Analysis of process parameters and common problems of wave peak furnace

Optimization of Wave Soldering Process Parameters and Solutions to Common Defects

In the mass production of the electronics manufacturing industry, wave soldering machines serve as core equipment for through-hole component soldering, where precise control of process parameters directly determines soldering quality and production efficiency. The wave soldering process encompasses multiple stages including flux application, preheating, molten solder immersion, and cooling solidification, with complex coupling relationships existing among the parameters of each stage—any minor adjustment in parameters may trigger chain reactions, leading to significant changes in solder joint morphology, strength, and reliability. Based on production practice data from thousands of batches, this article systematically outlines the wave soldering machine's process parameter system, solder wave characteristics, and strategies for addressing typical defects, providing actionable process optimization pathways for electronic assembly enterprises.

Flux Application Process and Solder Wave Profile Design

As a key auxiliary material in wave soldering, the application quality of flux directly affects the wetting effect in subsequent soldering. Depending on equipment structure and process requirements, mainstream application methods can be categorized into three types: foam, spray, and jet, each with distinct technical characteristics and applicable scenarios:

Foam Flux Application Process

This process uses compressed air to form uniform foam from flux in a foam chamber, with PCB coating achieved upon contact with the foam. Its core control point lies in the dynamic balance of flux concentration—due to the volatilization rate of alcohol solvents (such as isopropanol) in production environments reaching 0.5–1.2 g/h (at 25℃), failure to replenish diluent in a timely manner can result in a 5% concentration increase leading to a 30% rise in post-soldering residue, accompanied by yellowing on the PCB surface. In practice, specific gravity should be monitored with a hydrometer every 2 hours, and when concentration exceeds the standard value by 1.2%, specialized diluent (typically a mixture of ethanol and ethylene glycol monoethyl ether) should be added at a 1:10 ratio. Maintenance of the foam chamber is equally critical: weekly disassembly and cleaning of the foam tube are required to remove flux residue (primarily composed of rosin acid metal salts) adhering to the inner walls; otherwise, uneven foam formation with large bubbles > 3 mm in diameter may occur, causing coating amount fluctuations of ±20% or more. Practices at a consumer electronics company have shown that standardized foam system maintenance can increase solder joint qualification rates by 4.5%.

Spray Flux Application Process

This method employs pneumatic atomizing nozzles to uniformly sprayflux onto the PCB surface in the form of micron-sized droplets (5–50 mm in diameter), suitable for high-density, fine-pitch complex boards. Compared to the foam process, its advantages include: high application precision with film thickness control deviation ≤5 mm, meeting soldering requirements for 0.8 mm pitch components; high material utilization reaching over 85%, saving 30% flux compared to the foam process; and superior environmental performance with sealed pipeline design reducing VOC emissions by 60%.

The spray process has special requirements for flux characteristics: solid content must be ≤5% (rosin resin content ≤3%), otherwise nozzle clogging is likely (clogging rate is positively correlated with solid content, R²=0.91); viscosity should be controlled between 15–25 cP (at 25℃), as higher viscosity leads to poor atomization and droplet accumulation. An automotive electronics manufacturer reduced bridging defect rates from 2.1% to 0.3% by selecting specialized spray flux.

Jet Flux Application Process

This process uses a pressure pump to directly jet flux from small nozzles (0.3–0.5 mm diameter) to specified areas, previously used for localized application scenarios. However, due to inherent drawbacks—nozzle wear leading to application deviation (±0.1 mm per month), material waste rates as high as 40%, and high maintenance costs (30% nozzle replacement monthly)—its current market share is less than 5%, gradually being replaced by spray processes.

Rational selection of solder wave profile must match the component distribution characteristics of the PCB:

Single Wave System: Molten solder forms a single arched wave (height 15–25 mm) via a flow guide plate, suitable for pure through-hole PCBs or simple mixed-technology boards. Its advantages include simple structure, convenient maintenance, and 15–20% less dross generation compared to dual wave systems;

Dual Wave System: Comprises a composite wave of turbulent wave and smooth wave. The first wave (turbulent wave) height is 20–30 mm, ensuring solder fill for through-hole components via intense solder agitation; the second wave (smooth wave) height is 10–15 mm with stable flow, used to eliminate bridging and shape solder joints. In mixed-technology board soldering including SMT components, dual wave systems can increase solder joint qualification rates by 10–15%.

Comparative tests by a communication equipment company showed: through-hole fill rate for the same PCB was 82% with single wave soldering, while dual wave process achieved 98%, with significantly improved solder joint appearance consistency.

Scientific Setting of Core Process Parameters. Parameter settings for wave soldering machines must be based on a three-dimensional balance of material characteristics, equipment performance, and product requirements; isolated adjustment of any parameter may lead to process instability.

