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Final Report: Microstructural, Morphological and Electrical Studies of a Unique Dry Plasma Metal Deposition for Printed Circuit Boards (PCBs)
EPA Grant Number: R826119Title: Microstructural, Morphological and Electrical Studies of a Unique Dry Plasma Metal Deposition for Printed Circuit Boards (PCBs)
Investigators: Sampath, W. S. , Barth, Kurt
Institution: Colorado State University
EPA Project Officer: Richards, April
Project Period: September 1, 1997 through August 31, 2000 (Extended to September 30, 2000)
Project Amount: $200,001
RFA: Technology for a Sustainable Environment (1997)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:The objectives of this project were to conduct basic research for applying dry plasma metal film deposition to Printed Circuit Board (PCB) manufacturing. Plasma deposition would reduce the pollution intensiveness of the industry through reduction or elimination of liquid waste treatment and the reduction of solid waste. Much of this study was accomplished using our unique air to vacuum to air (AVA) apparatus for direct substrate insertion into high vacuum. The AVA system eliminates the need for load locks. Summary/Accomplishments (Outputs/Outcomes):
Following our proposal, our initial investigations began with an attempt to gain an understanding of the thermal aspects of plasma (magnetron sputter) deposition on the polymeric substrates used for PCBs. Initial depositions were thin films of nickel to evaluate the deposition and adhesion characteristics. The preliminary results were promising; however, when larger thicknesses were attempted the thermal constraints of the PCB material were exceeded and decomposition was observed. Sputtering through fine featured masks for selective deposition was attempted. The fine metal mask also became hot and lost dimensional tolerance. The ion bombardment, in addition to the latent heat of condensation of the sputtered material, adds a great amount of heat input to the substrate in long-duration depositions. Building the required 25 micron thickness of material was technically intractable; however, we were encouraged by the uniformity, and quality of the sputtered films on the PCB substrates.
We have investigated utilizing planer magnetron sputtering to deposit a thin initial layer on a bare PCB substrate as a means of through hole activation (formation of a conductive medium in drilled through holes in the PCB to enable subsequent electroplating). This would utilize the AVA sputtering technology in an additive PCB process rather than the current subtractive process. The highlights of the proposed PCB fabrication process would be as follows:
Currently, PCBs are manufactured in a "subtractive" process. The initial starting substrate is epoxy/glass board with a relatively thick copper cladding. Most of the copper is removed (or subtracted) to form the circuit pattern. In contrast, the AVA sputter process is an "additive" process. This methodology involves depositing a very thin coating on a bare, unclad board then using industry standard photolithography to form the circuit pattern. The thin layer provides a conductive path on the surface of the board and in the drilled holes for subsequent electroplating. Because it is an additive process, this technique would eliminate or greatly reduce many of the pollution intensive processing steps in PCB manufacturing, and greatly reduce the number of wet processing steps. The hole activation steps, including the electroless copper deposition, and rinses could be eliminated. The final etching step (the subtraction step) could be greatly reduced, while allowing traditional manufacturing equipment to be used.
The thickness of the copper removed by etching by the current industrial
method is at least
0.001 inches and the thickness of our sputter metalized
layer is 4 x 10 -6 inches. Because this etching step in PCB manufacturing
generates the largest amounts of waste, our process could reduce chemical etch
waste by more than 99 percent. The different steps for this fabrication
methodology have been demonstrated.
Following the methodology outlined above, we progressed on demonstrating the following steps:
1. AVA Sputter Ni and Cu on FR-4 PCB substrates.
2. Lithograph onto
sputtered films.
3. Electroplate onto sputtered films.
4. Flash etch to
remove the very thin sputtered film.
Both nickel and copper have been magnetron sputtered utilizing the AVA system onto FR-4 substrates. (Most PCBs utilize FR-4 substrates.) Initially, experiments were conducted with about 20 millitorr (mt) of argon as the background gas in the vacuum chamber. This led to good quality films with poor adhesion to the substrate when tested by the ASTM D-3359 method. When copper films were deposited with a 2:1 Ar to O2 ratio, the adhesion improved greatly and the films were able to pass the ASTM test. A deposition methodology was developed to utilize the oxygen to promote adhesion at the board to film interface, while not causing large film resistance degradation through the deposition of copper oxide in the bulk of the film. The methodology involves utilizing the unique capabilities of the AVA system. Completed printed circuit boards (with through holes and wiring patterns) were fabricated. The resistance and film characteristics of the AVA magnetron sputtered films are adequate for electroplating. Furthermore, through holes in the PCB are adequately activated by AVA magnetron sputtering to allow electroplating inside the holes with good coverage and mechanical properties when analyzed with optical microscopy. This is a significant result in that it allows for the elimination of waste and pollution associated with the traditional hole activation techniques, including carbon, palladium, tin/palladium, or electroless copper methods.
