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Carbon Method

In carbon processes, a conductive layer of carbon black particles is deposited onto the substrate surface and the through-holes. A pre-treatment conditioner solution removes oil and debris from the substrate and creates a positive charge on the glass and epoxy walls of the vias. After conditioning, the substrate is placed in a carbon black dispersion. A noncrystalline structure of carbon black particles is adsorbed onto the positively charged surfaces, creating a conductive layer coating the entire panel. A copper microetch then removes the carbon from the copper surface while cleaning the surface for plating. Because the microetch does not attack the glass and epoxy surfaces of the through-holes, a conductive carbon layer remains only on the through-hole surfaces.

A typical carbon process has six chemical process steps (cleaner, carbon black, conditioner, carbon black, microetch, and anti-tarnish) and two air knife/oven drying steps, as shown in Figure 1. The system is configured as an enclosed, conveyorized (horizontal) process. The specific number and location of rinses and air knife stations depends on the type of product run at a facility and the condition of the rinse water used.

Information is presented on the following carbon method:

  • Blackhole® (MacDermid, Inc.)

Blackhole® (Macdermid, Inc.)

To date, approximately 135 Blackhole® systems, distributed by MacDermid, Inc., have been installed worldwide. Blackhole® customers run a variety of substrates including Teflon®, polyimide, and rigid flex, with holes as small as 0.008 inches in diameter. Most Blackhole® customers run multi-layer boards. MacDermid has not identified any limitations in the types of boards that can be run through the Blackhole® system.

Figure 1. Typical Carbon Process Steps

Figure 1. Typical Carbon Process Steps

Implementation at Specific Facilities
For this implementation guide, two facilities in the U.S. were interviewed about their experiences with their Blackhole® systems. Both facilities process primarily multi-layer boards. Facilities A and B both run boards with up to sixteen layers and boards with aspect ratios of 8:1. Facility B has also successfully processed boards with a 10:1 aspect ratio, and runs a wide range of thicknesses from 0.001-inch thick flex to 0.250-inch thick back panels. When processing high aspect ratio boards, Facility B runs them through the Blackhole® system twice, as "insurance," although the facility engineer thinks this step is probably unnecessary. Both of these facilities are quick-turn shops, so reducing cycle time was the primary factor in their decisions to switch from electroless copper to the Blackhole® system. Both, however, noted other potential benefits, including reduced time and expenses for waste treatment system maintenance.

  • Reduced cycle time
  • Reduced waste treatment
  • Decreased maintenance requirements
  • Wider process window

Experiences with the Blackhole® Process
Both facilities interviewed completed their Blackhole® installations within the last few months and had very different experiences. Facility A installed Hollmuller equipment. After an installation period of two to three weeks, the facility put product on the line and ran the system for one month to qualify it. There were very few problems during the installation and debug period at Facility A, other than some rollers that were redepositing carbon on the panel surface. The type of roller used was changed and the problem was eliminated.

Facility A may have avoided other problems for two reasons. First, this facility uses deionized water for the Blackhole® line. According to MacDermid, there have been problems at other facilities where the water is very high in salts, which may contaminate the Blackhole® dispersion. In these cases, the incoming water may need to be deionized. Second, the engineer at Facility A noted that the capacity of their Blackhole® system far exceeds their throughput. This excess capacity makes it easy for the facility to keep the process controls tight and within MacDermid's parameters.

The installation at Facility B did not go as smoothly. The facility installed the first MacDermid brand equipment manufactured in the U.S. Both "minor irritations" and "major problems" occurred during the installation, all of which were equipment-related. MacDermid made all the on-site modifications necessary to get the system working well, including changing the microetch pumps, the cooling coils, the chiller, and the air knives. The engineer at Facility B believes that the MacDermid equipment manufactured in Germany is superior, since manufacturers there have had much more experience. Facility B's installation spanned several months, followed by one month of running boards to qualify the system. Once all equipment-related problems were solved (there were no issues with the chemistry), the facility was pleased with the system performance.

Facility B has made one equipment modification and one change in the process chemistry. The facility found that propagation improved by running a lower microetch rate. The facility adjusts the microetch daily to 35 to 40 microinches (&in), instead of the 30 to 50 &in given in MacDermid's specification sheet. The facility has also modified the first air knife section. The system had holes in the lid for heat exhaust; however, the carbon solution was blowing off of the boards and out of the holes in the cover. The facility mounted a piece of standard fiberglass door screen to the lid, which let the heat out but kept the carbon in.

As a quick turn-around shop, Facility A deals primarily with engineers. Because of this relationship, the facility did not have any problems obtaining customer acceptance of the new system. The process engineer at Facility A noted that there may be additional hurdles to acceptance for those companies that process a higher volume of boards. Facility B, also a quick turn-around shop, had no problem gaining customer acceptance of the new technology. Facility B invited its major customers (including NASA, the nuclear industry, and the military) to review the data, look at the process, and evaluate the product. These two facilities advise others going through this process to "let your customers tell you what they need to see" and then to supply it.

