Applying Digital Printing in the PCB Industry

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Traditional printing on PCBs includes screen-printing and photoimaging. These printing technologies involve high production costs, time-consuming procedures and several production steps. In today’s competitive PCB industry, where production runs are getting shorter, it is needless to say that cutting production costs and making procedures more efficient are import for the PCB manufacturer to remain competitive. PCB manufacturers therefore willingly accept this new technology to the PCB industry, and are quickly shifting from conventional to digital printing.

Applications for Digital Printing

Printing technologies are involved in numerous PCB manufacturing steps, and digital printing is applicable for many of them. A few companies today are active in the area of digital legend printing, usually by adapting laser plotters or electrical testers and modifying them for legend printing. It is not surprising that these companies offer printing systems for legend printing, since this forms the easiest digital printing application for PCB manufacturing.

Legend printing involves only approximately 2-3% of the total PCB manufacturing costs, while other applications, such as soldermask printing, accounts for up to 10-15% of the total PCB manufacturing costs. The use of digital printing is important for all production steps, however, its application will be most significant for the expensive process steps.

So, while the actual introduction of digital printing to the PCB industry is important, mainly to change its work processes, it is necessary to analyze the printing technologies offered in context to other more demanding applications such as soldermask printing. This is the real challenge for digital PCB printing technologies. The next chapter deals in-depth with the challenges PCB printer suppliers face to provide the best possible solutions to the industry, and discusses problems such as inkjet print-head reliability, PCB attachment, contamination and ink types, and describes possible solutions to the different problems.

Major Technological Issues in PCB Inkjet Printing:

1. Satellites and Tails

Figure 1. Schematic drawing of a print head chamber, showing a tail breaking down and forming a satellite (left to right).

Satellites and tails (or ligaments) are a major issue in the design of high quality ink jet systems, and pose special requirements both to ink developers and the printing system designers.

Satellites. When the ink droplet is ejected out of the nozzle, a tail is formed. This tail may break into small fragments, which tend to form yet smaller droplets. These smaller droplets follow the main drop, hence the term “satellites.” The satellites, having lower mass and momentum, tend to have lower velocities and hit the PCB surface in different random locations. The development of suitable ink for PCB inkjet printing will solve this problem.

Tails. If the relative speed between the PCB and the print head is too high, the tails will be evident on the PCB, even when it does not break into satellites. The speed of a PCB inkjet system has to be very carefully chosen to avoid this problem. Using multiple print heads for printing will comply with the requirements for high throughput of the PCB manufacturers, without having to use high relative velocities, which result in the tails showing on the PCB.

2. Reliability of a Moving Ink-Jet Print Head

Figure 2. Positive pressure causes a drop to form and drop down at a random location on the PCB, potentially causing a fatal defect.

Ink-jet printing is based on a delicate balance between very small negative pressures vs. the meniscus formed inside the nozzle (Figures 2-3). The pressure required is approximately 0.001 ATM, with a tolerance of 0.0001 ATM. A mechanical impulse is activated on the ink in the firing chamber to eject a single droplet, and then the meniscus is almost instantaneously formed again.

Photo 1. Printing by a single head machine. The head moves too fast, causing the tails (or ligaments) to hit the SMT pads, creating fatal defects. This picture also shows multiple satellites on the PCB surface.

The acceleration (and de-acceleration) of a moving ink-jet print head creates a positive or negative pressure variance, depending on acceleration direction. This pressure difference is calculated as the acceleration times the dimension of the body of liquid in the acceleration direction. Since the tolerance window for pressure is very tight, two possible effects might occur, depending on acceleration direction. Both effects might cause fatal defects.

Figure 3. Negative pressure causes air ingestion, resulting in inconsistent drop shooting which may lead to drop placement error, potentially causing a fatal defect.

First, positive pressure will cause unintentional leaking of ink from the nozzle, which might hit the PCB at random, including at SMT and BGA pads.

Negative pressure will cause air ingestion (intake) through the nozzle, and will form an air bubble inside the ink-jet firing chamber. In the better case this can cause a sporadic and temporary firing problem, manifesting itself in a “missing nozzle.” In the worst case, the nozzle will fire the drops sideways, causing potentially fatal defects.

