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Conductive Polymer Methods
This process deposits a conductive polymer layer on the substrate surface of the via. A cleaner/conditioner step coats the glass and epoxy surfaces in the through-holes with a water-soluble organic film. A permanganate catalyst solution then deposits manganese dioxide (MnO2) on the organic film (through oxidation). This only occurs on the film-coated glass and epoxy surfaces. Polymerization takes place when a conductive polymer solution containing the pyrrole monomer is applied to the surfaces coated with MnO2. The polymerization continues until all of the MnO2 oxidant is consumed, resulting in a layer of conductive polymer (polypyrrole) that coats the through-holes. The through-holes are then flash-plated with copper.
A typical conductive polymer process has six chemical process steps (microetch, cleaner/ conditioner, catalyst, conductive polymer, microetch, and copper flash-plate), as shown in Figure 4.
Information is presented on the following conductive polymer method:
- Compact CP (Atotech U.S.A., Inc.)
Compact CP (Atotech U.S.A., Inc.)
Background
The Compact CP process was introduced in 1988 and is used primarily in Europe. Eleven units are currently in full production, but only one unit, which is still in its trial phase, has been installed in the U.S. Compact CP is available only as a conveyorized (horizontal) plating unit. The volatility of the conductive polymer precludes its use in an open system, because it would deposit a black coating on the surrounding area. Facilities using Compact CP typically produce high volumes of FR-4 boards with four, six, or eight layers for the consumer electronic and communications industries. According to Atotech, "the buyers of these boards are demanding about product quality, have high technology demands, and typically require small holes." Companies that purchase or evaluate Compact CP are usually looking for a fully integrated system that can handle higher technology demands.
No limitations have been identified in terms of number of layers or hole size. The substrates, however, are limited to those that react well with permanganate. FR-4 is best suited for the Compact CP process. Polyimide also works but is not commonly used. Teflon¨ does not work with this technology.
Implementation at Specific Facilities
One facility (Facility T) that is experiencing success during the trial
phase of implementing Compact CP was interviewed for this report. According
to the Vice President of Process and Quality Control, the facility was
motivated to make a switch to direct metallization because the company
"looked at where the industry was going in the next five years or so,
and it was not electroless." The company wanted a conveyorized (horizontal)
process line to reduce handling and cycle time. The company anticipated
spending less money on waste treatment with the Compact CP system. In
addition, the company was concerned that the colloidal dispersions utilized
in graphite and palladium systems would not move well through the holes.
The company felt that a conductive polymer method would work well with
0.010 inch drilled (0.006 to 0.007 inch finished) holes. Atotech U.S.A.,
Inc. provided the advantage of supplying the chemistry and equipment as
a package. Atotech is the only supplier in the U.S. offering a complete
system. Facility T recently opened a new facility and installed the Compact
CP process in a fully automated, conveyorized (horizontal) configuration.
Acid copper plating is done in-line with Compact CP using an equipment
package developed and sold by Atotech under the trade name Uniplate. An
Atotech employee from Germany worked on-site full-time for six weeks to
assist with the start-up phase. Installation took nearly three months
to complete. At the time of the interview, the system had been running
for approximately two months. Currently, the majority of boards running
through the system are double-sided, but it is anticipated that this proportion
will change primarily to four, six, and eight-layer boards over the next
six months.
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Experiences with the Compact CP Process
In the vendor's experience, integrating equipment, especially equipment
from different suppliers, in an automated line with the conveyorized (horizontal)
Compact CP system is often the greatest challenge during the debugging
process. In general, it often takes three to five months to get the system
running at desired levels. Autoloaders bringing boards into the system
from deburring and unloaders moving boards to in-line copper plating are
computer-controlled, and it can be a time-consuming process to integrate
these positioners into the system.
Facility T experienced minor problems during installation of the system. One problem the facility faced after start-up was the sensitivity of the system's ventilation. Vapor from one tank was mixing with vapor from another tank within the enclosed unit. Engineers had to alter the air balance within the system by changing the belts on the blower on the roof to reduce air flow in the entire building. The facility also had to add and re-route plumbing, which had not been anticipated. In addition, the facility thought rinse water after the cleaning step would not require treatment for metals, but it did.
Comparisons to Electroless Copper
There is no direct basis of comparison to a previous system because the
Compact CP system was installed at a new facility. The following comparisons
between electroless copper and Compact CP were made based on experience
at a sister facility employing electroless copper. The amount of time
spent on preventive maintenance with Compact CP is a little more demanding
because of additional cleaning requirements and other tasks such as changing
filter cartridges. The process engineer at Facility T commented that "working
with permanganate tends to be pretty messy. We have to be very conscious
of cleanliness." Less time is spent on lab analysis and bath maintenance
because tank pump-outs, chemical additions, and most other bath maintenance
tasks are automated. Some analysis continues to be done manually for verification.
It is too early for Facility T to tell if there is a change in board failure rates, but they stated that "there is no indication that defect rates are any higher (with the new system)." The cycle time is about two-thirds faster than for the electroless line at the other facility. Whereas it takes two hours for the electroless line alone at the other facility, the new system takes only sixty minutes from deburring through electroplating. Water usage is greatly reduced with Compact CP. At the other facility water is used at a rate of 30 gallons per minute (gpm). For the same level of production, the new facility uses just 8 to 10 gpm. Facility T has not tracked copper discharge from the Compact CP line, but it plans to do this in the near future. Overall, Facility T feels that Compact CP will be very cost-effective, but it does not yet have adequate cost data to draw definitive conclusions.
Keys to Success
"Getting customer buy-in" is key to successfully implementing the Compact
CP system, according to Facility T. A company considering using this system
also"needs workers to commit to being on the line for installation, start-up,
and debugging to understand changes that have been made along the way."
This is essential because it is not possible to simply hire someone with
experience setting up this system; conductive polymer technology is new
in the U.S., and the learning curve is considerable. Facility T recommends
that other PWB manufacturers implementing the system "take time to become
familiar with the process, and take care of mechanical issues before running
product at full production level." Finally, start-up each day is a complex
process, so a facility needs to run enough product to make it cost-effective.
For Facility T, the system "runs a lot better at 400 panels a day than
100."
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Atotech stressed that management needs to make a firm decision and commit to switching to the alternative technology. Management must support all phases of the implementation, from installation to debugging to full production. Also, a substrate type appropriate for the Compact CP system must be used.
For more information on the Compact CP system, contact Mike Boyle of Atotech U.S.A., Inc. at 803-817-3561.