Analysis of 20 Common Defects in Wave Soldering and Their Causes

1. PCB Surface Contamination

• The flux solids content is too high, leaving excessive nonvolatile residue that cannot be completely removed after soldering.
• The PCB is not preheated before soldering, or the preheat temperature does not meet the process requirement. In dip-soldering applications, the immersion time may also be too short, preventing the flux from being sufficiently activated and volatilized.
• The PCB conveyor speed is too high, so the flux cannot adequately volatilize during preheating and a large amount remains on the board surface.
• The solder pot temperature is below the required soldering temperature. The reaction between the flux and molten solder is incomplete, making residue more likely to form.
• The impurity content in the solder pot is excessive, or the solder purity is too low. The reaction between these impurities and the flux produces additional residue.
• Antioxidants or anti-oxidation oils added to prevent solder oxidation adhere to the PCB surface during soldering and form contamination.
• The flux application volume exceeds the appropriate range, and the excess cannot be removed by subsequent processing.
• The PCB contains many sockets or open-frame components, but no dedicated preheating process is provided for these components. Localized flux residue remains as a result.
• The component lead diameter does not match the PCB hole size. If the holes are too large, flux can rise with the molten solder and remain on the board surface.
• The PCB was coated with rosin during manufacture. When additional flux is applied during soldering, the combined residue level increases.
• During tinning, the flux has excessive wetting capability and spreads too extensively over the pads, increasing the likelihood of residue.
• The PCB design contains process-related deficiencies. For example, an insufficient number of vias can restrict the paths through which flux vapor escapes, leaving a large amount of residue on the board.
• During manual dip soldering, the PCB is inserted into the solder at an unsuitable angle, obstructing the flux vapor path and causing residue.
• Diluent is not replenished during flux use. The flux concentration gradually increases, the proportion of nonvolatile components rises, and the amount of residue increases.

2. Soldering Fires

• The flux flash point does not meet the process safety requirement, and no flame-retardant component has been added to improve fire resistance. The flux can ignite when exposed to high temperatures.
• The soldering equipment is not equipped with an air knife. Excessive flux drips onto the heating tubes during preheating and ignites at high temperature.
• The air-knife installation angle is incorrect, producing an uneven flux coating. Excess flux accumulates in localized areas and can drip onto the heating tubes during preheating.
• Too many adhesive strips are applied to the PCB, or the adhesive has poor high-temperature resistance. The strips can ignite during preheating or soldering.
• Excess flux is applied to the PCB and is not evenly distributed or volatilized by the air knife. It drips onto the heating tubes and ignites on contact with the hot surface.
• The conveyor speed is abnormal. At excessive speed, the flux enters the soldering zone before it has fully volatilized and may drip and ignite. At excessively low speed, the PCB remains in the high-temperature environment for too long, raising the board temperature and increasing the risk of ignition.
• The preheat temperature is set too high and exceeds the flux temperature tolerance, causing the flux to burn prematurely.
• The process design is defective. Examples include a PCB material with inadequate high-temperature resistance or insufficient clearance between the heating tubes and PCB, resulting in localized overheating.

3. Corrosion: Green Components and Black Solder Joints

• Copper in the circuit board reacts chemically with the flux and forms green copper-based compounds, causing components to turn green.
• The lead-tin alloy in a solder joint reacts with the flux and forms black lead-tin compounds, causing the solder joint to turn black.
• Preheating is insufficient because the preheat temperature is too low or the conveyor speed is too high. The flux cannot volatilize adequately, increasing surface residue and creating conditions for corrosion.
• Flux residues are hygroscopic. If the water-soluble components in the residue do not meet the required conductivity limit, absorbed moisture forms a conductive liquid that accelerates corrosion.
• A flux intended for post-solder cleaning is used, but the PCB is not cleaned as required or is cleaned too late. The remaining flux stays in contact with components and solder joints and causes corrosion.
• The flux is excessively active. Even a small amount of residue remaining after soldering can react with component and solder-joint materials.
• Some electronic components use materials with poor compatibility with the active substances in the flux and are therefore susceptible to chemical corrosion.

4. Electrical Bridging and Leakage

• Ionic flux residue forms on the PCB surface, or the residue absorbs moisture from the air. Once it becomes conductive, it can cause electrical bridging or leakage.
• The PCB layout is unsuitable. For example, traces are spaced too closely, allowing residue or solder beads to create an unintended conductive path after soldering.
• The solder mask does not meet quality requirements and has poor insulation performance. During long-term operation or exposure to humid conditions, it can become conductive and cause electrical bridging or leakage.

