Where do soldering solder beads come from? How to solve it effectively?

Formation mechanism and systematic prevention and control technology of solder bead defects in SMT production

In the context of the continuous evolution of the electronic information industry towards high density and miniaturization, surface mount technology (SMT) has become the core process support of modern electronic manufacturing. According to the 2024 annual report of the China Electronics Manufacturers Association, the proportion of electronic products using SMT process has reached 93.7%, of which the application coverage of 0201 (0.6mm×0.3mm) and below size components has increased by 58% compared to 2018. However, as component pinout breaks from 0.5mm to 0.3mm or even 0.2mm, the quality risk posed by solder bead defects becomes more pronounced. The production data of a large EMS company shows that in the SMT process of 5G base station motherboards, the rework rate caused by tin beads is as high as 19.2%, which directly causes an average daily loss of 4.2 hours of working hours per production line, equivalent to a production capacity loss of about 230 pieces per day.

1. Technical definition and quality risk assessment of tin beads

As one of the most common defect types in SMT production, the definition of solder beads has evolved from empirical description to standardized quantification. According to the IPC-A-610H (2023 edition) acceptability standard for electronic components, solder beads are clearly defined as independent solder particles formed during reflow soldering that break away from the main solder joint, where particles with a diameter of ≥ 0.13mm or particles with a diameter of < 0.13mm but a spacing of ≤0.1mm from the conductor are considered defective. Combined analysis of a 3D solderpaste inspection system (SPI) with X-ray inspection equipment, solder beads can be divided into three categories according to morphology and risk level:

Low-risk solder beads (Class I): 0.13-0.2mm in diameter, isolated in the non-conductor region, ≥0.2mm from the nearest conductor. These solder beads are acceptable in non-critical areas of consumer electronics, such as PCB edges, but remain unacceptable defects in safety-related circuits in automotive electronics, such as ABS control modules. The reliability test of an automotive electronics manufacturer showed that even Class I solder beads still had a displacement probability of 11.3% after a temperature cycle of -40°C~125°C (1000 times).

Medium risk solder beads (Class II): 0.2-0.3mm diameter, or <0.2mm diameter but 0.1-0.2mm spacing from the conductor. In the power circuit of a communication device (operating voltage > 12V), the probability of such solder beads causing a short circuit is as high as 4.2%. A failure analysis by a base station equipment manufacturer showed that power module failures caused by Class II solder beads accounted for 18.7% of the total failures in 2023.

High-risk solder beads (Class III): > 0.3mm in diameter, or any diameter spacing of < 0.1mm between the solder beads and the conductor, or forming more than 3 clusters. Vibration tests (10-2000Hz, 20G acceleration) at an aerospace research institute showed that Class III solder beads had a 68.5% incidence of short circuits after 100 hours of testing. In 2022, a satellite attitude control system failure event was triggered by a 0.29mm Class III tin bead, resulting in direct economic losses of more than US$130 million.

The hazards of tin beads show significant industry differences: in the field of consumer electronics, visible tin beads mainly affect the appearance quality of products, and a user survey of a smartphone manufacturer shows that visible solder beads will increase the product return rate by 23.5%; In the field of medical electronics, any solder beads in implantable devices are classified as fatal defects, and the ISO 14971 risk assessment standard requires that their incidence must be controlled below 1 in million.

2. The influence mechanism of raw material characteristics on the formation of tin beads

The formation of solder beads is essentially a physicochemical process in which the pasta solder flows and separates under the action of heat, and the microscopic characteristics of the raw materials determine the controllability of this process. Systematic studies of scanning electron microscopy (SEM) and energy spectroscopy (EDS) can reveal the correlation between the parameters of each raw material and the solder beads:

2.1 Key Quality Parameters of PCB Substrates

As the basic carrier of soldering, the design and manufacturing quality of PCB have a fundamental impact on solder bead control. Failure analysis data from a PCB manufacturer showed that solder beads due to PCB problems accounted for 32.6% of total defects:

Geometric constraints of pad design:

Size match: Finite element simulations show that when the pad length of the 0201 component exceeds 0.7mm (component length 0.6mm), the "spillage risk factor" after solder paste printing jumps from 1.2 to 2.9. It is recommended to follow the design guideline of "pad length = component length ×1.05-1.1", and experimental data show that this design can reduce the incidence of solder beads by 42%;

Solder mask morphology: The verticality of the window edge of the laser-cut solder mask should be ≥ 85°, when the verticality is < 75°, it is easy to form a "wedge-shaped accumulation" on the inclined surface after solder paste printing, and the probability of solder beads after reflow increases by 53%;

Surface Roughness: The surface roughness (Ra) of electrolytic nickel-plated gold pads should be controlled at 0.1-0.3μm, and when exceeding 0.5μm, the aggregation of solder paste in the pit will cause local overheating splashes and increase the number of solder beads by 2.1 times.

