Micro structures with nano pastes

Citation

(2010), "Micro structures with nano pastes", Assembly Automation, Vol. 30 No. 4. https://doi.org/10.1108/aa.2010.03330dad.002

Micro structures with nano pastes

Article Type: New products From: Assembly Automation, Volume 30, Issue 4

New screen printing process prints fine structures with high cross sections

Conductive lines, 50 μm wide, can now be screen printed due to nano pastes and a new printing process, which is implemented in Essemtec’s new SP900-S printer. This system surpasses the capabilities of expensive offset or flex printers because screen printing is inexpensive, flexible and allows the printing of very high thickness layers.

OLEDs, organic FETs, organic batteries, plasma displays or fuel cells: all these products require multilayer high precision printing processes for their manufacturing. Until now, these applications have been limited due to the printing processes used that only allowed thin layers. This is now changing. With nano pastes and a new printing process, micro structures also can be produced using screen printing and can be built into multiple layers up to very high layer thickness.

“Screen printing now enters completely new applications,” said Joachim Biegel, Product Manager at Essemtec. During the last few years, he worked with the PV Lab of the EPFL Neuchâtel to develop a process and machine. PV Lab (photovoltaic and thin film electronics laboratory) researches and develops Heterojunction Solar Cells. Using the new printing process and printing machine, these cells are now more efficient than others by several percentage points. However, the innovation from the solar industry can also bring new wind into the thick film technologies.

50 μm wide structures with 40 μm thickness

Conductor lines with widths from 100 to 150 μm and a height of approximately 12 μm have been the state of the art for a long time when using screen and stencil printers. However, Essemtec’s SP900-S can print structures that are only 50 μm in width and 40 μm or more in height. The PV Lab in Neuchâtel uses this machine to produce its Heterojunction Solar Cells. These cells now belong to the best cells in the world and the most promising future technologies.

The efficiency of many products depends on the optimal design of the conductors, as do solar cells. The traces on these cells are called bus bars and fingers. They should be as thin as possible to maximize the active area. The cross section also should be as big as possible to minimize the inner resistance of the cell.

Using the SP900-S, solar cell manufacturers can maximize the height/width ratio of the conducting traces. No other printing machine allows such a high layer thickness to be produced and such high cross sections to be achieved. This is advantageous not only in the solar industry but also in other products that require thin lines with high electrical current capacity.

Multilayer screen printing

Narrow but high traces can be produced by printing multiple layers on top of each other until the desired height is reached. The idea is not new, but until now, almost no one could print such thin lines so precisely on top of each other. The SP900-S has been optimized for this task. Design engineers have implemented a new type of printing control and have developed new vision, drive and damping systems to fulfil the demanding requirements of such a process.

“It is similar to newspaper printing,” said Joachim Biegel. The paper printing process also requires an exact positioning of the four colors relative to each other; otherwise, the cover story only gives headaches. There, the printing machines make use of so-called registration marks to line up the colors. Therein lies the difference: on solar cells, there are no such marks as this would reduce the active area.

Therefore, the SP900-S must use structures on the wafer itself for alignment. For the first printing cycle, are edges and corners of the wafer. When printing a further layer, the printing screen must be lined up with the pre-printed fingers or bus bars. The vision system on the SP900-S can use both alignment methods. Screen and substrate are positioned with an accuracy of ± 8 μm to one another.

From force control to position control

High profile conducting lines are printed in multiple layers as previously described. In the past, a reduced aperture technology was used for such processes. For the top layer, the screen apertures were slightly smaller than for the first layer and were positioned on top of the pre-printed structure. However, local bleed-out often occurred that floated down on the bottom layer, resulting in improper printing. Additionally, the production of two different screens was expensive.

The new process uses the same screen for the first and the second layers. For the second print, the screen is lifted up several micrometers or the substrate is lowered, depending on the printing machine. The printing head no longer regulates to a pre-defined printing pressure but to an exactly defined location in the vertical axis.

In this way, the distance between the screen and the substrate is adjustable within microns. No more bleed-out occurred and the print thickness is precisely controllable. Of course, this process requires a perfect parallelism of squeegee, screen and substrate and also places special demands on the vertical drive of the print head. Normal printers are not suitable; however, the SP900-S is optimized for this.

Outlook

The uniquely thin and high traces on solar cells produced with the SP900-S printer have brought the researchers of the PV Lab a big step further. Furthermore, the industry now possesses a machine that has been developed using practical experience. It features a very compact design with a small footprint, an easy-to-use software, a high flexibility and a low cost of ownership. It is very well suited for use in pilot production lines, laboratories and small productions.

Figure 2 SP900-S printer is robust, precise and compact: ideal for pilot lines and small batch manufacturing

Development is not at an end yet. The next step is already targeted, which is another reduction of the printable structures down to 25 μm. This will bring screen printing into an area of applications that until now have required expensive offset or flex print technologies. Screen printing, however, will reduce the cost for production, will be much more flexible and will enable higher electrical currents to be transmitted through thinner traces (Figure 2).