How to buy solder wire on the market correctly?

Guide to suitable materials for solder wire and refined soldering processes

1. Scientific classification of solderable materials and solder adaptation

The solderability of a metallic material is essentially the ability of its surface to form intermetallic compounds (IMCs) with molten solder, which is determined by the material's crystal structure, oxidation tendency, and surface energy. The solderability of common metals in electronics manufacturing varies significantly, forming clear application boundaries.

1.1 High-quality weldable material system

These materials can be reliably soldered without special treatment, and their surface oxide film (if present) can be effectively removed by conventional fluxes:

1. Copper and copper alloy family: copper (purity > 99.5%): The surface oxide film is Cu₂O (thickness < 5nm), which can be removed within 3 seconds under the action of rosin-based flux, forming a continuous IMC layer (Cu₆Sn₅).Brass (Cu-Zn alloy): The presence of zinc accelerates oxidation, but grades above 60% copper still maintain good solderability, and medium active flux(RMA grade) is recommended, phosphor bronze (Cu-Sn-P): The tin content of 5-10% increases the tin affinity of the material itself, and the soldering temperature can be reduced by 10-15°C

2. Precious metal materials: gold (purity > 99%): the surface oxide film is extremely thin (<1nm) and unstable, will decompose on its own above 250°C, suitable for bonding and welding of precision electronic components, silver (purity > 92.5%): silver oxide (Ag₂O) is easily reduced at soldering temperature, and the wetting angle can be as low as 15°, But the cost limits it is only used for high-end RF components.

3. Surface treatment metal: tinned copper: tin layer thickness 5-10μm forms a physical barrier to prevent copper substrate oxidation, tin layer and solder fuse to form a homogeneous connection during soldering, suitable for high-frequency connectors, nickel-plated copper: nickel layer (3-5μm) needs active flux (RA grade) to assist soldering, forming a Ni₃Sn₂ IMC layer, corrosion resistance is better than pure copper solder joints 。

1.2 Special treatment scheme for difficult-to-weld metals

These materials require targeted material and process innovation due to the formation of stable oxide films on the surface or their own chemical inertness:

According to the practical data of a new energy battery company, the welding yield rate of aluminum lugs increased from 65% to 98.5% of the traditional method by using special solder (Sn92Zn5Ga3) containing gallium aluminum alloy with ultrasonic welding, and there was no false welding phenomenon after 1000 charge-discharge cycles.

2. Refined welding process control of the whole process

The stability of soldering quality depends on every detail control, from surface preparation to soldering iron extraction, and establishing a standardized operating process is key to achieving a highly reliable connection.

Technical specifications for surface pretreatment of weldments

Surface cleanliness directly determines the welding success rate, and different contaminants need to be treated in a targeted manner: Mechanical method: Use silicon carbide sandpaper (grit size 400-600 mesh) to cross-sand at a 45° angle until the metallic luster is revealed, and then use compressed air to remove debris (pressure 0.3-0.5MPa).Chemical method: Copper is soaked in 5-10% dilute sulfuric acid solution for 30-60 seconds, stainless steel is etched with 10% nitric acid + 2% hydrofluoric acid mixture for 15-30 seconds, and after treatment, it needs to be rinsed with deionized water to pH=7, non-polar oil (mineral oil): ultrasonic cleaning with isopropyl alcohol (purity ≥99.5%) (frequency 40kHz, time 3-5 minutes).

Polar oil stains (fingerprints, flux residues): use acetone or special electronic cleaning agent, wipe with a dust-free cloth (wipes per square centimeter≥3 times), the cleaned weldment should be welded within 1 hour, exposed to air for more than 2 hours needs to be reprocessed, the temporary storage environment needs to be controlled: temperature 20-25°C, relative humidity 30-50%, The concentration of pollutants in the air (ISO level 8 cleanliness), quality traceability in an avionics workshop showed that the incidence of early solder joint failure (< 1000 hours) was reduced from 2.3% to 0.15% after strict implementation of surface treatment specifications.

2.2 Technical points of the pre-welding process

Pre-soldering (tinning) is the quality cornerstone of hand soldering, and at its core is the formation of a uniform and continuous tin layer (thickness 5-15μm):

1. Pre-soldering temperature curve heating stage: 2-3°C/ms rate to solder temperature (280-300°C for leaded solder, 330-350°C for lead-free solder), constant temperature stage: hold for 2-3 seconds (to ensure IMC layer formation), cooling stage: natural cooling to room temperature (to avoid stress concentration caused by forced air cooling).

