Coated Ultra-Thin Copper on Printed Circuit Laminates
Nano copper-based electronic ink and associated processing have been developed to create ultra-thin copper on flexible PCB substrates. Using a unique nanomanufacturing process, nanomaterial is created, dispersed, and sintered, resulting in copper foil of one micron or less. The process is rapid and sufficiently low-temperature to allow deposition onto temperature-sensitive substrates such as polyimide, plastic and paper. The resulting foil can be etched and processed as well as electroplated or electrolessly plated without the use of palladium.
Applying thinner copper coatings to flexible PCB substrates than are currently available will allow for much finer etch resolution and increased circuit density. The roll-to-roll lamination process in use today limits the copper foil thickness due to manufacturing and handing ultra-thin copper. A coating process eliminates these problems.
High-volume roll-to-roll flexible circuit manufacturers use plasma vapor deposition (PVD) to deposit a 0.5 micron copper thin film onto unclad flexible laminates. This process provides several advantages over using copper clad laminates. One such advantage is the ability to metalize pre-drilled holes simultaneously with the surface providing good adhesion, ease of handling, and very fine feature definition. However, this is a very expensive method for applying copper and requires very specialized equipment when using today’s industry standard processes.
Nanomanufacturing incorporates the plasma generation process to tightly control particle size and distribution and create very pure copper. Furthermore, the manufacturing process allows the copper particles to be encapsulated with a coating, thus preventing oxidation and agglomeration. For the purpose of coating printed circuit laminates, nanomaterial particles of around 90 nm are the optimal size. Smaller particle size distributions can affect the metal loading and larger (too large) will require excessive energy to sinter. The nanoparticles can then be dispersed into solvent based inks that can be used to create very thin and uniform coatings. The ink dispersions have long shelf and pot life without special handling or storage.
The deposition equipment used to create ultra-thin copper coatings includes coaters, inkjet printers, and screen printers that are commercially available and typically used in the electronics industry today. The deposition is accomplished in open air without special environmental constraints because the coating on the nanoparticles prevents oxidation. This nano technology is being used to create copper coatings onto various substrates that, in the past, could only be accomplished by processes such as PVD.
The data presented herein was derived from nanomaterial that was slot die coated on a commercial system and air-impingent dried. It has been determined that care must be taken to thoroughly dry the material once applied to the laminate to remove all residual solvent. Failure to properly dry material can result in poor adhesion after sintering and failures in post-processing. This process has been successfully demonstrated on commercially available coating and sintering equipment. It is expected that spray, dip, and other coating processes will all show similar favorable results and allow custom tailoring of coating systems to meet specific needs.
Once deposited, the applied material is sintered either photonically or in an oven using a reduction environment. In the case of photonic sintering, a millisecond flash from a Xenon or broadband source transforms the coated material from insulting to conductive. Once sintered, the material adheres to the substrate and can be processed. For the data and images presented in this article, the coated roll of 60 feet of Kapton was then sintered using a roll-to-roll Xenon pulsed-light sintering system (Xenon Corporation S-5000).
Commercially available film coating systems would need to be retrofitted with a production quality broad band flash system or an oven system with a reducing environment to sinter the nano into pure copper. This equipment is commercially available as well. Roll-to-roll application of the nano copper can replace the previously mentioned PVD method in the manufacturing of flexible circuit materials with much thinner copper foil than is currently available.
Preliminary testing has been conducted by Intrinsiq to assess the quality of the ultra-thin film material and the ability to use these ultra-thin film laminates in standard PCB production processing, including pre-cleaning, imaging, etching and plating. The steps in creating the material and the steps taken in the PCB fabrication process are outlined below.
Standard polyimide flexible circuit material (0.005” thick) was coated with a one-micron dried thickness of nano copper ink in a roll-to-roll process using commercial slot die coating from the photographic film industry. The dried coating was then sintered into a 0.5-micron copper film using a production broad band flash unit designed to be integrated inline for true roll-to-roll processing. The conductivity of the resultant ultra-thin film copper was measured with a four-point probe at pre-set intervals to monitor the level of sintering during the flash process. The bulk resistivity of the sintered thin-film copper material is on the order of 10X that of bulk copper and achieved a 5B adhesion. The thin-film copper will electroplate readily due to the very low resistance of the coating as described herein. Shown in Figure 1 are examples of electrolessly plated coated copper.
Several 6” x 12” pieces (panels) were cut from the produced roll of polyimide with the ultra-thin film of copper and processed at a local PCB fabricator. The materials were pre-cleaned with standard chemistry and then laminated with photo-resist. The panels were imaged with several IPC Peel Test Coupons, tape test coupons, and standard PCB circuit patterns.
Figures 2, 3 and 4 show examples of the raw coated material, followed by sintering and the finished circuit, respectively. The samples were pattern plated with copper, then tin etch resist. The photo resist was stripped off; the circuit was etched in industry standard equipment with ammonia based solution. The tin was stripped off after etching resulting in a copper circuit. The coupons were tested for peel strength and showed a 1.2 N/cm average. Note that the coupons used for the test are industry standard patterns as designated by IPC.
The electroplated copper was also subjected to pull testing to verify adhesion to the base coating of copper. The result of this test was that the pull strength exceeded the detection limit of the device.
The sintered nano-copper, thin-film coating will also auto catalyze in standard PCB electro-less copper solutions. This seed layer technology replaces the palladium catalyst currently used to initiate electro-less copper in the PCB industry.
Similar process development is underway for rigid laminates and alumina as well. Initial testing shows comparable pull and peel strength to currently available laminates. This thin film coating also allows lower temperature processing of the ceramic laminates than current firing methods using precious metal pastes.
Nanocopper based copper foil has great promise for high-density microcircuits. Utilizing conventional coating capability, volume production is easily scalable and cost-effective.
David Ciufo is the PCB program manager at Intrinsiq Materials Inc. a recognized leader in nanomaterials development.