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Widening the SMT Print Process Window
Enclosed media print head cross-section.

About 15 years ago, the new mantra in stencil printing was to "take the art out of the art of screen printing." It was the age of the "lights-out turnkey" manufacturing "flowline". Screen printing solder paste had become stencil printing, and advances in PCB assembly technology now made it possible to better automate equipment to achieve higher yields with better quality consistency without the constant intervention of operators. We were told, "We have to take the artful operator out of the process; it's not about art anymore. If we want high-volume repeatability, it's got to be all about automation."

Today, one of the biggest issues with PIH (Paste-in-Hole printing, a.k.a. through-hole printing, pin-in-paste, and many others) is getting enough metal into the hole. On top of that, PIH has become tougher with shrinking board topographies because often, high paste volume PIH holes are too close to low-volume requirement fine pitch SMT pads. Fine-pitch aperture fill volume percentages are a function of paste pressure, generated by the rolling bead of paste driven across the stencil foil surface by the metal printing blade. More pressure results in better aperture fill for fine pitch, and may be required for more thorough print-filling of PIH holes, but when there is a high paste volume requirement such as an array of through-holes next to fine pitch, the paste pressure in the bead will be reduced by the through-hole requirement and thus rob needed pressure from the fine pitch apertures, with the result being poor aperture fill.

Enclosed Media Solution
Enclosed media print head technology was designed to make printing easier and more repeatable with fewer delicate variables than squeegee blade printing. It compensates for many of the shortcomings associated with metal blade printing for more challenging and changing applications including fine pitch printing and PIH.

Enclosed media technology was developed to provide the same required pressurization of the solder paste as the squeegee blade's angle of attack, but uniformly across the length of the print head while printing, regardless of the amount of paste in the enclosed chamber. The key to consistency and uniformity in printing results, where the stencil or print application requirements were anything but uniform, was determined to be direct, fast-response control of paste pressure. This would have to be closed-loop controlled so that pressurization could respond to such demands, for example, as are made by through-hole fills that require a very high volume of paste.

The enclosed media print head maintains solder paste in a closed print chamber that can be pressurized to force the solder paste out of the chamber and onto the substrate. Conventional squeegee blades are replaced with metal blades that function as both scraper and seal. These blades are aligned in a leading edge configuration at 45°, as opposed to a typical trailing edge squeegee configuration. The inwardly-inclined blades are 10mm apart at the point of contact with the stencil. When the head is lowered onto the stencil, the stencil foil will close the entire print chamber during operation and allow a positive pressure to be generated inside. Silicone dams seal the opposite ends of the chamber.

Mechanical Pressurization
Rather than indirectly or pneumatically, pressurization is accomplished mechanically by a motor-driven plunger or piston that is separated from the paste by a membrane. The plunger applies pressure directly to the volume of paste in the chamber, and a series of transducers sense the pressure and are part of a closed-loop system of very tight pressure control, which maintains chamber pressure actively at ±0.1 psi from set point. With enclosed media printing, solder paste isn't exposed to the air, so it doesn't have the opportunity to dry out. Paste savings of up to 50 percent are commonplace. Results with fine pitch printing are excellent; for example, whereas with metal squeegee blades, the finer the aperture, the less percentage of desired volume could be depended upon, with enclosed media, the trend was just the opposite, with print volumes over 50 percent for 150 micron apertures with a 0.375 Area Ratio.

It's a forgiving process; recovery is typically immediate after printing pauses, because the print head rests on the stencil and does not allow paste inside to dry out. Even with varying set-up settings by different operators, consistent, good results are obtained, often from the first print, and without further adjustment or "tweaking".

Controlled Uniformity
The controlled uniformity of solder paste pressure that is provided by enclosed media printing simplifies PIH printing. In many instances it can eliminate the need for step stencils, overprinting, and 2-print methods, all less than optimum "fixes" for getting enough solder into the through-holes . Enclosed media printing saves considerable material costs and paste waste over step stencil printing. Throughput is increased through less frequent underwiping, which also saves solvent (in solvent systems) and paper. In an enclosed media system, paste pressure is mechanically controlled and maintained so that aperture fill is optimum even for fine feature device patterns in close proximity to paste-hungry PIH holes. Paste pressure is not dependent on squeegee angle of attack or speed. It is a closed system where paste replenishment in the plenum is automatic via a closed-loop sensor system. Since the pressure can be adjusted, hole fill can be optimized, and other process variables maintained as constant values without the need to tweak. Chamber pressure is actively maintained at ±0.1 psi from set point.

It may not be possible to completely remove the need for the "artful operator" from the printing phase of PCB assembly. But because 70 percent of soldering defects can be traced to the SMT solder paste printing process, it's vital to widen the print process window, compensate for varying operator skill levels, and bring higher process repeatability back to the print process through enabling innovations such as enclosed media printing technology.


Contact: Speedline Technologies, 16 Forge Park, Franklin, MA 02038 508-520-0083 fax: 508-520-2288 E-mail: info@speedlinetech.com Web:
http://www.speedlinetech.com

 
 
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