PCB Imaging technology continues to evolve in the manufacturing of electronic devices, from contact printing using phototools, exposure equipment and photoresist, to alternatives such as DI and LDI, the best known and most widely practiced.
The term imaging in manufacturing of electronic devices such as circuit boards usually refers to the formation of a circuit pattern from digital data by means of photolithography, typically by contact printing, involving the use of a phototool, exposure equipment and photoresist.
Alternatives to contact printing have also found acceptance, notably digital, direct imaging technologies, of which laser direct imaging (LDI) is the best known and most widely practiced. It might be best to use the broader term DI (direct imaging) to label this technology since not all DI units employ lasers. Hayao Nakahara (N.T. Information Ltd) estimates that (cumulative) installations of DI units used in PCB fabrication were about 1,230 at the end of 2011.
The benefits of DI technology may be summarized as follows. Since phototool generation and conditioning are omitted, there is the advantage of shorter lead time. Small lots can be customized at no extra cost (e.g., with added date and lot number information). There may be an advantage in fine line imaging of surfaces with poor co-planarity because of the depth of focus of the laser beam. The biggest advantage may be the ability to scale, i.e, to change the dimension of each individual exposure for best fit to reference points on an underlying pattern of a multilayer structure. This is of particular value in soldermask LDI applications.
Inkjetting is a unique form of DI which has been accepted in PCB fabrication as a viable alternative to screen-printing of legend print. MicroCraft and Orbotech are well known suppliers. Other applications such as inkjetting of an etch resist pattern or inkjetting of a soldermask pattern are less common. Camtek1 has introduced a soldermask inkjet system that allows the selective coating thickness adjustment, depending on features to be covered and board surface location. The system uses a proprietary solder ink formulation, a fact that might limit the acceptability of the system. Since Camtek acquired Printar, the company that pioneered soldermask inkjetting, it is a reasonable assumption that Camtek’s system evolved from the Printar technology.
Use of Texas Instruments’ DMD (digital micromirror devices) The number of companies that offer DI systems that make use of Texas Instruments’ DMD (digital micromirror devices) is growing.
• ORC DI-Impact (formerly by Pentax). The DI-Impact offers a minimum feature size of 15um L&S, position accuracy (repeatability) of +/- 7.5 um, and a side-to-side registration of +/- 10 micron. The exposure time is about 35 seconds for 340x510mm, depending on the photospeed of the resist. Use of 405 nm wavelength. Highpressure
mercury lamp light source. Doublesided exposure, automated loading/unloading.
• Hitachi’s DE imagers DE-H, DE-S, and DE-F series differentiate in their resolution capabilities. The imagers use a 405 nm diode laser, and make use of Texas Instrument’s DMD micromirror array model XGA. The DE-H has the highest resolution, capable of producing 10 micron lines and spaces. The overlapping pixels
have an addressability (center-to-center spacing) of 1.2 micron. The DE-S is capable of doing 20 micron L/S (2.6 micron addressability), and the DE-F does 40 micron L/S.
• Miva 2600X (MIVA Technologies GmbH, Germany).
Higest resolution model: 2600X- 16, 16,000 dpi, image time 125 sec (15mj/ cm2; 18x24“). Also offered by Printprocess AG(Switzerland), as system Apollon-DI. Distributed by KuperTek in the USA.
• Maskless Lithography (USA)
• Aiscent Technologies Inc. (Canada) A variety of light sources can be used with DMD™ technology, both laser and non-laser, since the pixel size and shape is defined by the
micromirror and not by the foot print of a laser beam. It should be noted that the early laser systems that used wasteful, short-lived argonion lasers (Figure 1) have been replaced by more efficient solid-state lasers. Only two of the argon-ion laser emission lines were actually used for LDI.
Use of LEDs
One of the fastest growing light sources for both conventional and DI applications in 2011 have been LEDs (light emitting diodes). It is worth reviewing the intrinsic advantages of LED light sources and looking at some of the exposure systems that are being offered. But first, a brief look at the packaging (interconnect) options for LEDs and LED arrays, in particular high-intensity LEDs.
High-brightness LEDs produce a lot of heat that needs to be removed, otherwise the junction temperature gets too hot, which is detrimental to the light output and the LED lifetime.
Additionally, it can result in an undesirable spectral shift. Most LED packages are more complex, with large arrays of LEDs, and possibly more than one circuit layer to fan out interconnections.
The thermal laminate for such a package may also be referred to as metal core PCB (MCPCB); sometimes the terms thermal interface material (TIM) or IMS (insulated metal substrate) are used.
LEDs are known to have a longer life, lower power consumption, and easier maintenance compared to alternative power sources such as mercury lamps or lasers.
Dainippon Screen Manufacturing Co. (Japan) introduced its LED powered DI system at the 2011 TPCA Show in Taiwan .
The company started commercialization in January 2012 and expects to sell about 100 units during the first year. Dainippon Screen claims that the Ledia 5 is the fastest DI system due to its high intensity, multi-wavelength UVLED light source. It can use standard dry film photoresist and photoimageable soldermasks. Resolution down to 30 micron lines and spaces is achieved.
Miva Technologies (Germany) introduced its LED Direct Imaging System 2600X in 2010. Miva is represented in the U.S. by KuperTek and cooperates with Printprocess AG (Switzerland) in Europe. The 2600X-5 model achieves a minimum line width resolution of 2.5 mil (62 micron) and requires a 30-second exposure time @ 15 mJ/cm2. It works with conventional dry film resist and soldermask, as well as LDI dry films, liquids and soldermasks. LED arrays with emissions at 365 nm, 390 nm, and 405 nm are available.
The system makes use of DMD™ dynamic photomasks. Bacher (Germany), a well known supplier of exposure units, is also offering an LED powered exposure unit called SupraLight. This is a conventional contact printer. Illumination uniformity across the exposure area is +/- 5%. Resolution of 50 micron features is achieved. There is
no temperature change of the phototool during long production runs. Dry film photoresist exposure time is 6-8 seconds depending on the resist’s photospeed.
Limata GmbH2(Germany), also through representation by Walter Lemmen GmbH, has introduced its UV-LED direct imager at productronica 2011. The UV-P100 UV-LED Direct Imager was developed for prototype and short run production. It can use conventional dry film and soldermask. The light source life is >10,000 hours.
Max panel size: 650 x 540 mm2. Resolution: 50 micron.
Automated load/unload are available.
Side-to-side registration makes use of cameras and registration holes.
Gray-Level Imaging Technology
Maskless Lithography, Inc.3 was the first to introduce gray-level imaging technology to the direct imaging of PCBs. The Model 2027 uses a mercury arc light source and DMD arrays .
Each of the multiple DMD images is carefully aligned and oriented so that its mirror columns are parallel to the stage motion during the exposing scans. This means that, at some time during the scan, each point on the substrate passes under an imaged column of 768 DMD mirrors.
As it passes it receives doses, in serial, from some fraction of the 768 mirrors in the column Whether or not a pixel receives a dose from an individual mirror depends
on whether that mirror is on (“flipped” to direct light through projection lens) as the point passes under its projected image.
Thus, in theory any point on the substrate can receive partial energy in discrete graylevel increments up to a maximum of 768, or the total number of mirrors in a DMD column. This gray-level control of exposed image pixels allows for fine placement of feature edges using relatively large projected mirror pixels (34 um).”
As read in the PCB magazine.