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SMT reflow oven soldering possible defects that may occur in daily production

  • Time:2023-07-26
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SMT reflow oven soldering possible defects that may occur in daily production


SMT reflow oven soldering defects can be divided into main defects, secondary defects, and surface defects. Any defect that renders the SMA function ineffective is referred to as the main defect; Minor defects refer to defects where the wetting between solder joints is good and will not cause loss of SMA function, but may affect the product's lifespan. Surface defects refer to those that do not affect the functionality and lifespan of the product, and are influenced by many parameters such as solder paste, paste accuracy, and soldering processes. We are well aware that reasonable surface assembly technology plays a crucial role in controlling and improving the quality of SMT products during our research and production of SMT processes.


1. Tin beads in reflow soldering

(1) The mechanism of tin bead formation in reflow soldering

The solder beads (also known as solder balls) that appear in reflow soldering are often hidden between the sides or fine pitch pins of rectangular chip components. During the component placement process, solder paste is trapped between the pins and pads of the chip component. As the printed circuit board passes through the reflow soldering furnace, the solder paste melts into liquid. If it is poorly wetted with the pads and device pins, the liquid solder particles cannot aggregate into a solder joint. liquid solder will flow out of the weld seam, forming tin beads. Therefore, the poor wettability between solder and solder pads and device pins is the fundamental reason for the formation of solder beads.

In the printing process, due to the offset between the template and the solder pad, if the offset is too large, it will cause the solder paste to flow out of the solder pad, making it easy to produce solder beads after heating. The pressure on the Z-axis during the SMT process is an important cause of solder beads, which is often overlooked. Some SMT machines, due to the Z-axis head being positioned based on the thickness of the component, may cause the component to press the solder bud out of the solder pad at the moment it is attached to the PCB. This part of the tin will obviously cause solder beads. In this case, the size of the tin beads produced is slightly larger, and usually, simply adjusting the height of the z-axis can prevent the formation of tin beads.


(2) Cause analysis and control methods

There are many reasons for poor wettability of solder, and the following are the main reasons and solutions related to relevant processes:

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 rise rate in the preheating zone is too fast and the time is too short, which causes the moisture and rolling agent inside the solder paste to not completely evaporate. When it reaches the backwash soldering temperature zone, it causes the moisture and rolling agent to boil and precipitate tin beads. Practice has proven that controlling the rate of temperature rise in the preheating zone at 1-4 ℃/S is ideal.

If tin beads always appear in the same position, it is necessary to check the design structure of the metal template. The corrosion accuracy of the template opening size does not meet the requirements, and the solder pad size is too large. If the surface material is relatively soft (such as copper templates), it will cause the outer contour of the printed solder paste to be unclear and connected to each other. This situation often occurs when printing solder pads for fine pitch devices, and reflow will inevitably cause a large number of solder beads to be produced between the pins. Therefore, suitable template materials and manufacturing processes should be selected based on the different shapes and center distances of solder pads to ensure the quality of solder paste printing.

If the time from SMT to reflow soldering is too long, due to the oxidation of solder particles in the solder paste, the flux will deteriorate and its activity will be reduced, which will cause the solder paste to not reflow and produce solder beads. Choosing a solder paste with a longer working life (usually at least 4 hours) will reduce this impact.

In addition, insufficient cleaning of printed boards with solder paste misprints can result in solder paste residue on the surface and in the air. When attaching components before reflow soldering, the printed solder paste may deform. These are also the reasons for the formation of tin beads. Therefore, it is necessary to accelerate the sense of responsibility of operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process.


2. Vertical Plate Problem (Manhattan Phenomenon)

One end of a chip component is soldered onto a solder pad, while the other end is raised, a phenomenon known as Manhattan phenomenon. The main reason for this phenomenon is uneven heating at both ends of the component, and the melting of solder paste occurs sequentially. Uneven heating at both ends of the component can occur in the following situations:

The design of component arrangement direction is incorrect. We imagine a reflow soldering limit line that spans across the width of the furnace in the reflow soldering furnace, and once the solder paste passes through it, it will immediately melt. One end of the rectangular component is first limited by reflow soldering, and the solder paste melts first, completely wetting the metal surface of the component end with liquid surface tension. However, the other end has not reached the liquid phase temperature of 183 degrees Celsius, and the solder paste has not melted, only the adhesive force of the flux, which is much smaller than the surface tension of the reflow soldering solder paste, making the unmelted end of the component stand upright. Therefore, it is necessary to ensure that both ends of the component enter the reflow soldering limit line at the same time, so that the solder paste on the solder pads at both ends melts simultaneously, forming a balanced liquid surface tension and keeping the ink position of the component unchanged.

