Graphite methods disperse graphite (another form of carbon) onto the substrate surface. Similar to the carbon method, a conditioner solution creates a positive charge on the substrate surface, including the through-holes. Graphite particles are then adsorbed onto the exposed surfaces. In contrast to the amorphous structure of the carbon black crystallites, graphite is a three-dimensional, crystalline polymer. This crystalline structure creates a conductive layer covering both the copper and the nonconductive surfaces of the outside layer and interconnects. A copper microetch removes the unwanted graphite from the copper surfaces, leaving a conductive, graphite layer on the glass and epoxy surfaces in the vias.
A typical graphite process has three or four chemical process steps (cleaner/conditioner, graphite, fixer [optional], and microetch) and one air knife/oven drying step, as shown in Figure 2. The number and location of rinses needed between process steps will vary by facility.
Information is presented on the following graphite methods:
- Graphite 2000 (Shipley Company)
- Shadow (Electrochemicals, Inc.)
Background The Graphite 2000 process uses a patented shear pump to keep the graphite suspended in solution. The Graphite 2000 process is run on conveyorized (horizontal) equipment. Customers using this process run predominantly double-sided boards (approximately 65 to 90 percent double-sided and 10 to 35 percent multi-layer). Most of the customers run between 2,000 and 5,000 surface square feet per day. According to the vendor, the Graphite 2000 system is limited to boards that are 0.125 inches thick or less, holes that are 0.013 inches in diameter or greater, and aspect ratios of 8:1 or less.
Implementation at Specific Facilities
Two facilities (Facilities C and D) that have successfully implemented the Graphite 2000 process were interviewed for this report. Primary motivations for switching to the Graphite 2000 system in both cases included the elimination of formaldehyde, hydrazine, and cyanide; reduced operating costs; and improved worker safety. The facilities chose the Graphite 2000 system for several reasons: the vendor had a strong reputation in the industry, a good relationship had been established with the vendor, and both facilities were beta sites for the technology.
Facility C was the alpha-beta site; it was the first facility to install the Graphite 2000 system. Facility C took three months to install and debug the Graphite 2000 system; Facility D took six weeks. Both facilities installed new equipment from Finishing Services Limited (FSL), one of the vendors recommended by Shipley. To reduce water usage, Facility C installed a chiller to provide a closed-loop cooling system for the conveyorized unit. Facility D modified its equipment by removing the scrubbing unit and adding an anti-tarnish module and a high-pressure (125 pounds per square inch) water blast at the end of the graphite line. According to Shipley, equipment installation requires a week, chemistry evaluation takes another week, and then the system needs a trial month before a facility can go to full production on all substrates and work types.
|REASONS WHY FACILITIES SWITCHED TO GRAPHITE 2000|
Experiences with the Graphite 2000Process
During debugging, equipment problems outnumbered chemistry problems. For example, Facility C experienced problems with plugged nozzles that caused weak coverage on the board. By replacing the spray manifolds with fluid wedges, the facility improved the coverage and solved the problem. Occasionally, the squeegee rollers do not remove enough water from the board (after the graphite tank) during full production. Daily preventive maintenance helps minimize equipment problems. "Sometimes," reported Facility C, "small holes can be an issue. In these cases, we run the boards through three times." The facility believes that Shipley is improving the process so that smaller hole sizes can be run. Approximately 97 percent of Facility C's customers accepted the new technology immediately; the remaining 3 percent needed more data, testing, and in-house inspections before accepting it. Customers of Facility D had no problems with the new technology.
With its previous electroless copper line, Facility D contracted out its multi-layer production so that the facility would not need a permanganate desmear operation in-house. When Facility D installed its new Graphite 2000 line, the facility decided to continue to contract out its multi-layer production.
Comparisons to Electroless Copper
Facility C spends more time on equipment maintenance for the Graphite 2000ª process than for the electroless copper process, but less time on lab analysis. Facility D has not experienced any major changes in time spent on maintenance or lab analysis. Both facilities report reduced cycle time and water usage with the new system.
