Analysis and solution of welding tin bead problem

Systematic analysis and whole-process prevention and control strategy of solder bead defects in electronic welding process

In the wave of continuous upgrading of electronic manufacturing processes, the post-weld "no-clean" process has become a mainstream development direction due to its significant advantages in reducing process steps (2-3 cleaning steps), reducing production costs (saving about 30% annual cleaning agent costs), and reducing volatile pollutant emissions (reducing VOCs emissions by up to 40%). However, this process places demands on weld quality – any visible solder bead defects will result in the product not meeting the no-cleaning standard. These metal balls, usually 0.1-0.5mm in diameter, not only affect the appearance consistency of board-level products, but more seriously, on modern electronic circuit boards with dense components, they may cause short circuits between conductors of different potentials, directly threatening product reliability. The reliability test data of an automotive electronics manufacturer shows that after 1000 temperature cycles (-40°C~125°C), the probability of board failure with tin beads is 8.3 times that of tin-free products.

1. Morphological characteristics and industry standard system of tin beads

The diverse application scenarios in electronics manufacturing have led to differentiated solder bead control standards that reflect the specific requirements of different industries for product reliability:

Quantitative indicators of international common standards

Military Standard (MIL-STD-2000): Adopts the principle of zero tolerance, clearly stating that "no solder beads are allowed", which stems from the high reliability of military equipment in extreme environments (such as nuclear radiation, strong vibration); Electronic Components Standard (IPC-A-610C): Innovatively introduces the "quantity-size" two-dimensional control, allowing less than 5 solder beads per square inch, but strictly limiting the size - the minimum insulation gap is 0.13mm, and solder beads with a diameter less than this value are considered qualified, and corrective measures must be taken if they exceed it; Lead-Free Soldering Standard (IPC-A-610D): Removed quantity restrictions and focused more on the safety margin between solder beads and circuits, emphasizing that "solder beads must not cross or contact conductors of different potentials," a change that reflects the new requirements for process control in lead-free solder properties.

Stringent requirements of special industries

The "zero tin beads in functional areas" is implemented in the automotive electronics (ISO 16949 system) and medical devices (ISO 13485) areas:

Automotive electronics: In the power circuit area of the ECU (electronic control unit) (voltage > 12V), the diameter of the solder beads should be ≤0.1mm, the signal circuit area (voltage < 5V) should be allowed to ≤ 0.15mm, but it must be 0.2mm from the edge of ≥the conductor, and medical devices: implantable devices (such as pacemaker circuits) require "zero solder beads in the whole area", and the diameter of the solder beads in key areas of non-implantable devices (such as sensor interfaces) should not exceed 0.1mm and must pass 1000 Hourly humid-heat test (85°C/85% RH) verifies its stability.

Through 40-fold stereoscopic microscopy, the solder beads in electronic soldering present three typical morphology: spherical solder beads: 0.1-0.3mm in diameter, smooth surface (Ra≤0.5μm), mainly produced in the SMT reflow process, formed by molten solder that has not fully retracted after the collapse of thepasta solder; Irregular spatter beads: mostly ellipsoidal or polyhedral with a diameter of 0.05-0.2mm, often with oxidation spots on the surface (EDS analysis shows SnO₂ content > 5%), common in the contact spatter stage of wave soldering; Contiguous solder beads: composed of 2-5 small solder beads (diameter 0.05-0.1mm) connected with blurred edges, mainly caused by improper manual soldering operation and excessive solder overflow.

2. The cause and prevention and control system of solder beads in the SMT surface mount process

As the core process of high-precision electronic assembly, SMT (Surface Mount Technology) involves multi-dimensional collaboration such as solder paste characteristics, equipment parameters, and environmental control, which is one of the most technically difficult links in electronic manufacturing.

(1) The key influence and control method of solder paste characteristics

Critical value control of metal content

The mass ratio of metal content in solder paste should be strictly controlled at 89-91% (about 50% by volume), which has been verified by numerous experiments to balance printing performance with collapse resistance: for every 1% decrease in metal content, the incidence of solder beads increases by 8-10%. When the metal content is below 88%, the internal void rate of solder paste increases, and the "bubble-splash" effect is easy to form when the solvent volatilizes, resulting in a surge in the number of micro solder beads (< 0.1mm diameter). A comparative experiment by a pasta solder manufacturer showed that the metal content was increased from 88% to 90%, the number of solder beads in the 0402 component area was reduced by 62%, and the solder joint strength (push and shear force) was increased by 15%.