Utilizing standard ASTM techniques, the trace adhesion (adhesion of the completed PCB wiring trace after AVA magnetron sputtering, Cu electroplating and Pb/Sn electroplating) is 1 to 1.5 lb/inch. This is low compared to the industry accepted 7 to 8 lb/inch. The adhesion of the sputtered film to the substrate is excellent and degrades only during subsequent processing at the PCB manufacturing facility. Research has shown that the cause of the adhesion degradation is the caustic nature of chemicals used in electroplating. The chemicals degrade the oxygen bond between the polymer and the copper. Of particular concern is the sulfuric acid used in the copper electroplating bath. Higher O2 levels have been tried to promote adhesion, but also are attacked and have resulted in unacceptably high film resistance. High sputtered film resistance can cause the electroplated films to be non-uniform.
Many techniques were attempted to promote adhesion, including mechanically abrading the bare board, chemical etching the board, plasma etching the board, and specialized cleaning procedures. They resulted in only a small increase in adhesion. As a result, subsequent investigations have centered on improving the chemical bond between the AVA sputtered films and the polymeric substrate.
A detailed literature review of metal to polymeric bonding was conducted. Improving the chemical bonding of the sputtered metal to the PCB substrate can be accomplished using an adhesion promoting polymer interlayer (Shanefield, et al., U.S. Patent #4582564, 1986). Experiments have been conducted with epoxy interlayers having a high degree of unsaturation in the polymer chain. The unsaturated bonds promote chemical bonding with the sputtered metal. A plasma cleaning or mechanical abrasion of the interlayer before sputter deposition helps to promote adhesion of the sputtered film to the substrate during PCB processing. The sputtered film has demonstrated excellent adhesion to the interlayer (up to 10 lb/inch) before electroplating. When deposited on a plasma cleaned or abraded interlayer, the sputtered film is adherent and survives the PCB manufacturing process. Circuit trace adhesion has improved to 4 to 6 lb/inch after copper electroplating. When tested for adhesion, the films de-bond between the sputtered and the electroplated films.
In the next phase of the research, it was decided to concentrate the efforts on promoting adhesion by the use of an epoxy layer containing unsaturated bonds. The use of epoxy with unsaturated bonds is industrially viable for the PCB industry. This is because most PCBs are made from a glass fiber epoxy board like FR-4. The epoxy in these boards could be replaced with an epoxy such as 872-X-75. The epoxy 872-X-75 contains unsaturated bonds and could be used in the primary manufacture of the FR-4 boards. The results described below were all obtained on FR-4 boards coated with a very thin layer of 872-X-75. The epoxy was diluted with xylene for applying a thin coating. Further optimization of the sputtering process resulted in increasing the adhesion of the nickel film to the substrate to 30 lbs/inch. A 1-minute in situ plasma cleaning of the epoxy coating gave the best results. The adhesion was measured by ASTM 3359 score and peel test. Even after electroplating copper onto the sputtered nickel, the adhesion of the nickel film to the substrate was still greater than 15 lbs/inch. Thus, it was demonstrated that by using epoxy with unsaturated bonds, the adhesion of the nickel film can exceed the requirement of the PCB industry ( 7 to 8 lbs/inch).
Subsequently, the research was focused on improving the adhesion of the electroplated copper to the sputtered nickel film. Because we did not have the facilities for electroplating in our laboratory, the electroplating was performed at nearby PCB shops. The adhesion values between the sputtered nickel film and the electroplated copper film could not be improved above 6 lbs/inch. The films de-bonded between the nickel and the copper. Also, significant variations in the nature of the electroplated copper films were observed. In a few cases, the copper films had high residual stress, and the peeled trace in the adhesion tests would curl. In other experiments, the curling of the peeled trace was not observed.
It was concluded that further research on optimizing the copper electroplating with highly controlled bath conditions is needed to obtain the required adhesion (7 to 8 lbs/inch) between the electroplated copper film and the sputtered nickel film. The bath chemistry also may need to be modified to improve the adhesion. Because the electroplating facilities in local PCB shops are not adequate for these studies, installation of such facilities in our laboratory would be necessary to complete these studies.
Journal Articles:No journal articles submitted with this report: View all 7 publications for this project
Supplemental Keywords:chemicals, pollution prevention, innovative technology, waste reduction, waste minimization, environmentally conscious manufacturing, engineering, electronics. , Industry Sectors, Air, Sustainable Industry/Business, Scientific Discipline, RFA, Technology for Sustainable Environment, Sustainable Environment, air toxics, Manufacturing - NAIC 31-33, Environmental Engineering, cleaner production/pollution prevention, Environmental Chemistry, New/Innovative technologies, Economics and Business, Engineering, plasma-based dry plating, process modification, treatment, cleaner production, in-process changes, printed circuit boards, waste reduction, coating processes, dry plasma metal deposition for PCBs, PCBs, waste minimization, environmentally conscious manufacturing, electronics, electrochemical, industrial innovations, industrial process, alternative materials, electrochemical techniques, innovative technology, production processes
Relevant Websites:
http://www.engr.colostate.edu/depts/me/index.html
(click "facilities" then "Materials Engineering Lab")
Progress and Final Reports:
1999 Progress Report
Original Abstract