Today, the SMD parts become smaller and smaller. Their sizes can be developed to 0402, 0201, and even 01005 (Note 1). Moreover, the pitch between the pins of a common IC (integrated circuit) is shorten to fine pitch of 0.5mm, even 0.3 mm. The improvement becomes a challenge for the SMT process.
A great challenge for each SMT engineer is how to weld these thin electronic parts on circuit boards without any non-wetting or short-circuit. In addition, it is more difficult to solder few larger parts and a lot of thin ones on a circuit board than to solder thin parts without any larger ones.
Generally, to ensure solderability of the part pins on a circuit board, the required soldering tin depends on the size of each electronic part. The transfer efficiency of solder paste should be controlled precisely to avoid the short-circuit problems between pins. For example, a connector or a part of power supply requires more solder volume to prevent the parts from separation because of frequent insertion and extraction. Some small parts, however, are generally designed around or near this kind of larger parts to avoid ESD, voltage surge, or other problems. Therefore, the selection of stencils with appropriate thickness and aperture becomes a major issue.
Generally, 0402 means that the length and width of electronic passive element is 0.04″x0.02″ (about 1.0mmx0.5mm). Likewise, 0201 indicates an electronic passive element with size of 0.02″x0.01″ (about 0.6mmx0.3mm), and 0603, 0805, and 1206 are similar.
Not only to deal with the solder of parts with different sizes but also to ensure excellent soldering quality, various stencil thickness on a stencil is required in order to control solder paste volume precisely. Therefore, one kind of specific stencil, so called step-up and step-down stencil, is developed for the purpose. The kind of stencils increases/decreases solder paste volume through the step-up/step-down the stencil thickness. Moreover, a step-up stencil can also cover the co-planarity issue of part pins, and a step-down stencil can resolve short-circuit issue for fine pitch parts effectively.
The Left picture shows that the step-up/step-down is processed on the other side of the squeegee blade. It means that the recommended step-up/step-down side should be closed to the circuit board to avoid the damage of the squeegee blade. Few step-up/step-down, however, is processed on the same side of the squeegee blade in accordance with some specific considerations.
Of course, this kind of stencils is more expensive than a common stencil since a specific stencil should not only adopt a thicker stencil but also thin the required section by laser. Therefore, a step-down stencil should be easier to manufacture than a step-up one. If there is the requirement of only increasing solder paste volume, it is suggested to increase the aperture size of stencil first to save the cost.
Some limitations for the kind of specific stencils are listed as follows:
The thickness difference of stencil is recommended within 0.03mm. It means that the thickness of step-up stencil should not be more than 0.18mm if the thickness of the original stencil is 0.15mm. Similarly, the thickness of step-down stencil should not be less than 0.12mm. Although the thickness of stencil can be adjusted based on the requirements, the larger difference of stencil thickness is easier to cause problems in controlling solder paste volume. Since the height of squeegee blade is not easy to be adjusted in accordance with the thickness of stencil during printing, the larger thickness difference of stencil may cause much higher/lower pressure of the squeegee blade, resulting in the excessive/inadequate solder paste volume issue, even distortion problem.
It is better that there is same thickness of step-up/step-down stencil for the area squeegeed by the blade since the blade will prop up at the area. Otherwise, the solder paste volume may be uneven. To avoid the issue, CAD engineers should be requested to design the layout of electronic circuit well. They can arrange the location of parts in accordance with their solder paste volume requirements. In fact, most of step-up stencils will thicken partial area instead of the whole area. Therefore, it should be more careful to print solder paste with these stencils.
In the edges of step-up/step-down on a stencil, 3~5mm space should be reserved at least in order to mount other small parts requiring less solder paste volume, or larger ones requiring more solder paste volume. Since the squeegee blade may not fit the variation of stencil thickness in time, the solder paste may not be printed into the aperture well. Therefore, the space of edge is designed to avoid the insufficient solder paste volume issue.
When operating the kind of step-up/step-down stencils, the speed of squeegee blade should be slower to ensure good transfer efficiency of solder paste. Also, the downward pressure of the squeegee blade should be controlled well.