Guide for Framed vs Frameless SMT Stencils
SMT assembly practices for solder paste printing using SMT stencils vary widely based on the end market served and the machinery the stencil will be used on.
Framed stencils come in a variety of styles, colors, and materials. A framed stencil is one in which the outline of the stencil supplies tension to the patterned foil and is "ready to go" when it is delivered. Typically, it is glued onto the mesh of the frame. In a framed SMT stencil the frame enables the foil to be stretched tautly so that the stencil can gasket to the board. This will ensure that the solder paste will release properly from the stencil onto the circuit board. This gluing process can either be done by the manufacturer providing the SMT stencils or by one of their sub suppliers. SMT framed stencils can be made from lightweight aluminum for minimizing weight in order to reduce the shipping cost. For some high duty cycle applications, they can be made from cast aluminum in order to increase the robustness of the stencil. Frames can be color-coded for a variety of purposes including the type of solder paste alloy being run on the line or the point of the assembly in the development cycle (i.e. proto, alpha, beta or volume assembly). While not all stencil sizes come in all material types, the most common stencil sizes are generally available in a variety of material types. For example, the most common size of 29 x 29" SMT stencils, can be procured in the cast or extruded aluminum. Less common sizes would not be available in varied frame materials. Some highest demand stencil frame sizes come in compact frame designs. These frames have the smallest outline dimensions which provide the lowest possible storage requirements.
The term "frameless" SMT stencil refers to a stencil which is mounted in a proprietary fixture which emulates a framed stencil. The proprietary (e.g. VectorGuard™ and QTMS are common types) re-useable fixture is produced by a variety of different vendors who each either own the rights to the pattern, sell the fixtures or both. SMT stencil manufacturers can then cut the stencil apertures with the appropriate modifications surrounded by this proprietary pattern for the "frameless" SMT stencil system. The pneumatic frames then provide the proper tension for the printing process. The advantage of the frameless SMT stencil systems is lower cost and storage space as well as convenience. SMT frameless systems are lower in cost than their framed counterparts as the frame material costs are not passed along. In addition, a few pounds in shipping weight and therefore the overall cost is eliminated from each purchase. Frameless Stencils are easier to store and catalog as they take up less room than their framed stencil counterparts.
Other "frameless" SMT stencil options are appropriate for prototype assembly. These stencils are sheets of stainless-steel cut with the properly engineered aperture sizes. Some SMT prototype stencils are sheets of stainless with the proper apertures along with their edges or "flaps" folded up. These flaps allow the hobbyist or lab technician to squeegee the solder paste effectively to the flap edges thereby preventing the smearing of solder paste onto non-printed surfaces. Most of these prototype frameless SMT stencils are supplied with a manual squeegee blade.
The machinery used for the SMT assembly process has an impact on the type of SMT stencils that are used in the assembly of the printed circuit boards. Processes, where a semi-automated or fully automated stencil printing machine is used, utilize some form of a framed stencil. These framed SMT stencils can be either be glued into a frame or can be of the frameless variety with a pneumatically operated frame supplying the tension required for printing.
Framed SMT stencils as well as their counterpart "frameless" stencils have their place in the SMT stencil market. Framed stencils are appropriate for printing higher volume production runs as well as in situations when it is the only one type of stencil your printer can use. "Frameless" stencils are appropriate for short-run, high changeover or prototype PCB assembly operations.
Both the framed as well as frameless stencils have their place in the SMT assembly market.