Common problems and solutions of SMT solder joints

1. Common defects and solutions in dispensing process

1.1. Drawing/Trailing

1.1.1. Wire drawing/trailing is a common defect in dispensing, which can be caused by small inner diameter of the nozzle, high dispensing pressure, large distance between the nozzle and the PCB, expired or poor quality of the adhesive, high viscosity of the adhesive, failure to recover to room temperature after removal from the refrigerator, and excessive dispensing volume

1.1.2. Solution: Replace the rubber nozzle with a larger inner diameter; Reduce dispensing pressure; Adjust the "stop" height; Change the adhesive and select the appropriate viscosity of the adhesive; After removing the patch adhesive from the refrigerator, it should be restored to room temperature (about 4 hours) before being put into production; Adjust the dispensing amount

1.2. Glue nozzle blockage

1.2.1. The fault phenomenon is that the glue output from the nozzle is too low or there are no glue points coming out. The cause is generally that the needle hole is not completely cleaned; Impurities are mixed in the patch adhesive and there is blockage phenomenon; Incompatible glue mixed together

1.2.2 Solution: Replace with a clean needle; Replace with high-quality patch adhesive; The label of the adhesive should not be mistaken

1.3 Air strike

1.3.1. The phenomenon is that there is only dispensing action, but no glue output. The reason for this is that the adhesive is mixed with bubbles; The rubber nozzle is blocked

1.3.2. Solution: The glue in the injection cylinder should undergo bubble removal treatment (especially for the glue loaded by oneself); Replace the adhesive nozzle

1.4 Component displacement

1.4.1. The phenomenon is that after the adhesive is solidified, the components shift, and in severe cases, the pins of the components are not on the solder pad. The reason for this is that the adhesive output of the adhesive is uneven, such as one more or one less adhesive in two points of the chip component; Component displacement or low initial adhesion of the adhesive during mounting; The PCB is left for too long after dispensing, and the glue is semi cured

1.4.2. Solution: Check if the glue nozzle is blocked and eliminate the phenomenon of uneven glue discharge; Adjust the working status of the SMT machine; Change the glue; The PCB placement time after dispensing should not be too long (less than 4 hours)

1.5. After wave soldering, chips may fall off

1.5.1. The phenomenon is that the bonding strength of the cured components is insufficient, lower than the specified value, and sometimes chips may fall off when touched by hand. The reason for this is due to inadequate curing process parameters, especially insufficient temperature, excessive component size, and high heat absorption; Aging of UV curing lamps; Insufficient glue amount; The component/PCB is contaminated

1.5.2 Solution: Adjust the curing curve, especially increase the curing temperature. Usually, the peak curing temperature of the thermal curing adhesive is around 150 ℃, and failure to reach the peak temperature can easily cause peeling. For the photo curing adhesive, it is necessary to observe whether the photo curing lamp is aging and whether the lamp tube is blackened; The quantity of glue and whether the components/PCB are contaminated should be considered

1.6. Floating/shifting of component pins after solidification

1.6.1. The phenomenon of this fault is that the component pins float or shift after solidification, and the solder material enters the solder pad after wave soldering. In severe cases, short circuits and open circuits may occur. The main reasons for this are uneven patch adhesive, excessive amount of patch adhesive, or component displacement during patch

1.6.2 Solution: Adjust the dispensing process parameters; Control the dispensing amount; Adjust the patch process parameters

2、 Solder Paste Printing and Chip Quality Analysis

Quality Analysis of Solder Paste Printing

There are several common quality issues caused by poor printing of solder paste:

① Insufficient solder paste (partial or even overall) will lead to insufficient solder content at the solder joints of the components after welding, open circuit of the components, component misalignment, and component erection

② Adhesion of solder paste will cause circuit shorting and component misalignment after soldering

③ The overall deviation of solder paste printing will lead to poor soldering of the entire board components, such as lack of tin, open circuit, deviation, vertical parts, etc

④ Pointing solder paste can easily cause short circuits after welding

1. The main factors leading to insufficient solder paste

1.1. During the operation of the printing machine, solder paste was not added in a timely manner

1.2. Abnormal quality of solder paste, mixed with foreign objects such as hard lumps

1.3. The previously unused solder paste has expired and has been reused

1.4. Quality issues with circuit boards, such as inconspicuous covers on solder pads, such as solder resist (green oil) printed on them

1.5. The fixing and clamping of the circuit board in the printing machine are loose

1.6. Uneven thickness of solder paste leakage screen plate

1.7. There are pollutants on the solder paste leakage screen or circuit board (such as PCB packaging, screen wiping paper, foreign objects floating in the ambient air, etc.)

