Rachel Miller Short, William E. Coleman Ph.D.
Photo Stencil, Colorado Springs, CO
Parmi, Marlborough, MA
There has been recent activity and interest in Laser-Cut Electroform blank foils as an alternative to normal Electroform stencils. This study investigates and compares the print performance in terms of % paste transfer as well the dispersion in paste transfer volume for a variety of Electroform and Laser-Cut stencils with and without post processing treatments. Side wall quality will also be investigated in detail.
A Jabil solder paste qualification test board will be used as the PCB test vehicle. This board has a wide range of pads ranging from 75 micron (3 mil) squares and circles up to 300 micron (12 mil) squares and circles. There are also long rectangular pads with spacing's as low as 75 micron (3 mil). A total of 12 stencils, four stencils of different stencil technologies with three different coating configurations, will be tested as described in 1-4 below:
1- Electroform w/o Nano Coat and with and Nano Coat A and Nano Coat B
2- Laser-Cut Electroform foil w/o Nano-Coat and with Nano Coat A and Nano Coat B
3- Laser-Cut Fine Grain SS w/o Nano Coat and with Nano Coat A and Nano Coat B
4- Laser-Cut Fine Grain SS with Electropolish and Nickel plating, w/o Nano Coat and with Nano Coat A and Nano Coat B
A 100 micron (4 mil) thick stencil is used for all 12 stencils yielding Area Ratios ranging from .31 to .1.21.
Figure 25 shows the rankings of the 12 stencils in all five categories. The Stencil 1, Electroform, stencil with Nano Coat B had the best overall ranking. Stencil 2, Laser-cut Electroform foil, scored second in the rankings. The Stencil 2 with Nano Coat B provided the cleanest print after 10 prints without stencil wiping. Stencil 1 with Nano Coat B demonstrated the lowest Area Ratios (.33-.39). Mask defined pads generally provided lower area ratios for all 12 stencils compared to copper defined pads.
Square apertures provided lower area ratios compared to circular apertures. Rectangles having the same aperture widths as squares and circles provided better paste transfer and lower standard deviations mainly due to their higher area ratios.