How to handle solder beads?

Formation mechanism and system prevention and control technology of solder bead defects in wave soldering process

In the large-scale production of electronics manufacturing, wave soldering, as the mainstream soldering process for through-hole components and mixed circuit boards, directly determines the reliability of products. As one of the most common quality problems in the wave soldering process, solder bead defects can lead to a series of serious consequences such as short circuits and reduced insulation performance despite the small size of a single person (usually 0.1–0.8mm in diameter). Statistics from an automotive electronics production line show that solder bead defects generated by the wave soldering process account for 23.7% of the total soldering defects, of which 15% of on-site failures can be traced back to hidden failures caused by solder beads. In-depth analysis of the unique formation mechanism of wave solder beads and the construction of a targeted prevention and control system have important practical value for improving the quality of electronic assembly.

Dynamic formation mechanism of wave solder beads

Different from the "solder paste migration-local solidification" mode of solder beads in the reflow soldering process, the generation of wave solder beads is closely related to the dynamic flow characteristics of the solder liquid, presenting two typical formation paths, and the physical process can be clearly captured by high-speed camera technology (1000 frames per second). The formation process of contact splash solder beads When the circuit board first touches the surface of the tin liquid, the instantaneous heat exchange and material reaction are the key links in the formation of solder beads

Thermal shock stage: There is a temperature difference of more than 200°C between the temperature of the PCB solder surface (usually 25–35°C) and the temperature of the solder solution (240–260°C), so that the solvent in the flux and the adsorbed water of the board quickly reach the boiling point within 0.1–0.3 seconds. Experiments show that 5–8 bubble nuclei can be generated per square centimeter of welding surface, and the internal pressure can reach 0.3MPa. Burst splash stage: When the overpressure bubble breaks through the surface tension constraint of the tin solution, tiny tin droplets will be ejected at a speed of 15–20m/s, of which solder beads with a diameter of ≤0.2mm can splash up to 3–At a height of 5mm, the solder droplets that fall on the non-pad area of the PCB solidify within 0.5 seconds, forming a typical splash solder bead. Morphological characteristics: These solder beads are mostly irregular spherical in shape, with oxidation marks on the surface (SnO₂ film can be seen in SEM observation), mainly distributed within 2mm around the component pins, usually 0.1–0.3mm in diameter.

The formation mechanism of separation and drag solder beads

The interface separation process of the circuit board when it is separated from the solder wave is another important source of solder beads: Tin column stretching stage: When the PCB leaves the solder solution at an inclination angle of 3–5°, a continuous tin column is formed under the guidance of the pins and pads, and its diameter is positively correlated with the pin spacing (0.8mm spacing corresponds to the diameter of the tin post of about 0.5mm), and the stretching length can reach 4–6mm; Fracture retraction stage: When the length of the tin column exceeds the critical value (about 8–10 times the diameter), necking fracture occurs under the action of surface tension, and the fracture position is 2–3mm away from the PCB surface. The high-speed camera shows that 2–3 tin droplets will be produced at the moment of fracture; Fallback attachment stage: About 60% of the detached tin droplets will fall back into the tin cylinder due to the combined action of gravity and surface tension, and the remaining 40% of the solder beads with a diameter of 0.2–0.5mm may be adhered to the surface of the PCB by flux residues, forming separate solder beads. The comparative experiment of a communication equipment company shows that the contact splash solder beads account for 65% of the total, and the separation and drag type accounts for 35%, and the proportion of the two types of solder beads is dynamically adjusted with the change of PCB design and process parameters.

Influence of flux characteristics on solder beads and control strategies

As a key auxiliary material for wave soldering, the composition and performance parameters of flux directly affect the probability of solder beads, and it is necessary to establish a whole process control system from selection, inspection to use.

