By Bob Wettermann
When printing solder paste onto a PCB an SMT stencil presents in aperture patterns lining up the holes in the stencil with corresponding pads on the PCB requiring solder paste. These holes have solder paste rolled through them using a squeegee which should be parallel to the board being printed and pushes down with a consistent force perpendicular to the board.
SMT stencils can be made from a variety of materials including stainless steel, nickel and plastic. Stainless steel is the most commonly-used material, has good paste release properties to make sure the solder paste is properly released from the apertures. In addition stainless steel is reasonably priced and has good dimensional stability which is required for exact placement of the solder paste. Finally, the heat withstand properties of stainless steel make it well suited for laser machining which can quickly and very accurately and economically machine the contours and the patterns required.
When printing solder paste onto a PCB an SMT stencil presents in aperture patterns lining up the holes in the stencil with corresponding pads on the PCB requiring solder paste. These holes have solder paste rolled through them using a squeegee which should be parallel to the board being printed and pushes down with a consistent force perpendicular to the board.
SMT stencils can be made from a variety of materials including stainless steel, nickel and plastic. Stainless steel is the most commonly-used material, has good paste release properties to make sure the solder paste is properly released from the apertures. In addition stainless steel is reasonably priced and has good dimensional stability which is required for exact placement of the solder paste. Finally, the heat withstand properties of stainless steel make it well suited for laser machining which can quickly and very accurately and economically machine the contours and the patterns required.
Another material which is used in very high volume production is nickel which has all of the properties of stainless steel except for the fact that it has better wear characteristics than stainless. In addition, the wall characteristics of nickel is such that it can easily “let go” of the paste. The only downside to the use of such a material is that is very expensive vis-à-vis stainless steel.
The final standardized material used for SMT stencils are plastic films, the best of which are made from Kapton™. Prototyping stencils can be fabricated from Kapton™ with relatively larger pitches and with larger distances between subsequent apertures. This material is reasonably cost effective and can be machined on lasers requiring a much smaller capital investment than their stainless steel counterparts.
Whatever the SMT stencil material the objective is still the same-repeatable solder paste volume in the right location.
The final standardized material used for SMT stencils are plastic films, the best of which are made from Kapton™. Prototyping stencils can be fabricated from Kapton™ with relatively larger pitches and with larger distances between subsequent apertures. This material is reasonably cost effective and can be machined on lasers requiring a much smaller capital investment than their stainless steel counterparts.
Whatever the SMT stencil material the objective is still the same-repeatable solder paste volume in the right location.