Test Series for Challenging Paste Printing of Ultra-fine Structures in the Processingof MiniLEDsusedas Back-lightingforLCDPanels

Partnership-based cooperation ensures manufacturing success

BY STEFAN MAUSNER, HERAEUS DEUTSCHLAND; DR. SEBASTIAN FRITZSCHE, HERAEUS DEUTSCHLAND; LUKAS SÄNGER, ASYS AUTOMATISIERUNGSSYSTEME; AXEL LINDLOFF; KOH YOUNG EUROPE; SEBASTIAN BECHMANN, CHRISTIAN KOENEN GMBH

Since LEDs replaced CFL tubes as the back-light for LCD screens, there has been little fresh innovation in the display industry. This is now changing with the application of mini LEDs in back-light modules, which render LCD panels compatible with OLED screens in terms of contrast ratio. MiniLEDs will also be used in so-called LED-based video walls. The use of mini LEDs makes it possible to realize large-area displays whose performance far exceeds that of previous technologies.

Bild 1: The Christian Koenen Application Center during the miniLED print test series (FigureChristian
Koenen GmbH).

The greatest challenge in using mini LEDs lies in the processes of assembly and soldering of these tiny components onto PCBs, which accommodates the electronic circuit required for driving them of assembly and soldering of these tiny components on the printed circuit board (PCB), which accommodates the circuit electronics required for driving them. Typical mini LEDs have an edge length of <240 µm; their solder pads are required for the electrical and mechanical connection, which establish contact with the circuit board via the “classical” solder paste printing and soldering process, and are of course even smaller. Stencil printing is the proven and most efficient method of applying solder paste to PCBs. The paste deposit required for the assembly of the mini LEDs is specified by the size of the opening (aperture) in the stencil.

In order to apply the extremely small solder paste deposits evenly and reliably to the circuit board, solder pastes are required whose solder powder particles are smaller than 15 µm in diameter; similarly, the stencils must be manufactured with the highest precision. Paste application in this area with very minimized geometries only succeeds efficiently if the printing system features high-precision alignment and uses a special print head with optimum release behavior for the finest structures. Also, an integrated measurement system that can reliably detect such small geometries is necessary for high process reliability and control.

With its expertise, the Application Center of the Christian Koenen company contributes to the technological development of both board assemblies and manufacturing processes in electronics. The Application Center serves as a collaborative platform so that the best possible solutions for demanding requirements can be developed together with the customer. The Application Center is, of course, air-conditioned and classified as a clean room in accordance with the high requirements for such an installation. These are fundamental prerequisites for ensuring that process conditions can be reliably maintained at a constant level, as was the case in the test series described in the following.

In this series of experiments, each variant was printed 10 times on the printed circuit boards without using solder resist. The stencil featured about 1000 apertures so that a statistically significant analysis was reliably possible. The bottom side was never cleaned during the printing process. In this way, it was possible to reduce the number of influencing factors and thus improve the comparison of the data. The prints were checked directly in the MeisterS inspection system. To verify the results in the series of experiments, we also used different microscopes and a Cyber laser scanner. The printed circuit boards were then neither assembled nor soldered.

In the partnership of the compa- nies Heraeus, Koh Young, ASYS, and Christian Koenen, a reliable experimental setup was jointly realized. The tests were carried out under real-world conditions as in a manufacturing line, and commercially available components and materials were used in the process.

Systems and materials used for the series of experiments:

  • ASYS SMD line consisting of EKRA                   

SERIO 5000 printers together with transport and handling modules

  • MiniLED solder pastes Heraeus LED131 (type 6) and LED100 (type 7)
  • CK Nanovate™Nickel stencil with Plasma treatment
  • Koh Young Meister S solder paste inspection (SPI) system

