Screen Printing Application Guide for Thermal Interface Materials

Screen Printing Application Guide for Thermal Interface Materials

Table of Contents • • • •

Screen printing hardware Screen specifications Squeegee applicators Manual screen holder mechanism o o o o

Setting up Spacing and leveling Setting offset gap Screen printing mechanism

• Applying Thermal Interface Materials (TIMs) to the screen • Screen printing TIMs o General parameters for controlling pad thickness

• Final Considerations

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Screen Printing Hardware

Screen holder for manual screen printing

Screen holder with typical Screen holder, screen, screen attached and squeegee applicator

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Screen Specifications • Screen mesh type o Polyester, nylon, also stainless steel

• Screen mesh size (threads per inch) o ~80 mesh common, 60 to 100 typical

• Wire diameter and weave thickness o These can control printed pad thickness

• Emulsion thickness (back side coating) o Also can control pad thickness

• Screen mesh offset angle o Controls edge smoothness

• Any style of pad design can be made • Variety of frame sizes available

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Squeegee Applicators for Printing TIMs • Squeegees can be custom made for specific screen and print pattern sizes. o Aluminum handles o Rubber squeegee blade

• Rubber squeegee blade can range in hardness. o 60 to 80 durometer range is good for most TIMs o 70 durometer common o 80 durometer good for higher viscosity TIMs

• Blade edge can be smoothed to increase screen life.

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Screen Holder Mechanism •

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Place the screen frame into the screen holder/hinge. Clamp the screen frame using the screen locking nuts. Use hinge adjustment screws to change screen height. Use hinge locking nuts to fix the hinge position.

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Typical screen holder (manual printing):

• • • •

Screen locking nuts Counter weight Hinge adjustment screw Hinge locking nut

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Screen Printer Set-up Depending on the specific screen holder design, a spacer may be required to hold the screen frame when the screen is placed down. • •

Use a heat sink of similar height Additional thin shims may be required to adjust offset gap

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Screen Printer Set-up – Leveling • Use the hinge adjustment screws to level the screen frame. The screen must be parallel with the printing surface. • A correct offset gap between the screen and the print surface must be used (see next slide).

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Screen Printer Set-up – Setting Gap An initial offset gap of 2-3 mm should be used with a screen. o Gap depends of screen tension, mesh type and mesh size. o Pressure is required when printing to bring screen down into contact with the receiving surface such as a heat sink. o The screen mesh shears TIM material and deposits it onto the heat sink. o Adjustment and optimization of the off-set gap can effect pad thickness, deposit weight, and weight variability. Gap

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Screen Printing Mechanism TIM is deposited on to the heat sink surface as the squeegee travels over the screen openings. o The screen immediately peels away behind the squeegee thus transferring the TIM to the heat sink via the screen pattern.

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Screen Printing Suggestions Maintain constant pressure, angle, and speed when drawing squeegee across screen. Printing direction

Too steep

Suggested start angle ~45o

High viscosity grease

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Applying TIM to The Screen Apply a small amount of TIM to the upper part of the screen, in front of the screen opening.

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Screen Printing TIM • Place the squeegee down on the screen behind the TIM and tilt it down to ~45o angle. o Make sure enough pressure is applied to force the screen into contact with the heat sink surface.

• Draw the squeegee across the screen opening towards the opposite screen end. o Make sure to maintain constant applied pressure and speed. o Make sure screen remains in contact with the heat sink while drawing the squeegee.

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Screen Printing Completed

Once complete, you will have a nice, clean print.

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Printed Pad Thickness Parameters Adjusting these parameters can help control pad thickness/volume, pad dispense weight, and weight variability. In general:

Screen mesh # Emulsion Thickness Squeegee Pressure

PAD Thickness

Squeegee Speed

Screen Gap

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TIMs can be applied to many different styles

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Final Considerations • Manual screen printing hardware, screen holders, screens, and squeegees are relatively inexpensive. o Dow Corning can provide vendor contacts in your area

• Screen parameters and offset gap can be adjusted to control the pad dispense weight and pad weight variability. o The optimum conditions will be different for each application and may require fine tuning

• Custom screens can be designed for challenging printing applications. • Proficiency in screen printing can be developed rapidly with minor training and practice. • Also refer to Dow Corning Stencil Printing Guide for more information. • Contact your local Dow Corning representative with questions.

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Form Number 11-3407-01

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