When a hole is drilled in the back of a large pad of a MOSFET
In order to improve the heat dissipation of the MOSFET, we punched the MOSFET pad. In this case, we need to have vias on the pads. Of course, when processing large via holes, uniform holes need to be provided to ensure even heating of the pads.
Two small package resistors and capacitors, do not open the hole in the pad
The resistors and capacitors of general labels prevent erection and we need to do windowing.
"Tombstoning" phenomenon often occurs in the reflow process of CHIP components (such as chip capacitors and chip resistors). The smaller the component volume, the easier it is to occur. In particular, in the production of 1005 or smaller fishing 0603 SMD components, it is very difficult to eliminate the “tombstone†phenomenon. In the reflow soldering process of the surface mount process, the patch element will have a defect that it will be removed due to tilting and it will be referred to as a “tombstone†phenomenon.
The “tombstone†phenomenon occurs when the solder paste on the pads at both ends of the component is melted at the time of reflow. The surface tension of the two soldering ends of the component is unbalanced, and the end with a larger tension pulls the component and rotates along its bottom. There are also many factors that cause tension imbalance.
For example: pad size, electrode size and wettability, placement offset, temperature rise, solder paste performance, etc.
So in general, we will open windows to the copper-plated pins to prevent the phenomenon of tombstoning. Similarly, we cannot hit the SMT pad to prevent the surface tension of the two solder terminals of the component from being unbalanced.
Three ways to prevent tombstoning:
1, the pad, component surface without oxidation.
2, select the appropriate substrate material to ensure quality.
3, the correct design and layout of the pad, pad design consistent with no over the top of the pad.
4. Correctly set the process parameters during the warm-up period, and adjust the appropriate temperature profile of the oven temperature according to each different product.
5. Properly increase the temperature in the preheating zone and extend the time to the upper limit, so that the tin at both ends can be fully melted at the same time.
6. Use a higher active solder paste to improve the solder paste printing parameters, especially the window size of the template.
7. Adjust the placement accuracy of the components to avoid large patch deviations. When mounting, ensure that placement accuracy is over 90%.
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