SMT Stencil Printing

Screens and stencils are functionally same. In stencils, instead of screen mesh, the desired opening is chemically etched out either in metal sheets or in wire mesh covered with emulsion, such that the stencil opening does not obstruct solder paste flow. Thus, a stencil provides 100% opening for the paste to be printed through, whereas the screen provides only about 50% opening. Stencils can be classified into three major categories.

            1. Selectively etched mesh / emulsion screen
            2. Metal (Alloy 42, brass, or stainless steel) mask
            3. Flexible metal mask.

The selectively etched mesh /emulsion type is made the same way as the screen, but the openings are completely etched out. The main disadvantage of this type of stencil is high cost. Also, because the wire mesh is etched, the screen is less stable.

All metal mask stencils are not under constant tension and must be printed on contact only. The opening are generally etched by chemical milling from both the sides using two positive images. The advantage is long life of the stencil. The disadvantage is that, because all metal mask stencils allow only on-contact printing, there is a possibility of solder smear while lifting the stencil from the work area. Stainless steel or alloy 42 are desirable for SMT applications. Brass is too soft and therefore likely to deform under higher printing pressure. When deformed the small interconnects between lands allow paste to bleed and bridge between lands and they are easily broken. This is particularly so for FPT components.

Metal	Advantages	Disadvantages
Brass	Easy to etch Uniform wall finish	Least durable
Stainless steel	Strong and durable for fine pitch apertures More accurate etch than Brass Chemical resistance Cost	Harder to etch Difficult to obtain smooth, uniform surfaces
Molybdenum	Vertical and smooth walls Easy paste release	Expensive

The flexible metal mask combines the advantages of both all metal mask and the flexibility of screens. The etched pattern will be contained in the metal mask and the metal portion of the mask is held in tension by the border of the flexible mesh to allow off-contact printing. Typical fabrics are coarse polyester mesh or stainless steel. Polyester is the preferred fabric since it is highly durable and maintains its tension over a long period of time. This is most widely used type of stencil.

parameter	Flexible metal mask stencils	Screens
setup	Easy to mount due to clear visibility of the pads on the board	Difficult because the pads are not clearly visible due to wire mesh
Printing	On-contact with all metal masks and off contact with flexible stencils	Off-contact printing, this is also called offset printing
Print flexibility	can print by hand or automatically	Only automatic printing, cannot print by hand
Durability	High	Low
Cleaning	Easy because the openings are clear	Difficult
Cost and turn around time	high	lower cost and turn around time

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