Cause analysis and preventive measures of stele erection of reflow soldering components

Date：2022-05-17 15:32:01 Views：1253

In the reflow soldering process of surface mount process, the chip components will have the defect of off soldering due to warping, which is vividly called "monument" phenomenon (also known as "Manhattan" phenomenon). Reflow soldering is also a very important link in SMT process, but in practical operation, we often see that there are many small chip components, especially chip resistors, which have the monument defect of desoldering and erection of chip components. This "monument" phenomenon usually occurs in the reflow welding process of chip components (such as chip capacitor and chip resistor). The smaller the component volume, the more likely it is to occur. For example, this phenomenon occurs most in 0603 and 0402 components. There are many reasons for this phenomenon, and it is difficult for us to completely eliminate the "monument" phenomenon. However, we can minimize this undesirable phenomenon as far as possible.

The "monument" phenomenon is caused by the imbalance of the surface tension between the two welding ends of the element when the solder paste on the bonding pads at both ends of the element melts in the reflow furnace, and the higher end of the tension pulls the element to rotate along its bottom. There are many factors causing this tension imbalance. The following will introduce the cause analysis and preventive measures of reflow soldering component monument.

1. Preheating period

When the preheating temperature is set low and the preheating time is set short, the probability of different melting of solder paste at both ends of the element will greatly increase, resulting in the imbalance of tension at both ends to form a "monument". Therefore, the process parameters in the preheating period should be set correctly. According to our experience, the preheating temperature is generally 150 + 10 ℃, and the time is about 60-90 seconds.

2. Pad size

When designing sheet resistance and capacitance pads, their overall symmetry should be strictly maintained, that is, the shape and size of the pad pattern should be completely consistent, so as to ensure that the resultant force acting on the solder joint on the element is zero when the solder paste melts, so as to form an ideal solder joint. Design is the first step in the manufacturing process. Improper pad design may be the main reason for the erection of components. For specific pad design standards, please refer to ipc-782 standard for surface mount design and pad layout. In fact, too many pads with more components may allow the components to slide during solder wetting, resulting in pulling the components out of one end of the pad.

For small sheet elements, designing different pad sizes for one end of the element, or connecting one end of the pad to the ground plate, may also cause the element to stand up. The use of different pad sizes may result in unbalanced pad heating and solder paste flow time. During reflow, the element is almost floating on the liquid solder and reaches its final position when the solder solidifies. Different wetting forces on the pads may result in a lack of adhesion and rotation of the components. In some cases, prolonging the time above the liquefaction temperature can reduce the erection of elements.

3. Thickness of solder paste

When the thickness of solder paste becomes smaller, the phenomenon of Monument erection will be greatly reduced. This is because: (1) the solder paste is thin, and the surface tension when the solder paste melts decreases. (2) The solder paste becomes thinner, the heat capacity of the whole pad decreases, and the probability of simultaneous melting of solder paste on two pads increases greatly. The thickness of solder paste is determined by the thickness of the template. Table 2 shows the comparison of Monument erection using o.1mm and 0.2mm thick templates. 1608 elements are used. Generally, when components below 1608 are used, formwork below 0.15mm is recommended.

4. Mounting offset

In general, the component offset generated during mounting will be automatically corrected by pulling the component due to the surface tension when the solder paste melts during the reflow process. We call it "adaptive", but if the offset is serious, the pulling will make the component stand up and produce a "monument" phenomenon. This is because: (1) the solder end with more contact with the element gets more heat capacity, so it melts first. (2) The adhesive force between the two ends of the element and the solder paste is different. Therefore, the placement accuracy of components should be adjusted to avoid large placement deviation.

5. Component weight

The occurrence rate of "monument" phenomenon of lighter components is higher because unbalanced tension can easily pull the components. Therefore, when selecting components, if possible, priority should be given to components with large size and weight.

There are still many welding defects. This paper lists only the three most common defects. There are many measures to solve these welding defects, but they often restrict each other. For example, increasing the preheating temperature can effectively eliminate the stele, but it is possible to produce a large number of solder balls due to the faster heating speed. Therefore, when solving these problems, we should consider many aspects and choose a compromise scheme, which we should keep in mind in practical work.

Precautions to prevent monument erection of circuit board components after reflow soldering:

1. Clean the steel mesh (the operator is required to clean the steel mesh on time. If necessary, it must be blown with an air gun. It is strictly prohibited to wipe the steel mesh with paper, and it must be wiped with a dust cloth);

2. Adjust the distance between PCB and steel mesh (PCB must be parallel to steel mesh);

3. Clean nozzle (clean nozzle on time according to the regulations on the maintenance record sheet of the mounter. Note that nozzle can be cleaned with alcohol and dried with an air gun after washing);

4. Adjust Feida center point;

5. Correct the machine coordinates. (at the same time, clean the mirror / internal and external LED light-emitting panels of the flight camera) attention Do not use alcohol to clean the LED light-emitting board, otherwise it may cause short circuit of the machine);

6. Redesign the pad (or close the patch coordinates to the place with fewer pads);

7. Reset the temperature of reflow soldering and test the temperature curve.

The above is the cause analysis and preventive measures of the monument of the reflow soldering elements compiled by the core detection team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing organization for electronic components in China. It has three standardized laboratories with an area of more than 1000 square meters. The scope of testing services covers: testing and verification of electronic components, identification of IC authenticity, product design, material selection, failure analysis, function testing, factory incoming material inspection, X-ray testing of components, tape braiding and other testing items.