Photo 2. Stationary print heads ensure immunity from temporary nozzle failure caused by accelerations.

To eliminate pressure variation problems in ink-jet print heads all together, PCB inkjet printers should be designed with stationary print heads, where the PCB has to perform all the X-Y motions. Ink heads should only perform slow movements in the Z-direction to compensate for various PCB thicknesses. The trade-off for stationary heads is the relatively bigger size of the systems. However, a bigger footprint of the printer is the better choice and less expensive than the costs of fatal defects on PCBs.

3. PCB Attachment and Flattening

Figure 4.

An ink-jet PCB print head must keep a constant distance from the PCB, maintaining a tolerance of approximately ±0.2 mm to avoid gradual printing quality degradation that can lead to fatal PCB defects.

If the distance between the print head and the PCB is too big, the small inkjet droplet will lose momentum due to air friction and start wandering around, much like an aerosol droplet. It might eventually be dropped on an SMT or a BGA pad and destroy a $1,000 populated PCB. To get the feeling of it, try to visualize throwing a feather. When it leaves your hand the feather still has some velocity and momentum, but soon enough it will begin floating in the air, landing at random.

If the distance is too small, one of two things might happen. In the extreme case when the print head hits the PCB, it will smear the fresh printing, destroy the print head, and scratch the PCB. If the head is very close, but not touching the PCB yet, high velocity droplets impacting the PCB surface may be splashed on the PCB.

Legend or soldermask printing is performed on the outerlayers of the PCB. By then the PCB is nearly finished, and is typically quite rigid and in many cases far from being perfectly flat. In addition, even the PCB thickness itself may vary to some extent. All that has been mentioned clearly indicates that adequate PCB attachment and flattening is a major, critical requirement for keeping the necessary constant distance between the PCB and the print head.

Using mechanical clamps that hold the PCB edges down can solve PCB edge deformation. Commonly, a vacuum table should solve the problem of bows occurring in the center of a PCB, however, pressure is lost through the PCB holes. In addition, the air that passes through the holes deflects the ink-jet droplets and might cause fatal defects.

The combination of vacuum table and clamps forms one possible solution for PCB attachment, provided that the printing system delivers the appropriate solution for the drop deflection problem. Air flow spoilers are one possible solution: whenever there is a free flow path through a PCB hole which overlaps with a vacuum table hole, the increased flow will meet an obstacle creating increased turbulence. This turbulence will significantly decrease the flow. Another, far better solution, is to design a table where the vacuum is enabled only at locations not being currently printed.

4. Print Head Contamination

Figure 5.

In industrial PCB manufacturing plants—especially in the legend printing room—there is typically some level of contamination, particularly dust particles and fibers. Since inkjet printing requires a distance of about 1-1.5 mm between the print head and the PCB, any dust particle, hair, or a paper fiber that somehow gets attached to the print head will cause smearing of the just-printed ink on the PCB, causing potentially fatal defects.

The print-head is always covered with a thin liquid layer which readily absorbs any such particle. Therefore, the system designer must enable the system to be configured, optionally, to deal with the problem. A solution may include a filter that creates inside the machine a positive pressure of clean air, an external filtered air cover and, to make sure that the PCB itself does not introduce contamination into the machine, an air knife and electrostatic discharge rode to clean the panel.

5. Finding Suitable Ink for PCB Legend Printing

Choosing the suitable inkjet ink is always closely tied to the nature of the substrate. The PCB is a non-absorbing substrate, which creates a fixation problem for the ink drop after it is ejected from the print head and hits the substrate.

Using UV curable ink is the most straightforward solution for PCB printing. However, inkjet UV inks have three inherent problems: poor adhesion; low pigment density, and toxicity.

This solution can cause potential health problems. Inkjet UV ink, not like non-inkjet UV ink, must have very low viscosity (around 10 CPS) to be inkjet compatible (“jetable”). To reach this low viscosity, inkjet UV ink contains a high percentage of reactive monomers, which are small polymer molecules having low molecular weight. However, this attribute enables the monomers to be easily absorbed through the human skin and lungs into the blood stream, where it activates the immune system and responds by creating antibodies. The level of sensitivity is individual. It might gradually increase over the course of months or even years, until this person reaches a state of high sensitivity. The person is then “sensitized.” This fact is irreversible, and the injured person must be banned from exposure to such materials. In some cases this means forced early retirement and increased severance payments. Sometimes the reaction to monomers is almost immediate. On the other hand, there are people who show little or no sensitivity at all.