Comparisons to Electroless Copper
The two facilities interviewed experienced similar benefits of the Blackhole® system over electroless copper. Both saw notable reductions in cycle time ("dramatically reduced" and "significant decrease"). For example, the electroless copper line at Facility B took 1.5 hours to get the first product through after running a load of dummies. After start-up, the line could process approximately 60 panels per hour. Using the Blackhole" system, it takes only six minutes to get the first panel through, dummies are not required, and product can be processed at an average speed of 75 panels per hour.

Although quantitative data are not yet available on changes in board failure rates at either facility, engineers from both facilities have noticed improvements. Facility A noted that the boards are "far superior in terms of hole wall reliability." Facility B has seen increased capability in addition to improved board quality. For example, Facility B's engineer noted that the facility can process smaller holes with the Blackhole® system than with electroless copper. Also, when running electroless, the Facility B operator had to visually scan every panel for problems. Because of the improved quality of the Blackhole® system, this time-consuming step is no longer necessary.

Both facilities made changes to other parts of their process after the Blackhole® line was installed. Facility A has benefited from a more consistent surface than on panels processed with electroless copper. This has enabled the facility to eliminate one of the two acid cleaners from its pre-clean line in the plating process. At Facility B, a downstream change was needed in electrolytic plating operations. When using electroless copper, the facility would start with a low current density and ramp up. With Blackhole®, this facility starts with a high current density for fifteen minutes to get propagation through the hole, and then reduces the density to the lower level. Facility B was able to eliminate the scrubbing process it previously used with the electroless copper line. This has improved the facility's processing efficiency as the panels travel automatically from the Blackhole® line into imaging. It should be noted that the Blackhole® line is a" no-scrub" system; that is, scrubbing is not needed and should not be done after Blackhole® processing.

Although cost was not a primary motivation for installing the Blackhole® system, Facility A estimates that its production cost per square foot has decreased with the new system. The cost savings are the result of reduced chemical and labor costs. The labor savings come from a reduction in the time required for lab testing and maintenance. The electroless system at Facility A required from two to three hours per day of testing and maintenance, compared to two to three hours per week for the Blackhole® line. Facility B has seen similar reductions. The facility now spends about thirty minutes daily on lab analysis with the Blackhole® system, instead of over 2.5 hours per day with the old electroless copper system.

The two facilities have also realized cost savings through reduced maintenance requirements. Weekly maintenance tasks include approximately two hours per week for chemistry changes and four hours per week for equipment maintenance such as cleaning rollers and strainers, and inspecting filters, nozzles, and air knives. As part of their system maintenance, the operator at Facility B completes a 10-minute equipment check list and cleans the pinch rollers every morning. Without cleaning, the carbon can get baked onto the pinch rollers over time, so the rollers are removed daily and cleaned with water. MacDermid also stresses the importance of equipment maintenance. The vendor recommends performing preventive maintenance for two to three hours per week, including cleaning the carbon from the rollers and the nozzles twice a week.

Both facilities have also seen improvements in their waste treatment. Facility A has eliminated batch treatment of chelated wastewater at 700 to 800 gallons a month. There was also a "significant simplification" of waste treatment at Facility B. With the electroless copper line, Facility B's wastewater discharge contained 2.5 parts per million (ppm) copper and "it was a job to keep it there." With the Blackhole® system, the only treatment concern is the copper in the microetch, which is less than 1 ppm with less treatment. Facility B also no longer has to treat the manganese, several forms of copper, palladium, and chelated copper that were in the electroless copper wastestream. Total water use at Facility A appears to be about the same for Blackhole® as it was for its electroless system, whereas Facility B has seen a "considerable decrease" in its water use.

"Don't skimp on the equipment...It will end up costing a lot more for a lot less quality."
-Facility A

Keys to Success
Both the facilities and the supplier emphasized the importance of quality equipment. The process engineer at Facility A advises other facilities, "Don't skimp on the equipment. You'll end up with a sub-standard system that will require all kinds of on-site modifications. It will end up costing a lot more for a lot less quality." Facility B experienced such a string of on-site modifications. After several difficult months, the engineer at Facility B now considers the system to be a "surprisingly pleasant experience." Having used and removed a palladium system prior to Blackhole®, he advises facilities considering a change to talk to as many current customers as possible and to run some product at a Blackhole® customer's facility. He believes that Blackhole® has significant advantages over palladium processes, including a wider process window and a shorter cycle time. According to the engineer interviewed, Blackhole® may have a wider operating window than electroless.

Facility A also advises that facilities changing to direct metallization need to identify and understand the current and anticipated problems in all parts of their production process. This is because quality problems in other parts of the process can surface when a facility switches to direct metallization. It may be that these problems always existed, but could not be detected until direct metallization was installed. For example, it is important that problems in drilling or desmear operations are corrected prior to installing the Blackhole® process. "Electroless copper can be a band-aid over problems in other parts of the manufacturing process," according to Facility A. His advice is to know where your problems lie and don't be "too quick to point the finger at direct metallization."

MacDermid concurred with these observations, stressing the importance of working with the vendor to evaluate the application before implementing any changes. Most vendors will help a facility to determine if its line is suitable for direct metallization.

For more information on the Blackhole® process, contact Bill Sullivan of MacDermid, Inc. at 203- 575-5659.

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