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Analysis and solution of component tombstone after reflow soldering

Time:2019-07-20   |   Source:   |   Compile :9

After reflow soldering, one end of the rectangular chip component is soldered to the pad and the other end is erected. This phenomenon is called the Manhattan phenomenon. The main cause of this phenomenon is that the two ends of the element are unevenly heated, and the solder paste is melted one after another. Thermal unevenness at both ends of the component can occur in the following cases:

SMT monument

1、Defective component arrangement direction design. We envision a reflow line in the reflow oven that spans the width of the furnace and melts as soon as the solder paste passes through it. One end of the chip-shaped rectangular component first passes through the reflow soldering limit line, the solder paste melts first, completely wets the metal surface of the component, and has a liquid surface tension; and the other end does not reach the liquid phase temperature of 183 ° C, the solder paste is not melted, only The adhesion of the flux, which is much less than the surface tension of the reflow solder paste, thereby erecting the ends of the unmelted end up. Therefore, the two ends of the component are simultaneously brought into the reflow soldering limit line, so that the solder paste on the both ends of the pad is simultaneously melted to form a balanced liquid surface tension, so as to keep the position of the component unchanged.

2、The printed circuit assembly is not preheated sufficiently during vapor phase soldering. Vapor phase welding uses an inert liquid vapor to condense heat on the component leads and PCB pads to release the solder paste. In the vapor phase welding equilibrium zone and the saturated steam zone, the welding temperature in the saturated steam zone is as high as 217 & deg; C. In the production process, we found that if the welded component is not preheated enough, it will withstand a temperature difference of more than one hundred degrees, steam phase welding. The vaporization force easily floats the chip components of less than 1206 package size, resulting in a slab phenomenon. We preheated the welded component in the high and low chambers at a temperature of 145 ° C - 150 ° C for 1-2 minutes, then preheated for about 1 minute in the equilibrium zone of the vapor phase welding, and finally slowly entered the saturated steam zone for welding. Eliminate the phenomenon of standing.

c) The impact of pad design quality. If the pair of pads of the chip component are different in size or asymmetrical, the amount of solder paste that is missing may be inconsistent. The small pad responds quickly to temperature, the solder paste on it is easy to melt, and the large pad is reversed. Therefore, when the solder paste on the small pad is melted, the component is straightened and raised under the surface tension of the solder paste. If the width or gap of the pad is too large, a slab phenomenon may occur. Pad design in strict accordance with standard specifications is a prerequisite for addressing this defect.