Keys to Success
Both the vendor and the manufacturers thought that commitment and dedication at all levels, from management down to the line operators, is vital for the successful implementation of the Graphite 2000 system. According to Facility D, "The selection of high-quality equipment, and its daily maintenance, are extremely important. Also, management must be patient. It takes four to six weeks to get everything working smoothly." Shipley believes that a company needs to view the Graphite 2000 process as part of the bigger picture. "Often, changes need to be made upstream or downstream to optimize the graphite process. A company needs a willingness and commitment to change their process and to maintain better control of the process." For example, a facility switching to the Graphite 2000 process might need to adjust and optimize the process window in the electrolytic plating step downstream from the graphite step.
For more information on the Graphite 2000ª system, contact Hal Thrasher of Shipley Company at 508-229-7594.
The Shadow process uses a patented binder system in the graphite mixture to promote hole wall adhesion and colloid stability. The process also includes a fixer step immediately following the graphite bath; the patented fixer promotes a uniform graphite coating of the hole wall. Almost all Shadow systems are conveyorized (horizontal). The Shadow process has successfully run multi-layer boards and exotic substrates (e.g., Teflon®, rigid flex). According to the vendor, the limitations of the system are related to the quality of the incoming boards; drilling quality is especially important. According to one facility interviewed (Facility E), boards that are thicker than 0.093 inches are run at a slower conveyor speed, and Teflon® boards go through two passes at the slower conveyor speed.
Implementation at Specific Facilities
Three facilities (Facilities E, F, and G) were interviewed for this report--two that have successfully implemented Shadow and one that has switched back to electroless copper. Primary motivations for switching to the new process included the elimination of formaldehyde, reduced cycle time, and decreased water usage. One facility interviewed chose the Shadow system because it was the most affordable conveyorized system available at the time (March 1995). A second facility interviewed helped the supplier develop and test the Shadow system. Installation of the system took approximately two to five days (not counting delays due to missing equipment parts), and the debugging period ranged from one to two months. All three facilities purchased new, conveyorized (horizontal) equipment for the system.
|REASONS WHY FACILITIES SWITCHED TO SHADOW|
Experiences with the Shadow Process
All the facilities encountered some problems during debugging and/or full operation, most of which were equipment-related. Facility E now experiences only occasional mechanical problems during full production; "there are always little problems." For example, if one roller is out of place, it creates dragout which could contaminate the other tanks. Another problem occurs when the squeegee rollers sometimes develop hard spots where solids collect. "We are thinking of having a second set of rollers immersed at all times, so that changeover is more efficient," the facility reported. Most equipment problems can be minimized by aggressive preventive maintenance.
During debugging, Facility F found that graphite left on the board surface due to excessive dragout created drying problems. To reduce dragout from the graphite tank, the facility increased the tension of the squeegee rollers. This facility also had graphite build-up at the "knee" of the holes during drying. To solve this problem, the facility installed a bigger blower motor in the dryer to create sufficient air flow through the holes. Facilities E and F have not had any issues with customers accepting the Shadowª process.
Facility G switched back to electroless copper after running the Shadow line for less than one year. During debugging, Facility G switched from a sulfuric peroxide microetch to a sodium persulfate microetch, which increased the copper discharge concentration. In addition, the facility had problems maintaining the agitation needed to keep the graphite in suspension. This resulted in unexpected sludge generation and plugged nozzles. During full production, the facility's customers found that solder joints would fail during circuit board assembly.
According to the vendor, Electrochemicals, Inc., almost all of the companies that have pulled out of the Shadow process had problems with equipment. The vendor stated that it is critical for manufacturers to follow the vendor's equipment recommendations.
Comparisons to Electroless Copper
Facility F spent less time maintaining the Shadow line than maintaining their previous electroless system, and spent a lot less time on lab analysis. Facility F reduced cycle time from 90 minutes to approximately 10 to 15 minutes, while significantly reducing water consumption. Chelated copper was eliminated from the wastestream, and the copper concentration in the discharged wastewater was reduced.
Keys to Success
Electrochemicals, Inc. emphasizes the importance of quality production practices. "If a facility has quality problems with electroless copper, it will still have those problems with direct metallization." The vendor believes it is important that facilities follow the vendor's recommendations for equipment purchases. The facilities report that daily chemical analysis, preventive equipment maintenance, and a commitment to eliminating formaldehyde are necessary to successfully implement the Shadow process.
For more information on the Shadow system, contact John Myers of Electrochemicals, Inc. at 612-479-2008.