What materials can be soldered by solder wire and the details to pay attention to

Full analysis of solder wire technology: from composition properties to application practice

Introduction: The material cornerstone of electronic connections

In the sophisticated world of electronics manufacturing, solder wires act as "bridge architects" at the microscale, connecting electronic components to circuit boards through metal bonding. According to data from the China Electronic Materials Industry Association in 2023, the annual consumption of global wayar solderreaches 180,000 tons, of which the Chinese market accounts for 58%, and is widely used in consumer electronics, automotive electronics, industrial control and other fields. From 0.3mm pitch chip soldering on smartphone motherboards to power device connections in large server power supplies, the performance of solder wires directly determines the reliability of electronics – statistics show that 62% of early electronic device failures are related to soldering quality.

With the full implementation of the EU RoHS 2.0 directive and the deepening of China's "Administrative Measures for the Control of Pollution of Electronic Information Products", solder materials have completed the industrial transformation from leaded to lead-free, which not only involves the adjustment of material formulas, but also promotes the upgrading of soldering processes, equipment and even the entire electronic manufacturing system. This article will systematically analyze the material composition, performance differences, purchase standards and safety specifications of solder wires, and provide comprehensive technical guidance for electronic manufacturing and maintenance practitioners.

1. The material composition and scientific nature of solder wire

The excellence of the solder wire stems from its exquisite composite structure design, which achieves the triple function of "wetting, joining, and stabilizing" through the synergy of metal alloys and functional additives.

1.1 Scientific analysis of core ingredients

The solder wire adopts a two-component structure of "alloy core + flux", of which alloy accounts for 90-95% andflux accounts for 5-10%:

Metal Alloy System: As the main body of the conductive connection, its composition determines the melting point, strength, and conductivity of the solder. As a base element, tin (Sn) has a unique low melting point (231.9°C) and excellent ductility (elongation > 30%), which can form a continuous metal bonding layer under moderate heating. By adding other metal elements, the alloy properties can be precisely adjusted: Lead (Pb): A key component of traditional lead-containing sold, which can reduce the melting point to 183°C (eutectic point), but is restricted by the EU RoHS directive due to its biological toxicity (LD50=30mg/kg), silver (Ag): an important alloying element for lead-free solder, adding 3% silver can increase the tensile strength by 20% and reduce the interfacial resistance by 15%, Copper (Cu): The core component of tin-copper lead-free solders, 0.7% copper content can effectively inhibit the growth of "tin whiskers" in solder joints

Flux system: a functional material hidden in the center of the alloy core that guarantees weld quality through a "physical-chemical" dual action. High-quality fluxes usually contain: rosin resin (60-70%): provides basic viscosity and oxidation capacity, activator (5-10%): organic acid or organic amine salt, removes oxide film on the metal surface, solvent (20-30%): controls viscosity to ensure uniform distribution, additives (1-5%): includes corrosion inhibitors, surfactants, etc., Optimized wetting performance.

Scanning electron microscopy (SEM) observation shows that the flux distribution uniformity of high-quality solderwire should be > 95%, and the cross-sectional bubble rate should be < 3%, otherwise it will lead to splashing and voiding during soldering.

1.2 Material properties and historical evolution of tin

Tin, as the foundational element of solder wire, has unique physicochemical properties that make it ideal for electronic connections:

1. Physical characteristics: appearance: silvery-white metal with a slight bluish tint, fresh cross-section with mirror luster, crystal structure: body-centered cubic structure (β-Sn) at room temperature, and transformed into diamond-shaped structure (α-Sn) below 13.2°C, but this phase change will cause volume expansion by 27%, which needs to be inhibited by alloying, and conductivity: conductivity is 9.17× 10⁶ S/m, which is about 15% of copper, but sufficient for electronic connection needs

2. Chemical stability: It can form a dense oxide film (SnO₂) with a thickness of about 3-5nm in dry air, preventing further oxidation. It is corrosive resistant to water and dilute acids, but is dissolved by concentrated hydrochloric acid and strong alkalis. Intermetallic compounds (IMCs) are formed with most metals, which are the microscopic basis for reliable welding.

The history of tin application is almost synchronized with the history of human civilization: 3000 BC: Ancient Egyptians made bronze artifacts from tin and copper alloys, Zhou Dynasty period (1046-256 BC): Pewter has been widely used in China, and archaeological discoveries such as tin pots and tin tripods have a tin purity of more than 95%, and late 19th century: With the invention of electron tubes, tin-lead alloys began to be used in electronic welding In the 80s of the 20th century: the emergence of refined solder wires promoted the development of surface mount technology (SMT), and at the beginning of the 21st century: the wave of lead-free gave birth to new alloy systems such as tin, silver, and copper, and the application of modern tin showed obvious industry differentiation: the electronics industry consumed 52%, the packaging industry (mainly tinned steel plates) accounted for 28%, and other fields (such as chemicals, Automotive) accounted for 20%.