Preheating System Parameter Optimization

The core objective of preheating is to achieve "gradient heating"—controlling the PCB soldering surface temperature to 90–110℃ before solder contact, ensuring both sufficient flux activation (removing oxide layers) and avoiding excessive thermal shock on components. Its parameter settings must consider the following factors:

PCB Thickness Impact: 1.6 mm thick PCBs require 5–8℃ higher preheat temperature compared to 0.8 mm thick boards, as thicker boards have higher heat capacity and slower thermal conduction rates (approximately 0.8 W/(m·K) vs 1.2 W/(m·K)). When PCB thickness exceeds 2.0 mm, dual-zone preheating is recommended (zone 1: 60–80℃, zone 2: 100–120℃) to reduce the temperature difference between upper and lower surfaces;

Conveyor Speed Coordination: At standard speeds of 1.1–1.2 m/min, preheat zone length must be ≥1.5 m to ensure sufficient heating. If speed needs to increase to 1.5 m/min (increasing capacity by 25%), preheat temperature should be raised by 10–15℃, and the preheat zone extended to 2.0 m;

Component Heat Resistance Limits: For PCBs containing sensitive components like BGA and CSP, peak preheat temperature must be ≤100℃, and the heating rate controlled below 2℃/s to prevent reflow of internal component solder joints.

Typical consequences of insufficient preheating include: insufficient flux activity leading to cold solder joints (incidence increased 3 times), increased solder ball generation (average per board increasing from 15 to 40), and rise in solder joint icicle proportion to 12%. One company improved preheating process window stability to 95% by real-time monitoring of board surface temperature with an infrared thermometer.

Precise Control of Solder Pot Temperature

Solder temperature is a key parameter determining solder fluidity and joint strength. For 63/37 tin-lead solder (melting point 183℃), the optimal working temperature is 245–255℃, where solder viscosity is about 0.012 Pa·s, possessing optimal fluidity. Temperature control requires attention to:

Oxidation Balance: Above 260℃, solder oxidation rate increases exponentially (oxidation at 260℃ is 2.3 times that at 250℃), generated SnO₂ leading to solder joint inclusions and strength reduction (from 18 MPa to 14 MPa);

Temperature Uniformity: Temperature variation within the solder pot should be ≤±3℃, otherwise local overheating can cause solder segregation (lead content deviation ±0.5%). Regularly (monthly) calibrate thermocouple positions, ensuring measurement points are 5 mm above the wave formation area;

Lead-free Adaptation: If using SnCu0.7 lead-free solder, temperature needs to be increased to 260–270℃, but this requires upgrading solder pot material (using 316 stainless steel) to prevent high-temperature corrosion.

Comparative data from a power supply manufacturer showed: when solder pot temperature was stable at 250±2℃, solder joint qualification rate was 99.2%; when temperature fluctuated ±5℃, qualification rate dropped to 95.8%.

Conveyor Chain Parameter Setting

Chain angle and speed jointly determine the contact state between PCB and solder:

Angle Control: 5–6° is the optimal range, where the PCB forms tangential contact with the solder, contact time approximately 3–4 seconds. Angle <4° leads to excessive solder wetting (pad coverage > 120%), increasing bridging risk; angle >7° results in insufficient wetting (coverage < 80%), affecting conduction reliability;

Speed Matching: Forms a negative correlation with solder pot temperature—a 10% speed increase requires a 3–5℃ temperature increase to compensate for reduced contact time. When speed increases from 1.0 m/min to 1.4 m/min, contact time decreases from 4.2 seconds to 3.0 seconds, requiring a simultaneous temperature increase from 250℃ to 258℃.

Unstable chain operation (jitter > 0.5 mm) leads to periodic defects in solder joints, such as intermittent cold solder joints and uneven solder amount. Quarterly chain tension calibration is recommended, controlling operational deviation within 0.3 mm.

Fine Adjustment of Air Knife System

The air knife's role is "quantitative control"—removing excess flux (retained amount controlled at 0.8–1.2 mg/cm²) and ensuring uniform application. Its key parameters include:

Angle Setting: A 10° inclination (angle with horizontal line) allows flux to form a uniform film on the PCB surface; excessive angle (>15°) leads to insufficient flux at edge areas, while too small an angle (<5°) causes excessive residue in central areas;

Air Speed Control: Adjusted according to flux viscosity, typically 30–50 m/s. For high viscosity flux (>30 cP), air speed should be increased to 60 m/s, otherwise droplet accumulation is likely;

Distance Optimization: Distance between air knife nozzle and PCB surface should be 10±1 cm; too close causes local blow-off, too far reduces effectiveness.

A PCB manufacturer improved coating uniformity to 90% (film thickness deviation ≤10%) by detecting flux film thickness with a laser thickness gauge and implementing closed-loop adjustment of air knife parameters.

