The conventional image transfer system makes use of artworks and photoplotters. Photoplotters are generally considered infallible in terms of dimensional accuracy, as they are periodically calibrated and routinely maintained. The best positional accuracy the photoplotter can offer is 28 microns, which is without the effects of varying temperature and humidity. Inaccuracies are observed as both linear and non-linear errors in each axis along with the associated rhombic distortion, wherein the deviation across one diagonal is greater than across the other.
Artworks used for PCB manufacture are made from extruded polyester and, therefore, have different characteristics in the direction of extrusion and across the extrusion. In addition, the humidity characteristics for artwork material are approximately 14 ppm %RH. The temperature characteristic for artwork material is approximately 16 ppm /°C.
Punched tooling systems also tend to add further errors due to the accuracy and repeatability inconsistencies of the punch. Some print frames heat up during use and can cause further distortion of upto 25 microns.
When these errors are compounded during the inner layer manufacturing process, the mean error can be upto 130 microns. This would require a design rule of 175 microns to guarantee alignment when the layers are stacked for bonding. This means that high yielding HDI designs would be difficult to achieve on the large panels required for high volume cost-effective manufacture using conventional imaging technology. The removal of artworks from the imaging process would obviously substitute all sources of errors associated with their use. The use of laser direct imaging technique offers an artwork-free manufacturing route, instantly removing the large errors associated with environmental control during both production and use of the artwork. The use of the LDI system has shown that the total mean error is reduced form 130 microns to 12 microns.
LDI is a process of imaging printed circuit boards directly without the use of photo-tool. The exposure of the photo-sensitive resist is done using a laser beam, i.e. scanned across the panel surface and switched on and off by means of a computer control system. The laser used in this process is in the UV spectrum region, as this tends to suit most of the commonly available photo- resists. However, systems exist that operate in both the visible and infra-red spectrum, working with specially formulated photo-resist.
LDI systems first started to find their way into the printed circuit manufacturing arena in the late 1980s. These early systems were much slower than the conventional contact printing, and it has only been in the last few years with a new generation of faster LDI systems that the process has become a viable threat to contact printing with a photo-tool. With the availability of high-powered lasers, giving 4 watts of power at the work piece, and with the introduction of a new generation of photo-resists requiring exposure levels of the order of 8-10 mJ/sq.cm along with faster computers for data rasterization, it is now possible to get LDI exposure and handling times down below 30 seconds per side for an 18" ¥ 24" panel.
A Laser Direct Imaging system has the following important components (Sallan and Wiemers, 1999):
a The laser system;
a The optical system;
a The mechanical system; and a Data processing system.
The Laser System
The laser system is used as the light source and consists of a water-cooled argon-ion laser with a capacity of 1.5 W. The exposure takes place in the UV range at 2 wavelengths of 351 nm and 358 nm because of the maximum sensitivity of the common photo-resist within this range. The pixel diameter is 28 mm. Since the intensity within the pixel is different, an exposible pixel diameter of 10mm remains.
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The resist exposure by a laser beam is effected in the horizontal direction with a velocity of 240 m /s. During the scan movement, an exposure clock pulse ensures the light /dark scan of the laser beam. The vertical movement of the material to be exposed towards the scanning causes a line per line image structure all over the whole surface. The exposure grid is 10 mm. In case of a maximum exposure area of 340 ¥ 600 mm, a data quantity of about 2 billion pixels has to be processed.
The Optical System
Figure 7.14 shows the optical system of an LDI system. The system consists of an acousto-optical modulator, a quartz in which an acoustic wave rectangular to the laser beam is generated. It is situated in the optical axis of the laser beam. The incoming laser light is diffracted and its direction changes depending upon the frequency of the acoustic wave. The light reaches either a beam trap or the optical beam path. The scanning is achieved by deflection via a ten-face polygon whose mirror faces have an angle error to each other of less than 2". A line of 340 mm is exposed per each mirror face.
Table PCB Divider
Divider AGM
Laser
X-measuring system Laser diode Polygon
Objective lenses
X-measuring rule
Fig. 7.14 The optical system of direct laser imaging system (redrawn after Sallan and Wiemers, 1999)
In order to achieve a precise rotation speed and smooth running, the polygon is placed on an air slot of 10 mm and is synchronized with the table drive. The table moves on 10 mm with each new mirror face. The optical system consists of eight lenses, which ensure a telemetrically beam path and therefore, a clear pixel image.
After the laser beam passes through the lens system, it is guided via a semi-reflecting mirror down to the resist-laminated PCBs or inner layer laminates. The X-measuring system ensures the exact positioning of the pixel. The system consists of a laser diode of 685 nm wavelength, a glass measuring rule and several deflection and filter units as well as a reference grid.
The red laser beam of the measuring system is guided parallel to the blue laser beam of the exposure system via the polygon through the lens system and through a 50 per cent-mirror on an etched X-measuring rule. There, the image is reflected back through the mirror and the lens system to the polygon where it is de-coupled and mapped on a reference grid.