2. Comparison of classification system and performance of solder wire

Based on alloy composition and environmental protection standards, solder wires have formed a clear classification system, with significant differences in performance, process adaptability, and application scenarios among various categories.

Leaded solder wire: the last position of traditional technology

Despite environmental regulatory constraints, leaded wayar solderis still used in specific fields, and its core advantage lies in its proven process properties: Typical alloy ratios: Sn63Pb37: eutectic alloy with precise melting point of 183°C, no mushy zones during solidification, preferred for manual welding, Sn60Pb40: melting point 183-190°C, slightly less fluid but less costly, suitable for coarse wire diameter (>1mm) applications,Sn50Pb50: melting point 183-216°C, high strength, used in power equipment and other occasions that require mechanical strength, wettability: the wetting angle on the copper surface is < 25°, which is much better than most lead-free alloys, operating temperature: the soldering iron temperature is usually set at 280-320°C, which is less thermal damage to the PCB, Mechanical properties: Tensile strength of about 50-60MPa, elongation of 15-20%, application restrictions: Environmental regulations: EU RoHS directive allows exemptions for medical devices, aerospace, and other fields, market share: from 90% in 2006 to 15% in 2023, mainly in the maintenance and military industriesThe test data of a military electronics company shows that in the temperature cycling test of -55°C~125°C, the fatigue life of Sn63Pb37 solder joints is 1.8 times that of SAC305 lead-free solder joints, which is also an important reason why it is still retained in extreme environment equipment.

2.2 Lead-free solder wire: the mainstream technology in the era of environmental protection

Lead-free solder wire has formed a multi-series alloy system, which is optimized for different application scenarios:

Tin, silver, copper (SAC) series: representative models: SAC305 (Sn96.5Ag3.0Cu0.5), SAC0307 (Sn99.0Ag0.3Cu0.7) Melting point range: 217-220°C (SAC305), 217-226°C (SAC0307) Performance characteristics: The higher the silver content, the better the strength but also the higher the cost. SAC305 has a tensile strength of 75MPa, making it suitable for high-end products such as smartphones; SAC0307 30% lower cost for cost-sensitive areas such as consumer electronics

Tin Copper Series: Standard Model: Sn99.3Cu0.7, Melting Point: 227°C, Performance Characteristics: The cost is only 60% of SAC305, but the wettability is poor (wetting angle > 35°), and it needs to be used with more active fluxes, suitable for cost-sensitive high-volume products such as LED strips

Special Function Series: Low Temperature Lead-Free: Sn42Bi58 (Melting Point 138°C) for flexible PCBs and heat-sensitive components, High Reliability: Sn95Sb5 (Melting Point 232°C), Tensile Strength up to 80MPa, for automotive electronics, Fast soldering: Sn96Ag4 (Melting Point 221°C), 20% faster wetting speed than SAC305

The process challenges of lead-free solder wire are mainly reflected in the increasing soldering temperature of 30-50°C, higher requirements for the heat resistance of PCBs and components, a decrease in wettability of about 30%, the need to optimize flux formulation and soldering parameters, increased solder joint brittleness, accelerated growth rate of the interface IMC layer, and strict control of soldering time

2.3 Performance Comparison and Selection Guide

Through the quantitative comparison of key parameters, a scientific selection basis can be established:

When selecting, it is necessary to consider comprehensively:

Product environmental protection requirements: Export to the EU must comply with RoHS 2.0 (2011/65/EU)

Working environment: Automotive electronics need to pass the temperature cycle test of - 40°C~125°C (≥1000 times)

Production efficiency: Priority is given to alloy systems with fast wetting speed for mass production

Cost structure: Fluctuations in silver prices have a significant impact on the cost of the SAC series, so you need to pay attention to market conditions

3. Safety protection and health management system

The safety of solder operations is often underestimated, and in fact involves multiple risks such as metal exposure, chemical volatiles, and physical injuries, requiring the establishment of a systematic protection system.

Lead exposure risks and control measures

The main risk of leaded solder comes from the toxicity of lead (it is a cumulative poison, half-life of about 30 days): Inhalation: Although the evaporation temperature of lead (1749°C) is much higher than the operating temperature, lead microparticle aerosols (particle size < 5μm) may form during soldering, skin contact: ingestion when contaminated fingers touch food or smokeDigestive tract: Lead residues from work surfaces and tools are ingested through diet, acute poisoning: symptoms such as abdominal pain and anemia at blood lead concentrations > 400μg/L, chronic effects: long-term low-dose exposure (blood lead > 100μg/L) damage to the nervous system and reproductive system, occupational standard: Chinese GBZ 2.1-2019 The limit for lead smoke in the workplace is set at 0.03mg/m³

Protective measures:

1. Engineering control: local exhaust system: set up an air suction hood 30cm above the working point of the soldering iron, wind speed ≥0.5m/s, air purification: welding fume purifier equipped with HEPA filter, the number of air changes per hour≥10 times Personal protection: respiratory protection: N95 or above dust mask, change every 4 hours,Hand protection: Chemical-resistant gloves (nitrile recommended) to check for breakage every 2 hours

2. Management measures: prohibit eating, drinking, smoking in the work area, set up a special hand washing area (equipped with lead removal hand sanitizer containing EDTA), regular testing: blood lead test every six months, new employees need to take baseline measurements before taking up their posts, a rectification case of an electronics foundry shows that after the implementation of the above measures, the lead concentration in the air in the workshop dropped from 0.05mg/m³ 0.01mg/m³, and the average blood lead level of employees decreased by 62%.

Health risks of lead-free solder

Lead-free solder is not completely harmless, and its risks mainly come from flux volatiles: Rosin-based volatiles: Main components: terpenes (such as α-pinene), pungent odor, health effects: long-term exposure may trigger respiratory irritation and allergic reactions (occurrence of about 5-8%)

, control criteria: The ACGIH recommends an 8-hour time-weighted average (TWA) of 50mg/m³ in the United States. Active additives: Organic acid activators (such as adipic acid) may cause skin irritation, halogen-containing compounds (such as ammonium chloride) may produce corrosive gases at high temperatures, environmental requirements: EU REACH regulation restricts the use of certain brominated flame retardants

Protection suggestions: Ventilation requirements: Even if lead-free solder is used, it is still necessary to maintain an exhaust air speed of more than 0.3m/s, personal protection: For sensitive people, it is recommended to use gas masks with activated carbon, and material selection: preferential choice of "halogen-free" flux (chlorine + bromine content < 900ppm).

4. Purchasing standards and quality evaluation system

The identification of solder wire quality requires multi-dimensional evaluation, and the establishment of scientific purchasing standards can significantly improve the welding quality and production efficiency.

Quantitative evaluation of appearance quality

Quality grade can be preliminarily judged by visual and tactile inspection: Quality indicators: smooth surface, uniform metallic luster, no oxidation spots (1 spot < 0.5mm diameter per meter is allowed ≤), oxidation blackening: indicates improper storage or expiration (oxidation depth > 5μm will seriously affect soldering), flux bleeding: white or yellowish crystals are visible, Poor sealing (bleeding length > 5mm is a serious defect), uneven wire diameter: measured with a micrometer, the deviation of high-quality products should be <±0.02mm (20μm), ductility test: 0.8mm diameter tin wire can be bent 180° for at least 5 times without breaking (the characteristic of tin content > 95%),Wipe 10cm length with white filter paper, leaving no visible marks (grayscale value < 30, measured with a colorimeter).

High-magnification microscopy (50x) inspection should meet the following requirements: surface scratch depth < 2μm, no visible pits (> 5μm diameter pit ≤ 1 per centimeter), flux core position deviation of < 0.1mm (to ensure uniform flux release during soldering).

Core indicators of welding performance

Actual soldering performance is the ultimate criterion for evaluating the quality of the solder wire: wettability test (refer to IPC-TM-650 2.4.12): Test method: 0.5g of solder is placed on a clean copper-plated specimen and heated with a 350°C soldering iron for 5 seconds, qualifying criteria: wetting area ≥80%, wetting time < 2 seconds (time from contact to complete wetting),Poor wetting: The solder is spherical (contact angle > 90°), indicating insufficient flux activity or insufficient tin purity with uneven expansion: This may be due to uneven flux distribution or segregation of alloy composition

Process Adaptability: Splash rate: High quality wayar solder When soldering at 350°C, the spatter particles (diameter > 0.1mm) should be < 3 / solder joint, Residue: The residue after soldering should be easy to remove (more than 95% can be removed by wiping it 2 times with isopropyl alcohol), Solder joint appearance: bright and uniform, no pinholes (allow ≤ 1 pinhole with a diameter of < 0.1mm per solder joint)

Reliability verification: Temperature cycle test: -40°C~125°C, no cracks on the solder joint after 1000 cycles, humidity and heat test: 85°C/85% RH for 1000 hours, solder joint resistance change rate < 10%

Specification requirements for packaging and labeling

The packaging of a regular product should contain complete information: alloy composition (e.g., Sn63Pb37 or SAC305) and exact ratio, flux type (e.g., RA type, RMA type) and content (%). Melting point range or eutectic point temperature, wire diameter (mm or AWG gauge) and net weight, production date and shelf life (typically 12 months).

In the next five years, the solder materials market will show the following trends:

The global market size is expected to grow from $2.8 billion in 2023 to $3.9 billion by 2028 (CAGR 6.8%)

Demand surges in automotive electronics, accounting for 35% of total solder usage

The annual growth rate of solder paste products reached 9.2%, much higher than the 3.5% of traditional welding wires