1.8. The solder paste scraper is damaged, and the mesh plate is damaged

1.9 The pressure, angle, speed, demoulding speed and other equipment parameters of the solder paste scraper are not set properly

After the solder paste printing was completed, it was accidentally knocked off due to human factors

2. The main factors causing solder paste adhesion

2.1. Design defects in the circuit board, such as small pad spacing

2.2. Mesh board issue, incorrect hole position

2.3. The mesh plate is not wiped clean

2.4. The problem with the mesh plate caused poor detachment of solder paste

2.5. Poor performance of solder paste, unqualified viscosity and collapse

2.6. The fixing and clamping of the circuit board in the printing machine are loose

2.7 The pressure, angle, speed, demoulding speed and other equipment parameters of the solder paste scraper are not set properly

2.8. After the solder paste printing is completed, it is squeezed and adhered due to human factors

3. The main factors causing overall deviation of solder paste printing

3.1. The positioning reference point on the circuit board is not clear

3.2. The positioning reference point on the circuit board is not aligned with the reference point of the screen board

3.3. The fixing clamp of the circuit board in the printing machine is loose, and the positioning ejector pin is not in place

3.4. The optical positioning system of the printing machine malfunctions

3.5. The solder paste leakage screen board opening does not match the design document of the circuit board

4. The main factors causing printing solder paste to tip

4.1. There are issues with performance parameters such as solder paste viscosity

4.2. There is an issue with the setting of demolding parameters when separating the circuit board from the stencil screen board,

4.3. There are burrs on the hole wall of the stencil plate

Patch quality analysis

Common quality issues with SMT patches include missing parts, side parts, flipped parts, misalignment, and damaged parts

1. The main factors causing missing parts in the patch

1.1. The component feeder is not feeding properly

1.2. The air path of the component suction nozzle is blocked, the suction nozzle is damaged, and the suction nozzle height is incorrect

1.3. The vacuum circuit of the equipment malfunctions and becomes blocked

1.4. The circuit board undergoes poor procurement and deformation

There is no solder paste or too little solder paste on the solder pad of the circuit board

1.6. Component quality issue, inconsistent thickness of the same variety

1.7. There are errors or omissions in the program called by the SMT machine, or the selection of component thickness parameters during programming is incorrect

1.8. Inadvertently touching due to human factors

2. The main factors causing flipping and side parts during SMC resistor mounting

2.1. Abnormal feeding of component feeder

2.2. The suction nozzle height of the mounting head is incorrect

2.3. The height of the gripping material on the mounting head is incorrect

2.4. The loading hole size of the component ribbon is too large, causing the component to flip due to vibration

2.5 Loose materials are placed in the wrong direction when weaving

3. The main factors causing component placement deviation

3.1. When programming the SMT machine, the X-Y axis coordinates of the components are incorrect

3.2. The reason for the SMD suction nozzle is that the suction material is unstable

4. The main factors causing damage during component placement

4.1. The positioning ejector pin is too high, causing the circuit board to be too high and the components to be squeezed during installation

4.2. When programming the SMT machine, the Z-axis coordinates of the components are incorrect

4.3. The suction nozzle spring of the mounting head is stuck

3、 Factors affecting the quality of reflow soldering

1. Factors affecting solder paste

The quality of reflow soldering is influenced by many factors, the most important of which are the temperature curve of the reflow soldering furnace and the composition parameters of the solder paste. The commonly used high-performance reflow soldering furnace can easily and accurately control and adjust the temperature curve. In contrast, in the trend of high-density and miniaturization, the printing of solder paste has become the key to the quality of reflow soldering

The particle shape of solder paste alloy powder is related to the welding quality of narrow spacing devices, and the viscosity and composition of solder paste must also be appropriately selected. In addition, solder paste is generally stored in cold storage and can only be opened after being restored to room temperature. Special attention should be paid to avoid mixing solder paste with water vapor due to temperature difference. If necessary, a mixer should be used to evenly mix solder paste

2. The impact of welding equipment

Sometimes, excessive vibration on the conveyor belt of reflow welding equipment is also one of the factors affecting welding quality

3. The Influence of Reflow Welding Process

After excluding the quality anomalies of solder paste printing and SMT processes, the reflow soldering process itself can also cause the following quality anomalies:

① Cold welding usually involves low reflow temperature or insufficient time in the reflow zone

② The temperature climb speed in the preheating zone of the solder bead is too fast (generally, the slope of temperature rise is less than 3 degrees per second)

③ If the circuit board or components are damp and contain too much moisture, it can easily cause tin explosion and lead to tin connection

④ Cracks are generally caused by rapid temperature drop in the cooling zone (usually with a temperature drop slope of less than 4 degrees per second for lead welding)

4、 SMT welding quality defects

Quality defects and solutions of reflow welding

1. In the process of reflow soldering, the phenomenon of standing up often occurs in chip components. The root cause of the phenomenon is that the wetting force on both sides of the component is unbalanced, resulting in an unbalanced torque on both ends of the component, leading to the occurrence of the phenomenon

The following situations can cause an imbalance in the wetting force on both sides of the component during reflow soldering:

1.1. Unreasonable pad design and layout. If the pad design and layout have the following defects, it will cause an imbalance in the wetting force on both sides of the component

1.1.1. One of the solder pads on both sides of the component is connected to the ground wire, or one side of the solder pad has an area that is too large, resulting in uneven heat capacity at both ends of the solder pad;

1.1.2. The temperature difference on the surface of the PCB is too large, resulting in uneven heat absorption on both sides of the component solder pad;

1.1.3. Large devices such as QFP, BGA, and heat sinks may experience uneven temperature at both ends of the solder pads of small chip components

Solution: Change the pad design and layout