During the rotation of the polygon, the image of the X-measuring rule passes the reference grid and generates a light signal from which the exposure clock pulse can be deduced. The exact position of the red laser beam can be determined by counting the impulses of the exposure clock. The position of the blue laser beam is determined at the same time due to the coupling of the red laser beam with the blue one.
Mechanical System
The main mechanical components of the LDI are the Y-measuring system, the Z-positioning system, the registration and the air ventilation system for clean room facility.
The work table is directly connected with the Y-measuring system. Since the table movement and the polygon are synchronized, the table receives a positioning pulse after each mirror face. This impulse is given to a stepper motor situated on a ball screw, which is also connected to the work table. The positioning is carried out by counting the increments referring to a reference mark.
Before the exposure process, the PCB must be lifted in the focus area and registered. The work table is equipped with three stepper motors for this Z-positioning. These motors lift the table until a proximity sensor detects the copper clad of the PCB. The table is therefore always situated within the focus area with a rectangular alignment to the laser beam.
For the registration of the PCBs, two service holes can be found on the production panel. This procedure enables proper adjustment of the fitting for the pattern structure to the drilling pattern to be produced. There are LEDs beneath the panel and there are also four quadrant sensors above the panel. The sensors measure the light quantity per quadrant and adjust via the positioning motors in X, Y and Z-direction as long as each quadrant captures the same light intensity.
The work table with the optical system and the laser unit has air bearings to eliminate the effect of external vibrations caused by vehicular circulation and other machinery in the vicinity. This vibration damping system ensures a correct exposure quality.
Data Processing System
CAD or CAM data usually exist as GERBER data which is normally used for photoplotting. Due to compatibility reasons and the need to simplify the processing, the LDI system also prefers to process these data formats.
Figure 7.15 illustrates the typical data flow arrangement in which the external data of the CAD system is received by ISDN or by e-mail. CAM reads out the data from the modem, processes it and makes it available via an internal LAN system on the LDI system.
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PCB
Data format: Gerber
Transfer
Internet ISDN
eMail LAN
CAD CAM LDI
Fig. 7.15 Typical data flow arrangement for laser direct imaging
Since vectors are described in the GERBER format, which is inappropriate for a line per line presentation, the data have to be transformed into information for 10 mm grid pixels. The original amount of data is therefore multiplied. The compilation of the data is performed in real time by multiple simultaneously working transputers. The data are read line per line and stored on a 250 MB RAM. The RAM is sufficient to store exposure data for approximately five double-sided PCBs.
7.8.1 Benefits of LDI
A comparison between film and laser exposure clearly reveals the advantages that the laser system offers in terms of quality, production tolerances and savings in time and cost (Barclay and Morrell, 2001). The savings in cost of manufacture can obviously be seen from any one or all of the following factors:
a Elimination of photo-tools and the cost involved in their manufacture and storage;
a Reduced job set up time between prints and manufacture of PCB; Manufacture can start as soon as the data leaves the engineering department. The time saved (from 10 hours for films compared to three hours by laser) is very important during the production of prototypes;
a Possibility of adopting a flexible manufacturing route to meet the varied demands of production without impact on throughput; and
a Reduced manufacturing lead times by enabling manufacture to start as soon as the data leave the engineering department.
Similarly, quality improvements offer several benefits such as:
a Elimination of film- and printing-related defects; and
a Elimination or reduction of temperature-and humidity-induced effects on the product due to the controlled environment employed within the laser imaging systems.
Over and above the cost saving and quality improvement benefits, there are a number of technical advantages, which are detailed below.
a Resolution: LDI systems offer improved resolution due to the small laser spot size. Sub 50mm features can be easily resolved. With process optimization it is possible to produce fine lines of the order of 35 mm in a 40 mm resist. With the future LDI systems improvements, 25mm lines and spaces are likely to be realized.
a Registration: Improvements in registration are achieved by eliminating the photo-tool, which has always given alignment problems, especially as the tools move anisotropically with temperature and humidity changes. With the LDI system, it is possible to use a CCD (charge coupled device) camera system and target fiducials on the panel to align the print image and panel. It is also possible to use these target positions to calculate any panel or drilling movement, thus achieving an improved registration.
a Tolerances: A qualitatively different tolerance class is achieved by using a LDI system.
The industrial standard of 0.1 mm permissible misalignment from drilling pattern to conductive pattern can be reduced to 0.03 mm in the ideal case.
Kelley and Jones (2002) illustrate the application of laser direct imaging. The LDI systems are bringing about fundamental changes in the organization of sequences, and in the logistics and data storage facilities in the PCB industry. One LDI system is replacing all exposure systems in PCB manufacture. Both the tasks that are being currently carried out by conventional film exposure systems and those of a photoplotter, can be taken over by LDI systems. The software control adaptation of the machine to the required task can easily be carried out and all CAD systems would work with a uniform table and every PCB would have the same material specifications. The use of LDI systems, in future, is likely to drastically reduce the development time for electronic assemblies, a sensational but possible solution. Vaucher and Jaquet (2002) provide an update on Laser Direct Imaging and Structuring.