Properties of solder paste

Panorama of flux technology: from basic functions to high-end applications

1. Key media in soldering systems: the core value of flux

In electronics manufacturing, flux acts as a "multi-functional coordinator" that goes far beyond simple surface cleaning. In modern electronic assembly, thousands of electronic components are often integrated on a single PCB substrate, involving a variety of metals such as copper, brass, nickel, silver, etc., ranging in shape from 0402 micropackages to large BGA chips, and even complex stacked structures containing 4-12 layers. In such complex systems, flux ensures soldering quality through a triple core function:

Surface Purification: Precise removal of 0.5-2nm thick oxide layers (mainly CuO, Cu₂O) and organic contaminants from metal surfaces, creating a clean interface for the combination of solder and substrate

Interface Regulation: Reduces the surface tension of the molten solder from 70-80mN/m to 35-50mN/m, significantly improving wettability (contact angle from > 90° to < 30°)

Thermal field equalization: Compensate for the heat conduction differences between different materials through phase transition and flow characteristics (for example, brass has a thermal conductivity of only 1/3 of copper), and control the temperature difference in the welding area within ±5°C

Test data from an electronics manufacturing company showed that SMT soldering yield was only 62% without flux, but with adapted flux, the yield jumped to 98.5%, which fully confirms the irreplaceability of flux in soldering systems. With the development of lead-free and high-density packaging, the performance requirements of flux have been upgraded from a simple "soldering aid" to a "process stability control core".

2. Heat conduction mechanism: the invisible regulation ability of flux

The essence of the soldering process is the precise distribution of thermal energy at the microscopic scale, and the flux becomes the "intelligent regulator" of the temperature field through its unique thermal response characteristics.

The multi-dimensional role of heat conduction

In multilayer PCB soldering, the difference in heat accumulation in different regions can reach 30-50°C, and the flux is balanced through a three-stage thermal response mechanism: solid-state stage (<60°C): acts as a thermal insulation layer to protect heat-sensitive components and delays heat conduction (thermal conductivity < 0.1W/m·K).Melting stage (60-200°C): During the phase transition from solid softening to completely liquid state, heat is absorbed and transferred, and thermal conductivity is increased to 0.2-0.5 W/m・K, active stage (>200°C): Heat is released through chemical reactions, and its fluidity ensures that heat energy is transferred to the low-temperature regionThis dynamic adjustment capability is particularly critical in batch welding. When the reflow oven heats the entire board, the flux reduces the difference in the heating rate between the copper pad and the plastic package pins from 2:1 to 1.2:1, ensuring that all solder joints reach solder temperature in the same time window.

2.2 Visual diagnosis of temperature distribution

The diffusion form of the flux is a "natural recorder" of the soldering temperature field, and experienced technicians can determine the temperature status by the residue distribution:

High-temperature area characteristics: The flux diffusion range is large (2-3 times the diameter of the low-temperature region), presenting a uniform fan-shaped distribution with clear edges

Characteristics of low-temperature areas: small diffusion range, residue aggregation in clumps, and may be accompanied by incompletely melted particles

Hot Spot Identification: The presence of flux carbonization (brownish-black) in localized areas indicates temperatures exceeding 300°C, potentially leading to substrate damage

In automatic welding, this feature is translated into the basis for optimizing process parameters. A brand of automatic soldering robot can adjust the soldering speed (range 2-10mm/s) and soldering iron temperature (fluctuation ±5°C) in real time by image recognition of the flux diffusion area (accuracy ±0.1mm), improving the soldering consistency of complex PCBs by 40%.

2.3 Chain reaction of heat-related phenomena

Temperature changes trigger a series of physicochemical changes in the flux, forming an interlocking reaction chain:

These changes are particularly noticeable in wave soldering. When the temperature of the flux is too high (>220°C), its viscosity will drop below 100cP, and it is easy to detach from the solder joint under the impact of the solder wave, resulting in poor wetting; However, when the temperature is too low (<180°C), the viscosity remains above 500cP, and the fluidity is poor, unable to cover all pins, forming a false solder. On a wave soldering line, the solder joint defect rate was reduced from 3.2% to 0.8% by stabilizing the fluxtemperature at 200±5°C.

3. Material system and performance optimization: from tradition to innovation

The composition design of flux is the art of balancing functionality, craftsmanship, and reliability, and its evolution reflects technological advancements in electronics manufacturing.

Fine regulation of the traditional rosin system

As the core component of flux (60-80%), the performance of rosin can be optimized by: Grading screening: grading according to the softening point (60-100°C), high softening point rosin (>80°C) suitable for high-temperature soldering (e.g., lead-free process), modification treatment: through hydrogenation or polymerization reactions, the thermal stability of rosin is increased by 30%, and the decomposition temperature is increased from 250°C to more than 280°C, purity control: The acid value of premium rosin should be stable at 160-180mgKOH/g, and the ash content should be < 0.03% to avoid ionic contaminationIn real-world applications, the surface insulation resistance (SIR) of the soldered PCB can be increased by 1-2 orders of magnitude for every grade of rosin purity. An automotive electronics manufacturer used 99.9% pure hydrogenated rosin flux to pass a 1000-hour SIR test at 85°C/85°C/1000% RH (requiring > 10¹⁰Ω).

3.2 Technological breakthrough of halogen-free flux

To meet environmental regulations (e.g. EU RoHS 2.0) and high reliability requirements, halogen-free fluxes have developed a well-established technology system:

Core formula: organic acid activation system (OA content 5-8%), mainly including adipic acid, sebacic acid and other diacids, key indicators: thermal decomposition temperature > 280°C (30-50°C higher than traditional halogen-containing products), ionic residue < 0.1μg/cm² (total amount of Na⁺+K⁺), volume resistivity > 10¹⁰Ω・cm (cured), process fit: Higher activation temperatures (10-20°C higher than halogen-containing products) are required for reflow rather than wave soldering, and test data shows that halogen-free fluxes extend the salt spray test life of BGA solder joints (5% NaCl solution) from 200 hours More than 500 hours, especially suitable for automotive electronics and outdoor equipment.

