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Discussion on the technical requirements of perforated reflow soldering

Discussion on the technical requirements of perforated reflow soldering

SMT Technical literature
2014/10/22 10:55
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Perforated reflow is an emerging technology in international electronics assembly applications. When there are both mounting components on the same side of the PCB and a small number of sockets and other components, we generally take the method of first patching through the reflow oven and then manually inserting the wave soldering. However, if perforated reflow soldering is used, it is only necessary to insert the plug-in components into the reflow oven after the patch is completed, before entering the reflow oven.
   From this comparison, the superiority of perforated reflow soldering to conventional processes can be seen. The first is to reduce the process, eliminating the wave soldering process, which can save a lot of money. At the same time, the required staff has been reduced and the efficiency has been improved. Secondly, reflow soldering is much less likely to produce bridging than wave soldering, which increases the pass rate. Perforated reflow technology has great advantages in economy and advancement compared with traditional processes. Therefore, the perforated reflow soldering technology is an innovation in electronic assembly and will inevitably be widely used. However, if you want to apply the reflow soldering technology, you need to put forward some requirements different from the traditional process for the device, PCB design, and stencil design.
First, the components:
   The perforated element is required to withstand the reflow temperature of the reflow oven, with a minimum of 230 degrees and 65 seconds. This process involves applying a solder paste over the holes (which will enter the holes during reflow). In order to make this process feasible, the component body should be 0.5 mm from the surface of the board. The length of the selected component should be the same as the thickness of the board. There should be a square or U-shaped cross section (better than the rectangle).
Second, calculate the hole size
   The finished hole should be sized to be 0.255 mm (0.010 in.) larger than the maximum measured dimension of the pin, usually with the cross-section of the lead diagonal, without the retention feature. The size of the drill hole is 0.15 mm (0.006 inch) larger than the finished hole, which is the plating compensation, so that the calculated hole is the smallest acceptable size.
Third, calculate the screen: (the amount of solder paste)
   The first part of the calculation is to find out the amount of solder paste required for soldering, the volume of the hole minus the volume of the pin plus the volume of the solder fillet. (What kind of welding fillet is needed). Multiplying the required weld volume by 2 is the amount of solder paste required because the metal content of the solder paste is 50% by volume (take ALPHA's UP78 solder paste as an example). During the silk screen printing process, the solder paste is printed on the PCB through the mesh. The pressure can generally be pressed into the hole by 0.8 mm (when the blade is at a 45 degree angle to the screen). We calculate the volume of solder paste entering the hole and subtract it from the amount of solder paste required to obtain the volume of solder paste left in the mesh. This volume is divided by the thickness of the stencil to determine the area required for the mesh.
Fourth, stencil design:
   The location of the stencil will depend on several factors:
1. The minimum distance from one side of the mesh to the center of the hole is equal to the radius of the hole.
2. The mesh is always larger than the pad, so the solder paste will be applied on the solder mask. After reflow soldering, it will be confirmed that no solder paste remains on the resist pad. The edge of the mesh is straight, because when reflow soldering The process solder paste enters the hole and there will be no solder paste reflow on the surface.
3. There is a design limitation on the shape of the lower mold on the bottom surface of the device, and a space of 0.2 mm is required between the lower bottom surface and the screen printed solder paste. (must be included in the design)
4. On the socket, many meshes provide straight and narrow silkscreen, so the component positioning and test points beside the punched socket leave a certain amount of space for the solder paste layer.
5. General components such as crystals have enough space under the components to meet the area required for silkscreening, which means that it is not necessary to apply solder paste to the outside of the component.
Fifth, the preparation of the component pins:
It is important that the pins have the correct length. They must be pre-cut to achieve a condition of 1.5 mm more than the plate thickness before they enter the process. All variations in pin size and mesh size will be included in the amount of solder fillet, so some variations will be reflected in the height variation of the solder fillet. The temperature profile of the reflow oven is set to: smoothly rise to 165 + 20 degrees in 4.5 minutes, only one temperature zone from 165 to 220 + 5 degrees, and 50 seconds at 220 + 5 degrees.
Sixth, welding:
For practical reasons, there is always a change in solder paste when reflow soldering, so there is a weld fillet designed to solve a series of changes. The varying fillets are always below the component, from flat to full to meet inspection criteria.
Perforated reflow soldering is relatively simple in application, but it requires more preparatory work than the “injection-wave soldering” process. The main point of perforated reflow soldering is the design of the mesh shape and size. This calculation process must take into account factors such as the solid content of the solder paste, the volume of the lead, and the size of the aperture. Therefore, to get a perfect solder joint, you need to go through detailed calculations and repeated tests. This article only explains the basic technical requirements for perforated reflow soldering. There are many details to be further tested and summarized.