Photo 3. An example of adhesion problem in UV ink.

Poor adhesion is caused by two factors. First, UV ink shrinks during curing, which creates a gradual shift and dislocation at the interface with the substrate. Second, pigment scatters the UV radiation, especially in white UV inks. The interface between the ink drop and the substrate is covered by the bulk of the whole drop, which reflects UV radiation, and therefore curing at the interface is relatively poor.

Low pigment density is a necessary fact for inkjet UV printing on PCBs. High pigment loading would prevent the UV light propagation in the bulk of the ink drop, and would disable the curing process.

The development of heat-curable ink-jet for PCB legend is a much more complex task. The most challenging task is causing the ink to stop its flow. However, the use of heat-curable ink would solve all the above-mentioned problems that exist for UV-curable inks. Heat-curable ink is not toxic and enables high pigment loading. It has excellent adhesion, since no shrinkage occurs; curing is based on heat, and heat propagates easily to the PCB-ink interface.

A practical heat-curable PCB legend ink solution is as follows: the flow of the ink is restricted by controlling its rheological properties, and by drying the ink immediately after printing.

Considerations for Soldermask Inkjet Printers

Once the industry adopts soldermask inkjet printing, very strict requirements will dictate the engineering design of printing systems. Adaptation of plotters or electrical testers will not be able meet these requirements, and application-specific printing systems need to be developed. Below is a list of soldermask-specific requirements and possible inkjet printer solutions.

Quality and reliability. Reliability and printing quality requirements for soldermask inkjet printing are much stricter, and dictate the use of stationary print heads. In soldermask printing, most of the panel, except the pads, has to be coated. Even a small “pin-hole,” exposing two adjacent conductors, can cause a critical defect, such as during wave soldering. Side firing of the ink, when printing very close to the pads, such as solder dams in fine pitch SMT or BGA pads, will cause a fatal defect.

The combination of higher speed and higher resolution. The IPC soldermask roadmap specifies solder dams of 50 microns; this dictates the use of high-resolution print heads. Throughput requirements of the industry are commonly 120-150 sides per hour. These combined requirements on one hand, and the relative speed limitation imposed by the ligament problem for inkjet print heads on the other hand, strongly suggest the use of multiple print head printers. Printers with stationary big printing bridge are easily scalable to any required number of print heads.

Material quantity. Soldermask printing requires large amounts of material: 50 to 100 kilograms per day. This strongly suggests the use of a stationary printing bridge and a stationary ink supply system that complies to the requirement for a very high ink flow rate.

Material considerations. Soldermask, as a critical element in the final construction of the PCB, prevents shorts between adjacent SMT and BGA pads, and protects the circuit from any environmental influences. Materials comparable in performance to currently used soldermask materials are necessary to meet these requirements and dictate the use of heat curable epoxy based materials.

The design and qualification of a heat-curable inkjet soldermask requires major R&D focus and investment, due to a set of challenging requirements.

Other Major Considerations in PCB Inkjet Printing

There are many other aspects in PCB inkjet printing system and ink design—most notably regarding reliability and print quality. Much proprietary technology is employed under the surface without the customer being necessarily aware. Some of the obvious aspects in the design of high reliability inkjet systems include print head capping, wiping, and exercising facilities, ink circulation, etc. Inkjet and digital printing pioneers in the Graphic Arts industry have been laying the foundations for the current state of industrial inkjet technology.

The fact that the PCB industry is now embracing digital printing is actually an old and established technology migration path. (It is no coincidence that the PCB manufacturing process is so similar to the process of manufacturing the offset printing plate).

But as always, there is work to be done when adopting a new printing technology from the traditional graphics arts to the PCB industry, which has its own special requirement. Speaking on behalf of PCB inkjet printers designers, I believe we can say, quoting Sir Isaac Newton, “We could see so far ahead, only because we stood on the shoulders of giants.”