For assembling surface mount devices or SMDs on a PCB, assemblers require a stencil. They use the stencil as a guide to depositing the correct amount of solder paste on the footprint pads on a circuit board. Rush PCB UK recommends using laser-cut stencils to achieve high-quality soldering.
Most SMD stencils are made of thin stainless steel foils with openings for the solder paste to pass through. The operator places the stencil on the board and registers the openings to match with the component pads. They apply a small amount of solder paste on the stencil and drag it across with a metal squeegee. The solder paste passes through the openings in the stencil and deposits on the pads on the board.
To cut the openings in the stencil accurately, manufacturers use laser beams. The tiny beam of a laser is smaller than any metal tool, and therefore, can make more accurate cuts. Electro-polishing the edges of the cut make them smooth enough to release the solder paste easily.
Most laser stencils are made of stainless steel, although nickel stencils are also available. Manufacturers make the openings in the stencil using lasers. Using laser has the advantage of achieving high precision and low processing time, as there are no photo films involved. The apertures in the stencil are highly accurate when cut with lasers.
Even when cut with high-precision lasers, the walls of the apertures usually have tiny burs. These hamper the smooth transfer of solder paste from the stencil to the board. A process of electro-polishing or nickel plating removes the tiny burs, improving the transfer of solder paste.
The amount of solder paste deposit required on the pads defines the stencil thickness. SMD stencils are typically 0.006 to 0.010 inches thick. The thickness of the stencil is important to achieve quality solder joints.
If the SMD stencil is thicker than necessary, it will deposit a high volume of solder paste. During reflow, this may cause problems for fine pitch SMDs, as the excess solder may join to create shorts. On the other hand, the high volume of solder paste may also make it stick to the edges of the opening on the stencil, affecting its transfer efficiency, and subsequent transfers. if the stencil is thin, the solder paste it deposits may not be enough to properly form a proper solder joint.
To improve the transfer efficiency of SMD stencils, manufacturers follow stencil aperture rules. Ideally, the amount of solder paste held in the opening of a stencil should transfer totally and completely to the pad, after the operator lifts the stencil. However, this does not happen in reality. A small amount of solder paste sticks to the stencil opening walls. The ratio of the solder paste volume deposited by the stencil to the calculated volume is the transfer efficiency of the stencil, and should ideally be 1 for a specific stencil.
Stencil manufacturers follow a mathematical relationship between the stencil thickness and the stencil opening, to maximize the transfer efficiency. The aperture is usually trapezoidal, such that the bottom opening is wider than the opening at the top of the stencil. This helps to increase the transfer efficiency.
Care of Stencils
After using a stencil, the operator must thoroughly clean it to get rid of any remaining solder paste. Rush PCB UK recommends storing stencils vertically in a suitably protective environment.