Analysis of flux in soft soldering

In the soft soldering process system of electronic assembly, the wetting quality of solder on substrates is a key factor determining the mechanical strength and electrical performance of solder joints. When molten solder aggregates into spherical shapes on the surface of solid substrates instead of spreading uniformly, it not only significantly reduces the effective contact area of solder joints but may also cause fatal defects such as cold solder joints and bridging. The core cause of poor wetting lies in the surface tension at the interface between the liquid solder and the substrate. As a key functional component in flux, surfactants, due to their unique molecular structure, can significantly reduce interfacial tension, creating thermodynamic conditions for the full spreading of solder. This article will start from the basic theory of surface tension, systematically explain the mechanism of surfactants and their application characteristics in different soft soldering processes, providing technical references for solving wetting issues in actual production.

The Physical Nature of Surface Tension and Solder Joint Interface Behavior

The fundamental cause of surface tension is the force imbalance of molecules at the phase interface. Among the five types of interfaces—gas-liquid, gas-solid, liquid-liquid, etc.—the tension phenomenon at the liquid surface is particularly significant: molecules inside the liquid are subjected to uniform attractive forces from surrounding molecules, maintaining a state of force balance; whereas surface molecules are only attracted by the liquid molecules below, with negligible attraction from the gas molecules above. This force difference causes surface molecules to tend to contract inward, ultimately causing the liquid to exhibit spherical characteristics that minimize surface area (a sphere has the smallest surface area for a given volume). This unit-length force that promotes surface contraction is surface tension (unit: mN/m), and its magnitude is directly related to the intermolecular forces of the substance—the stronger the intermolecular attraction, the greater the surface tension.

In the temperature field of soft soldering (typically 183°C–260°C), the solder undergoes a solid-liquid-solid phase transition, with its wetting behavior mainly occurring in the liquid phase. At this stage, molten solder forms a solid-liquid interface with the substrate surface (such as copper, nickel, and other metals), while the liquid solder forms a gas-liquid interface with the surrounding atmosphere. The tension balance of these two interfaces determines the final wetting morphology. Without any intervention, the natural wetting angle of tin-lead solder (melting point 183°C) on a copper surface is typically greater than 90°, showing obvious non-wetting; whereas lead-free solder (e.g., SnAgCu, melting point 217°C), due to its higher surface tension (approximately 500 mN/m, higher than the 450 mN/m of tin-lead solder), faces even greater wetting difficulty.

Poor wetting manifests in various soldering processes:

In wave soldering, it appears as insufficient pad wetting, with "meniscus missing" at pin roots; in manual soldering, it shows as solder joint spikes, where solder wire fails to spread smoothly after melting; in reflow soldering, it causes "tombstoning" of chip components, where small components like 0402 stand upright due to unbalanced tension at both ends; after solder paste printing, it results in solder balls, where tiny solder particles aggregate due to surface tension to form isolated spheres. The common feature of these defects is the failure of solder to adequately fill the gap between pads and component leads, leading to insufficient effective connection area. Failure analysis data from an automotive electronics company shows that solder joint failures due to poor wetting account for 38% of total failures, with 60% attributable to improper surface tension control.

Molecular Structure of Surfactants and Interfacial Regulation Mechanism

The ability of surfactants to significantly reduce surface tension stems from their unique amphiphilic molecular structure—molecules have both hydrophilic groups (polar parts, such as hydroxyl, carboxyl groups) and hydrophobic groups (non-polar parts, such as hydrocarbon chains, fluorocarbon chains) at their ends. This asymmetric structure allows them to orient at phase interfaces, thereby altering interface properties. Based on the dissociation characteristics of polar groups, they can be classified into four main types: anionic (e.g., carboxylates), cationic (e.g., quaternary ammonium salts), nonionic (e.g., polyoxyethylene ethers), and amphoteric (e.g., betaines). Among these, fluorocarbon nonionic surfactants are most widely used in fluxes due to their high thermal stability.

Their action process can be divided into three stages:

Interface adsorption: When surfactants dissolve in flux solvents (e.g., isopropanol, ethylene glycol monoethyl ether), hydrophobic groups spontaneously migrate to the gas-liquid interface due to the "hydrophobic effect," while hydrophilic groups remain in the liquid phase, forming a monomolecular adsorption layer. At this point, the interface originally dominated by solvent molecules is covered by hydrophobic groups, intermolecular forces weaken, and surface tension rapidly decreases. Micelle formation: When the surfactant concentration reaches the critical micelle concentration (cmc), interface adsorption becomes saturated, and excess molecules spontaneously aggregate in the solution to form micelles—hydrophobic groups form the core inward, while hydrophilic groups face outward to contact the solvent. Micelle formation maintains surface tension at the minimum value (rcmc) and reserves active molecules for subsequent wetting processes. Dynamic equilibrium: In the high-temperature environment of soldering, micelles disintegrate, releasing surfactant molecules that continuously replenish the interface adsorption layer reduced by evaporation and decomposition, ensuring effective surface tension reduction during the critical phase of solder melting (217°C–240°C).

