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Time of issue:2020-07-15 00:00:00
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SMT patch tombstone reasons and solutions

(Summary description)The tombstone phenomenon often occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component size, the more likely it is. Especially in the production of 1005 or smaller Diao 0603 SMD components, it is difficult to eliminate "Tombstone" phenomenon. In the reflow soldering process of the surface mount process, the chip components will have the defect of unsoldering due to the uplifting. In the reflow soldering process of the surface mount process, the chip components will be caused by the uplifting. The defect of desoldering is called the "tombstone" phenomenon...

SMT patch tombstone reasons and solutions

(Summary description)The tombstone phenomenon often occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component size, the more likely it is. Especially in the production of 1005 or smaller Diao 0603 SMD components, it is difficult to eliminate "Tombstone" phenomenon. In the reflow soldering process of the surface mount process, the chip components will have the defect of unsoldering due to the uplifting. In the reflow soldering process of the surface mount process, the chip components will be caused by the uplifting. The defect of desoldering is called the "tombstone" phenomenon...

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Today, Sanjing will take you to understand the "stone erection" and the reasons for the "stone erection":

 

The tombstone phenomenon often occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component size, the more likely it is. Especially in the production of 1005 or smaller Diao 0603 SMD components, it is difficult to eliminate "Tombstone" phenomenon. In the reflow soldering process of the surface mount process, the chip components will have the defect of unsoldering due to the uplifting. In the reflow soldering process of the surface mount process, the chip components will be caused by the uplifting. The defect of desoldering is called the "tombstone" phenomenon (Manhattan phenomenon). The reason is that the solder paste on the pads at both ends of the component will have surface tension on the two solder ends of the component when it is reflowed and melted. Unbalanced. The end with greater tension pulls the component and rotates along its bottom.

 

There are also many factors that cause tension imbalance. Here is a brief analysis of these factors:

 

1. Uneven heating, uneven temperature distribution in the reflow furnace, uneven temperature distribution on the board surface-pay attention to the preheating period

 

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 will not melt at the same time will greatly increase, which will cause the imbalance of tension at both ends to form a "tombstone", so the preheating period process 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. 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-pay attention to the size of the pad

 

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 component rotation. 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.

 

3. Poor placement accuracy and serious component deviation-pay attention to placement deviation

 

Under normal circumstances, the component deviation generated during mounting 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, 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 misalignment is serious-pay attention to the thickness of the solder paste

 

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. Pay attention to 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 many welding defects, and we only list the three most common defects. 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.

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