We all know that not all the reasons will definitely lead to monument erection. Now we will analyze several reasons with high probability in the actual SMT patch processing and production, and put forward preventive measures.
1、 Pad design
1. The excessive pad spacing is not due to the mismatch between the pad and the component, but the component size and the overall size of the pad meet the reliability requirements, but the intermediate spacing between the two pads is too large, which will cause the element to offset and separate from the solder paste when the solder wetts the element terminal. Generally, in order to avoid the problem of monument, it is recommended that the pad size of components, especially the inner spacing, should meet certain requirements.
2. The pad size is inconsistent and the heat capacity is different
The area of the welding area of the two pads at position C33 is different. The pad at the upper position is formed by opening a window on a large piece of copper foil, and the area of the welding area will be larger than that of the pad at the lower position. Moreover, because the pad at the upper position is connected with a large piece of copper foil, its temperature rise rate during reflow will be relatively smaller than that of the lower pad, so the melting and wetting rate of solder paste will be different, This is very easy to cause offset or monument problems. According to Xiaobian's understanding, many Guangzhou SMT patch processing plants have experienced such a nightmare. After the reflux, the offset, Monument and flying parts of 0402 components have occurred, which is impossible to prevent. Generally, designers should be advised to adopt the same design method of pads at both ends and the same SMD or nsmd design at DFM stage, rather than SMD at one end and nsmd at the other end. It is better to design as R12 or R16 in the figure. If it is really necessary to connect large copper foils, thermal isolation design can be considered. As shown in the right figure below, thermal relief can balance the temperature rise of pads at both ends. The width of this isolation is equivalent to a quarter of the pad diameter or width.
3. Solder mask thickness
In fact, the influence of the thickness of the solder mask is generally not too much, because in the design of most components below 0201, the solder mask between the pads has been cancelled. If it does exist, it is recommended that the designer cancel the intermediate solder mask.
2、 Process design
1. Solder paste printing offset. If the spacing between pads is increased in order to avoid solder bead problems, or solder paste printing offset, the probability of Monument after reflow will increase greatly. Therefore, controlling the problem of solder paste printing is very key. Of course, it is easy to find in actual production. However, it is difficult to find the opening design of steel mesh. Generally, it is recommended to keep the opening spacing of steel mesh consistent with the C value in the table.
2. Mounting offset
In fact, the mechanism of SMT chip processing and mounting offset is the same as that of solder paste printing offset, that is, component offset, resulting in insufficient contact between welding terminal and solder paste and uneven wetting or wetting force. Monument erection is naturally difficult to avoid. This requires optimizing the mount program.
3. Nitrogen concentration
As we all know, nitrogen is an inert gas. It isolates the influence of oxygen and improves the solderability of welding terminals, the wettability of solder paste, and the climbing ability of solder paste on PCB pads and component terminals after melting. This is very helpful for the welding quality, whether it is the yield of the production line or the reliability of the solder joint. It is very necessary to put aside the cost factor. Of course, if the solderability of components or PCB pads is good, there is no problem, but if there is a difference in the solderability of two terminals of components, nitrogen may amplify the difference, which is why sometimes there is a monument problem when the nitrogen concentration is high and the oxygen concentration is below 1000ppm. The experience of many SMT patch processing manufacturers in Guangzhou shows that when the oxygen concentration is lower than 500ppm, the monument problem is very serious. However, there is no standard value. According to practical experience, the oxygen concentration is usually controlled at 1000-1500ppm, which is not only conducive to the completion of welding, but also not easy to cause monument problems.
4. Improper profile setting
The temperature setting mainly needs to consider the thermal balance of the whole PCB. During reflux, if the heat difference on the PCB is large, it may lead to thermal shock. When the temperature rises too fast, more than 2 ° C per second, monument erection may occur. Therefore, it is generally recommended that the temperature rise slope should not exceed 2 ° C per second, and try to maintain the temperature balance of the PCB before reflux.
3、 Material problem
The solderability of two welding terminals of components can be determined according to IPC j-std-002, solderability tests for
Component leads, terminations, lugs, terminals and wires test the terminal solderability of components. The evaluation criteria are shown in the table below. Through such a test, it can only show that there is no problem with the solderability of the component terminal. However, if there is a monument problem, it is necessary to determine the time when the two terminals reach the same wetting force or the wetting force reached in a specific time period. The large difference between the wetting rate or wetting force is likely to lead to the monument problem. Recommended reading: process and application of rapid proofing of sheet metal products
4、 Conclusion
1. The root cause of tablet component monument is the uneven wetting of solder on two welding terminals;
2. In terms of design, the same design method is adopted for the two pads. If the size, inner spacing and heat capacity are balanced, the occurrence of Monument erection can be prevented, and the thermal relief design can avoid the problem of Monument erection;
3. In the design of steel mesh, maintaining an appropriate value on the inner distance of the two mesh holes can effectively prevent the occurrence of Monument erection;
4. If the installation quality is poor and the equipment accuracy is limited, excessive deviation may also lead to the problem of Monument erection;
5. Rapid temperature rise may lead to the problem of Monument erection due to uneven heat distribution on the PCB. The temperature rise slope less than 2 ° C per second can prevent the problem of Monument erection;
6. Nitrogen reflux can effectively improve the welding quality, but too low oxygen content may lead to monument erection. Generally, the oxygen content greater than 1000ppm will not lead to monument erection;
7. In solderability testing, SMT patch processing should pay attention to the balance of wetting time and wetting force in the two terminals of the component, otherwise the stele problem is inevitable.
