Here's a great technical paper from DuPont:
Process for Plugging Low to High Aspect Ratio Through-Holes with Polymer Thick Film Conductive Ink in Production Volumes
Abstract
With an increasing number of designers specifying conductive material for plugging through-holes, a more robust process to produce 100% fill is needed for the whole range of aspect ratios. With via sizes ranging from 8 to 20 mils in diameter and board thicknesses approaching and sometimes exceeding 200 mils, a method to plug reliably and consistently with ease is necessary for the future of the conductive filling process.
It is very common in the industry to use a vacuum table to assist in "pulling" the material into the through hole. Typically this requires several passes with a squeegee to attempt to fill the through hole completely. Because this vacuum assist is enabled throughout the process, air ingression is inevitable. Further aggravating this issue is the fact that many production houses still squeegee this material by hand, offering no control over other important process parameters such as squeegee angle, speed and pressure. The end result is an inconsistent process, with incomplete fill of the though-hole and large air pockets.
This paper will identify the process variables that affect the through hole fill quality and offer solutions to control them. It will also investigate an alternative production method that eliminates the need for "vacuum pull" to draw the material (and air) into the through hole. Rather it utilizes a "direct imaging" method that applies positive pressure within an enclosed print head to press the material into the through hole while maintaining control over all process parameters. With this process, it is now possible to fill through holes more quickly and efficiently, providing a more reliable process that enables a high-volume application of conductive through-hole.
See the full pdf here.
Process for Plugging Low to High Aspect Ratio Through-Holes with Polymer Thick Film Conductive Ink in Production Volumes
Abstract
With an increasing number of designers specifying conductive material for plugging through-holes, a more robust process to produce 100% fill is needed for the whole range of aspect ratios. With via sizes ranging from 8 to 20 mils in diameter and board thicknesses approaching and sometimes exceeding 200 mils, a method to plug reliably and consistently with ease is necessary for the future of the conductive filling process.
It is very common in the industry to use a vacuum table to assist in "pulling" the material into the through hole. Typically this requires several passes with a squeegee to attempt to fill the through hole completely. Because this vacuum assist is enabled throughout the process, air ingression is inevitable. Further aggravating this issue is the fact that many production houses still squeegee this material by hand, offering no control over other important process parameters such as squeegee angle, speed and pressure. The end result is an inconsistent process, with incomplete fill of the though-hole and large air pockets.
This paper will identify the process variables that affect the through hole fill quality and offer solutions to control them. It will also investigate an alternative production method that eliminates the need for "vacuum pull" to draw the material (and air) into the through hole. Rather it utilizes a "direct imaging" method that applies positive pressure within an enclosed print head to press the material into the through hole while maintaining control over all process parameters. With this process, it is now possible to fill through holes more quickly and efficiently, providing a more reliable process that enables a high-volume application of conductive through-hole.
See the full pdf here.