5. Open Solder Joints and Cold or Weak Solder Joints

• The flux activity is insufficient to remove the oxide layer from the pads and component leads.
• The flux has inadequate wetting performance and cannot sufficiently wet the pads and leads, preventing uniform solder adhesion.
• The flux application volume is too low, leaving some pads and leads without soldering medium.
• The flux coating is uneven, so some pads or component leads do not contact the flux.
• Some PCB areas cannot be properly coated because of solder mask coverage, surface oil, or other contamination, resulting in local open joints.
• Some pads do not receive sufficient solder because of pad oxidation or poor molten-solder flow.
• Severe oxidation on pads or component leads cannot be completely removed by the flux, preventing the molten solder from forming a joint.
• The PCB layout is unsuitable and the components are arranged irregularly, restricting the flow and wetting range of the molten solder.
• The PCB travel direction does not match the wave direction, preventing some leads from making effective contact with the solder.
• The tin content in the solder is insufficient, or copper and other impurities exceed acceptable levels. The liquidus temperature rises, solder flow decreases, and weak solder joints are more likely.
• The flux foaming tube is blocked or leaking, producing uneven foam and an unbalanced flux coating on the PCB.
• The air-knife parameters are unsuitable, so the flux is not evenly distributed and some areas receive too much or too little flux.
• The conveyor speed and preheat temperature are not properly coordinated. If the speed is too high while the preheat temperature is low, the flux is not activated. If the speed is too low while the preheat temperature is high, the flux may become inactive too early.
• Manual dip-soldering is performed incorrectly. An unsuitable insertion angle or dwell time prevents sufficient contact with the molten solder.
• The conveyor-chain inclination is unsuitable and affects the contact pressure and contact area between the PCB and molten solder.
• The wave surface is uneven. In areas where the solder height is insufficient, effective soldering cannot take place.

6. Abnormal Solder-Joint Color

• Flux-related causes:
  • The problem can sometimes be improved by adjusting the flux additives. In essence, the selected flux does not match the actual process requirements, causing a deviation in solder-joint color.
  • The flux has a slight corrosive effect on the solder joint and changes its surface color after soldering.
• The tin content in the molten solder is too low, or the impurity content is too high, causing abnormal color after the solder joint solidifies.

7. Short Circuits

Molten-solder-related causes

• Solder bridging occurs during the process but is not detected during inspection, allowing the finished product to contain a short circuit.
• The molten solder has not reached its normal operating temperature and therefore has poor flowability. A thin solder bridge can form between adjacent joints.
• Fine solder beads are present between solder joints and connect adjacent joints, creating a hidden short circuit.
• Solder bridging occurs directly during soldering, connecting adjacent joints with molten solder.

Flux-related causes

• Low flux activity or poor wetting causes uneven solder distribution between joints and can produce solder bridging.
• The flux insulation resistance does not meet the required level. Poor insulation allows flux residue between solder joints to form a conductive path.

PCB-related causes

• Solder-mask delamination or loss on the PCB creates an unintended conductive path and causes a short circuit.

8. Excessive Smoke and Odor

• Resin composition: Ordinary resin types can generate relatively large amounts of smoke during soldering.
• Solvent composition: Some flux solvents have strong or irritating odors that volatilize and spread when heated.
• Activator composition: Some activators generate considerable smoke and irritating odors when heated.
• The exhaust and ventilation system is inadequate.

9. Solder Spatter and Solder Beads

Flux-related causes

• The moisture content of the flux is too high or exceeds the specified range. Water vaporizes rapidly during heating and causes solder spatter and solder beads.
• The flux contains high-boiling-point components that cannot fully volatilize during preheating. They vaporize in the soldering zone and cause solder spatter.

Process-related causes

• The preheat temperature is too low, so the solvent cannot fully volatilize. The remaining solvent vaporizes during soldering and drives solder spatter.
• The conveyor speed is too high, so the PCB does not reach the intended preheat condition. Incomplete flux volatilization then produces solder beads.
• The conveyor-chain inclination is unsuitable. Air can become trapped when the PCB contacts the molten solder, and bursting bubbles can cause solder spatter and beads.
• The flux application volume is excessive because the equipment does not have an air knife or the air knife performs poorly. Excess flux vaporizes during heating and causes spatter.
• Manual dip soldering is performed incorrectly. Excessive insertion speed or an unsuitable angle can draw air into the molten solder and form bubbles.
• The ambient humidity is too high. Moisture adheres to the PCB or solder surface and causes spatter during heating.

PCB-related causes

• The board surface is damp, has not been fully preheated, or contains generated moisture.
• The PCB vent-hole design is unsuitable, allowing air to become trapped between the PCB and molten solder.
• The PCB layout is unsuitable, and overly dense component leads create pockets where air can become trapped.
• The PCB through-holes are defective.

10. Poor Solder Coverage and Incomplete Solder Joints

• The flux has poor wetting performance and cannot sufficiently wet the pads and component leads, preventing the solder from spreading uniformly.
• The flux activity is too weak to remove oxide from pad and lead surfaces, making solder adhesion difficult.
• The temperature required for flux wetting or activation is too low, and the effective operating range is too narrow to cover the entire soldering area.
• In a dual-wave soldering process, the active flux components may have fully volatilized during the first wave. When the PCB reaches the second wave, no effective flux assistance remains, resulting in poor solder coverage.
• Excessive preheat temperature activates the flux too early. By the time the PCB reaches the solder wave, the activator may have become inactive or significantly weakened.
• The conveyor speed is too low, keeping the PCB in the preheat zone for too long and causing the flux to become inactive before soldering.
• The flux coating is uneven, leaving some pads or component leads without flux and reducing solder adhesion.
• Severe oxidation on pads or component leads cannot be completely removed by the flux, resulting in poor solderability and incomplete joints.
• The flux application volume is too low to fully wet the PCB pads and component leads.
• The PCB layout is unsuitable. Component placement affects the molten-solder flow path and obstructs solder coverage on some components.