Precise control of oxide content

The thickness of the oxide layer on the surface of the tin powder should be ≤ 3nm (detected by XPS), and when the oxidation degree (DO) exceeds 0.15%, the surface tension of the molten solder will increase by 15-20% (from 0.5N/m to 0.6N/m), resulting in poor wetting: laser particle size analyzer and coulomb method are used to ensure that the oxidation degree ≤ 0.15%; Production practices have shown that nitrogen protection stirring can reduce the oxidation of tin powder by 40%, and an SMT factory reduced the defective rate of solder beads from 1.2% to 0.35%.

Scientific matching of metal powder particle size

According to the IPC-J-STD-005 standard, different spacing components need to match the corresponding particle size of tin powder: Type 5 tin powder (10-25μm) is selected for a pitch of less than 0.4mm; Type 4 (20-38μm) is selected for 0.4-0.8mm pitch;

Experimental data show that in 0.3mm pitch component soldering, the use of Type 5 solder powder reduces solder beads by 55% compared to Type 4, as fine-grained solder powder is more likely to form a uniform coating during printing.

Quantitative evaluation of thermal collapse resistance

In a thermal collapse test at 120°C/30min, the collapse of high-quality solder paste should be ≤ 15% (rate of change in initial height):

By adding polyamide wax thixotropic agent (3-5% addition), the solder paste can maintain structural stability during the preheating stage; A consumer electronics company has verified that the number of solder beads around BGAs is reduced by 70% after using high-collapse solder paste.

(2) Collaborative optimization technology of process parameters

Four-stage regulation of the reflow curve

The temperature-time curve of the reflow soldering process is the core of solder bead prevention and control, and it adopts four-stage precise control: preheating section (60-150°C): heating rate 1-1.5°C/s to remove solvent and avoid violent volatilization (weight loss rate ≤5%/min); Constant temperature section (150-180°C): Maintain for 60-90s to activate the flux and reduce the oxidation of the tin powder; Reflow section (180-245°C): peak temperature 220-240°C (lead-free), keep for 20-30s to ensure complete wetting; Cooling section (245-100°C): cooling rate 2-3°C/s to reduce internal stress.

Experiments by a telecommunications equipment manufacturer showed that reducing the preheating rate from 2°C/s to 1.2°C/s reduced solder paste spatter by 65% and the number of solder beads by 58%.

Golden combination of printing parameters

The optimal printing parameters were determined by DOE experiments: Template thickness: Follow the "component size-spacing-thickness" matching principle: 0402 components (0.4mm spacing) correspond to 0.1mm thickness; 0603 element (0.5mm pitch) corresponds to 0.12mm thickness;

Cut-out design: Adopts a "shrink + anti-solder bead" composite design, the cutout size is 10% smaller than the pad (e.g., a 0.3mm pad corresponds to a 0.27mm cutout), and a 0.05mm rounded corner is added to the edge of the cutout; Squeegee parameters: The pressure of the polyurethane scraper (hardness 80 Shore A) is set to 5-8N and the speed is 20-30mm/s.

A case study from an automotive electronics Tier 1 supplier showed that the solder paste printing accuracy increased from ±0.03mm to ±0.015mm and reduced the incidence of solder beads by 58% after using an optimally designed template.

Dynamic adjustment of placement pressure

Excessive placement pressure will cause excessive extrusion of solder paste, which needs to be accurately set according to the component size: 0402 Components: 20-30g (about 0.2-0.3N); 0603 Element: 30-50g (approx. 0.3-0.5N); QFP/BGA: 50-100g (about 0.5-1.0N), pressure deviation should be ≤± 5g. After introducing a pressure feedback system, a consumer electronics company reduced the number of defective solder beads due to improper placement pressure from 12% to 3%.

