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Article Excerpt
The driver for using a digital inkjet printer to lay down a UV
etch resist for PCB innerlayer fabrication is fairly straightforward:
elimination of all photolithography processes and equipment (i.e.,
artwork generation, photoresist exposure, and development). The
reduction in space requirements, material usage, cycle time, and
associated costs also makes the switch to digital inkjet printing very
cost effective. This article presents the latest results at achieving
this goal.
[ILLUSTRATION OMITTED]
Introduction
The photolithography process has been and continues to be the
workhorse of PCB manufacturing. As trace dimensions and tolerances have
become smaller and tighter, process improvements have kept pace. But
with manufacturing costs being factored more and more into the equation,
reducing processing steps and cycle time is a major focus of all PCB
fabricators. As the process step comparison in Table 1 demonstrates,
digital inkjet printing of a primary etch resist is very advantageous.
By directly translating the design file into a print raster file, the
inkjet printer enables the elimination of the artwork department in
innerlayer etching, as shown in Figure 1.
Laser direct
imaging (LDI) is currently the most advanced digital imaging method
being used for PCB manufacturing. LDI dry film photoresists are capable
of <50 [micro]m resolution with good edge definition. While this
technology eliminates the artwork generation step in Figure 1, it still
requires all the other photolithography process steps. More importantly,
LDI equipment/maintenance and the photoresists are very expensive.
[FIGURE 1 OMITTED]
The concept of inkjet printing for PCB manufacturing dates back over
20 years. Currently, inkjet printers have only been successfully used
commercially for legend or nomenclature printing due to the lower
requirements for print resolution and definition. High-resolution
requirements of PCB innerlayer imaging have traditionally outpaced the
capabilities of inkjet printhead and printer technology. In the last
year, however, the evolution of piezoelectric drop-on-demand (DOD)
printhead technology resulted in precise and repeatable drops, with
volumes down to 3 pL (10 to 12 L). Next, the printhead and printer have
to be fully integrated to accurately reproduce the digital image onto
the circuit board. Finally, the development of etch resist inks that are
compatible with the materials of construction of the printhead and
achieve good drop formation is critical for fine-line resolution.
This article presents results achieved to date with a newly developed
UV curable etch resist on a commercially available inkjet printer. To
print 100 and <100 [micro]m traces, the inkjet printer, printhead,
ink, and substrate surface interactions must all be optimized.
The Printer
The printing platform design has to meet the required needs of the PCB manufacturing process:
1. Rigid and flex capability;
2. Image translation software;
3. Substrate clamping system;
4. XY positioning accuracy;
5. Printhead alignment and height adjustment;
6. Front-to-back alignment;
7. Integrated UV curing;
8. Automatic ink delivery system; and
9. Printhead cleaning/maintenance station.
An inkjet printer that meets these requirements is shown in Figure 2.
[FIGURE 2 OMITTED]
Along with the positioning accuracy and image translation software of
the printer, the choice of printhead technology is crucial to placement
accuracy and printed image quality. In the case of PCB manufacturing,
piezoelectric DOD printhead technology, which produces precise and
repeatable drop volumes with minimal angular deviation, is the preferred
technology. State-of-the-art DOD printheads can eject drop volumes as
small as 3 pL (10 to 12 L).
The printer in Figure 2 is
currently equipped with a variable grayscale DOD inkjet head that can
print drop sizes of 6 to 42 pL in volume. This allows the printing
machine to match drop size with required resolution. As a result, large
features such as power/ground can be printed at lower resolution and
larger drop size, thereby increasing overall print speed.
Panel throughput depends on a number of factors, such as print speed,
number of printheads, and resolution/dpi. The near-term throughput goal
is 50 to 60 sides/hour, with a future goal of 120 sides/hour. Print
speed and drop placement have the greatest effect on the printed image
quality. Increasing the number of printheads and/or the native
resolution (nozzle spacing) of the printhead versus increasing the print
speed will result in better image quality with faster throughput.
In regard to flex manufacturing, inkjet printing can be easily
interfaced into the production process. Another model at the beginning
stages of development is a reel-to-reel version enabling direct flex
circuit printing. An earlier version is shown in Figure 3.
[FIGURE 3 OMITTED]
UV Curable Etch Resist
The chemical properties of the resist have to be optimized for inkjet
printing and PCB fabrication. The ink must be compatible with the
materials of construction of the printhead and stable over time and at
operating temperature. The viscosity and surface tension of the ink will
affect nozzle plate buildup, drop formation, and drop spreading.
Varying the jetting temperature and voltage will influence these
parameters, but only optimizing the ink chemistry will result in good
drop formation.
Figure 4 shows the results of ink formulation
optimization using a Drop Watcher station designed to monitor drop
formation using stroboscopic techniques. Figure 4a shows an ink with
poor drop and tail formation, leading to the formation of satellites.
These satellites will lead to excess copper and poor line formation.
Figure 4b shows good drop formation with optimized ink chemistry.
[FIGURE 4 OMITTED]
In addition to the native drop size of the printhead, the actual
printed resolution also depends on the drop dynamics of ink rheology,
surface roughness, and UV response. Once optimized, drop spreading can
be minimized.
Figure 5 shows the effect of improving the
substrate/ink interaction in the inkjet printing process. The top image
shows a pre-cleaned copper innerlayer with line growth using typical
non-optimized inkjet printing. The bottom image shows the results using
the optimized process with the identical drop volume.
[FIGURE 5 OMITTED]
The comparison of the lines show that the conventional surface and printing process... |