The tombstone is also known as the suspension bridge and the Manhattan phenomenon. It refers to the two Korean-end surface-mounted components. After reflow soldering, one of the ends leaves the surface of the pad. The entire component is inclined or upright, like a stone monument, as shown in the figure. As shown, one end of the rectangular chip component is soldered to the pad, and the other end is upright.
Monument
The analysis of several common tombstone status is as follows.
(1) The placement accuracy is insufficient. Under normal circumstances, the component deviation generated during mounting will generate surface tension due to the melting of the solder paste during reflow soldering, and the component will be pulled for automatic positioning, that is, self-alignment. However, if the deviation is severe, the pulling will cause the components to stand up, resulting in a tombstone phenomenon. In addition, the difference in viscosity between the two ends of the component and the solder paste is also one of the reasons for the tombstone phenomenon. The solution is to adjust the placement accuracy of the placement machine to avoid large placement deviations.
(2) The design of pad size is unreasonable. If the pair of pads of the chip component is asymmetrical, it will cause the amount of missing solder paste to be inconsistent. Small pads respond quickly to temperature, and the solder paste on the pads is easy to melt. The opposite is true for large pads. After the solder paste on the pad is melted, under the action of surface tension, the component is straightened and erected, resulting in a tombstone phenomenon. The solution of the device is to design the pad strictly according to the standard specification to ensure that the shape and size of the pad pattern are exactly the same. At the same time, when designing the pad, under the premise of ensuring the strength of the solder joint, the size of the pad should be as small as possible, and the tombstone phenomenon will be greatly reduced.
smt patch processing
(3) The solder paste is applied too thickly. When the solder paste is too thick, the probability that the solder paste on the two pads will not melt at the same time will greatly increase, resulting in an imbalance in the surface tension of the two solder ends of the component, resulting in tombstone phenomenon. On the contrary, when the solder paste becomes thinner, the probability of the solder paste on the two pads being melted at the same time is greatly increased, and the tombstone phenomenon is greatly reduced. The solution is: because the thickness of the solder paste is determined by the thickness of the template, a thinner module should be selected.
(4) Insufficient preheating. When the preheating temperature setting is lower and the preheating time setting is shorter, the probability that the solder paste at both ends of the component cannot be melted at the same time is greatly increased, resulting in an imbalance in the surface tension of the two solder ends of the component, resulting in tombstone phenomenon. The solution is to correctly set the preheating period process parameters and extend the preheating time.
(5) There are defects in the design of the component arrangement direction. If during reflow soldering, one solder end of the protagonist of the film passes through the reflow soldering area first, and the solder paste melts first, but the other solder end does not reach the melting temperature, then the solder end that melts first will be affected by the surface tension. The components are straightened and erected, resulting in a tombstone phenomenon. The solution is to ensure that the two solder ends of the chip component enter the reflow soldering area at the same time, so that the solder paste on the pads at both ends is melted at the same time.
(6) The components are lighter. Lighter components have a higher incidence of tombstones, because the uneven surface tension at both ends of the component can easily pull the component.