report lists the findings, and the corrective actions are scheduled within 30 days. The last audit found one minor finding: a worn gasket on the chamber door. The finding was corrected within the scheduled time. The audit record is filed in the quality system.

 

　　The coating bay spare parts inventory includes: high-voltage cables, cathode feedthroughs, fuses, and quartz crystal monitors. The inventory is stocked at 20 percent of the annual consumption, and is reviewed monthly. The inventory is stored in a locked cabinet, and the key is held by the bay supervisor. The parts are logged out when used, and the log is reviewed by the quality department. The inventory was once depleted when a batch of feedthroughs failed, and the stock was replenished within 3 days.

 

　　The coating bay operates two shifts per day, 6 days per week. The day shift runs the decorative film process, and the evening shift runs the smart-window glass process. The product changeover happens between shifts, and the target is swapped. The shift handover includes a status report, listing the completed runs, the film thickness, and the optical transmittance. The handover log is reviewed by the bay supervisor, who allocates resources for the next shift.

 

　　The vacuum coating process is qualified per ISO 9211, with the film thickness and optical properties tested. The test coupons are prepared alongside the production workpieces, and the coupons are tested after each batch. The film thickness is measured by a profilometer, and the optical transmittance is measured by a spectrophotometer. The test results are recorded in the quality system, and the batch is released only when all tests pass. The process has maintained the qualification for 5 years, with no major findings in the last 2 audits.

 

　　The coating chamber vacuum system is maintained by the maintenance department, with weekly inspection and monthly preventive maintenance. The turbomolecular pump is backed by a rotary vane pump, and the pump oil is replaced annually. The vacuum gauge is calibrated quarterly, and the calibration record is filed in the quality system. The vacuum system has a 99 percent uptime target, and the downtime is logged in the maintenance record. The system once had a vacuum leak, and the leak was fixed within 6 hours, with no production loss.

 

　　The coating bay operator training is conducted by the production department, with a 32 hour course covering the vacuum coating procedures, the safety rules, and the equipment operation. The operator must pass a written exam and a practical exam, before being authorized to run the coating. The operator certification is renewed annually, with a refresher course. The training record is filed in the quality system, and the certification cards are carried by the operators. The training has been conducted for 5 years, and 10 operators have been certified.

 

　　A batch of 120 stainless steel trim parts came back from the decorative-line customer with 6 percent of the parts showing a matte patch on the curved edge. The patch traced to an arc that struck between the silver target and the grounded shield during the ramp-up. The supply carries an arc extinguisher that drops the output current to zero for 50 microseconds on a detected arc, then ramps back in 50 microsecond steps. On the bad batch the arc detector threshold sat at 1.5 times the normal operating current, and weak arcs starting at the edge of the racetrack were not caught. The unextinguished arc heated a 2 mm spot on the shield, spitting copper material onto the parts, and the matte patch measured 15 to 20 percent lower reflectance than the surrounding silver. The threshold was lowered to 1.15 times operating current, and a second arc detector was added on the cathode voltage di/dt channel. After the change, the arc count on a 20 minute run rose from an average of 3 to 22, but the spitter events dropped to zero across the next 8 lots. The lower threshold adds 4 seconds of ramp-up per run, about 0.3 percent of the cycle time, and the trade was accepted because the customer reject rate fell from 6 percent to 0.2 percent. The arc log is now reviewed weekly, and more than 40 arcs per run opens a target cleaning stop.

 

　　The supply has run for 12 months. The high-voltage cable has been replaced once. The output capacitor shows a 3 percent capacitance loss, still within the 10 percent threshold. The unresolved issue is the target erosion rate. The silver target erodes 1 μm per hour of sputtering, and the target life is 500 hours. The customer asked for a longer target life, and a rotating cathode design is under evaluation.