The reason and Common Solutions of tombstoning for SMT assembly 01

- Jul 21, 2017-

The result is that the smaller the component volume is likely to occur during the SMT reflow process of CHIP components such as chip capacitors and chip resistors, especially in the production of 1005 or smaller 0603 patch components, In the process of reflow soldering of the surface mount process, the patch element will produce defects due to arrogance and desoldering. As shown in the figure, the image is called "tombstoning" (also known as " Manhattan" phenomenon".)." tombstoning "phenomenon is due to the solder bumps at both ends of the solder plate in the reflow melting, the two parts of the surface of the solder joint surface tension is not balanced, pull the end of the larger components along the bottom Rotation caused by the tension caused by a lot of factors are not balanced, the following will be a brief analysis of some of the main factors.

1. Preheating period

When the preheating temperature is set lower, the warm-up time is set short, the probability that the solder paste does not melt at both ends is greatly increased, resulting in the unbalanced tension at both ends to form a "monument", so the warm- parameter. According to our experience, the preheating temperature is generally 150 + 10 ℃, the time is about 60-90 seconds.

2. Pad size

For a small chip element, it is possible to design a different pad size for one end of the element, or to connect one end of the pad to the ground plate, and may cause the element to erect. The use of different pad sizes may cause unbalanced pad heating and solder paste flow times. During reflow, the element is simply floating on the liquid solder and reaches its final position when the solder is solidified. Different wetting forces on the pad may cause a lack of adhesion and rotation of the element. In some cases, prolonging the liquefaction temperature above the time can be reduced by erecting the element. Design of chip resistors, capacitor pads, should be strictly to maintain its full symmetry, that is, the shape and size of the pad pattern should be exactly the same to ensure that the solder paste melting, the role of the solder joint on the component to zero, to Conducive to the formation of the ideal solder joints. Design is the first step in the manufacturing process, improper pad design may be the main reason for the erection of components. Specific pad design criteria can be found in IPC-782 "Surface Mount Design and Pad Layout Standards".In fact, too many pads over the component may allow the element to slide in the wet process of the solder, causing the element to pull out one end of the pad.