Home:: Digital inkjet printing for etching circuits: putting the pieces in place for printed PCBs.

Digital inkjet printing for etching circuits: putting the pieces in place for printed PCBs.

Digital inkjet printing for etching circuits: putting the pieces in place for printed PCBs.

Publication: CircuiTree
Publication Date: 01-JUL-07
Format: Online
Delivery: Immediate Online Access

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.



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.


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.


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.


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.


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.


The comparison of the lines show that the conventional surface and printing process...