Insufficient preheating of printed circuit components during gas phase soldering. Gas phase is the process of using inert liquid vapor to condense on component pins and PCB pads, releasing heat and melting solder paste. Gas phase soldering is divided into equilibrium zone and steam zone. In the saturated steam zone, the soldering temperature can reach up to 217 ℃. During the production process, we found that if the welded components are not preheated sufficiently and subjected to temperature changes of more than 100 degrees Celsius, the gasification force of gas phase soldering can easily lift up chip components smaller than 1206 package sizes, resulting in the phenomenon of standing pieces. We preheated the welded components at a temperature of 145-150 ℃ in a high and low temperature box for about 1-2 minutes, and finally slowly entered the saturated steam zone for soldering, eliminating the phenomenon of flaking.

The impact of pad design quality. If the size of a pair of solder pads in a chip component is different or asymmetric, it can also cause inconsistent solder paste amount in printing. Small solder pads have a fast temperature response, and the solder paste on them is easy to melt, while large solder pads have the opposite effect. Therefore, when the solder paste on the small solder pads melts, the component is straightened and erected under the surface tension of the solder paste. If the width or gap of the solder pad is too large, it may also cause the phenomenon of chip standing. Strictly following the standard specifications for pad design is a prerequisite for solving this defect.


3. Bridging

Bridging is also one of the common defects in SMT production, which can cause short circuits between components and must be repaired when encountering bridging.

Quality issues with solder paste

The metal content in solder paste is relatively high, especially after a long printing time, which is prone to an increase in metal content; The solder paste has low viscosity and flows out of the solder pad after preheating; Poor slump of solder paste, which flows out of the solder pad after preheating, can cause IC pin bridging.

Printing system

The printing machine has poor repeatability accuracy, uneven alignment, and the solder paste is printed outside the copper clamp, which is more common in fine pitch QFP production; Poor alignment of steel plates and PCBs, as well as incorrect design of steel plate window size/thickness and uneven alloy coating in PCB pad design, can lead to excessive solder paste and cause solder joints. The solution is to adjust the printing machine and improve the coating layer of PCB pads.

Paste and place

Excessive sticking pressure and immersion of solder paste under pressure are common reasons in production, and the Z-axis height should be adjusted. If there is insufficient SMT accuracy, component displacement, and IC pin deformation, improvement should be made based on the cause.

preheat

The heating rate is too fast, and the rolling agent in the solder paste cannot evaporate in time.


4. Material suction/core suction phenomenon

Core suction phenomenon, also known as core pulling phenomenon, is one of the common soldering defects, commonly seen in vapor phase reflow soldering. The phenomenon of core suction can lead to severe virtual soldering when the solder detaches from the solder pads along the pins and between the chip body.

The reason for this is usually believed to be the high thermal conductivity of the original pins, which causes rapid heating, resulting in the solder preferentially wetting the pins. The wetting force between the solder and the pins is much greater than that between the solder and the solder pad, and the upward warping of the pins will further exacerbate the occurrence of core suction phenomenon. In infrared reflow soldering, the organic flux in the PCB substrate and solder is an excellent absorbing medium for infrared radiation, while the pins can partially reflect infrared radiation. In comparison, the solder preferentially melts, and its wetting force with the solder pad is greater than that between it and the pins. Therefore, the solder part will rise along the pins, reducing the probability of core suction phenomenon.

The solution is: during vapor phase reflow soldering, the SMA should be fully preheated before being placed in the vapor phase furnace; Careful inspection and assurance of the solderability of PCB pads should be carried out, and PCBs with poor solderability should not be used in production; The coplanarity of components cannot be ignored, and devices with poor coplanarity should not be used in production.


5. After soldering, the printed solder mask film will blister

After soldering, printed circuit board components may have light green bubbles around individual solder joints, and in severe cases, bubble shaped objects the size of fingernails may appear. This not only affects the appearance quality, but in severe cases, it can also affect performance, making it one of the common problems in soldering processes.

The fundamental reason for the blistering of the solder mask is the presence of gas/water vapor between the solder mask and the anode substrate. Trace amounts of gas/water vapor can be carried into different processes, and when encountering high temperatures, gas expansion leads to delamination between the solder mask and the anode substrate. During soldering, the temperature of the solder pad is relatively high, so bubbles first appear around the solder pad.

Nowadays, the processing process often requires cleaning and drying before proceeding to the next step. For example, after cutting, it should be dried before applying a solder mask. If the drying temperature is not enough, water vapor will be carried into the next step. Poor storage environment before PCB processing, high humidity, and lack of timely drying treatment during soldering; In the wave soldering process, water-based 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 first enter around the solder pad. When encountering high soldering temperatures, these situations will produce bubbles.

The solution is:

Strict control should be exercised over all aspects, and purchased PCBs should be inspected and stored. Under standard circumstances, there should be no bubbles.

PCB should be stored in a ventilated and dry environment, with a storage period not exceeding 6 months;

PCB should be pre baked in an oven at 105 ℃/4H~6H before soldering.


6. PCB distortion

PCB distortion is a common problem in SMT production. It will have a significant impact on assembly and testing, so it should be avoided as much as possible in production. There are several reasons for PCB distortion:

Improper selection of raw materials for PCBs can result in low Tg, especially for paper-based PCBs. If the processing temperature is too high, it can cause the PCB to bend.