2. Pre-soldering quality acceptance: Appearance: The tin layer is uniform and bright, no pinholes, bubbles (1 pinhole with a diameter of < 0.1mm per square centimeter is allowed ≤ per square centimeter) Bonding force: quickly peel off after pasting with 3M tape (model 610), the tin layer shedding area < 5%, thickness: detected with X-ray fluorescence thickness gauge (accuracy ±0.1μm), the deviation should be within ±20% of the set value。

3. Pre-soldering of special components: Thermal components (such as electrolytic capacitors): use low heat capacity soldering iron tips (below 2.4mm), pre-soldering time < 1 second, thin leads (diameter < 0.3mm): need to be fixed with tweezers during pre-soldering to prevent lead deformation (bending degree < 5°).

Parameter optimization of the welding process

The soldering process is a precise balance of heat, force, and time, which needs to be dynamically adjusted according to the type of component: the flux release of the rosin core wayar solder should be controlled at 2-3mg/cm (calibrated by the weighing method), and when additional flux is added, the amount per solder joint is 5-10mg for ordinary solder joints, and 2-5mg for precision solder joints (spacing < 0.5mm).It is forbidden to use excessive flux in the following components: relay contacts, microswitches, adjustable resistors (easy to cause poor contact), soldering iron power selection: small components (0402 package) 20-30W, medium components (SOP-16) 30-50W, large components (QFP-100) 50-80W, contact method: soldering iron tip forms a 30-45° angle between the soldering iron tip and the solder part, The contact area ≥ 60% of the cross-sectional area of the soldering iron tip, time control: 3-5 seconds for ordinary solder joints, 2 seconds ≤for heat-sensitive components, and good heat dissipation soldering parts (such as copper grounding tabs) can be extended to 5-7 seconds, and the solder joint volume calculation formula: V=k×d² (k is the coefficient: ordinary solder joints 1.5, dense solder joints 1.2, d is the solder joint diameter),Visual standard: Solder covers 90% of the pad area ≥ no more than 0.2mm at the edge of the pad (avoid bridging).

Soldering iron tip maintenance and condition management

The soldering iron tip is a key medium for heat transfer, and its state directly affects the quality of soldering: Before each use: wipe with a damp sponge to remove the oxide layer, dip a small amount of solder to form a protective film, in use: clean every 5-10 solder joints, and deal with oxidation blackening immediately, after use: dip it in solder in a heated state, and store it after cooling naturally (to prevent air oxidation),  The tip of the soldering iron tip is worn > 0.5mm (affecting the positioning accuracy), the peeling area of the coating (usually iron-nickel alloy) is > 30% (resulting in a decrease in heat transfer efficiency), cracks or deformations appear (regardless of size need to be replaced), long-term discontinuation (> 7 days): need to be disassembled and cleaned, Store after applying anti-rust oil for high-frequency use (> 8 hours a day): Soak for 10 minutes a week with a special cleaner (containing 5% formic acid) to remove internal oxidation.

3. Industrial welding quality standards and verification system

The soldering quality of modern electronics manufacturing needs to meet strict industry standards, forming a complete system from process control to final verification.

3.1 IPC-A-610G quality grade analysis

The standard classifies welding quality into three levels, adapting to different application scenarios:

Class 1 (General Purpose Electronics): Accepts minor pinholes, recesses (depth < 20% of tin layer thickness) Function: Requires only basic electrical connections, no reliability requirements, typical applications: toys, low-cost consumer electronics

Class 2 (dedicated service equipment): full solder joints, no defects affecting reliability (such as cracks, voids > 5%) Reliability: 1000 temperature cycles (-40~85°C) tested, typical applications: office equipment, communication terminals

Class 3 (High Reliability Product): Flawless, contact angle < 25°, solder joint profile according to IPC-782, Reliability: Solder joint resistance change rate < 5% through 2000 temperature cyclesTypical applications: aerospace, medical equipment, automotive safety systems, certification data from an automotive electronics supplier shows that the reliability of Class 3 solder joints reaches 99.99% over the life cycle of the vehicle (15 years / 200,000 km).

