More and more high-volume electronic manufacturing has been continuously transferred to low-cost countries. Western countries are seeking new technologies to establish a firm foothold in low-yield, high-profit production.
Digital printing of conductive materials is a flexible method of producing flexible and rigid PCBs without the need to provide expensive processing programs in advance. Non-repetitive processing greatly reduces the low production cost and turnaround time, and provides a way to replace the more labor-intensive processes that have traditionally been used for PCB production.
Traditionally, PCB production is a labor-intensive process with many steps, and each step requires some manual intervention. Traditional PCB production also relies on the initial costs of front-end non-repetitive engineering (NRE). It is usually embodied in the form of a production tool such as a photomask or a screen. This shows that the initial processing costs will have a serious impact on small-batch production operations, and the associated labor and infrastructure costs will determine the mass production costs. As a result, high-volume PCB production has to be transferred to low-cost countries to reduce costs.
Small-scale production operations often require the convenience of the production site to keep it relatively local. However, the processing and labor costs still indicate that this is an expensive process, and its low-cost unit parts cost is relatively high. Digital printing of conductive materials offers possibilities for non-labor-intensive production processes that also reduce costs by eliminating the need for initial processing tools such as screens or photomasks.
Digital deposition of conductive material
In the past decade, the feasibility of digitally printing conductive materials has caused great concern. The vast majority of this research stems from the field of organic electronics, where the premise of producing fully printed electronic devices and displays requires printable contact wires and signal buses. Most of the initial studies focused on conductive polymers such as polydiethoxythiophene/poly-p-phenylene sulphonic acid (PEDOT:PSS), and later solutions used carbon and metal particulate materials.
Conductive polymers have a relatively low conductivity problem, while thicker screen-printed silver materials can produce higher conductivities, but these materials must undergo a high-temperature sintering stage to obtain optimal conductivity, which limits the base material. The range of choices. However, the use of metallic inks generally does not allow ink jet printing of printed circuit boards (PWBs) because the particle size and direction of condensation can seriously affect the reliability of the printing process. New developments in the development of metal nanoparticles in inks have made inkjet printing possible, but the resulting thin ink layers and sintering requirements still limit the practicality of the process.
A low-temperature digital process that provides great conductivity will provide a fast and flexible way to complete rapid prototyping and low-to-medium volume production on low-cost flexible and rigid substrates.
New Technology
Printable conductive metal materials often have to trade off the rheological and conductive properties of the material. The binder and carrier used to provide flow during printing and adhesion to the substrate affects the conductivity of the final composite layer and prevents current flow through the wire.
However, there is a process which provides a way to make addition preparation and substrate adhesion independent of the printed portion's conduction requirements. The conductive ink technology (CIT) has developed a process in which a catalytic ink is printed on a substrate and the ultraviolet rays are cured to provide a rapidly processed adhesive substrate. The base layer itself is not conductive, but it acts as a catalyst for the electroless deposition of the metal layer.
The printed cured substrate is immersed in a commercially available electroless plating bath and the thick metal layer at the top of the base bath is deposited. This two-stage process allows the plating baths to be optimized for different substrate materials and different printing tools, respectively, without affecting the conductivity of the final process. The process can use most of the standard electroless plating metals, including nickel, cobalt, and palladium, but copper is the most commonly used and most widely used. The two stages of the process can be performed inline, or electroless plating can be performed as a batch process afterwards.
Copper typically grows in the range of 20 nanometers per minute to 90 nanometers per minute (equivalent to bulk copper), which produces a sheet resistance of 30 ohms during about 10 minutes of plating. Usually the resistivity is 2.5 times larger than the bulk metal (copper), but it depends on the bath and the conditions used.
The optimal conduction range for CIT processes is greater than 10 ohms (equivalent to 1.5 to 2 microns of bulk copper). It is suitable for a wide range of applications, including UHF RFID (radio frequency identification), keypad membranes, low-current PCB (signal), low-power heater assemblies, a wide range of sensor applications, and many Other flexible and rigid applications. For higher conductivity and greater current carrying capacity, post-process plating can also be performed.
Inkjet resolution
The CIT process is designed for piezoelectrically controllable print heads such as those manufactured by Xaar, Konica Minolta and Spectra. The typical native resolution of these printheads is from about 180 to 360 nozzles per inch, and is designed to print at a resolution higher than 360 dots per inch, with drops below about 40 pl. Such print resolution can generally provide a size equivalent to 100 micron line width on a polyester or polyimide substrate. However, a new generation of grey printheads supports variable drops of up to about 32 pl, which results in digital print sizes of about 50 microns or less.
Digital Manufacturing System
A wide range of systems can be used for digital production of flexible circuits. Located in the lower part of the range is a small development system such as Dimatix's DMP series. Such printers will produce A4 paper at different resolutions by using disposable 16-nozzle print heads. Due to the small number of nozzles on the print head, the yield of such systems is low, but it can be perfectly used for development and precise research.
Systems such as the X4000 series from Xennia Technologies or the XY100 from Konica Minolta will be more suitable for production. These systems are also based on the A4 format, which uses larger industrial print heads such as the Xaar Omnidot series or the Konica Minolta KM512 series. These systems have a print bandwidth of up to 70 mm and a productivity of up to 1 to 2 square meters per minute. Similar systems can also provide a width of 1 meter or more, depending on the printhead and the desired configuration.
CIT also teamed up with Preco to develop a narrow-web digital printing tool, the metalJet 6000, for in-line tape-and-reel production of flexible circuits and radio frequency identification (RFID) antennas. The system prints and cures on a 140 mm platform and executes our patented electroplating module, which significantly reduces the footprint and complexity required for in-line electroless plating of network materials. Current print head technology allows the system to produce flexible circuits at a rate of 0.56 (equals to 4.7 square meters per minute) per millisecond, which typically produces products such as UHF RFID antennas at a rate of 0.3 per millisecond (equal to 2.5 square meters per minute). . The system is modular and can be configured to increase production speed and/or deposition thickness.
These solutions, in most cases, submit the CAD drawing so that the usual turnaround time for a 10-m2 single-layer board does not exceed 1 hour.
to sum up
The new digital technology provides additive and no-processing methods for the production of small and medium-sized PCBs using the NRE's minimum rapid process. The ability to separate jetting characteristics and ink adhesion from the electrical properties of the material provides independent control of the conductivity of the wire. The use of inkjet printing as a production method provides high throughput and a short turnaround time without adding to the front-end processing costs.
Dr. Steve Thomas received a bachelor's degree in materials science and metallurgy. Steve completed his Ph.D. and postdoctoral research in organic semiconductors and equipment at the Cavendish Laboratory at Cambridge University. Following various R&D and engineering management positions at a telecommunications start-up company in Zurich, Switzerland, he returned to the UK for various consulting, intellectual property and technology transfer companies, including the University of Cambridge’s Technology Transfer Office. In September 2004, Steve joined Conductin Inkjet Technology as an application engineer where he participated in the development of digital deposition of various conductive materials.
Reprinted from: Electronic Production Equipment Network