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If you have been following my Short Scoop series, you will recall that we have focused on different types of stencils and their applications and what to look for when selecting a stencil. We talked about the importance of the blade in the printing process, various stencil fabrication techniques, and specialty coatings that drive paste release, particularly for fine pitch components. Another part of stencil selection that is often overlooked, but that is also quite necessary, is the selection of the frame. This month’s Short Scoop talks about the stencil frame and some of the options to consider when specifying the frame required for your printing process and needs.
What is a Frame?
For a stencil, the frame primarily provides a rigid support structure that allows the stencil to be used with your printing equipment. Without the frame, the stencil is no more than a thin foil with limited use due to the flexure of the stencil when left unsupported. The frame enables the foil to be stretched taut so that the stencil can gasket to the board, and thus solder paste will release properly from the stencil onto the circuit board.
Frames can range from large to small and thick to thin and be cast aluminum, welded aluminum, space-saving, or frameless systems, including VectorGuard™ and QTS. They can range from 8” x10” die cast (DC) frames to 29” x 29” tubular frames with many options in between. It is usually the printer type and printing equipment that determines the overall frame dimensions/size to be used. Besides choosing the frame type that fits your printer, there are additional options. Frames can usually be purchased either from the printer manufacturer or from the stencil manufacturer.
The overall size of a stencil frame is dictated largely by the size of the outside diameter of the stencil itself. Frame specifications should take into account the stencil’s inside dimension, outside dimension, corner hole locations (if applicable), hole thread size, frame thickness, flat and parallel dimensions, and the maximum print area. If you have a lot of option variability with the printer you are using, make sure to pick a frame size large enough to accommodate the foil size (stencil size), the glue border to mount the foil, and allow an additional 2” square spacing around the outside of the image which is necessary to print successfully.
Read the full column here.
Editor's Note: This column originally appeared in the June 2014 issue of SMT Magazine.