Solder Impurity Control

Impurities such as copper and aluminum in the solder significantly degrade soldering performance. When copper content exceeds 0.3%, solder melting point increases from 183℃ to above 190℃, fluidity decreases by 40%, and "copper embrittlement" occurs—solder joint shear strength drops below 12 MPa.

Impurity control measures include:

Regular Testing: Monthly sampling and analysis using atomic absorption spectrometry to determine impurity content, initiating alerts when copper content reaches 0.25%;

Process Optimization: Minimize PCB solder immersion passes (≤2 times), avoiding excessive dissolution of component leads;

Pot Maintenance: When copper content > 0.3%, perform comprehensive pot cleaning and replace solder (add 0.5% pure tin before first soldering after cleaning to stabilize alloy composition).

An automotive electronics company reduced soldering defect rates caused by impurities from 3.2% to 0.8% by establishing a solder impurity early warning mechanism.

Systematic Solutions for Common Soldering Defects

Defect analysis in wave soldering should employ the "Fishbone Diagram" method, investigating root causes from the five dimensions of Man, Machine, Material, Method, and Environment, avoiding simplistic attribution.

Insufficient Solder Fill and Cold Solder Joints

80% of such defects stem from the following reasons:

Low solder temperature (<240℃) leading to insufficient solder fluidity; confirm actual temperature with an infrared thermometer rather than relying on gauge display (typically has 5–8℃ deviation);

Insufficient flux activity, manifested as uncleared pad oxide layers (appearing dark brown); replace with flux containing higher activator content (organic acid content increased from 3% to 5%);

Excessive preheat temperature (>120℃) causing premature flux decomposition, losing wetting function; reduce zone 2 preheat temperature by 10℃ and extend zone 1 preheat time.

One case showed: by increasing solder temperature from 240℃ to 250℃ and adjusting flux activator ratio, the cold solder joint rate decreased from 7.5% to 1.2%.

Bridging and Short Circuits

Unintended connections between adjacent solder joints are mainly caused by:Excessive flux application (>1.5 mg/cm²); increase air knife speed by 5 m/s or increase angle to 12°;Excessively small chain angle (<4°) causing excessive PCB-solder contact area; adjustment to 5.5° can significantly improve;Unstable solder wave (fluctuation > 2 mm); inspect solder pump impeller and remove foreign object blockages.In 0.65 mm pitch pin soldering, bridging rates can be reduced from 5% to 0.3% through the above measures, with the key being controlling surface tension balance during solder separation.

PCB Surface Contamination

Countermeasures for excessive post-soldering residue or white crystalline formation:Pre-coated PCBs require matching specialized flux: rosin-based flux can effectively avoid "whitening"; if no-clean is required, choose modified alcohol-based flux compatible with the pre-coating rosin;

Hot air leveling PCBs should use low solid content flux (<3%), and increase cleaning water temperature to 60℃ to enhance residue dissolution;Control flux solid content: each 1% increase in solid content increases residue by approximately 0.2 mg/cm²; cleaning is required when exceeding IPC standard (≤0.5 mg/cm²).A communication equipment company improved PCB surface cleanliness qualification rate from 82% to 99% by switching to a compatible flux.

Dross Residue and Rough Solder Joints

Excessive impurities in solder (copper > 0.3%, iron > 0.05%) are the main cause; solution steps include:Emergency Treatment: Add dross reducing agent (e.g., specialized antioxidant), which can reduce floating dross by 50%;Root Cause Management: Schedule pot cleaning to thoroughly remove bottom sediments (primarily composed of Cu₃Sn, FeSn₂);Preventive Measures: Install solder filtration device (50 mm filter), clean weekly.Post-cleaning solder joint roughness (Ra) can decrease from 3.2 mm to 1.6 mm, significantly improving solder joint reliability.Process Synergy and Continuous Improvement System

Quality control in wave soldering cannot rely on single parameter optimization but should establish a closed-loop management mechanism of "Parameter–Defect–Adjustment":Establish Process Database: Record optimal parameter combinations for different products (e.g., PCB thickness 1.6 mm corresponds to preheat 100℃, speed 1.1 m/min, temperature 250℃), directly callable and fine-tunable during new product trial production;

   Implement Statistical Process Control (SPC): Real-time monitoring of key parameters (temperature, speed, flux application amount), initiating correction procedures when CPK value < 1.33;Supplier Collaboration: Establish joint testing mechanisms with flux and solder suppliers, conduct new material validation quarterly, introducing superior performance auxiliary materials.A large EMS enterprise shortened new product trial production cycles by 30% and stabilized soldering qualification rates above 99.5% by building a process knowledge management system.The optimization of wave soldering processes is endless. With the trends towards lead-free and high-density assembly, future focus should be on: temperature window adaptation for low-silver solders, thermal shock protection for miniaturized components, and application of intelligent parameter self-adjustment systems. Only by transforming process parameter control into systematic knowledge assets can technological advantages be maintained in the fierce competition of the electronics manufacturing industry.