Process innovation for welding of special materials

Continuous breakthroughs in welding technology for difficult-to-soldering materials have formed a series of solutions Aluminum substrate welding: Material combination: Sn-Cu-Ni-Ge alloy (melting point 227°C) + fluoride-based flux, process parameters: nitrogen protection (oxygen content < 50ppm), soldering temperature 350±10°C, time 4-5 seconds,Quality verification: Peel strength ≥8N/cm, thermal cycling (-40~125°C) after 500 times No delamination High-Density Interconnect (HDI) Board soldering: Solder selection: SAC305 ultra-fine wayar solder (diameter less than 0.3mm), equipment requirements: precision soldering iron with microscope (positioning accuracy ±0.01mm), detection method: X-ray detection (resolution 5μm), Ensure blind hole filling rate > 95%. Rare earth reinforced solder application: New formulation: Sn96Ag3Cu0.5Re0.5 (Re is mixed rare earth), performance improvement: fatigue life is 3 times that of ordinary SAC305, conductivity is increased by 5%, application case: satellite communication module, performance degradation after 10,000 temperature cycles < 10%

4. Welding defect analysis and prevention system

Quality control of the welding process requires the establishment of a "prevention-detection-improvement" closed-loop management, and the root cause analysis of common defects is as follows:

Control of insufficient and excessive solder

1. Insufficient solder: characteristics: the solder joint is pointed, the pad coverage rate is < 70% Root cause: the solder wire feed is insufficient (< 70% of the calculated value), or the soldering iron is evacuated too early, prevention: use a quantitative wire feeding tool to set the minimum wire feed amount (ordinary solder joints ≥ 0.5mm length).

2. Excessive soldering: Characteristics: solder joints are spherical, beyond the edge of the pad > 0.3mm, root causes: too much wire feed or too long soldering time (> 150% of the recommended value), prevention: use a graduated solder wire (marked every 5mm), train operators to master the "visual quantitation" skill

Identification and prevention of virtual welding and cold welding

1. Virtual welding: microscopic characteristics: IMC layer discontinuity (coverage < 50%), there are obvious gaps, detection method: infrared thermal imager observation (abnormal increase of solder joint temperature > 5°C at working temperature), prevention: ensure that the weldment is clean, and the welding temperature reaches the standard (fluctuation <±10°C).

2. Cold welding: appearance characteristics: the surface is dull, in the shape of tofu dregs, formation mechanism: vibration (amplitude > 5μm) during the solder solidification process, prevention: anti-vibration pads (vibration attenuation rate > 90%) are installed in the welding area, and contact is prohibited during the solidification time

4.3 Solutions for bridging and blowholes

Bridging (short circuit at adjacent solder joints): High incidence areas: Pin tight components with pin spacing < 0.8mm (e.g., QFP, BGA)

Solution: Use a fine diameter solder wire (less than 0.3mm), clean up with a solder belt (width < pin spacing), and optimize the process: the soldering sequence is from the middle to both sides, and the adjacent pins are checked after each soldering

Pores (holes inside solder joints): Cause: Insufficient flux volatilization (low soldering temperature) or oil stains on the surface of the weldment, control standard: pore area < 5% of the total area of the solder joint, and not centrally distributed

Improvement method: Preheat the solder to 100-120°C (accelerated flux volatilization) and optimize the heating curve

By establishing a soldering defect database and root cause analysis (RCA) mechanism, a consumer electronics foundry reduced the soldering defect rate from 3000ppm to 150ppm, saving more than 2 million yuan in annual rework costs.

epilogue

The selection of solder wires and the control of the soldering process are the cornerstones of electronics manufacturing reliability, and their technical requirements increase exponentially with product complexity. From the scientific classification of weldable materials to the optimization of refined welding parameters, every link requires a combination of engineering experience and scientific methods. With the in-depth development of high-density packaging and lead-free technology, the soldering process is evolving in the direction of intelligence (automatic parameter adjustment), green (low VOC flux), and high reliability (long-life solder joints).

For electronic manufacturing practitioners, it is recommended to establish a quality control system of "material-process-environment": select the appropriate solder according to the material of the solder, ensure process consistency through standardized operations, and verify the welding quality with the help of precision testing equipment. Only by controlling the details throughout the entire welding process can the quality of electronic products leap from "working" to "stable working".