The U of Inkjet Printing Technology for Fabricating Electronic Circuits – The
Promi and the Practical
Brian Amos – Engineering Manager, Dow Electronic Materials, Marlborough, MA, USA Thomas Sutter – Emerging Technologies R&D, Dow Electronic Materials, Marlborough, MA,
USA
Abstract
Manufacturers of electronic devices are always arching for new technologies that can improve process, extend capabilities and lower costs. The drivers, along with the needs of new markets like Printed and Plastic Electronics, have brought process like inkjet printing to the forefront. This paper explores the promi of what inkjet printing can bring to process simplification, cost reduction and improved capabilities. It also takes a critical look at the practical issues and concerns of this new technology.
THE DIGITAL FABRICATION PROCESS
Digital Fabrication refers to a process by which data in digital form is ud to directly fabricate a part without intermediate tooling. In addition to inkjet printing, some examples of digital fabrication are: lective lar sintering, lar cutting, stereo lithography, lar induced printing and lar direct imaging.
Drop-on-demand inkjet printheads u either thermal or piezoelectric modes to eject droplets. Since most industrial fabrication is done with piezoelectric heads, this will be the focus for this paper. A typical piezoelectric head is constructed of a micro-machined chamber with one or more walls fabricated from a ceramic, such as PZT (lead-zirconate-titanate), which will mechanically deflect when an electric field is applied. The flexure of the wall creates a volume change within the chamber and an acoustic wave which drives a droplet of liquid through the hole in the nozzle plate. (See Figure One)
THE PROMISE OF INKJET PRINTING
Process Simplification and Faster Turnaround
丧句子The promi of inkjet printing is that it is an additive process, greatly reducing material waste as compared to a traditional lithographic process. The digital nature of the technique allows for direct CAD to board processing and in-process image compensation. Photomasks are eliminated along with the process costs and storage requirements. Inkjet printing is also a non-contact method (the head is placed about 1 mm above the surface during printing) and so is an ideal technique for fragile substrates.
折纸花篮
夏至是几月几号Figure 2 – LithoJet TM Inkjet Resist Printed on Copper Figure 3 – After Etch and Strip
Waste Reduction
In addition to elimination of the photomask generation process, the downstream developing process step is also eliminated which can save on water, energy, waste treatment process and maintenance down time. Overall it is a much more environmentally friendly process than traditional process.
Inkjet imaging is inherently better environmentally than traditional lithographic techniques, in materials and process. As an example, a dry film etch resist will be described.
For a standard dry-film process, the resist itlf must be produced by casting the lacquer onto the polyester sheet from solvent carriers like acetone, alcohol or MEK. Dry film lacquer is 30% to 50% solids, meaning 50% to 70%is volatile organic content (VOC) that must be evaporated and treated, usually by burning. Even liquid photo-imageable (LPI) resists contain up to 60% solvents. Inkjet inks, like Dow’s LithoJet™ inks for example, are 100% solids so no VOCs are evolved during manufacture or u.
When a dry film is ud as an etch resist, approximately 50% of the material is developed away as waste. Inkjet ink is deposited only where it is needed, so the waste is minimal. In addition, the typical thickness of an applied inkjet ink is 10 to 15 microns, compared to 15 to 25 microns for dry film. Bad on the numbers, an inkjet process us only about 30% of the material of a dry film process, which is 70% less material to waste treat.
贷款流程一语天然万古新Dry film process also requires photomask generation and resist developing, along with the associated water and chemical u, energy u and labor. Additional packagings such as boxes, plastic cores end supports and cover sheets add to the total material bill.
妇科常见病Improved Registration
Alignment of congruent images is a major challenge for PCB producers, especially for the soldermask process. The biggest potential advantage with digital imaging is the ability to correct for registration error due to distortion. In conventional contact imaging, you are limited to rigid body shift corrections (i.e. - X, Y and rotation). If there is any stretch, shrink or shear in either the mask or the substrate, getting the two patterns congruent, whether they are image-to-image or image-to-hole, becomes difficult. Some fabricators generate multiple masks with varying compensation factors in order to obtain a best fit. This, of cour, is costly and time consuming.
A digital imaging system, properly outfitted with image capture cameras, is able to acquire fiducial positions and scale the data to fit. This could be done on a full panel basis or even on a board level within a panel. The maximum benefit would be
realized when digital process are utilized throughout the board building process.