The unique advantages of fluorocarbon surfactants (e.g., perfluorooctyl sulfonate derivatives) include: Hydrophobic groups are fluorocarbon chains (-CF2-), whose intermolecular forces are much smaller than those of hydrocarbon chains, allowing surface tension to be reduced to lower levels (as low as 15 mN/m, whereas hydrocarbon types typically only reduce to 30 mN/m); Excellent chemical stability, with carbon-fluorine bond energy (485 kJ/mol) much higher than carbon-hydrogen bonds (414 kJ/mol), maintaining structural stability even at 260°C soldering temperatures; Good compatibility with flux systems, less likely to undergo chemical reactions with activators (e.g., organic acids, amines), ensuring formulation stability.

Young's Equation can quantitatively describe the effect of surfactants: γsv = γsl + γlv・cosθ, where γsv is the solid-vapor interfacial tension, γsl is the solid-liquid interfacial tension, γlv is the liquid-vapor interfacial tension, and θ is the contact angle. When surfactants reduce γlv and γsl, the cosθ value increases, the contact angle θ decreases, and the solder's spreading ability enhances. Experimental data show that adding 0.1% fluorocarbon surfactant can reduce the contact angle of SnAgCu solder on a copper surface from 65° to 35°, fully meeting the wetting requirements of electronic assembly (θ ≤ 45°).

Application Characteristics of Surfactants in Soft Soldering Scenarios

As the carrier for surfactants, flux formulation design must balance multiple requirements including surface activity, oxide removal, and post-soldering residues. A typical flux consists of five components: solvent (60%–80%, e.g., alcohols, ethers), activator (5%–15%, e.g., organic carboxylic acids, amine hydrohalides), surfactant (0.1%–1%), film-forming agent (5%–20%, e.g., rosin resin), and additives (<5%, e.g., corrosion inhibitors, defoamers). The synergistic effect between surfactants and activators is particularly critical—activators are responsible for removing oxide layers (e.g., CuO, Cu2O) from the substrate surface, while surfactants ensure uniform spreading of the flux on the fresh metal surface and reduce the surface tension of the solder.

During flux preparation, the addition process of surfactants directly affects their dispersion: Dilution treatment: Due to high purity (typically above 95%) and high viscosity, fluorocarbon surfactants need to be diluted with isopropanol to 10% concentration first to avoid local aggregation from direct addition; Addition sequence: Should be added after the activator is completely dissolved, slowly dripped in under stirring conditions, with stirring speed controlled at 300–500 rpm to ensure uniform dispersion; Compatibility testing: Each batch requires thermal stability testing (baking at 120°C for 2 hours), observing for phenomena like stratification or precipitation, and can only be put into production after passing.

Core technical requirements for surfactants in electronic assembly include:

Efficiency: Significantly improves wetting at very low addition levels (0.5‰–1.5‰). One experiment showed that 0.8‰ addition increased solder spread ratio from 60% to 90%, with diminishing returns upon further increase. Thermal stability: Can maintain activity within the solder melting temperature range (217°C–260°C). The ST series fluorocarbon surfactants show only 25% mass loss at 305°C (5°C/min heating rate), fully covering the temperature requirements of lead-free soldering. Low residue characteristics: Removed post-soldering through volatilization or decomposition, avoiding conductive ion formation. Fluorocarbon surfactants gradually decompose above 260°C, with final products being harmless fluorine-containing gases, not forming ionic residues on PCB surfaces (insulation resistance ≥ 10¹¹ Ω). Chemical inertness: Does not adversely react with activators or metal surfaces. Compared to traditional amine surfactants, fluorocarbon types do not undergo displacement reactions with tin powder, effectively preventing solder paste "gelling."

Different soldering processes require different surfactant selections:

Wave soldering flux: Requires rapid spreading to cover large PCB areas, suitable for low-foam surfactants (e.g., silicon-fluorine copolymers), addition amount 0.5‰–1‰;

Solder paste flux: Requires high wettability to ensure fine-pitch solder joint quality, recommended to use high-activity fluorocarbon surfactants, addition amount 1‰–1.5‰;

Manual solder wire core flux: Needs to balance fluidity and wettability, can use hydrocarbon-fluorocarbon composite surfactants to balance cost and performance.

