Laser Direct Imaging: A User's Perspective
LDI is a process of imaging printed circuit boards directly without
the use of a phototool or mask. The exposure of the photo-sensitive
resist is done using a laser beam that is scanned across the panel
surface and switched on and off by means of a computer control system.
The laser used in this process is often assumed to be in the UV
spectrum, as this tends to suit most of the commonly available
photoresists. However, systems exist that operate in both the visible
and infrared spectrums, working with specially formulated photoresists.
Papers have been written on both alternatives over the years, but it is
the UV-based systems that interest most of the PCB manufacturers as it
offers the opportunity to be a direct plug-in replacement into the
"yellow room" environment. Consequently, we concentrate here to our
experiences with a UV laser system.
LDI systems first started to find there way into the
printed circuit manufacturing arena in the late 1980s (Ref. 1). 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 phototool. With the availability of
higher-powered lasers, giving in the order of 4 watts at the work
piece, and with the introduction of a new generation of photoresists
requiring exposure levels in 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 x
Benefits of Laser Imaging
With imaging times now getting close to that of Contact Printing, the use of LDI now offers the manufacture an alternative. With
the current LDI systems, using high speed Dry Film resists,
productivity rates of 60 double-sided panels per hour are not
It is now that the system can start to be viewed as
a production tool and the benefits given serious thought. Much has been
said about the lower cost of production with the use of LDI. It is not
too difficult to see how we may reduce the cost of manufacture from any
one or all of the following:
- The cost savings from the elimination of Phototools and the cost in their manufacture and storage.
- Savings in the cost of manufacture of the Printed Circuit by minimising the time to set up between prints and jobs.
- Elimination of the need to produce First Offs for the proving of the Set-Up and Phototools.
- Offering the opportunity for a flexible manufacturing route
and giving the opportunity of printing varied panels to meet the
demands of production without impact on through put.
- Providing the opportunity to reduce manufacturing lead times
by allowing manufacture to start as soon as the data leaves the
In addition benefits are gained from potential quality improvements, such as:
- Elimination of film related and 'Printed-In' defects.
- Due to the controlled environment within the Laser Imaging
Systems it is possible to eliminate or reduce temperature and humidity
effects on the product and minimise the ingress of dust.
As well as the cost saving benefits and the potential yield
improvements there are a number of technical advantages and it is these
that we will discuss and explore. These benefits include:
- Improved resolution with the small laser spot size, sub 50um features are easily resolved
Improved registration are also achieved by the use of CCD
- Camera automatic alignment coupled with the opportunity to scale the image for best fit.
Whilst to the board fabricator yield improvements and cost reductions
in the manufacturing process are essential in an ever-increasing
cost-conscious supply chain, it is the technical benefits that LDI
offers that will give him the advantages in the future. This will be
either by widening the operating window or allowing more advanced
design rules to be used to provide higher density products.
On installation, LDI will give all but the most advanced
fabricators an immediate improvement in their dry film image
resolution. The printing of fine lines and spaces of 50um and below
starts to become a reality, without the risk of low yields due to
printed and film related defects. With process optimization, it should
be possible to produce fine lines in the order of 35um in a 40um
resist. With the predicted future LDI system improvements, 25um lines
and spaces will become a reality.
Figure 2 shows a typical result of fine line printing
using a standard 5 kW UV light source and silver halide films. It is
easy to see the track widening that occurs and the under-developed gaps.
In Figure 3, however, it shows an example where it has
been possible to resolve the lines and spaces but an area of local
track widening has caused a print defect. Also visible is the loss of
line clarity and the tendency for the track width to vary slightly.
Compare this with the image shown in Figure 4. Here
50um tracks and gaps are shown having been printed on the LDI system.
Here in the vertical, horizontal and diagonal axes, the image
resolution appears sharp with straight sidewalls and good development
of the gaps.
Another major benefit that LDI can give us is in improved
registration. Improvements are achieved by eliminating the phototool,
which has always given rise to issues, especially as the tools move
anisotropically with temperature and humidity changes (Ref. 2). This
coupled with the tendency for contact printers to run hot, the
alignment of film and panel has been an ongoing challenge. With LDI,
however, it is possible to use a CCD 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,
and scale the rasterized print image so that we improve the
registration and achieve best fit.
The LDI systems are able to use a variety of fiducials
to allow registration and scaling. Figure 5 shows some that may be
used, such as a matrix of mechanically drilled through holes, a matrix
of laser ablated microvias or a laser drill alignment fiducial on layer
X-1. In addition, the CCD camera system must be flexible and able to
recognise target fiducials that may vary in the way they are prepared.
