SMT 0402 component tombstone improvement case analysis
Oct 14, 2021
The "tombstone" phenomenon often occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component, the more likely it is. Especially in the production of 1005 SMD components, it is difficult to eliminate the "tombstone" phenomenon. In the reflow soldering process of the surface mount process, the chip components will have the defect of unsoldering due to tilting. In the reflow soldering process of the surface mount process, the chip component will have the defect of desoldering due to the tilting. People vividly call it the "tombstone" phenomenon (that is, the Manhattan phenomenon).
The "tombstone" phenomenon often occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component, the more likely it is. Especially in the production of 1005 SMD components, it is difficult to eliminate the "tombstone" phenomenon. In the reflow soldering process of the surface mount process, the chip components will have the defect of unsoldering due to tilting. In the reflow soldering process of the surface mount process, the chip component will have the defect of desoldering due to the tilting. People vividly call it the "tombstone" phenomenon (that is, the Manhattan phenomenon).
The reason is that when the solder paste on the pads at both ends of the component is reflowed and melted, the surface tension on the two solder ends of the component is unbalanced. The specific analysis has the following 7 main reasons:
1. Uneven heating, uneven temperature distribution in the reflow oven, uneven temperature distribution on the board surface
2. The problem of the component, the shape and size of the soldering end are different, the solderability of the soldering end is different, and the weight of the component is too light
3. The material and thickness of the substrate, the thermal conductivity of the substrate material is poor, and the thickness uniformity of the substrate is poor
4. The shape and solderability of the pad, the heat capacity of the pad is quite different, and the solderability of the pad is quite different
5. Solder paste, the uniformity of the flux in the solder paste is poor or the activity is poor, the thickness of the solder paste on the two pads is quite different, the solder paste is too thick, the printing accuracy is poor, and the dislocation is serious
6. The preheating temperature, the preheating temperature is too low
7. Poor placement accuracy and serious component deviation.
The "tombstone" phenomenon occurs because the solder paste on the pads at both ends of the component is reflowed and melted, and the surface tension of the two solder ends of the component is unbalanced, and the end with greater tension pulls the component and rotates along its bottom. There are also many factors that cause tension imbalance. The following will briefly analyze some of the main factors.
1. Warm-up period
Table 1 is the test statistics results of tombstone phenomenon in infrared heating and gas phase heating reflow soldering. In the test, 1608 and 2125 chip capacitors are used. The test is infrared and hot air reflow soldering and gas-phase heating reflow soldering without preheating. From the table It can be clearly seen that the occurrence rate of tombstone phenomenon in the latter is much greater than the former. This is because the gas-phase heating has no preheating zone, which makes the temperature rise very fast. As a result, the probability that the solder paste at both ends of the component does not melt at the same time is greatly increased. When the preheating temperature setting is lower and the preheating time setting is shorter, the probability that the solder paste on both ends of the component will not melt at the same time will greatly increase, which will cause the imbalance of tension at both ends to form a "tombstone". Therefore, the preheating period must be set correctly. parameter. According to our experience, the preheating temperature is generally 150+10℃, and the time is about 60-90 seconds.
2. Pad size
For small chip components, designing different pad sizes for one end of the component or connecting one end of the pad to the ground board may also cause the component to stand up. The use of different pad sizes may cause unbalanced pad heating and solder paste flow time. During reflow, the component simply floats on the liquid solder and reaches its final position when the solder solidifies. Different wetting forces on the pads may cause lack of adhesion and rotation of components. In some cases, extending the time above the liquefaction temperature can reduce component erection. When designing chip resistors and capacitor pads, the overall symmetry should be strictly maintained, that is, the shape and size of the pad pattern should be exactly the same to ensure that the total force acting on the solder joints on the component is zero when the solder paste is melted. Conducive to the formation of ideal solder joints. Design is the first step in the manufacturing process, and improper pad design may be the main reason for component erection. For specific pad design standards, please refer to IPC-782 "Surface Mount Design and Land Layout Standards". In fact, too many pads exceeding the component may allow the component to slide during the solder wetting process, causing the component to be pulled out of the pad. One end.
3. Mounting offset
Under normal circumstances, the component deviation generated during placement will be automatically corrected during the reflow process due to the surface tension pulling the component when the solder paste melts. We call it "adaptive", but the deviation is serious and the pulling will cause it The erection of the component produces a "tombstone" phenomenon. This is because: (1) The solder terminal that is in contact with the component gets more heat capacity and melts first. (2) The two ends of the component have different adhesion to the solder paste. Therefore, the placement accuracy of the components should be adjusted to avoid large patch deviations.
4. Solder paste thickness
When the thickness of the solder paste becomes smaller, the tombstone phenomenon will be greatly reduced. This is because: (1) The solder paste is thinner, and the surface tension of the solder paste is reduced when it melts. (2) The solder paste becomes thinner, the heat capacity of the entire pad is reduced, and the probability of the solder paste on the two pads being melted at the same time is greatly increased.
5. Component weight
Because the tension generated by the melting of solder paste is generally about 1g to 3g, the incidence of "tombstone" phenomenon of lighter components is higher, and the probability of occurrence is higher; this is because uneven tension can easily Pull the component to the ground. Therefore, when selecting components, if possible, the components with larger size and weight should be selected first.
There are still many welding defects, and there are many measures to solve these welding defects, but they often restrict each other. Increasing the preheating temperature can effectively eliminate the tombstone, but it may produce a large number of solder balls due to the faster heating speed. Therefore, when solving these problems, we should consider multiple aspects and choose a compromise method.