Sensitivity of PCB Storage Environment:

Humidity adsorption law: Lead-free PCBs can absorb water up to 0.033g/m² when exposed to a 60% RH environment for 24 hours, which is 3.2 times higher than that of a 30% RH environment. According to the IPC/J-STD-033D standard, these PCBs need to be baked at 125°C±5°C for 4 hours, with an 11.5% increase in the incidence of solder beads for each 1 hour of baking;

OSP film age: After 3 months of storage, the film thickness of alkyl benzimidazole OSP film will drop from the initial 0.3μm to less than 0.15μm, at which time the oxidation rate of the pad will be accelerated, resulting in a decrease in the wettability of the solder paste, and the number of solder beads will be 2.4 times that of the new board. OSP plates must be replaced after 3 months of storage due to baking that destroys the organic membrane structure.

2.2 Material science characteristics of solder paste

As the core material for forming solder joints, the formula design and production process of solder paste play a decisive role in the control of solder beads. R&D data from a Japanese solder paste manufacturer shows that optimized solder paste formulations can reduce solder bead defects by 69%:

Grading technology of metal powder:

Particle size distribution: The three-stage scheme of "coarse powder (Type3, 38-53μm) 60% + medium powder (Type4, 20-38μm) 30% + fine powder (Type5, 10-20μm) 10%" was adopted, which reduced the collapse rate by 41% compared with the pasta solder printing of single-particle powder.

Oxidation degree control: The oxidation degree (DO value) of tin powder produced by inert gas atomization method can be controlled below 0.08%, while the DO value of tin powder produced by air atomization method is usually > 0.15%. For every 0.05% increase in DO value, the number of solder beads increases by 28% during reflow;

Sphericity requirements: The sphericity of the tin powder produced by the airflow shaping process can reach 0.95 (ideal value 1.0), which reduces the surface area by 29% compared to the tin powder produced by the mechanical grinding method (0.75 sphericity), thereby reducing the risk of oxidation.

Microstructure design of flux:

Resin system: The composite system of rosin (50%) + modified phenolic resin (30%) + polyamide resin (20%) can be stabilized at 4.5-5.0, which is 36% higher than that of a single rosin system.

Solvent volatilization gradient: Configure mixed solvents with boiling points of 80°C (ethanol, 20%), 140°C (propylene glycol methyl ether, 50%), and 200°C (diethylene glycol butyl ether, 30%), so that the weight loss rate in the preheating stage shows a "slow-fast-stable" change to avoid violent boiling.

Activator concentration: When the content of organic acids (such as adipic acid) is 4.2%, the oxide layer of the pad is removed the best, and the incidence of solder beads is the lowest. Levels above 6% can corrode the PCB substrate, which in turn increases defects.

3. Collaborative optimization system of process parameters

A DOE experiment by a German electronics manufacturer showed that optimizing only a single parameter can reduce solder beads by up to 36%, while multi-parameter co-optimization can achieve a 73% defect reduction:

3.1 Parameter matrix of printing process

As the first process of forming solder joints, solder paste printing directly determines the initial distribution state of solder paste:

Precise design of formwork openings:

Thickness Selection Formula: For pin spacing P(mm), the template thickness T(mm) should meet T=0.2+0.1×P. When P=0.4mm, T=0.24mm is the optimal value, and the deviation of the amount of solder paste can be controlled within ±8%.

Anti-solder bead structure: Adding a 0.08mm "flow limiting groove" in the direction of the long side of the opening corresponding to the 0402 element, the experimental data shows that the solder beads on the side of the element can be reduced by 59%;

Hole wall treatment: The composite process of electropolishing + Teflon coating is used to reduce the friction coefficient of the hole wall from 0.35 to 0.18, and the pasta solder release rate is increased to 98.5%.