A leap in performance for nanocomposite fluxes

The introduction of nanotechnology has achieved a qualitative breakthrough in flux performance: adding 2-5% nano alumina (particle size 50-100nm) increases thermal conductivity by 40-60% to 0.8-1.2W/m·K, and the catalytic action of nanoparticles shortens the soldering time by 30%, from the traditional 5-8 seconds to 3-4 secondsPrecise control of IMC layer thickness (±0.2μm) to avoid overgrowth of brittle phases such as Cu₃Sn

In the 0.3mm pitch soldering of 5G modules, nanocomposite fluxes exhibit unique advantages. Through the "bridging" action of nanoparticles, the spread speed of solder in extremely narrow gaps is increased by 50%, and the first-pass rate is increased from 95% to 99.97%, significantly reducing rework costs.

4. Process adaptation and quality control: targeted solutions

There are significant differences in the performance requirements of fluxes for different soldering processes, which require customized matching.

Flux regulation for wave soldering

The core challenge of flux in wave soldering is "dynamic adhesion", which needs to be met at the same time: 120-180cP (25°C) to ensure that it can be applied evenly without being lost under the impact of solder waves, coating amount control: 1.5-3.0g/m², precisely adjusted by foaming ratio (1:10-1:15), pre-baking parameters: 100-120°C/60-90 seconds, Solvent is removed but remains active

An optimization case of a consumer electronics foundry shows that the solid content of wave soldering flux was adjusted from 10% to 8%, and the conveyor belt speed was reduced by 10%, reducing the bridge defect rate from 2.1% to 0.5%, saving 1.2 million yuan in annual repair costs.

4.2 Special requirements for SMT process

In surface mount technology, fluxes (often included in solder paste) need to address the challenge of "no holding force": Anti-monument design: The wetting force of the flux needs to be balanced (< 10% left and right) to prevent small components such as 0402 from lifting due to uneven forces

Slow heating: The heating rate from room temperature to 150°C should be controlled at 1-2°C/s to prevent component shifts caused by rapid flux boiling, and solder paste compatibility: Mixing stability with solder powder (usually SnAgCu alloy, particle size 20-38μm) takes > 24 hoursFor 01005 ultra-miniature components (size 0.4mm×0.2mm), the viscosity of the special flux needs to be precisely controlled at 800-1000cP and the thixotropic index > 3.0 to ensure that the shape remains good after printing, without collapsing or drawing.

Precise control of selective welding

In selective soldering, the "localization" of the flux is crucial: Injection accuracy: flux droplet diameter is controlled at 0.5-1.0mm (error < 0.1mm) to avoid contamination of non-soldered areas, and continuous activity: under local heating conditions, the activity retention time takes > 10 seconds to ensure that the solder can still be effectively wetted when it arrives, and residue characteristics: hardness after curing > 60 Shore D to prevent contamination from subsequent processes to prevent contamination from falling off, a medical device manufacturer used a piezoelectric jetting flux coating system with a low solids content (<2%) formulation to increase the selective soldering yield rate of cardiac monitor PCBs from 92% to 99.5%.

5. Technology trends and future prospects

Flux technology is developing rapidly in the direction of "precision, multi-functional, and green":

1. Intelligent Responsive Flux:

Formulation with temperature grading response characteristics was developed, mainly playing a thermal equalization role at 150-180°C, and focusing on activating surface activity above 200°C, reducing the cold weld defect rate of 0402 components from 1.2% to 0.3%.

2. Bionic structure design:

Mimicking the self-healing properties of biofilms, microencapsulated activators (5-10μm diameter) are added and released on demand during soldering, extending the effective active window of flux to 2 times that of conventional products.

3. Sustainable material system:

Replacing traditional rosin with plant-based rosin (such as masson pine extract) reduces carbon footprint by 40%; Develop degradable activators to achieve natural decomposition after welding and meet the requirements of the EU ecolabel.

4. Digital process integration:

Through machine learning to analyze the spectral characteristics of flux residues (infrared or Raman spectroscopy), the correlation model with soldering quality is established, and the prediction accuracy can reach more than 95%, providing key data support for intelligent manufacturing.

These technological innovations are reshaping the boundaries of flux applications. In the micro-interconnect (<50μm pitch) in the chiplet package, the new flux has achieved 100% pad coverage; In the soldering of automotive-grade power devices, high-temperature flux ensures long-term reliability (> 10,000 cycles) at -40°C~150°C temperature cycles.

epilogue

Flux, as the "invisible steward" of the soldering process, has always resonated with the advancement of electronic manufacturing. From traditional rosin-based formulations to modern nanocomposite systems, from simple auxiliary functions to intelligent process control, every upgrade of flux promotes the improvement of electronic manufacturing accuracy and reliability.

For industry practitioners, a deep understanding of the thermal conductivity characteristics, material behavior, and process adaptability of flux can not only solve quality problems in daily production, but also grasp technical opportunities under trends such as high-density packaging, lead-free, and green manufacturing. In the future, as semiconductor packaging develops in the direction of three-dimensional integration and heterogeneous integration, flux will play a key role at a more microscopic scale (nanoscale) and become an important support for advanced manufacturing technology.

As a senior process engineer said, "If you understand the flux residue, you will understand the story of the entire soldering process." This insight into the details is a vivid embodiment of the spirit of excellence in electronic manufacturing.