What are the quality defects of reflow soldering? How to deal with it?

Date：2022-05-17 15:02:00Views：1074

After reflow soldering of electronic product circuit board, due to various reasons such as reflow soldering equipment, circuit board, worker operation, solder paste, chip mounter, etc., it is often seen that some products have various adverse phenomena. If these adverse phenomena are not strictly controlled, they will cause serious consequences to the company. The following is a detailed analysis of the adverse phenomena of reflow soldering quality defects and puts forward corresponding solutions. If you are interested in what this article will cover, read on.

1、 Analysis and solution of solder beads on circuit board after reflow soldering

1. Mechanism of tin bead formation in reflow soldering

Solder beads (or solder balls) in reflow soldering are often hidden on the side between the two ends of rectangular chip components or between thin spaced pins. In the process of component paste, the solder paste is placed between the pin and pad of chip components. As the printed board passes through the reflow furnace, the solder paste melts into liquid. If it is not well wetted with the pad and device pin, the liquid solder particles cannot aggregate into a solder joint. Some liquid solder will flow out of the weld to form tin beads. Therefore, the poor wettability between solder and pad and device pin is the root cause of tin bead formation.

In the printing process of solder paste, due to the alignment offset between the template and the pad, if the offset is too large, the solder paste will flow out of the pad, and solder beads are easy to appear after heating. The pressure of z-axis is an important reason for causing tin beads in the process of chip placement, which is often ignored. Because the z-axis head of some mounters is positioned according to the thickness of the components, it will cause the phenomenon that the components are pasted on the PCB and the tin buds are squeezed out of the solder pad in an instant. This part of tin will obviously cause tin beads. In this case, the size of tin beads is slightly larger. Generally, the generation of tin beads can be prevented by readjusting the height of Z axis.

2. Cause analysis and control method

There are many reasons for poor solder wettability. The following mainly analyzes the reasons and solutions related to relevant processes:

(1) Improper setting of reflux temperature curve. The reflow of solder paste is related to temperature and time. If sufficient temperature or time is not reached, the solder paste will not reflow. The temperature in the preheating zone rises too fast and the time is too short, so that the water and solvent in the solder paste are not completely volatilized. When reaching the reflow temperature zone, the water and solvent boil and splash solder beads. Practice has proved that it is ideal to control the rising speed of preheating zone temperature at 1 ~ 4 ℃ / s.

(2) If tin beads always appear at the same position, it is necessary to check the design structure of metal formwork. The corrosion accuracy of the opening size of the template can not meet the requirements, the size of the pad is too large, and the surface material is soft (such as copper template), which will cause the outer contour of the printed solder paste to be unclear and connected with each other. This situation often occurs when printing the pad of devices with fine spacing, and a large number of tin beads between the pins will be generated after reflow. Therefore, appropriate template materials and template manufacturing process should be selected according to the different shapes and center distance of pad graphics to ensure the quality of solder paste printing.

(3) If the time from patch to reflow soldering is too long, the solder particles in the solder paste will oxidize, the flux will deteriorate and the activity will be reduced, resulting in no reflow of the solder paste and tin beads. Selecting solder paste with longer service life (generally at least 4h) will reduce this effect.

(4) In addition, if the printed board with wrong solder paste is not cleaned sufficiently, the solder paste will remain on the surface of the printed board and in the air. When components are pasted before reflow soldering, the printing paste will be deformed. These are also the causes of tin beads. Therefore, the sense of responsibility of operators and process personnel in the production process should be accelerated, the production should be carried out in strict accordance with the process requirements and operating procedures, and the quality control of the process should be strengthened.

2、 Analysis and solution of component monument after reflow soldering

One end of the chip element is welded on the pad and the other end is tilted. This phenomenon is called Manhattan phenomenon. The main reason for this phenomenon is the uneven heating at both ends of the element and the melting of the solder paste. Uneven heating at both ends of the element will be caused under the following conditions:

1. The component arrangement direction is not designed correctly. We assume that there is a reflow limit line across the width of the furnace in the reflow furnace. Once the solder paste passes through it, it will melt immediately. One end of the chip rectangular element passes through the limit line of reflow welding first, the solder paste melts first, and the metal surface of the end of the element is completely wetted with liquid surface tension; The other end does not reach the liquid phase temperature of 183 ℃, the solder paste is not melted, and only the adhesive force of the flux is much less than the surface tension of the reflow solder paste, so the end of the element at the unmelted end is upright upward. Therefore, keep both ends of the element entering the reflow limit line at the same time, so that the solder paste on the pads at both ends melts at the same time, form a balanced liquid surface tension, and keep the position of the element unchanged.

