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Stencil Tension
Should we have a tension meter in our SMT assembly area? What is the acceptable stencil mesh tension to maintain good solder paste prints?
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Stencil Tension

Our stencil supplier certifies that our stencils have 35 N/cm tension when delivered new.

Is it advisable to have a tension meter in our SMT assembly area to test our older stencils?

What is the acceptable stencil mesh tension in order to maintain good solder paste prints?

E. R.
Expert's Panel Responses

I would not recommenda tension meter for measuring the tension on your older stencils. The only placeto measuremeshtension on an existing stencil is out near the edges of the frame. A tension metermeasures deflection in a plunger andit is more difficult to get an accurate, and consistent, tension reading in this area.

Mesh tension should really be measured in the print area, which is why stencil suppliers typically measure mesh tension on stencil frames that have mesh, but no mounted foil. Mesh tension is typically measured near the center of the stencil frame and in both the X and Y directions.

If your stencils use polyester mesh, the tension is not that predictable to get 35 N/cm on every stencil. There has to be a tension range. Typically 36 +/- 4N/cm is fine for stencils with foils above 0.002" thickness. At, or below, a foil thickness of 0.002" requires lower mesh tension to minimize foil stretch. We use a mesh tension of 28 +/- 2 N/cm when the foil thickness is at, or below, 0.002".

Keep in mind thatthere are variations in the quality of tubularstencil frames. Many are using extruded aluminum frames with a very thin wall thickness to minimize cost. These frames can't be stretched to high mesh tensions because the sides of the frames will bow inward. This inward bow reduces the mesh tension.

We tested this inward bow years ago to determine the minimum wall thickness for the mesh tension listed above and only stock stencil frames that meet this minimum criteria.

The variation in mesh tension can be eliminated by looking at a stencil and foil system. I recommend DEK's Vector Guard system since it has no mesh and the foil is tensioned mechanically rather than through an air bladder system.

There was a white paper released in 2010 that tested the accuracy and repeatability of the DEK Vector Guard system. Since it is a mechanical tensioning system, the foil tension is extremely consistent and produces aperture registration accuracies equal to that of traditional framed stencils.

FCT Assembly did not participate in the testing, but I can pass along a copy of the white paper if you are interested.

Robert Dervaes
V.P. Technology & Engineering
Fine Line Stencil, Inc.
Robert Dervaes has worked in the electronics industry since 1992 in both design and manufacturing. Over the past 11 years he has established the technical foundation of Fine Line Stencil, Inc. - a premier stencil supplier to the electronics industry.

Under normal use, and assuming the metal, mesh, or glue has not sustained damage the stencil should not lose noticeable tension sitting on the shelf. Acceptable tension ranges for fixed frame stencils is 20-43 N/cm. Process adjustments will need to be made for tensions on both ends of this spectrum.

Factors that may affect the tension are: failure in the glue bonding the mesh to the frame or the plate to the mesh, tears in the mesh, or soft nickel alloys that deform and stretch under squeegee pressure (permanently affects the location of land patterns and stencil tension).

RST Gauge manufactures a SMT Foil Tension Analyzer for use in your facility.

image
Stephanie Nash
Director
Integrated Ideas & Technologies, Inc.
Stephanie Nash is the Director of Technical Services & Marketing for Integrated Ideas & Technologies, Inc., a premier manufacturer of SMT stencils. She has been instrumental in the stencil design and technical support.

What is more important is the tension of the stencil itself. As it is used it the stencil foil may be stretched and distorted. A measure of the stencil tension or tauntness is how much will it deflect with a known force.

A long bar extending one end to the other of the stencil with a plunger gage in the middle that has a known weight applied to it will measure the deflection. In this way it is possible to determine when the stencil is too loose for good printing.

image
Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

Yes it is good to have a tension meter in the SMT Assembly area to check the stencil in good condition.

Acceptable mesh Tension is between 30 - 40 N/cm, and also another important factor is the method we use to measure.

Need to check 5 points (4 Corners and the Centre of the Stencil).

image
Keshava Murthy
Process Engineer
Bosch Automotive Electronics India Ltd.
Keshava Murthy is a Process Engineer in the PCB assembly industry working in Bangalore, India.
Reader Comment
Yes, it is good to have a tension meter in SMT assembly and we have to follow a frequency to check the tension after every 15K stencil shots. Please check the stencil tension after 50K shots in any condition and if you find the tension below 20 then please discuss with your supplier.

Amit Tiwari
Barco Electronics System Pvt Ltd
India
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