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PCB Knowlesge-2(stencil for assembly)

Stencil Types
Important print quality variables include accuracy and smoothness of the stencil aperture sidewalls. Maintaining a proper aspect ratio between stencil width and thickness is important. The recommended aspect ratio (aperture width divided by stencil thickness) is 1.5. This is important for preventing stencil clogging. Generally, solder paste remains in the opening if the aspect ratio is less than 1.5. In addition to aspect ratio, it also is good to have an area ratio (area of pad divided by area of aperture walls) of greater than 0.66, as recommended by IPC-7525, Stencil Design Guidelines. (This document can serve as a good starting point for stencil design.)
The process by which the aperture is made controls both the smoothness and accuracy of aperture walls. There are three common processes for making stencils: chemical etching, laser cutting and the additive process.

Chemically Etched Stencils
Metal mask and flexible metal mask stencils are etched by chemical milling from both sides using two positive images. During this process, etching proceeds not only in the desired vertical direction but also laterally. This is called undercutting — the openings are larger than desired, causing extra solder deposit. Because 50/50 etching proceeds from both sides, it results in almost a straight wall tapering to a slight hourglass shape in the center.
Because electroetched stencil walls may not be smooth, electropolishing, a microetching process, is one method for achieving a smooth wall. Another way to achieve smoother side walls in the aperture is nickel plating. A polished or smooth surface is good for paste release but may cause the paste to skip across the stencil surface rather than roll in front of the squeegee. This problem can be avoided by selectively polishing the aperture walls without polishing the stencil surface. Nickel plating further improves smoothness and printing performance. However, it does reduce aperture opening and requires artwork adjustment.

Laser-cut Stencils
Laser cutting also is a subtractive process, but it does not have the undercutting problem. The stencil is produced directly from the Gerber data, so aperture accuracy is improved. The data can be adjusted to change dimensions as necessary. Better process control also improves aperture accuracy. Another benefit of laser-cut stencils is that the walls can be tapered. Chemically etched stencils also can be tapered if they are etched only from one side, but the aperture size may be too large. A tapered aperture with an opening slightly larger on the board side than on the squeegee side (0.001 to 0.002" to produce an angle of about 2°) is desired for easier paste release.
Laser cut is capable of producing aperture widths as small as 0.004" with an accuracy of 0.0005", so it is very suitable for ultra-fine-pitch component printing. Laser-cut stencils also produce ragged edges because the vaporized metal is transformed into metal slag during the cutting process. This can cause paste clogging. Smoother walls can be produced by microetching. Laser-cut stencils cannot make stepped multilevel stencils without prechemical etching of the areas that need to be thinner. The laser cuts each aperture individually, so stencil cost depends upon the number of apertures to be cut.

Electroformed Stencils
The third process for making stencils is the additive process, most commonly called electroforming. In this process, nickel is deposited on a copper mandrel to build the aperture. A photosensitive dry film is laminated on the copper foil (about 0.25" thick). The film is polymerized by ultraviolet (UV) light through a photomask of the stencil pattern. After developing, a negative image is created on the mandrel where only the stencil apertures remain covered by the photoresist. The stencil is then grown by nickel plating around the photoresist. After achieving the desired stencil thickness, the photoresist is removed from the apertures. The electroformed nickel foil is separated from the mandrel by flexing — a key process step. Now the foil is ready for framing as in other stencil making processes.
Electroforming step stencils can be done at added cost. Because of the close tolerances possible, electroformed stencils provide a good gasket effect, which minimizes under-stencil paste seepage. This means that the frequency of underside stencil wiping is reduced drastically, which reduces potential bridges.

Chemical etching and laser cutting are subtractive processes for making stencils. The chemical-etch process is the oldest and most widely used. Laser cut is a relative newcomer, while electroformed stencils are the latest rage.
To achieve good printing results, a combination of the right paste material (viscosity, metal content, largest powder size and lowest flux activity possible), the right tools (printer, stencil and squeegee blade) and the right process (good registration, clean sweep) are necessary.
Even the best pastes, equipment and application methods do not sufficiently ensure acceptable results. The user must control process and equipment variables to achieve good print quality. This is even more critical in fine-pitch and through-hole printing — the subject of my next column.



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