Arlon’s Howefill® HF-50 is a 250⁰C high glass transition temperature polyimide compound for use in filling clearance holes in metal core printed wiring boards, providing the user with an optimized material to fill holes that have been drilled through the metal for through hole interconnections. Based on a unique polyimide chemistry, this material provides full compatibility with other Arlon polyimide materials and processes. HF 50 has been formulated to improve thermal conductivity, crack resistance, and minimal resin shrinkage back into the holes.
HF 50 is a free-flowing powder. It is advised that the sample be tumbled prior to use to ensure thorough distribution of contents after receipt, as some settling may occur during shipping or storage. Bonds to metal can be enhanced by treating copper surfaces with an oxide process and aluminum surfaces by passivation and chemical film deposition.
- Howefill should be sifted onto the package on both sides of the core to be filled. Sift the HF 50 onto the material at the bottom of the core board as shown, above. Sift additional HF-50 on top of the core board itself, concentrating on the areas with holes or cutouts as required. A template may help with this. Level the surface.
- Use of one or more pieces of prepreg on either side of the sifted HF-50, (illustrated below), drive the powder uniformly into the holes.
- ) Laminate as standard polyimide. Heatup rate should be controlled at 4.5-6.5 °C/min (8-12 °F/min) between 95 and 150 °C (200-300 °F). Apply full pressure, 300-400 psi, depending on panel size. Cure at 218 °C (425 °F) for 120 minutes. When laminating a full package (core board plus laminate and prepreg) follow laminating instructions for the specific polyimide system being used to ensure full cure of all components.
- Cool very slowly in the press after cure. Suggested cooling rate is 2-4 degrees F per minute.
- Panel can be ground or sanded if needed prior to subsequent operations. Plating, drilling and other post-processing will be similar to polyimide.