(3) Material management and environmental control specifications

Scientific management process of solder paste

Establish the whole process control from storage to use: Refrigeration conditions: 0-10°C constant temperature refrigerator (temperature fluctuation ≤± 1°C), relative humidity ≤ 60%, avoid mixing with other chemicals; Reheating requirements: After taking out of the refrigerator, reheat at room temperature (23±2°C) for at least 4 hours in a sealed state, insufficient reheating will lead to water vapor condensation (25% increase in solder beads for every 1 hour of reheating); Stirring parameters: use a planetary agitator, rotation speed 1500r/min, time 2-3 minutes, ensure the uniformity of pasta solder (viscosity deviation ≤5%); Expiration date management: It should be used within 8 hours after opening, re-stirring if it is not printed for more than 30 minutes, and recycling and refrigeration if it is not printed for more than 1 hour.

Closed-loop control of environmental factors

The workshop environment has a significant impact on the formation of tin beads, and a linkage monitoring system is established:

Temperature control: 25±3°C, every fluctuation of 5°C will cause the viscosity of the solder paste to change by 15%, and the printing pressure needs to be adjusted synchronously; Humidity control: 45-65% RH, too high humidity (>65%) will increase the water absorption of the PCB, and the warm-up time needs to be extended; Cleanliness: Class 10000, HEPA filter is changed regularly (every 3 months).

In the humid region of southern our country, an electronics factory reduced the defective solder beads caused by moisture absorption by 68% by installing a dehumidification system (reducing humidity from 75% to 55%).

3. Solder bead prevention and control technology of wave soldering process

As the mainstream soldering process for through-hole components and mixed circuit boards, wave soldering is closely related to the dynamic flow characteristics of tin liquid, and it is necessary to achieve effective prevention and control through the accurate matching of material selection and equipment parameters.

(1) Analysis of the dynamic process of tin bead formation

High-speed camera technology (1000 frames per second) captures two formation paths of wave solder beads:

1. Contact splash solder beads

When the PCB (temperature 25-35°C) comes into contact with the tin solution (240-260°C), a violent heat exchange is generated within 0.1-0.3 seconds, and the solvent in the flux and the moisture of the plate boil instantly, forming bubbles with an internal pressure of up to 0.3MPa: when the bubble bursts, stin droplets are ejected at a speed of 15-20m/s, of which solder beads with a diameter of ≤0.2mm can splash to a height of 3-5mm; One test showed that 5-8 bubble nuclei could be generated per square centimeter of weld surface, resulting in about 3-5 splashing solder beads.

2. Separate and drag solder beads

When the PCB leaves the solder wave at a 3-5° inclination, the pins lead to form a 4-6mm long tin column: necking fracture occurs when the length exceeds 8-10 times the diameter, producing 2-3 tiny tin droplets; About 40% of the tin droplets do not fall back into the tin cylinder and are stuck to the surface of the PCB by flux residue to form solder beads, usually 0.2-0.5mm in diameter.

Statistics from a home appliance company show that contact spatter solder beads account for 65% of the total, and separation and drag beads account for 35%, and the proportion of the two varies with PCB thickness: 1.6mm thick PCB accounts for 72% of sputter type, while 0.8mm thin PCB drops to 58%.

Key control indicators of flux

1. Scientific ratio of solvent system

The ideal flux should be a three-stage mixed solvent system: low boiling point (60-80°C) accounts for 30-40% (such as ethanol); Medium boiling point (100-120°C) accounts for 50-60% (e.g., propylene glycol methyl ether); High boiling point (150-180°C) ≤ 10% (such as terpineol).

Experiments have confirmed that when the proportion of a single high boiling point solvent exceeds 20%, the residual amount increases after preheating, and the incidence of solder beads increases by 2.3 times.

2. Strict control of moisture content

The flux water content needs to be ≤ 0.5% (Karl Fischer method): for every 0.1% increase in moisture, the number of solder beads increases by 15-20%; Vacuum packed in aluminum foil for storage, with built-in molecular sieve desiccant (water absorption capacity ≥20%), must be used within 4 hours of opening.

