1999 Progress Report: Microstructural, Morphological and Electrical Studies of a Unique Dry Plasma Metal Deposition for Printed Circuit Boards (PCBs)EPA Grant Number: R826119
Title: 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 Period Covered by this Report: September 1, 1998 through August 31, 1999
Project Amount: $200,001
RFA: Technology for a Sustainable Environment (1997) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
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 to be 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.
Progress Summary:Currently, PCBs are manufactured in a 'subtractive' processes. The initial starting substrate is an 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 and 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 in and the thickness of our sputter metallized layer is 4 x 10-6 in. Since this etching step in PCB manufacturing generates the largest amounts of waste, our process could reduce chemical etch waste by more than 99 percent.
Completed printed circuit boards (with through holes and wiring patterns) have been fabricated using the AVA sputtering process. The resistance and film characteristics of the AVA 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 as 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/in. This is low compared to the industry accepted 7 - 8 lb/in. 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 due to 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. 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 has been conducted. Improving the chemical bonding of the sputtered metal to the PCB substrate can be accomplished using an adhesion promoting polymer interlayer. Experiments have been conducted with epoxy interlayers having a high degree of unsaturation in the polymer chain. 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/in) 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/in after copper electroplating. When tested for adhesion, the films debond between the sputtered and the electroplated films. Work will continue to understand and improve this metal to metal adhesion.