11. Poor Flux Foaming

• The selected flux is incompatible with the foaming equipment, such as the foaming tube, and cannot form stable foam.
• The foaming-tube hole diameter is unsuitable. In general, no-clean fluxes use smaller hole diameters, while resin-based fluxes use larger diameters. A mismatch causes poor foaming.
• The foaming area in the flux tank is too large for the capacity of the air pump and the foaming capability of the flux, resulting in sparse foam.
• The air-pump output pressure is too low to provide sufficient foaming force, so the foam height and density are inadequate.
• The foaming tube has blocked holes or an air leak, causing uneven foaming and insufficient foam in some areas.
• Too much diluent is added during flux use, reducing the flux concentration and preventing stable foam formation.

12. Excessive Foaming

• The air-pump output pressure is set too high, providing excessive foaming force and producing much more foam than required.
• The foaming area in the flux tank is too small. At the same pressure, foam accumulates in the smaller area and appears excessive.
• The total amount of flux in the flux tank is too high, allowing the foam to overflow during operation.
• Diluent is not replenished in time. The flux concentration gradually increases, its foaming capability becomes stronger, and excessive foam is produced.

13. Flux Discoloration

Some nontransparent fluxes contain a small amount of photosensitive additive. This additive changes color after exposure to light, but the discoloration does not affect the flux soldering performance.

14. Solder-Mask Delamination, Peeling, or Blistering

More than 80% of these defects originate during PCB manufacturing.

• The PCB surface is not cleaned thoroughly during manufacture. Residual oil, contaminants, or other debris prevents the solder mask from bonding firmly to the substrate.
• Low-quality solder-mask material has poor adhesion and inadequate high-temperature resistance, making it susceptible to peeling or blistering.
• The PCB laminate and solder-mask materials are incompatible. A large difference in thermal expansion causes delamination during heating.
• Metal debris or dust enters the interface between the solder mask and substrate during drilling and damages the bonding layer.
• The PCB passes through hot-air leveling too many times. Repeated heating accelerates solder-mask aging and peeling.
• Some flux additives are corrosive and may damage the solder mask, resulting in peeling or blistering.
• The solder-pot or preheat temperature is too high and exceeds the solder mask's high-temperature limit, causing it to soften, blister, or peel.
• The PCB passes through the solder wave repeatedly. Repeated thermal exposure ages the solder mask and reduces its adhesion.
• During manual dip soldering, the PCB remains on the molten-solder surface for too long and the solder mask is exposed to high-temperature heating for an extended period.

15. High-Frequency Electrical Performance Changes

• The flux has low insulation resistance and does not provide the insulation performance required by high-frequency circuits, interfering with signal transmission.
• Flux residue is distributed unevenly on the PCB surface, producing a nonuniform insulation-resistance distribution. The residue can introduce additional capacitive or resistive effects and change the designed electrical performance.
• The flux water-extract resistance does not meet the required level. The residue readily absorbs moisture from the air, reducing insulation stability in high-frequency applications.
• If a cleaning process is used, professional post-solder cleaning can remove flux residue and may prevent or resolve these problems.

16. Voids and Pinholes

• The temperature at the top of the PCB is too low.
• Excess moisture accumulated in the circuit board attempts to escape through a thin copper-plating layer.
• Similar component types are not oriented in the same direction, resulting in poor copper-plating conditions.
• The lead-to-hole ratio is either too small or too large.

17. Icicles

• The solderability of the substrate or component leads is poor.
• The flux specific gravity is too low.
• Large components do not absorb sufficient heat during preheating, or the soldering temperature is too low.
• The immersion time in the molten solder is too long.
• The angle of the cooling-air flow after the PCB leaves the wave is incorrect.
• The component leads contact oxide dross in the solder.

18. Cold Solder Joints

• The pad or component lead is oxidized.
• Mechanical vibration from the conveyor or guide rail disturbs the solder joint while it is cooling.
• The soldering temperature is too low, or the soldering time is too short.

19. Cracked Components

• The component was damaged before assembly.
• Thermal mismatch between the PCB material and component during soldering causes the component to crack.
• The soldering temperature is too high.
• Cooling is too rapid, causing stress concentration.

20. Lifted Pads

• The wave-soldering temperature is too high, or the PCB remains in the high-temperature zone for too long.
• The PCB laminate is of poor quality, and the bond strength between the copper foil and substrate is insufficient.
• The pad and trace layout is unsuitable, resulting in excessive mechanical stress.