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Factors of SMT processing template:
1. The material and engraving of the stencil
Usually two methods of chemical etching and laser cutting are used. For high-precision stencils, laser cutting should be used, because the laser-cut hole walls are straight, have a small roughness (less than 3μm) and have a taper. Some people have verified through experiments that for 01005 devices with the size of salt grains, solder paste printing has higher precision requirements. Laser cutting can no longer meet the requirements. Special electroforming, also called electroplating, is required.
2. The relationship between each part of the stencil and solder paste printing
1. The thickness of the screen
The thickness of the stencil and the size of the opening have a great relationship with the printing of solder paste and subsequent reflow soldering. Specifically, the thinner the thickness, the larger the opening, which is more conducive to the release of solder paste. It has been proved that good printing quality must require the ratio of opening size to screen thickness to be greater than 1.5. Otherwise, the solder paste printing is not complete. In general, for lead pitches of 0.3 to 0.4 mm, use a screen with a thickness of 0.12 to 0.15 mm, and for pitches below 0.3, use a screen with a thickness of 0.1 mm.
2. The direction and size of the screen hole
When the release of the solder paste in the length direction of the pad is consistent with the printing direction, the printing effect is better than when the two directions are perpendicular.
The dimensions of the opening. The shape of the openings on the stencil and the shape of the pads on the printed board are many sizes, which are often serious for the careful printing of solder paste. In the patching effort, the high precision patch function correctly controls the placement pressure. The policy also includes even if the solder paste pattern is not squeezed, broken, or destroyed, so as to avoid bridging and splashing in the reflow. The opening on the screen is mainly determined by the size of the corresponding pad on the printed board. In general, the size of the opening on the stencil should be 10% smaller than the corresponding pad. In theory, many companies reject the ratio of openings to pads at 1:1 in the manufacture of stencils. There is still a small amount of manual soldering for small batches and multiple types of consumption. The author has soldered many QFN devices and used manual soldering. The essentials of spot solder paste, and strictly control the amount of solder paste at each point, but no matter how to adjust the reflow temperature, use X-RAY detection, there are more or less solder balls on the bottom of the device. If the theoretical environment does not have the prerequisite for manufacturing the stencil, the method of first using the device to plant the ball has achieved a better welding effect.
Three types of packaging for SMT processing
Three types of packaging that SMT processing needs to know:
SMT processed strips (shipping tubes)-the main component container: the strips are made of transparent or translucent polyethylene (PVC) material, extruded into an applicable standard shape that meets current industry standards. The strip size provides proper component positioning and orientation for industry-standard automatic assembly equipment. The strips are packaged and transported in a combination of the number of individual strips.
SMT processed coils-main component container: typical coil structures are designed to meet modern industry standards. There are two generally accepted standards for covering tape and reel packaging structures.
The packaging and transportation of pallets is a combination of single pallets, which are then stacked and bundled together to have a certain degree of rigidity. An empty cover tray is placed on top of the installed components and stacked trays.
SMT processed trays-the main component container: the tray is made of carbon powder or fiber materials, which are selected based on the highest temperature rate of the special tray. Trays designed for components that require exposure to high temperatures (moisture-sensitive components) have a temperature resistance of typically 150°C or higher. The tray is cast into a rectangular standard shape and contains a uniform matrix of cavities. The recesses hold the components and provide protection for the components during transportation and handling. The spacing provides accurate component locations for standard industrial automated assembly equipment used for placement in the circuit board assembly process.