Influence of solvent system and volatile properties

The boiling point distribution and volatilization rate of the solvent in the flux are the core of controlling the contact splash solder beads: Boiling point matching principle: The ideal flux should adopt a mixed solvent system, with low boiling point components (60–80°C) accounting for 30–40%, which is used for preheating and volatilization in the early stage. The middle boiling point components (100–120°C) accounted for 50–60%, which volatilized in the middle stage of preheating. High boiling point components (150–180°C) ≤ 10% to ensure that they remain active during welding. Experiments have confirmed that when the proportion of a single high boiling point solvent (such as glycol ether, boiling point 135°C) exceeds 20%, the incidence of solder beads increases by 2.3 times. Volatile residue: The solid residue of flux after preheating should be controlled at 8–12% (mass ratio), which will lead to excessive flue gas during welding and increase the probability of bubble formation. By thermogravimetric analysis (TGA), the volatilization should be ≥ 85% at 120°C/60s. Moisture control: The water content of the flux needs to be ≤ 0.5%, and for every 0.1% moisture addition, the number of solder beads increases by 15–20%. It should be packaged in sealed aluminum foil when stored, with built-in desiccant (moisture content ≤3%), and should be used within 4 hours after opening.

Systematic verification method for flux quality

The establishment of scientific incoming inspection standards can effectively prevent batch solder bead problems: Simulation soldering test: use standard test plates (FR-4 material, 100mm×100mm) for actual soldering verification, and require the number of solder beads to be ≤ 3 per plate (diameter ≥0.13mm); Thermal stability test: The flux should be placed in a 120°C oven for 1 hour, and its viscosity change rate should be ≤ 10% (initial viscosity 150–). 250cP), otherwise it indicates that the solvent system is unstable. Supplier management: Suppliers are required to provide a component analysis report (COA) for each batch, focusing on verifying the boiling point distribution and moisture content of solvents. A consumer electronics company has reduced the defective rate of solder beads caused by materials from 8.6% to 2.1% by implementing a strict flux access system.

Collaborative optimization technology for wave soldering process parameters

The prevention and control of wave solder beads requires accurate matching of multiple parameters, and the incidence of solder beads can be significantly reduced by establishing a four-dimensional regulation model of "temperature-speed-angle-airflow". The core of the preheating process is to achieve smooth volatilization of the flux and avoid violent boiling: Temperature gradient design: Adopt three-stage preheating zones (zone 60–80°C, zone 2 90–110°C, zone 3 110–). 130°C), the temperature difference in adjacent areas is ≤30°C, and the total preheating length ≥ 1.5m. Experimental data show that linear heating can reduce bubble generation by 35% compared with step heating. Actual temperature monitoring: Use an infrared thermometer (accuracy ±2°C) to measure the PCB solder surface temperature to ensure a process window of 90–110°C, rather than relying on the device to display the temperature (usually with a 5–8°C deviation). calibrate the thermometer every 2 hours to ensure data accuracy; Humidity compensation mechanism: When the relative humidity of the workshop exceeds 60%, the preheating temperature needs to be increased by 5–10°C, and the preheating time should be extended by 10–15 seconds to compensate for the additional moisture absorbed by the plate.

Matching of conveying speed to tin wave parameters

The coordination of the plate speed and the flow state of the tin liquid is the key to controlling the solder beads:

Speed range selection: The standard speed is set at 1.1–1.4m/min, and the contact time between the PCB and the solder liquid is 3–4 seconds. For every 0.1m/min increase in speed, the preheating temperature should be increased by 3–5°C to ensure full volatilization of the solvent. Solder wave height control: The main wave height should be set to 1.5–2 times the thickness of the PCB (1.6mm thick PCB corresponds to 2.4–3.2mm solder wave), too high will increase the amount of solder liquid carrying, resulting in more sin droplets during separation; Crest stability: Through the pressure monitoring of the tin liquid circulation system (fluctuation ≤ 0.02MPa), ensure that the fluctuation amplitude of the crest surface is ≤ 0.5mm, and the violent fluctuation will increase the incidence of solder beads by 40%.

