Screen Printing Innovations to Tackle Fine-Pitch Surface-Mount Challenges

By Richard Vereijssen, Yamaha Motor IM Europe

The latest generations of ultra low-profile and highly miniaturised components, such as QFN ICs, LGA power devices, and 03015 (0.3mm x 0.15mm) chip resistors challenge surface mount manufacturers to control screen printing within a narrower window, particularly in respect of positional alignment and solder-paste volume. At the same time, however, manufacturers are also under pressure to minimise setup and changeover times, to maintain productivity. In high-volume scenarios, there is also pressure to sustain fast cycle times.

Improving the Print Process

The performance of the print process has a critical effect on end-of-line yield. It is known that around 70% of board defects detected at end of the line have their origins at the printing stage. Manufacturers cannot afford to cut corners when setting up their print processes, or sacrifice process control to boost cycle time. Yet speed is of the essence in order to meet tough cost targets and turnaround times. Something has to change.

Productivity is compromised by some aspects of printing that have remained the same for many years, such as the practice of printing a number of trial boards at the start of each batch to verify correct settings such as stencil to board alignment. If a fixed squeegee angle is used, the excursion speed is typically reduced for the first few print cycles after each time the stencil is cleaned. At the same time, printing is becoming more complex, for example with increased use of dual-thickness stepped stencils to achieve the extremely low paste volumes needed for the smallest component sizes such as 03015 while also ensuring adequate paste volumes for larger conventional packages on the board.

Some manufacturers are keen to use two separate stencils of different thickness to achieve this. Both practices have an impact on printing and printer settings that can be best addressed through substantial changes in the printer and squeegee.

As far as stencil alignment is concerned, the large variance between the smallest and largest components on the board is now challenging conventional fiducial alignment. Best practice is moving towards prioritising correct alignment of the smallest features, relying on a greater tolerance for error in relation to larger features. 

Improvements to aspects of the screen printer, such as the alignment procedure, squeegee mechanism, and process-monitoring systems can meet the challenges being presented by today’s tiniest component geometries, while also enabling manufacturers to streamline setup and changeover as well as maintain fast print cycle times in order to ensure a high level of productivity.

Process-in-Control, from Board One

Figure 1. The graphical tool provides greater flexibility to optimise stencil-to-board alignment. Figure 1. The graphical tool provides greater flexibility to optimise stencil-to-board alignment.

Productivity can be improved if the optimum stencil-to-board alignment can be achieved during setup without waiting for SPC analysis to calculate any necessary offsets. Yamaha’s graphical base-layer alignment function, as seen in figure 1, allows engineers to fine-tune alignment by optimising a composite image comprising images captured from the PCB and stencil recognition cameras. After making small X and Y adjustments to bring the two images into the desired alignment on the screen, the Teach function automatically calculates the offsets needed to replicate the desired alignment. This approach gives engineers the flexibility to prioritise alignment of the smallest board features, or to align according to fiducials or other features as required.

Figure 2 Front rail stencil vacuum system Figure 2 Front rail stencil vacuum system

To ensure optimum alignment is maintained throughout printing, after the optimum alignment has been calculated, Yamaha’s exclusive Rail Vacuum System (RVS – Figure 2) stabilises the metal area of the stencil against the conveyor rails. This significantly reduces alignment shift in the print direction as squeegee pressure is applied, which can be as much as 25µm in systems relying on conventional mechanical stencil clamps alone. Figure 3 illustrates the reduction in offset achieved using Yamaha RVS. 

Figure 3. The Rail Vacuum System significantly reduces stencil-board alignment shift in forward and reverse print directions

A standard configuration for many inline screen printers incorporates two squeegees for forward and reverse print strokes. The angle of attack for each squeegee is fixed during setup and is not changed unless the machine is stopped and the squeegee angle adjusted manually. Yamaha’s Swing Single Squeegee (3S) head, shown in figure 4, allows the printer to optimise the squeegee angle automatically and in real-time to adapt to changes in process conditions. This helps to avoid poor print results immediately after initial startup or after a pause in production, which are known to have a significant effect on the overall print defect rate.

Figure 4. The 3S head has a key role in boosting print-process productivity
Figure 4. The 3S head has a key role in boosting print-process productivity

Immediately after startup the paste exhibits non-Newtonian behaviour, as its viscosity changes under applied force. The first two to four boards are typically expected to show erratic variations in deposited paste volume, reported by SPC data, until the paste reaches its optimum working viscosity. Once this point is reached, inhibitors help maintain constant viscosity thereby allowing the process to stabilise. If production is paused the paste may return to its static state, and must regain its design viscosity before good print results can be expected.