For example the target may be electroless copper plated, electroplated
copper, coated with direct metalization, brushed, micro-etched or
pumice-scrubbed. To add further variability, the copper may have
tarnished to some degree. To the CCD camera, all these finishes have a
potentially different appearance, and are likely to need a different
algorithm to enable them to all be recognized, so it may be necessary
to set these variables up in the early stages.
Figure 6 shows some of the range of target fiducials
that may be experienced following electroplating with copper and some
with direct metalization.
Why Registration and Scaling Errors Exist
As mentioned before, the use of phototools with all their potential
inaccuracies makes the alignment of the phototool and the panel a
challenge for PCBs with tight tolerance specifications. Below, we have
elaborated on the potential causes of errors in the image creation
process, and tried to analyze the potential scale of the problem for a
board with 500 mm X and Y dimensions.
Temperature and Humidity Stability of the Plotting and Imaging Rooms
In every printed circuit facility, the photoplotting and imaging
areas are key areas where temperature and humidity control are
considered very important. The typical specification of these rooms is
a temperature of 22 degrees C with a variation of I2 degrees C and a
relative humidity of 50% with a variation of I5% RH. There are three
questions to consider. First, do these areas really stay within their
specifications all of the time? If not, what are the lengths and
magnitudes of the excursions? Finally, what exactly is the scale of the
effect of these variations on the phototools and the base substrates
With respect to the maintenance of specifications, every time the
humidifiers inject humidity, there is an excursion. When one looks at
graphs of temperature in a clean room, which has a heavy heat load from
printers, there are always excursions from the specifications. Yes, the
basic specifications are met most of the time, but certainly not all of
Photographic Film is Anisotropic
Although manufacturers try to eliminate this anisotropy, the fact
that the carrier sheet of the gelatine is an extruded sheet means that
there are different stresses in the axial and transverse directions.
This means that the reaction of the film to temperature and humidity is
different in size and timing in each direction. Simply put, for tight
tolerance PCBs, using the same scaling in both directions could lead to
Photographic Film Changes Size with Temperature and Humidity
The specification for temperature sensitivity for photographic film
is about 11 ppm per degrees centigrade. Thus, for a variation of 4
degrees C (I2?C) on a 500 mm panel, this gives a total potential
variation of 22 microns.
Sensitivity of film to humidity variations is about 12 ppm per %
relative humidity. Doing the same type of calculation as before, for a
variation of 10% RH, gives a total potential figure of 60 microns.
Base Material Changes Size with Temperature and Humidity
Although it is not commonly looked at since it is assumed that FR-4
only changes size slowly with temperature and humidity, there are
circumstances where perhaps it should not be ignored. For instance, the
difference in the temperature of the panel at different stages in the
manufacturing process will probably be different than that in the
cleanroom. Therefore, if the material has reached an equilibrium
temperature in any of these steps which affect registration, e.g.,
imaging and drilling, there will be a shift between the image and the
FR-4 has a temperature sensitivity of about 15 ppm per degree C. Going
through the same calculations and assuming only a maximum of 4 degrees
C difference in temperatures will give a total potential variation of
30 microns. We have not estimated the effect of humidity as its effect
is both smaller and appears to takes longer.
Other Potential Sources of Registration and Scaling Errors
There are other potential sources of variations in size of PCBs,
most particularly in bonded multilayer panels. Typically in MLB
manufacturing, the straightforward variation is size between one panel
and another from the same lot after bonding can be up to 100 ppm. This
would translate into a maximum potential error of 50 microns.
Non-linear variations can be considerably larger depending on the
materials used to build the MLB and the types of multilayer presses
used. Figures discussed in the industry put this at up to 500 ppm.
Drilling will also add some variability to the process. Drilling
machines in themselves have an inherent accuracy. However, panels which
are at the bottom of a stack might suffer from random drill wander of
up to I25 microns. This would give a total estimated potential
variation of 50 microns.
What Does this Mean for Production?
If you look at all the potential variations as are
shown in the chart below, it looks as though it should be impossible to
make a high tolerance circuit board.
Of course, this does not happen for several reasons. First, the
differences are never the maximum for every variable. Second, the
inaccuracies are usually random in the positive and negative directions
and so would rarely, if ever, all add up to the sum of all the values
listed above. However, what is does mean is that at the imaging stage
of the production process, especially for outer layers of MLBs, each
panel in a lot has a variation in size which can be quite significant,
and which is not the same in the X and Y directions. Therefore, if each
panel of a lot--of a tight tolerance MLB--is imaged with the same
scaling factor (even if different in X and Y), there is a high
probability that there will be unacceptable hole break-out and levels
This is borne out by the experience of PCB manufacturers who have
always produced difficult PCBs on small panel sizes in order to
minimize the problem of registration and scaling. It is also clearly
shown in the section "Using LDI In Manufacture" that results for the
LIDCAT project, where the tight tolerances on the annular rings of the
microvias would have been impossible to achieve with high yields
without the ability to differentially scale for each panel in X and Y.
