Tuesday, June 19, 2018
Publication Date: 09/1/2012
Archive >  September 2012 Issue >  Special Features: SMT and Production > 

Stencil Printing Process Tools for High Yield Processing
(Left) The untreated stencil's reduced contact angle allowed the flux to spread. (Right) The treated stencil's flux deposit exhibits a more amalgamated characteristic, indicating that the Nano coating has reduced the spread and therefore reduced the capillary action of the flux.

Miniaturization raises a number of issues for the stencil printing process. How small can we print? What are the tightest pitches? Can we print small deposits next to large for high-mix technology assemblies? How closely can we place components for high density products? And then on top of all of this, how can we satisfy some of the cost pressures through the whole supply chain and improve yield in the production process?

Today we are operating close to the limits of the stencil printing process. The area ratio rule (the relationship between stencil aperture opening and aperture surface area) fundamentally dictates what can and cannot be achieved in a print process. However, for next generation components and assembly processes, these established rules need to be broken.

New stencil printing techniques are becoming available which address some of these challenges. Active squeegees have been shown to push area ratio limits to new boundaries, permitting printing for next generation 0.3CSP technology. Results also indicate there are potential yield benefits for today's leading edge components as well.

Stencil coatings are also showing promise. In tests performed to date it is becoming apparent that certain coatings can provide higher yield processing by extending the number of prints that can be performed in-between stencil cleans during a print process.

The Printing Process
The SMT printing process can be broken down into two parts, filling and release. The hydrodynamic pressure (filling pressure) created by the squeegee system is a key element to successful aperture fill. To ensure the aperture is correctly filled, there are many trade-offs that the engineer has to make. The most simplistic approach is to increase all parameters that produce the highest filling pressure. Such a process could take the approach of squeegee angles of 45° and less, a high print pressure and low print speeds. This approach would indeed give high filling pressure but would be detrimental to the overall process.
Qualification test board.

The high filling pressure would cause print medium to breach the gasket between stencil and PCB land thus creating a "wet bridge" between neighboring apertures. The issue of wet bridging becomes more critical as the distance between apertures reduces. Unfortunately for the process engineer, the introduction of fine webbed 01005 and 0.3mm CSP technologies will produce even further wet-bridge opportunities.

The presence of wet bridging within an SMT print process is a catastrophic failure mode and therefore must be remedied. If a high pressure produces defective prints, the process engineer could set the print process to produce a low filling pressure, increased squeegee angle, lower print pressure and higher speeds. This approach would reduce the filling pressure but now the outcome would be one of incomplete aperture fill and deficient interconnect integrity.

The Filling Process
Thus we can see the filling process is a balance that has to be fully understood by the process engineer. In the real-world a perfect balance can never be achieved due to throughput requirements and exogenous factors, therefore the process engineer tends to take a cautious approach and choose a higher filling pressure setup. By choosing the higher pressure setup, the tendency to wet bridge increases. However today's fully automatic printing machines are equipped with under-stencil cleaners that have the capability to remove small amounts of wet bridged material from around the stencil apertures. Therefore by choosing the high pressure filling process the process engineer now has a process in which the occurrence of insufficiencies are minimized and wet bridging occurrences are managed by an under-stencil cleaning strategy.

While this compromise seems acceptable, the action of cleaning is one that reduces throughput and increases costs. The time penalty associated with the most basic under-stencil cleaning operation is approximately 20 seconds. If this cleaning action is instigated every 5 boards, then the cycle time overhead becomes significant. Also the under-stencil cleaner requires consumable materials — solvents and paper — and these materials increase the overhead cost of the printing process. Therefore running a print process which uses the under-stencil cleaner to "compensate" is a process has both time and cost penalties.

Wet Bridging
The phenomenon of wet bridging occurs when a print material breaks the gasket between stencil aperture and the printed circuit board land. The breach between this gasket usually occurs over time but once the underside of the stencil becomes contaminated it acts as a "tipping point" and thus the wet bridge quickly cascades. The cause of wet bridging is derived from the onset of under-stencil smear (this is derived from the breakdown in gasket between stencil and PCB). Therefore to reduce wet bridging we either reduce the occurrence of violations between stencil and PCB or contain the print material that creates the under-stencil smear. The first argument is of course the correct approach but due to the exogenous factors within the print process this approach can rarely be achieved. This leaves the latter line of reasoning; one such mechanism which generates a containment effect is Nano coating (Nano-ProTek®). This material is applied to the underside of the stencil where it chemically bonds itself to the stencil foil. The Nano coating modifies the surface tension of the stencil material such that a "barrier" around the aperture is created. This barrier prevents print medium migrating across aperture interspaces and thus creating a wet bridge.
0.3mm CSP results shows bridging on untreated stencil, even after cleaning.