2. Insufficient preheating of printed circuit components during gas phase welding. The vapor phase is to use inert liquid vapor to condense on the component pin and PCB pad, release heat and melt the solder paste. Gas phase welding is divided into equilibrium zone and steam zone. In the saturated steam zone, the welding temperature is as high as 217 ℃. In the production process, we found that if the welded components are not preheated sufficiently and undergo temperature changes above 100 ℃, the gasification force of gas phase welding is easy to be reduced

Chip components smaller than 1206 package size float, resulting in vertical chip phenomenon. By preheating the welded components at 145 ~ 150 ℃ in the high and low temperature box for about 1 ~ 2min, and finally slowly entering the saturated steam area for welding, the phenomenon of sheet erection is eliminated.

3. Influence of pad design quality. If the size of a pair of pads of chip components is different or asymmetric, the amount of solder paste printed will also be inconsistent. The small pad responds quickly to the temperature, and the solder paste on it is easy to melt, while the large pad is the opposite. Therefore, when the solder paste on the small pad melts, the components will be straightened and erected under the action of the surface tension of the solder paste. If the width or gap of the pad is too large, there may also be vertical flakes. The precondition to solve this defect is to design the pad in strict accordance with the standard specifications.

3、 Analysis and solution of bridging (continuous welding) between components after reflow soldering

Bridging is also one of the common defects in SMT production. It will cause short circuit between components and must be repaired in case of bridging.

(1) Solder paste quality problems

The metal content in the solder paste is high, especially after the printing time is too long, it is easy to increase the metal content; The viscosity of solder paste is low and flows out of the welding pad after preheating; Poor solder paste slump and overflow outside the pad after preheating will lead to IC pin bridging.

(2) Printing system

The printing machine has poor repetition accuracy and uneven alignment. Solder paste is printed outside copper and platinum, which is often seen in fine pitch QFP production; Poor alignment of steel plate and PCB, wrong size / thickness design of steel plate window and uneven alloy coating of PCB pad design will lead to excessive amount of solder paste, which will lead to grounding. The solution is to adjust the printing press and improve the coating layer of PCB pad.

(3) Paste and put

Excessive sticking pressure and immersion of solder paste after pressure are common reasons in production. The height of z-axis should be adjusted. If the accuracy of the chip is not enough, the component is displaced and the IC pin is deformed, it should be improved according to the reason.

(4) Preheat

The heating rate is too fast, and the solvent in the solder paste has no time to volatilize.

4、 Analysis and solution of material absorption / core absorption of components after reflow soldering

Wicking phenomenon, also known as core pulling phenomenon, is one of the common welding defects, which is often seen in vapor phase reflow welding. Wicking phenomenon is that the solder is separated from the lead along the pad and between the chip body, which will form a serious phenomenon of false soldering.

The reason is usually considered to be that the thermal conductivity of the original pin is high and the temperature rises rapidly, so that the solder wetting the pin first. The wetting force between the solder and the pin is much greater than that between the solder and the pad. The upturning of the pin will aggravate the wicking phenomenon. In infrared reflow soldering, the organic flux in PCB substrate and solder is an excellent infrared absorption medium, while the pin can partially reflect infrared. In contrast, the solder melts preferentially, and its wetting force with the pad is greater than that between it and the pin. Therefore, the solder part will rise along the pin, and the probability of wicking is much smaller.

The solution is: during vapor phase reflow welding, SMA should be fully preheated before being put into the vapor phase furnace; Carefully check and ensure the solderability of PCB pad, and PCB with poor solderability shall not be applied and produced; The coplanarity of components can not be ignored, and devices with poor coplanarity should not be used in production.

5、 Analysis and solution of blistering of solder resist film on printed board after reflow soldering

After the printed board assembly is welded, light green bubbles will appear around individual solder joints, and blisters the size of fingernail will appear in serious cases, which will not only affect the appearance quality, but also affect the performance in serious cases. It is one of the common problems in the welding process.

The root cause of blistering of solder resist film is the existence of gas / water vapor between solder resist film and anode substrate. A small amount of gas / water vapor will be entrained into different processes. When encountering high temperature, the gas expands, resulting in the delamination of solder resist film and anode substrate. When welding, the pad temperature is relatively high, so bubbles first appear around the pad.

At present, the processing process often needs to be cleaned and dried before the next process. For example, after etching, the solder mask should be pasted after drying. At this time, if the drying temperature is not enough, water vapor will be entrained into the next process. The storage environment of PCB before processing is not good, the humidity is too high, and there is no timely drying treatment during welding; In the wave soldering process, water containing solder resist is often used. If the PCB preheating temperature is not enough, the water vapor in the flux will enter the interior of the PCB substrate along the hole wall of the through hole, and the water vapor will enter around the pad first. In case of high welding temperature, bubbles will be generated.