For the evaluation of the LDI system and verification
of the CCD camera registration system, a special test panel was
constructed in a 1+2+1 build using an FR-4 laminate core with Resin
Coated Copper (RCCT) bonded to each side. Two panel formats were
produced, 406 x 508 mm and 457 x 610 mm. These were then mechanically
drilled and laser ablated as shown in Figure 7. The panel layout was
such that the features within the panel could also be used as secondary
alignment targets to ensure plenty of opportunity to look at target
variation if required.
Once drilled, the panels were then processed through the direct
metalization process, and half the panels were held at this stage. The
remainder were panel-plated with electrolytic copper. All the panels
were then prepared and coated with a suitable dry film photoresist
specifically developed for laser imaging.
Next, a series of tests, detailed below, were conducted to check the
performance of the CCD camera system and its operation in conjunction
with the laser imaging system.
1) CCD Image Processing Repeatability. With the CCD camera in a fixed
location, the system was set up to repeatability acquire the target
image and calculate the target center. This was designed to prove the
accuracy of the CCD systems in finding the target center.
2) CCD Image Processing Repeatability (Production Targets). This test
involved the location and analysis of targets produced in normal
manufacture, and was designed to look at the system accuracy when
dealing with process variation as would be expected in the normal
course of manufacture.
3) W/CCD Repeatability (Imaging Panels). This test looked at the system
printing accuracy when repeatedly printing the same panel without
removing the panel. This test is looking at the accuracy of the system,
ability to locate the target centres and print an image in register.
4) W/CCD Reproducibility (Imaging Panels). Here we looked at the
repeated imaging of panels but the panel was removed and reloaded
between each print. This was designed to measure the system accuracy in
a simulated production situation.
System Evaluation Results
CCD Image Processing Repeatability < 0.9 um
CCD Image Processing Repeatability (Production Targets)
- PTH 3 x 3 Matrix - <1.8 um (Direct Metalisation)
- PTH 3 x 3 Matrix - <1.7 um (Plated)
- uVia 5 x 5 Matrix - <2.3 um (Direct Metalisation)
- uVia 5 x 5 Matrix - <12.1 um (Plated) *
W/CCD Repeatability (Imaging Panels)
- PTH 3 x 3 Matrix - <6.0 um (Direct Metalisation)
- PTH 3 x 3 Matrix - <9.0 um (Plated)
- uVia 5 x 5 Matrix - <6.0 um (Direct Metalisation)
- uVia 5 x 5 Matrix - <6.0 um (Plated)
W/CCD Reproducibility (Imaging Panels)
- PTH 3 x 3 Matrix - <6.0 um (Direct Metalisation)
- PTH 3 x 3 Matrix - <6.0 um (Plated)
- uVia 5 x 5 Matrix - <6.0 um (Direct Metalisation)
- uVia 5 x 5 Matrix - <12.0 um (Plated)
* Includes some untypical results where the system failed to find target center.
W/CCD Reproducibility Accuracy (Capability with Scaling)
- PTH 3 x 3 Matrix (Direct Metalisation) -Inside 75um annular ring
- PTH 3 x 3 Matrix (Plated) -Inside 75um annular ring
- uVia 5 x 5 Matrix (Direct Metalisation) -~ Inside 50um annular ring
- uVia 5 x 5 Matrix (Plated) -Inside 50um annular ring
From the tests conducted in the system evaluation, it is possible
to achieve a "no break-out" condition on a production panel with a 75
um annular ring for mechanically drilled plated-through holes, and a no
break-out condition with a 50 um annular ring with laser ablated
microvias, when using the CCD camera registration and scaling feature.
During the trials, some issues arose with the CCD camera system failing
to find the true center of some panel-plated microvia targets. This
accounted for the non-typical results included in the CCD image
processing repeatability (production targets) results. Following the
trial, modifications to the algorithms were made to aid the plated
target location to resolve this problem.