For the miniaturized miniLED pad geometries, special ultra-fine solder powders with grain sizes of <15 µm are required to obtain the solder pastes that can be reliably used in this application. With stencil thicknesses between 15 and 30 µm and aperture openings smaller than80 µm, these powders can reliably reduce such potential printing errors as substantially fluctuating solder volumes or completely missing solder paste deposits. For the studies described here, special solder powders of type 6/7 were used, produced by the patented Welco® process. In this process, the solder alloy is first heated in a thermally stable dispersion medium above its melting range. With the utilization of a special rotor-stator procedure, molten, very homogeneous solder particles are produced. After cooling, we thus obtain the required solder powders with a very narrow particle size distribution (type 6:5-15 µm / type 7: 2-11 µm). They are characterized by very good sphericity (aspect ratio of ~1) and a very smooth surface. These solder powder parameters help to guarantee very good initial printability as well as additionally the required long-term print stability, which is of particular importance in the ramp-up of mini LED assembly technology.

Bild 1: Solder powder size as a function of paste-type (Figure Heraeus).

To ensure not only excellent printability but also the necessary solderability for these ultra-fine solder pastes, Heraeus has also developed an optimized NC flux formulation series (LED131/LED100) for these LED applications, with which the high requirements can be met. Due to the smaller grain size, the surface area of the solder powder spheres increases considerably (higher oxidation), so first, a reliable formulation for activation had to be designed. The aim was to achieve

With its expertise, the Application Center of the Christian Koenen company contributes to the technological development of both board assemblies and manufacturing processes in electronics.

excellent wettability of all joining partners involved (especially mini LEDs and substrates) as well as a low void rate, but at the same time to maintain the necessary insulation values (Surface Isolation Resistance SIR) of the remaining flux residues. In addition to this consideration and the standard requirements for solder pastes, such as low slump and low solder ball forming, particular attention was also paid to ensuring sufficient tackiness of the solder pastes prior to reflow. This guarantees that the dies of the mini LEDs with their significantly minimized connection areas remain in place until the reflow process was performed.

The CK Nickel stencil material has been decisively further improved by the Christian Koenen team in recent years. This resulted in the novel stencil materialCK Nanovate™ Nickel. The introduction of this super-alloy made it possible to cut nickel using a laser without the formation of micro-cracks. The material thus combines the advantages of stencils made of nickel and of stainless steel, but without their drawbacks.

If the physical properties of the possible stencils are compared, it becomes readily apparent that CK Nanovate™ Nickel yields significant advantages even with very thin material thicknesses.

The table shows an overview of the commercially available stencil materials. Due to the small grain size of the material and its high strength, stencils with a thickness of 15 µm or more can be manufactured under standard serial production conditions.

For the formation of the small solder deposits of the mini LEDs as well as for the verification of the limits of printability, we used stencils with material thicknesses of 15 to 30 µm for the printing experiments. This allowed us to evaluate different area ratios and filling degrees.

To ensure that we obtained representative results despite the selection of different sizes of mini LEDs, we decided to use the following aperture sizes: 45 µm x 45 µm; 60 µm x 60 µm; and 80 µm x 80 µm.

Bild 3: A stencil used in the series of experiments (Figure Christian Koenen GmbH).

A further improvement in the printability of the very small apertures resulted from the Plasma 3.0 coating of the underside of the stencil as well as of the inner walls of the apertures. The plasma 3.0 coating reduces the adhesion between paste and stencil, which results in a better release from the apertures and reduced volume dispersion between the individual depots and also between the subsequent printing cycles.

After optimizing the stencil layout and by using the optimal stencil technology (a frame-mounted CK Nanovate™ Nickel stencil with Plasma 3.0 coating), the past deposits on the pads of the mini LED could always be printed with high reproducibility.It is a widely known fact that modern paste printing systems are characterized by their high flexibility: EKRA printers can dynamically respond to changing requirement profiles. With the trend towards continuous miniaturization, the necessity arises to achieve a reliably reproducible paste transfer in the micrometer range – this of course also applies to the application described here for processing the mini LED. The scalable SERIO 5000 printer platform used for this series of tests is a system that will meet all current and future process requirements.

The key factors that determine the success of paste transfer are the flatness of the substrate and its coplanarity to the stencil. These directly influence the release behavior during the separation process.