The solution is:

(1) All links shall be strictly controlled. The purchased PCB shall be warehoused after inspection. Generally, bubbles shall not appear under standard conditions.

(2) PCB shall be stored in a ventilated and dry environment, and the storage period shall not exceed 6 months;

(3) Before welding, PCB shall be pre baked in the oven at 105 ℃ / 4H ~ 6h;

6、 Analysis and solution of PCB distortion after reflow soldering

PCB distortion is a common problem in SMT production. It will have a considerable impact on assembly and testing. Therefore, this problem should be avoided in production. The causes of PCB distortion are as follows;

(1) The raw materials of PCB are improperly selected, and the Tg of PCB is low. Especially for paper-based PCB, its processing temperature is too high, which will bend the PCB.

(2) Unreasonable PCB design and uneven distribution of components will cause excessive thermal stress of PCB. Connectors and sockets with large shape will also affect the expansion and contraction of PCB, and even permanent distortion.

(3) Double sided PCB, if the copper foil on one side remains too large (such as ground wire). Too little copper foil on the other side will cause uneven shrinkage and deformation on both sides.

(4) Too high temperature in reflow soldering will also cause PCB distortion.

For the above reasons, the solutions are as follows:

When the price and space allow, select the PCB with high Tg or increase the thickness of PCB to obtain the best aspect ratio; Reasonably design the area of copper foil on both sides of PCB, which shall be balanced. The place without circuit shall be covered with steel layer and appear in the form of network, so as to increase the stiffness of PCB. Preheat the PCB before mounting, and the condition is 105 ℃ / 4H; Adjust the clamp or clamping distance to ensure the space for PCB thermal expansion; The welding process temperature shall be reduced as much as possible; When slight distortion has occurred, it can be placed in the positioning fixture, heated and reset to release the stress, and generally satisfactory results will be achieved.

7、 Analysis and solution of open circuit / false soldering of IC pin after reflow soldering

Faulty soldering of some pins after IC pin welding is a common welding defect. There are many reasons. One is the poor coplanarity, especially QFP devices. Due to improper storage, the pins are deformed, which is sometimes difficult to be found (some mounters do not have coplanarity function). Therefore, attention should be paid to the storage of devices. Do not take components or open the package casually. Second, poor solderability of pins. The IC is stored for a long time, the pins are yellow, and poor solderability will also cause false soldering. During production, the solderability of components should be checked, and special attention should be paid to that the storage period should not be too long (within one year from the date of manufacture). During storage, it should not be subject to high temperature and humidity, and the packaging bag should not be opened casually. Third, the solder paste has poor quality, low metal content and poor weldability. The metal content of solder paste usually used for welding QFP devices shall not be less than 90% Fourth, the preheating temperature is too high, which is easy to cause IC pin oxidation and poor solderability. Fifth, the size of the template window is small, so that the amount of solder paste is not enough. Generally, after the template is manufactured, the window size of the template shall be carefully checked, which shall not be too large or too small, and attention shall be paid to matching with the PCB pad size.

8、 Analysis and solution of chip component cracking after reflow soldering

In SMC production, the cracking of chip components is common in multilayer chip capacitors (MLCC), which is mainly caused by effector force and mechanical stress.

(1) For MLCC capacitors, there is a great vulnerability in their structure. Generally, MLCC is composed of multi-layer ceramic capacitors, which has low strength and is extremely resistant to the impact of heat and mechanical force.

(2) In the process of mounting, the suction and discharge height of the z-axis of the mounter, especially for some mounters without z-axis soft landing function, is determined by the thickness of the chip element rather than the pressure sensor, so the tolerance of the thickness of the element will cause cracking.

(3) The warping stress of PCB, especially after welding, is easy to cause cracking of components.

(4) The PCB of some panels will damage components during splitting.

The preventive measures are: carefully adjust the welding process curve, especially the temperature in the preheating zone can not be too low; Carefully adjust the suction and placement height of the z-axis of the Mounter; The warpage of PCB, especially the warpage after welding, should be corrected pertinently. If there is a quality problem of PCB plate, it should be considered emphatically.

Reflow soldering defects of electronic products are divided into main defects, secondary defects and surface defects. Any defect that makes the SMA function of electronic products invalid is called the main defect; Secondary defects refer to the defects with good wetting between solder joints, which will not cause the loss of SMA function of electronic products, but may affect the service life of products; Surface defects refer to those that do not affect the function and service life of the product. In SMT process research and production, we know that reasonable surface assembly technology plays a vital role in controlling and improving SMT product quality.