Using LDI in Manufacture
Following the system evaluation, the LDI system was then tested on
a simulated production run. Here a test panel for a fifth framework RTD
project, IST-1999-10852, LIDCAT (www.lidcat.com) was selected. The test
panel was designed as a high density microvia design on a panel 405 x
508 mm of a 1+4+1 construction. The individual test structures are
manufactured 4 up in a subpanel, with 6 subpanels per manufacturing
panel, giving a total of 24 test structure panels per manufacturing
panels. The structures within the test panel are as follows:
- 800 um Full Array BGAs
- 500 um Partial Array BGAs
- 250 um Flip Chip Array
- 50 um Tracks and Spaces
- 45 um Laser Ablated Microvias
The design rules used in this board were as follows:
- 50 um Tracks and Spacings
- 50 um Pad to Pad
- 45 um Track to Pad
- 25um Annular Ring on Outer Layers
- 17.5 um Land Condition on Microvia Capture Pads on Inner Layers
A trial batch of these boards was then run through production, both
inner and outer layers being printed on the LDI system. Inner layer
yields at AOI were over 99%, but the design rules for these layers were
not challenging. Batch 1 yields at electrical test, reflecting the full
manufacturing process, was 28%. Following examination of the defects,
fine tuning of the laser ablation of the high aspect ratio holes,
plating and inner layer scaling was conducted before the release of
Batch 2. The inner layer yield of Batch 2 again was high, over 99%. At
electrical test, the full process yield was 96%.
Inspection of the finished panels after LDI showed excellent
registration. Even where only 25um annular ring existed, it was
possible to achieve a "no break out" condition. This can be clearly
seen in Figure 8. The good registration and the image quality of the
outer layer print were major factors in the high yield of Batch 2. The
yield figures do give an indication of the primary image quality, but
do not indicate the challenges that were faced in achieving a good
soldermask registration. Here, due to the tight registration and the
fact the primary image was scaled, it is fair to say that the
soldermask print was difficult at best. If we are to get the best from
our LDI, we will need laser imageable soldermasks as well as primary
image dry films.
Implications for HDI MLB Multilayer Board Primary and Soldermask Imaging
The results show that as the tolerances on annular rings and
positioning accuracy of HDI panels become ever tighter to allow for
greater wiring density and, as the number of HDI layers with their
higher inherent movement increase, the necessity to allow for this
panel movement with some form of registration and scaling becomes
imperative. LDI should be the leading candidate for this application
based on the results shown in this article.
We would also like to touch on the potential need for LDI of
soldermasks. There are two driving forces. The first is the same as for
LDI of primary images, and if the primary image is created by LDI, it
might be essential to use LDI for the soldermask image in order to
maintain registration accuracy. This was the conclusion from the LIDCAT
test boards discussed above as well. The other driving force is the
demand from electronic OEMs for smaller tolerances for soldermask
positioning around features on the PCB.
The convergence of these two forces has led several soldermask
manufacturers including Taiyo, Coates and Vantico to develop
soldermasks with photo sensitivities in the region of 30-50 mJ/sq.cm.
Although these will take about one minute per side to image with
today's LDI systems, they can confer great advantages for the user.
Also, the alternative of imaging several films for soldermask imaging
and trying to optimize the registration on each panel take a lot of
time and effort, while the net time taken may not be less.
We have seen that LDI has now started to reach maturity as a
process for the manufacture of PCBs. It is now possible to get
productivity rates in the same order as that of contact printing. With
potential cost savings and yield improvements over contact printing,
LDI has shown it is now a viable alternative. Whilst these benefits are
important to the PCB fabricator, the potential benefits in resolution
and registration offer most to the advancement in printed circuit
In the system evaluation, we were able to predict that we would be able
to achieve a "no break-out" condition on laser drilled microvias when
we had a 50um annular ring, but in production trials we were able to
manufacture product with an annular ring down to 25um in localized
areas. With this level of improvement in registration, it will not be
far into the future that we will be able to start to change design
rules and reduce pad sizes.
A reduction in the pad size or annular ring coupled with the use of LDI
could then offer the PCB fabricator a number of benefits:
- Better print quality giving rise to higher yield
- Reduced risk of breakout
- Reduced annular ring giving the opportunity for wider tracks and spaces
- Minimal lead-time from receipt of data to start of production,
with the elimination of silver halide film production and its required
stabilization before manufacture.
For the end-user and designer, there are other potential benefits.
Better registration could give rise to smaller pads, finer tracks and spaces, and higher density products.
Clearly, LDI will give us better control of our process with a much
wider operating window. To get full benefit of the process however we
must wait for the fully commercialized LDI soldermasks to become
1. "Laser Direct Imaging: A Cost-Effective Choice," Dave Stone, Barco Belgium. Proceedings EIPC Summer Conference 2001.
Direct Imaging: A Larger Volume-Users View," Steve Jones, Viasystems
(Tyneside, U.K.). Proceedings EPC Show Maastricht 2000.