Optimization scheme of wave soldering equipment parameters

1. Precise control of the preheating system

Three-stage preheating: 60-80°C in zone 1, 90-110°C in zone 2, 110-130°C in zone 3, total length ≥ 1.5m; Actual temperature monitoring: Use an infrared thermometer (accuracy of ±2°C) to measure the temperature of the PCB solder surface to ensure it reaches 90-110°C; Humidity compensation: When the humidity in the workshop > 60% RH, the preheating temperature is increased by 5-10°C and the time is extended by 10-15 seconds.

2. Coordinate conveying and tin wave parameters

Plate speed: 1.1-1.4m/min in the standard range, for every 0.1m/min increase in speed, the preheating temperature needs to be increased by 3-5°C synchronously; Solder wave height: The main wave height is 1.5-2 times the thickness of the PCB (1.6mm PCB corresponds to 2.4-3.2mm); Chain inclination: 5-6° is the optimal range, where the PCB forms tangential contact with the tin liquid, reducing the contact area by 60% compared to the horizontal state.

Fine adjustment of the air knife system

The angle between the air knife and the PCB is maintained at 10±1°, and the distance is controlled at 10±1cm; Wind speed 30-50m/s to ensure that the flux is evenly coated without excessive residue; Clean the air knife nozzle daily to ensure that the aperture deviation ≤ 0.1mm.

A power adapter manufacturer reduced the defective rate of wave solder beads from 4.2% to 0.8% through parameter optimization, saving about 1.2 million yuan in annual rework costs.

4. Prevention and control and operation specifications of solder beads in the manual welding process

As a supplementary process in electronic manufacturing, manual soldering has a low incidence of tin beads, but the quality fluctuation caused by high operational flexibility is large, and it needs to be effectively controlled through standardized operation and skill training.

(1) Typical causes of manual soldering beads

Video analysis can be summarized into three categories of root causes:

Splash caused by insufficient preheating: Solder is added without sufficient preheating (temperature < 180°C) of the solder joint, and the solder cools instantly when it touches the low surface, producing 0.1-0.3mm splash solder beads, accounting for 60% of manual solder beads;

Excessive solder spillage: The soldering tip stays for too long (>3 seconds) or too much solder wire is added, causing the molten solder to exceed the range of the pad, forming irregular solder beads after cooling, usually 0.2-0.5mm in diameter;

Poor cleaning of the tip: The oxide layer of the tip (thickness > 5μm) leads to a decrease in heat conduction efficiency, and the solder cannot be fully wetted, forming unfused solder bead particles with a diameter of 0.05-0.15mm.

(2) Standardized operation process of manual welding

Establishing a "five-step method" can effectively prevent solder beads from occurring:

1. Preparation stage: select a 30W thermostatic soldering iron (temperature 280±10°C), use a solder wire with rosin core (diameter 0.8-1.0mm);

2. Cleaning stage: Wipe the tip of the soldering iron with a special cleaning sponge to ensure that there is no oxide layer and residual solder;

3. Preheating stage: Place the soldering iron tip on the solder joint for 1-2 seconds to warm up the solder joint temperature to 180-200°C;

4. Soldering stage: Add solder wire from the opposite side of the soldering iron tip, and remove it after the solder evenly wets the solder joint (about 1-2 seconds);

5. Finishing stage: Remove the solder wire first, and then remove the soldering iron tip after 1 second to ensure that the solder is fully solidified.

A medical device company has reduced the defective rate of manual solder beads from 2.5% to 0.3% and the welding pass rate to 99.8% through the implementation of standardized operations.

5. The construction and practical effect of the whole process management and control system

The prevention and control of tin beads requires the establishment of a closed-loop management system of "prevention-detection-improvement" to achieve continuous improvement through systematic measures.

Prevention system: 55 standardized operating specifications

Covering the whole process from raw material entry to finished product inspection, the key control points include: incoming material inspection: the quality of the solder mask of the PCB (3 indicators), the oxidation and viscosity of the pasta solder (5 indicators); Equipment calibration: press squeegee pressure (monthly calibration), reflow oven temperature profile (weekly verification); Operating specifications: Solder paste reheating and stirring (4 steps), stencil cleaning (5 tools).