Dynamic Control of the Printing Process:

Segmented adjustment of scraper pressure: For 0.12mm thick formwork, the pressure curves of "5N in the starting section→ 6.5N in the stable section→ and 5N in the closing section are adopted, which reduces the edge overflow by 24% compared to the constant pressure;

Release Parameter Optimization: When the printing speed is 25mm/s, the release delay time is set to 80ms, and the release speed adopts a gradient mode of 0.8mm/s (initial) → 2.5mm/s (final section), and the shape retention rate of the solder paste reaches 92%;

Stencil cleaning cycle: Wet erase + vacuum dry erase every 50 PCBs printed, which can control the mesh clogging rate below 0.3% and avoid printing abnormalities caused by residual solder paste.

3.2 Force control technology for component placement

With the miniaturization of component size, the control accuracy of placement pressure has been improved from gram level to milligram level, and the pressure control accuracy of the latest model of a domestic placement machine manufacturer can reach ±1mg:

Grading criteria for pressure parameters:

01005 Components (0.4mm×0.2mm): The placement pressure is strictly controlled at 30-50mg, and the solder paste extrusion is increased by 15% for every 10mg;

LGA package (10mm×10mm): Uses a differentiated pressure pattern of "1.2N in the center area + 0.8N in the edge area" to reduce the solder beads at the edges;

BGA components: The compression amount of solder paste after placement should be controlled at 30%-40% of the initial height, and monitored in real time by laser height sensor.

Six-dimensional control of placement accuracy:

X/Y positioning: Uses vision + laser composite positioning, with a repeatability of up to ±12μm, reducing the risk of deviation by 40% compared to single vision positioning.

θ rotation: controlled within ±0.05°, when the rotation deviation exceeds 0.1°, the incidence of solder beads at the QFP pin increases by 28%;

Z-axis height: The distance between the placement nozzle and the PCB is maintained at 0.15±0.02mm, ensuring that the solder paste is evenly forced.

3.3 Thermodynamic control of reflow soldering

The temperature profile of the reflow oven is the "last mile" of solder bead control, and the KIC RPI furnace temperature tester enables temperature control accuracy of ±1°C:

Four-stage temperature curve optimization:

Preheating section (room temperature→150°C): the heating rate is 1.2°C/s, 70% of the solvent is removed at this stage, and the rapid heating will cause the solvent to boil (boiling point 100-120°C);

Constant temperature section (150°C→180°C): Hold for 75 seconds to fully activate the flux, at which time the solder paste viscosity should be maintained above 60% of the initial value;

Reflow section (180°C→245°C): The time from the liquid phase line (217°C) to the peak temperature (245°C) is controlled at 30 seconds to avoid excessive solder flow;

Cooling section (245°C→100°C): Nitrogen forced cooling at a rate of 2.5°C/s to reduce the thickness deviation of the IMC layer.

Precise control of nitrogen atmosphere:

Oxygen content zone control: the oxygen content in the preheating area is controlled at 1000-1500ppm (conducive to solvent volatilization), and the return area is reduced to 300-500ppm (to improve wettability);

Nitrogen flow distribution: Asymmetric flow design with a top of 30m³/h + bottom of 20m³/h to form stable laminar flow protection;

Energy saving mode: Using low-purity nitrogen with an oxygen content of 1000ppm in the non-precision component area, it saves 42% of operating costs compared to global high-purity nitrogen (300ppm).

4. Environmental control and process management system

The environmental factors and process management level of the SMT workshop directly affect the stability of solder bead control, and a Taiwanese electronics foundry reduced the standard deviation of solder bead defects from 0.82 to 0.35 by implementing digital management:

4.1 Closed-loop control of environmental parameters

Establish an environmental management system with real-time monitoring and automatic adjustment:

Dynamic balance of temperature and humidity:

Temperature control: Adopt inverter air conditioning system to stabilize the workshop temperature at 25±1°C, fluctuating no more than ±0.5°C per hour. Experimental data show that the viscosity of solder paste changes by 8% for every 2°C temperature fluctuation.