Analysis of Causes of Poor Wetting and Solutions

In actual production, the failure of surfactants to function fully often leads to various wetting defects, which can be attributed to three main categories: materials, process, and environment. Failure Mode and Effects Analysis (FMEA) can establish targeted resolution strategies:

Poor Wetting in DIP Process

Common defects like cold solder joints and pin solder entrapment in wave soldering or hand soldering are 30% related to surfactants: Insufficient dosage: When addition is <0.3‰, the wetting angle of solder at pad edges exceeds 50°, forming "undercut" phenomenon. Increasing to 0.8‰ can achieve a 90% resolution rate; High-temperature decomposition: If preheat temperature is too high (>140°C), surfactants volatilize prematurely, leading to insufficient activity in the soldering zone. Optimizing the preheat curve (120°C±5°C) can increase retention by 40%; Selection error: Using hydrocarbon surfactants with lead-free solder often leads to bridging due to insufficient surface tension reduction. After switching to fluorocarbon types, bridging rate can drop from 2.5% to 0.3%.

"Tombstoning" Phenomenon in SMT Process

Upright defects of small chip components like 0402, 0201, 70% stem from unbalanced surface tension at both ends: Asymmetric pad design: Excessive pad area on one side causes solder volume difference, making it difficult for surfactants to balance tension. Using equal area design with 1.2‰ surfactant can reduce tombstoning rate to below 0.1%; Temperature gradient: When the temperature difference between component ends is >5°C, solder melting times are unsynchronized. Optimizing the reflow profile (temperature difference in constant temperature zone ≤3°C), combined with the uniform spreading effect of surfactants, can eliminate such defects; Uneven solder paste printing: Stencil aperture deviation causes solder volume differences, surfactants cannot compensate for excessive tension differences. Improving printing accuracy (±5μm) is the fundamental solution.

Formation Mechanism of Dewetting

Isolated unwetted spots appear on the solder film, forming "island" defects, with causes including: Substrate contamination: Oil or oxide layers cause local non-wetting, making it difficult for surfactants to spread. Strengthening PCB incoming material cleaning (alcohol ultrasonic + plasma treatment) can increase yield to 99.5%; Gas interference: Flux volatiles or substrate decomposition gases form bubbles, hindering solder contact. Using low volatility rate solvents (e.g., diethylene glycol butyl ether) can reduce gas generation;

Insufficient activator: Unable to completely remove oxide layers, surfactants lose their action basis. Increasing activator content (from 8% to 12%) combined with surfactants can eliminate dewetting.

Wetting Challenges with OSP Treatment

The benzotriazole-type protective film formed by Organic Solderability Preservatives (OSP) (thickness 0.35μm–1.4μm) must balance oxidation resistance and solderability:

Excessive protective film thickness: Activators struggle to completely remove it, preventing surfactants from contacting fresh copper surface. Controlling OSP thickness ≤0.8mm is key;

Thermal shock failure: Multiple reflows cause incomplete decomposition of the protective film, forming carbide residues. Selecting high-temperature resistant OSP (260°C/10s tolerance) can improve;

Activator-surfactant ratio: Need to increase activator (15%) to ensure removal effect, while increasing surfactant (1.5‰) to promote spreading, recommended ratio 100:1. An improvement case from a communication equipment manufacturer showed that by switching from hydrocarbon to fluorocarbon surfactants in the OSP process and adjusting the activator ratio, cold solder joint rate dropped from 3% to 0.2%, with post-soldering insulation resistance maintained above 10¹² Ω.

Technology Development Trends and Application Outlook

The gap in flux technology between domestic and international levels is mainly reflected in basic research of core materials: Domestic companies mostly focus on formulation optimization and cost control, with insufficient investment in molecular design of surfactants; whereas foreign manufacturers (e.g., US Alpha, Japan Senju) have developed third-generation fluorocarbon surfactants with thermal stability up to 330°C (50% mass loss), addition amount reducible to 0.3‰, significantly improving performance and reducing costs. Future development directions include: Multi-functional composites: Combining surface activity with corrosion inhibition and oxidation resistance functions, developing "multi-effect" products to reduce component count; Environmentally friendly: Developing biodegradable fluorocarbon alternatives (e.g., fluorinated polyether derivatives) to reduce ecological risks; Smart response: Designing temperature-sensitive surfactants that release activity at specific soldering stages (e.g., melting peak), reducing preliminary loss. As the "invisible pillar" of soft soldering processes, performance improvement of surfactants directly drives progress in electronic assembly quality. With increasing demands for solder joint reliability in fields like 5G and automotive electronics, in-depth research on the mechanism of surfactants and optimization of their application technology in fluxes will become a key link in the high-quality development of the electronics manufacturing industry. Domestic enterprises need to strengthen basic material R&D, break through core technology bottlenecks, to achieve the leap from "following" to "leading."