By optimising the squeegee angle to suit the paste viscosity, the 3S head improves aperture filling while the paste viscosity is greater than optimum. This improves aperture filling after startup or after a pause, while allowing the squeegee speed to be kept constant. Constant squeegee speed maintains the shearing effect on the paste, thereby helping the paste reach its working viscosity quickly.

The 3S head is also able to re-optimise the squeegee angle immediately after stencil cleaning, which helps to prevent any cleaning solvent remaining in the stencil from inhibiting aperture filling.  

The swing mechanism of the head also allows a single squeegee to be used for both forward and return excursions. This enables pressure to be controlled using a single load cell thereby eliminating any variations between forward and return excursions, hence ensuring superior process control.

The squeegee blade has a sharp profile, which minimises paste contamination on the upper surface of the stencil and also delivers improved performance with dual-thickness stepped stencils. Moreover, a smaller volume of paste adheres to the squeegee, which helps to maintain a consistent mass on the stencil surface and helps minimise drying. A high-durability, low-friction protective coating helps maintain the squeegee profile throughout a long service life. This coating uses the same material technology as applied to the cylinder bores of Yamaha high-performance motorcycle engines. Its low-friction property minimises wear on the squeegee blade and stencil surface, and also helps prevent paste adhering to the squeegee.

Maintaining Print Stability

Typically, the setup procedure must determine how frequently the solder paste on the stencil surface must be replenished to ensure consistent aperture filling. If the quantity present is excessive, or insufficient, the paste will not roll, resulting in inadequate aperture filling. 
Determining the optimum interval for paste replenishment normally requires printing a number of boards until post-print inspection detects insufficient paste volume. Yamaha’s Print Stability Control (PSC) option continuously monitors the size of the paste roll and automatically dispenses extra paste when the lower limit of the target range is approached. This eliminates the effects of variation on stencil aperture filling. Setup time is reduced, since PSC saves the need to print trial boards, and productivity is increased since production can continue without interruption for manual paste replenishment. 

The combined effect of the 3S head, graphic layer base alignment and PSC enable the print process to be in control and stable from the very first board, which enhances productivity by maximising effective machine uptime and minimising print defects. These features are common to all Yamaha printers, including the YSP, the YSP20 with dual-stage and dual-stencil capability, and the new YCP10.

Dual-Lane Productivity

The dual-stage YSP20 can achieve line tact time of approximately 5 seconds using both stages to print alternately with dual stencils. The dual-stage layout also allows non-stop setups and changeovers, or can be configured for a speed advantage when sequentially printing PCBs using two different stencils of different thicknesses.

Non-stop operation is possible if the mounter is setup so as not to require feeder changes to change from one product to the next. If the printer is used in single-lane configuration , the unused stage can be prepared to print the next product. As soon as the current production run is completed, the mounters are reprogrammed with the next product and the printer is ready immediately for the first board of the new batch. This can significantly increase productivity when building a high mix of products in small batches.

As an alternative to using a stepped stencil, sequential printing involves a first print operation using a thin stencil to deposit the smallest paste volumes for fine features such as 03015 chip passives or ultra-fine-pitch area-array devices. The board is then printed a second time, using a thicker-gauge stencil to deposit paste for the remaining board features. The underside of the second stencil is routed to avoid interfering with the paste deposits from the first print cycle. The dual-stage YSP20 occupies less floor space than would be required to install two conventional printers in series to achieve similar throughput and flexibility.

Stencil cleaning

An important process step in solder paste screenprinting is the under stencil cleaning process and one of the key materials in this process is the cleaning roll. This, often neglected, material affects the cleaning process and thereby also the print quality.

When printing solder paste through very small apertures with low area ratios, the traction forces from the aperture walls will often be large enough to make much of the solder paste remain in the apertures. If this remaining solder paste is not removed, the deposited solder paste volume will be affected and potentially cause open solder joint, bridging and solder ball formation.

Hyperclean stencil cleaning rolls are proven to be a improve the cleaning performance compared to standard roll and is therefore recommended for use in Yamaha printers.

Conclusion

Yamaha Printer YCP10 Yamaha Printer YCP10

Many years of research has gone into developing stable screen-printing processes, in the context of ongoing miniaturisation of the smallest surface-mount components and IC interconnects, as well as other changes such as the switch lead-free solder pastes. This research informs not only the adjustment of process settings but also fundamental aspects of screen printer design.
New features of the latest generation of printers, like Yamaha new YCP10, enable manufacturing engineers to quickly optimise process settings for the ever-widening range of components from the smallest chip passives to larger components such as connectors. At the same time, the new features help to minimise the trial and error traditionally associated with setting up a new board, which increases productivity by allowing useful units to be produced from board one.

 

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