Unreasonable PCB design and uneven distribution of components can cause excessive thermal stress on the PCB, and larger connectors and sockets can also affect the expansion and contraction of the PCB, and even permanent distortion.

Double sided PCB, if the copper foil on one side is retained (such as the ground wire). If there is too little copper foil on the other side, it will cause uneven shrinkage and deformation on both sides.

Excessive temperature during reflow soldering can also cause distortion of the PCB.

The solutions for the above reasons are as follows:

When price and space allow, choose a PCB with a higher Tg or increase the thickness of the PCB to achieve the best aspect ratio; Reasonable design of the copper foil area on both sides of the PCB should be balanced, covered with steel layers in areas without circuits, and appear in a network form to increase the stiffness of the PCB. Preheating the PCB before SMT should be carried out under the condition of 105U/4H; Adjust the fixture or clamping distance to ensure the space for PCB thermal expansion; Lower the soldering process temperature as much as possible; When mild distortion has already occurred, it can be placed in the positioning fixture, heated and reset to release stress, and generally satisfactory results will be achieved.


7. Open circuit/virtual soldering of IC pins after soldering

Partial soldering of IC pins after soldering is a common soldering defect, which has many causes. The main reason is poor commonality, especially in QFP devices. Due to improper storage, the pins are deformed and sometimes difficult to detect (some SMT machines do not have the function of surface protection). Therefore, attention should be paid to the storage of the components, and do not take the components or open the packaging casually. The second issue is poor solderability of the pins. Long storage time, yellowing of pins, poor solderability can also cause virtual soldering. During production, the solderability of components should be checked, and special attention should be paid to the storage period not being too long (within one year from the manufacturing date). When storing, it should not be exposed to high temperature and humidity, and packaging bags should not be opened casually. The third reason is poor quality of solder paste, low metal content, and poor solderability. The metal content of solder paste commonly used for QFP devices should not be less than 90%. The fourth issue is that the preheating temperature is too high, which can easily cause oxidation of IC pins and deteriorate solderability. The fifth reason is that the template window size is small, resulting in insufficient solder paste. Usually, after template manufacturing, the window size of the template should be carefully checked, which should not be too large or too small, and attention should be paid to matching the PCB pad size.


8. Cracking of chip components

In SMC production, cracking of chip components is common in multi-layer chip capacitors (MLCC), mainly caused by effect forces and mechanical stresses.

For MLCC capacitors, there is significant structural fragility. Usually, MLCC is made of multi-layer ceramic capacitors, which have low strength and are extremely resistant to heat and mechanical impact.

During the SMT process, the suction and discharge height of the Z-axis of the SMT machine, especially for some SMT machines that do not have Z-axis soft landing function, is determined by the thickness of the chip components rather than pressure sensors. Therefore, the tolerance of component thickness can cause cracking.

The bending stress of PCB, especially after soldering, can easily cause cracking of components.

Some assembled PCBs can damage components during damage sorting

The preventive measure is to carefully adjust the soldering process curve, especially the temperature in the preheating zone should not be too low. When mounting, the suction and discharge height of the Z-axis of the mounting machine should be carefully adjusted; The warpage of PCBs, especially after soldering, should be corrected accordingly. If there are quality issues with the PCB board, special attention should be paid.


9. Other common soldering defects

Poor wettability

Poor wettability is manifested in poor soldering of PCB pads or poor soldering of component pins. Reason for occurrence: The FCB solder pads of the component pins have been oxidized and contaminated; Excessive reflux temperature; The quality of solder paste is poor. Both can lead to poor wettability, and in severe cases, virtual soldering may occur.

Very little tin content

The amount of solder is very small, manifested by incomplete solder joints and small meniscus at the root of IC pins. Cause: The printing template window is small; Lamp core phenomenon (temperature curve difference); Tin paste has low metal content. All of these will result in low tin content and weak solder joint strength.

Damaged pins

Pin damage, manifested as poor coplanarity or bending of device pins, directly affects soldering quality.

Cause of occurrence: Damage during transportation and handling. For this reason, components, especially FQFP, should be carefully stored.

Contaminants cover the solder pads

Contaminants cover the solder pads and occur from time to time during production.

Cause of occurrence: Paper from the site, foreign objects from the tape, manual touch of PCB pads or components, incorrect character printing positions. Therefore, attention should be paid to the cleanliness of the production site during production. The process should be standardized.

Insufficient tin content

Insufficient amount of solder paste is a common phenomenon in production.

Cause: Printing after the first PCB printing machine stopped; Changes in printing process parameters; Steel plate window blockage; The quality of solder paste has deteriorated.

One of the above reasons can lead to insufficient tin content, and the problem should be addressed in a targeted manner

The solder paste is angular in shape

The solder paste is angular in shape, which often occurs during production and is difficult to detect. In severe cases, it may be welded together.

Cause: The lifting speed of the printing machine is too fast; The template hole wall is not smooth, which can easily cause the solder paste to form a treasure like shape.