In a test in which a controlled quantity of rework flux was deposited onto the untreated stencil, the reduced contact angle allowed the flux to spread, observed in the overall dimensions of the flux deposit. Deposited onto a treated stencil, the flux deposit exhibits a more amalgamated characteristic, indicating that the Nano coating has reduced the spread and therefore reduced the capillary action of the flux. This simple test has shown that Nano technology has the ability to react and alter the flow characteristics of thixotropic materials, such as flux.

When nano technology is applied to the underside of the stencil, the barrier effect of does not stop the gasket breach, but it produces a barrier which maintains the paste deposit integrity. This barrier effect can be simply thought of as an area of higher "resistance" to print material thus print material will not migrate across the aperture webs.

Qualification Test Board
Testing was carried out using a qualification test board. The process parameters were chosen to reflect a standard setup, one in which aperture fill pressure would be medium to high. The under stencil cleaner was setup with standard materials and activated after the 14th print. Table 1 illustrates the test parameters.

Test Stencils
To contrast and compare the effect of the Nano coating solution, two stencils were used, one with the coating applied and one untreated. The solder paste used throughout the test was a SAC305, Type 4 commercially available material.
Engineer placing stencil in Europa, the type of machine used in testing.
A total of 20 boards were run for each stencil. The testing strategy was selected to ensure that wet bridging would be prevalent within the testing, thus guaranteeing the possibility to test the effects of a Nano coating technology. The strategy encompassed four warm-up prints to ensure the process was stabilized, ten prints were processed with the fourteenth print retained for visual inspection. An automatic under-stencil cleaning cycle was activated before the 15th print; the 15th print was also retained for visual inspection. The untreated stencil exhibits significant wet bridging on board 14, indicating that the gasket between stencil and substrate has breached and allowed the print medium to traverse across the stencil webs. The results also show the deposit after an automated stencil clean (board 15), remarkably the volume of wet bridging has overwhelmed the cleaning process and left the deposit contaminated. The print quality from the treated stencil shows the volume of wet bridging produced on board 14 to be minor. The reduction of wet bridge volume allows the automated clean cycle to suitably maintain the integrity of the process.

QFP Observation
A QFP device tends to possess additional process issues, namely an imbalance of filling between E-W and N-S apertures but also this imbalance exists regarding the under stencil cleaning process; N-S apertures will tend to have a greater cleaning period than the E-W apertures.

Deposits from print 14 and the untreated stencil show bridging on the N-S apertures; this indicates the extended fill period has forced a wet bridge situation. Interestingly after the clean cycle the defect moves to the E-W apertures indicating that reduced cleaning period has now caused additional process defects. These results illustrate the symbiotic relationship of the printing process.

The result from the treated stencil shows a process that exhibits no wet bridging before or after a clean cycle. This indicates that a significant process shift has taken place.

Nano Coating Works
The tests have found that the Nano coating has dramatically reduced the propensity for wet bridging. To ensure the Nano coating's efficiency was validated during the test, a test strategy was chosen to yield wet bridging. Therefore the wet bridging results from the untreated stencil were expected, although the untreated stencil did clearly show how one failure mode (wet bridge) can morph into an alternative failure (under stencil contamination).

The results from the treated stencil showed that even under a harsh test strategy a Nano coating inhibited wet bridging on 0.3mm pitch devices. This evidence verifies that the barrier created by the Nano technology has overcome the issues associated with a high squeegee filling pressure and stencil to board gasket violations.

The overall outcome from this test is to demonstrate that the process engineer has the ability to extend the number of prints between under-stencil cleans. This benefit is twofold:

  • Increased throughput as a consequence of reduced cleaning.
  • Reduced costs through decreased consumable consumption.
    Contact: DEK, 1785 Winnetka Circle, Rolling Meadows, IL 60008 847-368-1155 Web:
    http://www.dek.com or DEK Printing Machines Ltd, 11 Albany Road, Weymouth, Dorset YUDT4 9TH, UK +44 (0)1305 760760 fax: +44 (0)1305 760123 E-mail: sales@dek.com Web: http://www.dek.com

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