Typically, mini LED substrates are a few tenths of a millimeter thin and therefore susceptible to bending. If such applications are to be expected in the high-mix/ low-volume manufacturing environment, then the MultiClamp transport system is used in conjunction with a vacuum print nest: With its over-the-top and side clamping bar the mechanism supports the substrate to its edge beyond the end of the print nest, and then the retractable clamp draws the substrate flat. The result is a completely even surface for paste application. If only products of the types described are handled in production lines, dedicated transport systems are employed.

It goes without saying that reproducible high manufacturing quality also requires reliably repeatable operating conditions. For the test series, this was ensured by the special design of the printing table in theEKRA system. The integrated measurement system guarantees that the substrate is fed to the stencil within the micrometer – and as a result, optimum sealing is achieved for each individual printing process. The separation process has been finely optimized in speed and travel to suit the release behavior of the paste by the system’s high-resolution servo motor.

EKRA’s Advanced Print Head, which was specially engineered for ultra-fine-pitch applications, also made a significant improvement to the results: The pressure of the squeegee moves precisely within a narrow tolerance window. In addition, the stencil is not subjected to any pressure during the separation process, resulting in positive effects on coplanarity and the even distribution of forces during the paste release.

All relevant print parameters are continuously transferred to the Koh Young KPO inspection solution via the EKRA Advanced Closed Loop interface, recorded, and automatically optimized. In that way, the squeegee and separation behavior can be fine-tuned for applications such as the highly miniaturized mini LED, without any intervention by the operator staff.

In automated optical inspection, it is always necessary to find a feasible compromise between inspection speed and measurement accuracy. This is because that a camera can only resolve a certain number of pixels. Pixel size, in turn, correlates with accuracy. The smaller, the more precise.

Processing mini LEDs in an SPI system is also about pixel sizes. In this application, that means the smaller the LED, the better the resolution on the display. For solder paste printing and inspection, this implies that very small structures and low print heights must first be printed with high repeatability and then likewise inspected by excellent instrumentation. For a perfectly repeatable measurement, we need the Information of approximately 100 pixels. This means that one can quickly extrapolate from the pattern size to the required resolution of the inspection system. Another important aspect is the z- resolution. Optical measurement systems evaluate the gray-value phase shift between the single pixels. For this purpose, light is projected at a defined angle from the sides into the camera field. If one considers this in a simplified way as triangulation, it very soon becomes clear that the projection angle and the pixel size correlate with the height resolution.

The design of the Meister S inspection system is tailored precisely to these requirements. The pixel size was reduced here to 5 µm, also the projection angle was minimized. Although this results in a reduction of the height measurement range by a factor of half compared to a standard SPI, high precision at low heights is required in the specific applications for the Meister S system. In addition, the Plus version of the Meister machines is particularly optimized for reflective surfaces.

This investigation results underpin the system engineering approach and reveal the physical limits of optical inspection. The smallest aperture size of 45 µm does not meet the criterion of 100 pixels for the image. Therefore, we see a higher volume distribution of the measurement system as a result. For this reason, Koh Young is already designing new camera systems with a finer resolution of 3 µm. At the time of this test series, however, these cameras were not yet available. Such a system would reliably inspect aperture sizes >30 µm

During these series of tests reliably reproducible solder deposits could be printed with the paste formulations LED131 and LED100. With the stencil openings 80 µm and 60 µm, the best results were achieved on the EKRA SERIO 5000 and inspected in line with the Koh Young Meister S.

Bild 5: Printed solder deposits, paste-type versus stencil thickness (Figure Christian Koenen GmbH).

By using a Heraeus solder paste type 8+ (2-6µm powder size) in combination with a 15 µm thick stencil CK Nanovate™Nickel featuring Plasma 3.0 coating, in further feasibility trials, the printing of solder paste deposits smaller than 45µ x45 µm could also be achieved.

For such small depots and low stencil thicknesses, the sealing and release behavior of the paste in the printing process are of decisive importance for manufacturing success. Printing systems must therefore be able to provide outstanding solutions for maximum coplanarity between substrate and stencil. The squeegee printing, as well as the separation process, must be absolutely reliably repeatable and finely tunable. In a nutshell: the continuous advancement of SMT (surface mount technology) manufacturing will thus continue.