Figure 4 – Soldermask BGA Printed with Inkjet Masking
THE PRACTICAL OF INKJET PRINTING
The practical side is that inkjet imaging is a new technology with respect to circuit board fabrication and, like LDI before it, will take time to mature. Although inkjet-type printing was first developed in 1948, and home office and graphics printing has been mainstream for some years now 1, industrial digital fabrication is a complex process with more demanding requirements than just visual acuity. Magazines photos are printed at about 300 dpi 4, so for graphics applications this allows for less den drop placement and faster printing speed. This is far from what is required for producing continuous and functional electronic circuitry. In addition, as resolution increas, the complexity of the deposition system increas whilst the printing speed decreas.
Feature Resolution and dpi
Two key parameters affecting the acuity of the final features are drop volume and dpi or drop spacing and overlap. Drop volume is a critical factor for minimum resolution of a system. Without special tricks, the finest printed feature can not be smaller than the drop diameter, and it is typically larger by some multiple. Graph One shows how feature size is related to drop volume with high-spre
ading and low-spreading inks plotted to show the interaction. Low-spreading inks have the potential to produce finer lines. The modification of substrate surface energy, through chemical or physical treatments can influence the minimum dot size possible, but usually with an impact on how much drop overlap is required for smooth lines. Additionally the u of “on-the-fly” UV pinning can be employed whereby a UV exposure system follows cloly behind the
printhead and partially cures the ink in place.
Graph One – Feature Size vs. Drop Volume – (Single Drop Diameter)
A certain amount of drop overlap is required for smoothing lines and is a function of the ink properties (surface tension, viscosity, molecular weight, etc.) and surface properties of the substrate (roughness, surface energy, etc.). Let us assume that a 33% overlap of droplets has been determined to produce a smooth line with a particular ink. (See Figure Five) Graph Two shows how inks with different spreading characteristics affect the minimum dpi required as a function of drop volume.
Graph Two – DPI Required for Overlap vs. Drop Volume and Spreading
经常洗头好吗Depending on the spreading characteristics of the ink drops, the dpi will need to be adjusted to obtain the correct overlap. Increasing dpi in the scan direction is simply a function of increasing the fi
ring rate, or frequency, of the printhead relative to the table speed. There are some limitations to the maximum firing rate due to head designs and ink properties, but for our purpos we can operate within an acceptable range without sacrificing speed.
Increasing the dpi in the orthogonal direction is more complicated and can directly affect printing speed. Each style of printhead has a fixed spacing of the nozzles, which is referred to as native resolution. For example, the Dimatix SE-128 head has 128 nozzles on a 508 micron pitch, which gives it a 50 dpi resolution. The only ways to increa this dpi is to make multiple pass, offtting the head with each pass, or by adding additional heads that are interlaced. (See Figure Six) Often both techniques are ud in combination such that multiple heads increa the apparent native dpi and multiple offt scans increa the final dpi. This increa in dpi, however, comes with a cost. Increasing heads means that the print system is more complex and a greater number of nozzles must be maintained through proper preventive maintenance so that drops are not lost. Increasing scans means more time is required for the multiple pass over the panel, so productivity is decread. If smaller drops are ud to produce finer features, then the number of scans will increa further.
10 mils = 100 dpi interlaced
Figure 6 – Interlacing of Printheads
System Printing Speed
Resolution, printing speed and equipment complexity are intimately linked and understanding the trade-offs between the parameters is esntial in making decisions about which system configuration best address a target application. The impact on printing time for one side of an 18” x 24” panel is illustrated in the following graphs. Graph Three shows printing time at 500 dpi and Graph Four shows time at 5000 dpi with changing conditions of table speed and number of printheads. The printheads here have a native resolution of 50 dpi. One can e that at 500 dpi, print times of less than one minute are possible with a reasonable number of heads: five. Increasing the dpi to 5000 caus the print time to increa by an order of magnitude, meaning a time of 5 to 10 mi
nutes would be required.
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Print Time (min)创业计划书代写
Printing Speed for 18x24 in @ 500 dpi Resolution
Chart Three – 500 dpi Printing Speed (Printhead=50 dpi native)
