Printed circuit board (PCB) manufacturers using surface mount components have found themselves coping with a variety of undesirable issues related to the stencils they need for their process.
The primary problems encountered include:

• Stencil image shift or "Float" during print stroke, resulting in mis-aligned paste deposits
• Storage of current and archived framed stencils
• The life cycle costs associated with handling framed stencils
• Disruptions and inconveniences tied to mesh mounted stencil failures

A variety of universal framing systems have been developed to address the above issues. However, many of these systems have introduced a new set of difficulties related to ease of operation and foil connection time. Alpha Metals offers a frame stencil management system that addresses all of these issues by employing a universal frame and stencil foil design to provide a stable print platform.

The EMPF uses the Tetra Stencil Management System because of the following reasons:

• Ease of use - A stencils foil can be removed from the frame and another stencil foil mounted in less than 2 minutes.
• Compatibility and Interchangeability with all of the major stencil printer manufacturers. In most cases, this eliminates our need to keep duplicate sets of stencils mounted in different sized frames.
• Dual images in a single foil enable the EMPF to switch from one of the two images quickly and reduce storage space requirements.
• Stencil cleaning is simple and easy.
• No maintenance is required.

The Alpha Metals’ Tetra System is a combination of equipment, including: a frame, a loading terminal, a washing case, a loading station and a stencil archive storage unit. The loading terminal, in conjunction with the frame, allows a steel stencil, with a prepared image, to be mounted with a 4-way, equalized tensioning. The washing case allows the stencil, once removed from the frame, to be securely mounted for placement into a manual or automated stencil cleaner. The loading station is mobile, stores over 100 stencils in individual storage cassettes and facilitates foil loading and unloading activities. The stencil archive storage unit is a freestanding cabinet with storage capacity for over 400 stencils in individual storage cassettes.

For years, precise registration of a stencil and PCB has been challenging for the process engineers. Stencil shifting or "floating" is recognized as a major cause of incorrect registration. This float is caused by a combination of low stencil foil tensioning in traditional mesh-mounted stencils and the variation of the tension across the surface of the stencil. Many automatic printers, built in recent years, have the ability to compensate for stencil float by programming an offset feature. Some automatic stencil printers even have a dual offset capability that compensates for the difference in stencil shaft that occurs between the forward and the return squeegee strokes. However, mesh stability and tension deteriorate considerably with use and cleaning. Current mesh/foil stencils are limited in the amount of tension they can structurally maintain. The mesh-to-frame glue bond, mesh-to-foil glue bond and the mesh material itself are the limiting factors.

The Tetra system uses mechanical means for four-sided, pressure-equalized connections between the foil and the frame. This eliminates the movement associated with a steel stencil glued to a synthetic, elastic material. In addition, this mechanical connection allows the frame to apply a much higher degree of tension across the foil surface. Note: Foil tension for most mesh-mounted stencils averages 25N/cm, while the Tetra system averages 45N/cm of tensioning. Because foil tension directly impacts stencil float during printing, the Tetra design provides a much more stable print platform than traditional mesh-mounted stencils.

Stencil Storage:
There are two significant issues related to the storage of the framed stencil. The first involves the proximity of its storage to the printer. The second is the overall space required for storing both active as well as archived stencils.

Traditional stencil storage racks are not normally located in close proximity to the PCB assembly lines. This costs time and effort on the part of an operator to bring a used stencil back to the storage area or to go to the storage area to retrieve a new one. This inconvenience is also the root cause of stencil clutter when a framed stencil is placed somewhere near the printer, i.e., the floor, table etc, with the intention of being brought back to storage later.

Stencil storage racks also consume a great deal of valuable manufacturing floor space. In any business, floor space is a valuable commodity. A standard rack stores 18 framed stencils (29"x 29" x 1.5") in a 3'x3' area of floor space. In the same 9 sq. ft. of floor space, a user can store 65 Tetra stencils in the storage cassettes.

The Tetra workstation is designed to store over 100 stencils in 15 sq. ft. of floor space. This mobile cabinet can also be staged closer to the printer than most stencil rack systems, making stencil change outs faster and easier for the operator.

Stencil Failures:
The stress of mesh tensioning on the stencil-to-mesh and the mesh-to-frame adhesive bonding is substantial. This factor, combined with an additional 12 to 24 lbs. of continuous squeegee blade pressure and the deterioration caused by solvent or detergent-based stencil cleaners, can cause a failure of these critical bonds. There are generally no warning signs and these failures are usually discovered at the worst possible time, when boards need to be printed. The results are delays and downtime in your assembly line until a new stencil can be ordered, manufactured and shipped.

The Tetra stencil frames are manufactured in a variety of sizes designed to fit all of the popular and available stencil printers built for today's market. They are available in 29" x 29" front and centered justifications.

The benefits of the Tetra Stencil System will positively impact several areas of manufacturing, yielding savings in time and labor. The Tetra system is simple to implement and requires no maintenance. The overall stability of the stencil provides the greatest benefit - maximum print accuracy.

If you would like more information regarding the Tetra Stencil System, or to see a demonstration, please contact Jeff Stong (610.362.1200 x 224 or jstong@aciusa.org).