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Die Attach Adhesives


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Die Attach Adhesives Tutorial - Lab Supplies

Printing Theory

Recommended Lab Supplies

Cleaning Supplies:

Wipes: Non-woven wipes that are highly durable and absorbent are needed for cleaning the stencil. A strong, non-woven wipe is necessary so the wipe does not snag in the stencil, creating loose fibers. The wipe also needs to be absorbent so it will clean efficiently and not smear silicone into unwanted areas.

Dow Corning recommends BlueWipes, TX512, from TEXWIPE1.

Sponges: A strong, non-silicone-contaminating sponge is needed for cleaning the stencil after use. A sponge gets into the holes, cleaning areas that cannot be reached with a wipe. A strong sponge is required so it can withstand the metal edges on the stencil without tearing. It is important to note that many sponges contain materials that inhibit the cure of silicones. We highly recommend the Wilshire Ultrasov High Density Sponge, 0.375" by 3.625" by 5.375", P/N 4754HD from Wilshire Technologies2. This is the only rugged sponge that we have found that does not inhibit silicones. If another sponge is used, a test should be done to test its compatibility with silicones.

Gloves: Rubber and latex gloves can inhibit silicone cure. We recommend using polyurethane gloves. A polyurethane glove is available through Wilshire Technologies2. These are DuraClean gloves, part no: 13037 (lg.). For other glove types, we recommend performing a compatibility test to ensure there is no inhibition between the gloves and the silicone.

Cleaning solvent: We have found that Dow Corning® OS-10 Fluid is the most effective solvent for removing silicones from the stencil. However, ketones such as MEK and acetone also work well.

Printing Tools:

Stencil printers: Dow Corning recommends Speedline MPM4 as the supplier of choice for stencil printers. The Ultraprint 2000 HiE is the model of choice.

Stencils: Stainless steel, laser-cut stencils are recommended. Possible suppliers are Acumen5 and Southwest Stencil6.

Key factors to consider when purchasing stencils are:

  • Aperture size: The diameter of the aperture and thickness of the stencil foil determine the volume of the printed nubbin.
  • Geometry and smoothness of aperture wall: Poor geometries and rugged aperture walls will obstruct the paste, preventing clean transfer of the material to the TAB tape. The technology used to create the holes is the key factor in determining aperture wall quality.

 

Chem-etch process:

In this process a metal foil is laminated with photoresist on both sides, and a latent image is exposed into both sides of the photoresist. After resist development, the apertures are etched from both sides simultaneously using a dual-sided etching process.

Results:

jagged, “hour glass” shape

accuracy of approximately .003"

corner ridges

Laser-cut process:

In this process, the stencil foil is held flat and tight while a laser beam traces out and cuts the aperture.

Results:

straight, vertical, sharp cuts

accuracy of approximately .0002"

no corner ridges

natural trapezoidal wall

Information and drawings on chem-etch vs. laser-cut process courtesy of Southwest Stencil.

Area ratio: The ratio between the contact area of the board (area of the aperture opening) and the surface area of the aperture (the inner wall area). Assuming a circular aperture; the equation:

Area ratio = πr²/2πrt = r/2t

where: r = radius of the aperture

t = stencil

Squeegee blades: Polyurethane, 90-durometer squeegees are recommended for printing chip scale products. Blades can be bought from Speedline MPM4.

<  1  4 

 
  1. Die Attach Adhesive Tutorial - Printing Theory


  2. Die Attach Adhesives Tutorial - Lab Supplies


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