Humidity Regulation: Rotor dehumidifier is used during the Huangmei season to strictly control the humidity at 50±5% RH. When the humidity exceeds 65% RH for 2 hours, the water absorption of the PCB will exceed 0.04g/m², and the risk of solder beads will increase significantly.

Temperature and humidity partitioning: The humidity of the printing area is controlled at 45-50% RH (reducing moisture absorption of solder paste), and the reflow soldering area is controlled at 55-60% RH (reducing static electricity), and the area is isolated by the air curtain.

Cleanliness and microenvironment control:

Air cleanliness: Print area reaches Class 1000 (0.5μm particles≤ 35200 pcs/m³), HEPA filter is replaced regularly (replaced when resistance reaches 250Pa);

Local purification: Set up FFU (fan filter unit) at the 01005 component placement station, and control the wind speed at 0.45m/s to form a vertical one-way flow.

Electrostatic protection: The ground impedance is controlled at 10^6-10^9Ω, and the operator wears an anti-static bracelet (impedance 10^6-10^8Ω), and conducts an electrostatic test before going to work every day.

4.2 Digital transformation of process management

Realize the full process traceability of solder bead-related parameters through the Manufacturing Execution System (MES):

Solder paste lifecycle management:

Intelligent storage: Refrigerator with RFID tag is used to automatically record the warehousing time and batch information of pasta solder, and automatically warn 48 hours before expiration;

Temperature recovery monitoring: The temperature and humidity sensor and timer are built into the temperature cabinet, and the solder paste that has not reached the 4-hour temperature recovery time cannot be left the warehouse, eliminating human error.

Usage tracking: Each solder paste is bound with a unique QR code, which records the printing machine, usage time, remaining quantity and other information, and is automatically locked for more than 8 hours.

Preventive maintenance of equipment:

Printing machine: check the scraper wear amount (≤0.02mm) every day, calibrate the stencil tension (25-30N/cm) every week, and replace the transmission belt every month;

Placement machine: replace the nozzle every 2 million times after placement, calibrate the positioning accuracy of the servo motor every quarter, and carry out screw lubrication and maintenance every year;

Reflow welding furnace: use special test strips to detect the actual temperature of each temperature zone every week, clean the surface area of the heating tube every month to carbon, and replace the circulation fan bearing every six months.

Three-level verification of quality inspection:

First article inspection: AOI+X-Ray joint inspection is used to verify the solder beads between the bottom of the BGA and the QFP pin, and the first product can be mass-produced only if it is qualified.

Process sampling: 5 PCBs are extracted every hour, and the thickness of the solder paste is measured by 3D SPI (deviation ≤ 10%), and the abnormality is immediately stopped and adjusted;

Failure analysis: Slice analysis of defective solder beads, observe the cross-sectional structure of the solder beads through SEM, and analyze the components of EDS to determine the root cause.

5. Industry application cases and technological innovation

There are significant differences in the control requirements for tin beads in different industries, requiring targeted solutions:

Automotive electronics (ISO 16949 system):

Implement the "zero solder bead" control standard, use laser solder paste printing technology in key areas of the ECU motherboard (such as the periphery of the MCU), and achieve a position accuracy of ±5μm;

Application Results: A Tier1 supplier's engine control module solder bead defect rate was reduced from 2.1% to 0.15%, with zero failure through a 1000-hour vibration test (10-2000Hz).

Medical devices (ISO 13485 system):

For implantable medical devices, the composite process of nitrogen reflux + vacuum reflux is used to reduce the incidence of solder beads by 91%;

Validation data: The pacemaker circuit was tested at 85°C/85% RH for 1000 hours without any solder bead migration.

Technological innovation direction:

Intelligent prediction: Analyze the historical data of SPI and AOI through machine learning algorithms to establish a solder bead defect prediction model with an accuracy rate of 89%.

Material breakthrough: research and development of self-healing pasta solder, adding microencapsulated flux, when the formation of solder beads is detected, automatically released, the repair rate can reach 72%;

Process Innovation: Uses laser reflow soldering technology to achieve local precise heating, reducing solder beads by 56% compared to traditional hot air reflow.

Solder bead control is a systematic project that requires the entire chain of coordination, from PCB design, material selection, equipment parameters, to environmental management. With the continuous improvement of the precision requirements of electronic manufacturing, the prevention and control technology of tin beads will continue to develop in the direction of intelligence and refinement