12/17/2023 0 Comments Solder paste reflowAlso, but less likely, paste flux gasses may do the same thing, as they escape from under large parts that are close to the surface of the board. Such eruptions can "jet" gasses in all sorts of direction and push liquid solder around on the board. These trapped contaminants are is forced out when heated in reflow from between the layers and will erupt from anywhere it can escape. Air, moisture or even the alcohol you use to clean the board, can absorb into the board between the layers through the edges (delamination), open vias or any through-holes in the board whose plating is incomplete or cracked. Masters Degree in Industrial Engg, State University of New York-Binghamton. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Two to fours hours before use, place the paste in a normal temperature environment.Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Solder paste should be stored in a refrigerator at 0 to 5 degrees. If the solder paste is applied without a temperature recovery period, the water vapour in the surrounding environment will condense and penetrate the solder paste this will cause the solder to splash. The recommended drying temperature is 125 degrees for 4 hours. If the PCB moisture is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and creating solder balls.ĭry the PCB if it was fabricated over 6 months ago. The blade pressure is typically set at about 5N ~ 15N / 25mm. When the blade pressure is too high, it will result in more paste leakage. When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the stencil. The speed should be controlled to around 20 ~ 40 mm/s. A higher printing speed will cause insufficient solder to be applied. The best blade angle should be set around 45 to 60 degrees.Ī higher the printing speed means that less time will be spent in applying the solder paste through the stencil aperture surface. ![]() If the angle is too small, the solder paste will not be squeezed into the stencil apertures. The blade angle affects the vertical force applied on the solder paste. Its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow. It is best to use a circular-shaped stencil aperture design. When the stencil aperature size is too small, and insufficient solder paste will be applied. When the stencil aperature size is too big, a solder bridge short can occur. Conversely, a high flux content will result in an insufficient amount of solder applied.Ī stencil that is too thick will cause a solder bridge short.Ī stencil that is too thin will cause an insufficient solder to be applied. A lower flux content will result in excess amount of solder paste applied. The flux in the solder paste should be controlled to between 8 and 15 percent. When the external force stops, the viscosity will recover to ensure no deformation occurs. Since the viscosity decreases when the paste passes through the stencil apertures, the paste can be applied to the PCB pads rapidly. The particle size should be controlled to about 25 ~ 45 μm in order to meet the requirements for fine-pitch QFP soldering, If the partical size desired is 25 to 30 μm, it should applied with less than 20 μm solder paste for an ultra fine-pitch IC.įlux contains a thixotropic agent, which allows the solder paste to have pseudoplastic flow characteristics. It will have a high oxygen content and cause a solder ball after reflow. If the particle size is too small, the results will be poor paste adhesion. It can also cause solder ball and short bridge defects after reflow.Ī spherical shape is best, especially for fine-pitch QFP printing. This will cause a big slump after printing. ![]() The irregular shape of solder powder will easily clog stencil apertures. The following table lists an analysis of solder paste printing defects: So it is very important to understand what causes defects in solder paste printing. Statistics demonstrate that 60% to 90% of soldering defects are related to solder paste printing defects. ![]() Quality solder paste printing guarantees a quality solder joint and final product. Since the solder paste is used to directly form the soldering joint, the quality of solder paste printing affects the performance and reliability of the surface mount assembly. In SMT PCB production, solder paste printing is a critical step. An Analysis of SMT Solder Paste Printing Defects
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