Precision adjustment of the inclination angle with the air knife system

The slight adjustment of mechanical parameters has a significant effect on the prevention and control of tin beads:

Chain inclination angle optimization: 5–6° is the optimal inclination range, at this time, the PCB forms tangent contact with the tin liquid, the contact area is reduced by 60% compared with the horizontal state, and the amount of bubbles generated is reduced by 55%. The number of tinder beads varies by 20–25% for every ± of 1°. Air knife parameter setting: The angle between the air knife and the PCB is kept at 10±1°, the distance is controlled at 10±1cm, and the wind speed is 30–50m/s. Insufficient wind speed (<25m/s) will lead to excessive flux residue, and high wind speed (>60m/s) may blow away the flux in the pad area, resulting in poor soldering; Air knife cleaning and maintenance: Check the air knife nozzle before production every day to ensure that there is no blockage (pore diameter deviation ≤ 0.1mm), disassemble and clean it once a week to remove the internal flux residue (the main component is rosinate).

A power supply manufacturer optimized the parameter combination through DOE experiments, adjusted the inclination angle from 4° to 5.5°, fine-tuned the wind speed from 40m/s to 45m/s, and reduced the defective rate of solder beads from 3.8% to 0.9%. Based on the formation mechanism and influencing factors of solder beads, a whole-process prevention and control system including prevention, detection and improvement is constructed, which can realize the systematic management of solder bead defects.

Preventive measures at the source

PCB design optimization: set up a 0.1–0.2mm wide solder mask dam on the edge of the pad to reduce solder liquid overflow; When component layout, sensitive areas (such as connector pins) should be kept at a distance of ≥2mm from the large pad; Material matching verification: Each batch of flux and PCB combination needs to be tested for compatibility, focusing on evaluating the generation of solder beads under the standard process;

Environmental control: The workshop maintains a temperature of 23±2°C and a relative humidity of 50±5% to avoid excessive humidity leading to an increase in the water absorption of the board. Online AOI detection: 5 million pixel AOI equipment is installed at the wave soldering outlet, with a detection accuracy of 0.05mm, which can identify more than 99.5% of the solder beads, and the detection speed is synchronized with the production line (1.2m/min); Manual sampling: 3 samples are extracted every hour, and the solder beads in hidden areas (such as the bottom of the component) are inspected under a 20x microscope to make up for the blind spot of AOI detection; Statistical analysis: Establish a solder bead defect database, classify and count according to shift, product model, and process parameters, and start the early warning mechanism when the defect rate exceeds 1%.

Root cause analysis: The fishbone diagram method was used to investigate the reasons for the excess of solder beads from five dimensions: man, machine, material, method and ring, and the typical cases showed that 70% of the solder bead abnormalities were related to parameter deviations. Process discipline management: Formulate standardized operation instructions (SOPs) to clarify the allowable fluctuation range of key parameters such as preheating temperature and plate running speed (±3°C, ±0.1m/min), and parameter changes need to pass the three-level approval. Regular verification: Conduct a process capability analysis (CPK) once a month to ensure that the CPK value of the solder bead defect rate is ≥ 1.33, and implement special improvements when it is below 1.0. Through the implementation of this prevention and control system, a large EMS enterprise has stably controlled the defective rate of wave solder beads below 0.5%, reducing rework costs by about 1.8 million yuan per year, and reducing the failure rate of product sites by 27%.

epilogue

The prevention and control of wave solder beads is a systematic project that takes into account the synergy between material properties, equipment parameters and process environment. With the development of electronic products towards high density and fine spacing, the control difficulty of wave solder beads continues to increase, and in the future, we need to focus on the development of low-volatilization rate fluxes, intelligent parameter adaptive systems (real-time adjustment based on machine vision), and the application of nitrogen protection wave soldering technology. By transforming the solder bead prevention and control technology into standardized process specifications, it can effectively improve the stability of wave soldering quality and provide a solid guarantee for the high-quality development of the electronics manufacturing industry.