Summary of Types of Soldering Defects on PCB and Analysis of 15 Causes

In the field of electronics manufacturing, Printed Circuit Boards (PCBs) play a crucial role as one of the key components, facilitating the connection between electronic components and circuits. Among the processes involved, the soldering of surface-mounted components directly impacts the product's quality and reliability. However, as is inevitable in many manufacturing processes, the soldering process faces several challenges, leading to a decline in soldering quality and an increase in production costs. This article will summarize common defects that occur during the soldering process on PCBs and provide an in-depth analysis of their causes. By understanding the root causes of these issues, we will be able to take targeted measures to improve the stability of the soldering process and product quality, thereby better meeting customer demands.

Excessive residue on the PCB surface

After welding, there are many dirty residues on the PCB surface.

• The flux solid content is high, and there is too much non-volatile material.
• Insufficient preheating or low preheating temperature before soldering (during wave soldering, the time is too short).
• The board speed is too fast (the flux cannot fully evaporate).
• The soldering furnace temperature is not sufficient.
• There are too many impurities in the soldering furnace or the tin content is low.
• Caused by the use of anti-oxidants or anti-oxidation oils.
• Too much solder flux applied.
• Too many connectors or open components on the PCB, without preheating.
• Component leads and board holes are disproportionate (holes too large), causing flux to rise.
• The PCB itself has pre-coated rosin.
• In the tinning process, the flux wetting is too strong.
• PCB process problem, insufficient through-holes, causing poor evaporation of flux.
• Incorrect angle of PCB immersion in the solder bath.
• Diluent not added for a long time during the flux usage.

On Fire

• The flux has a low flash point,and no flame retardant has been added.
• Without the air knife, there is excessive flux coating, and it drips onto the heating tube during preheating.
• The angle of the air knife is incorrect, leading to uneven flux coating on the PCB.
• There are too many adhesive tapes on the PCB, causing them to catch fire.
• There is too much flux on the PCB, and it drips down onto the heating tube.
• The board's travel speed is either too fast (flux not completely evaporated, leading to dripping) or too slow (causing excessive heating on the board surface).
• The preheating temperature is too high.
• Process issues (poor PCB material, the heating tube is too close to the PCB).

Corrosion (components turn green, solder joints turn black)

• Copper reacts with FLUX to form green copper compounds.
• Lead-tin reacts with FLUX to form black lead-tin compounds.
• Insufficient preheating (low preheating temperature, fast board transit speed) results in a high amount of FLUX residue, leading to excessive harmful substance residues.
• Residues absorb water (water-soluble ionic conductivity not meeting the standard).
• Using FLUX that requires cleaning, but not cleaning or not cleaning in a timely manner after soldering.
• FLUX has excessively strong activity.
• Electronic components react with active substances in the FLUX.

Connection issues, leakage (poor insulation)

• Ions from FLUX residue on the board; or FLUX residue absorbs moisture, leading to conductivity.
• Unreasonable PCB design, such as routing too close.
• Poor quality of PCB solder mask, prone to conductivity.

Cold soldering, insufficient soldering, continuous soldering

• Insufficient flux activity.
• Insufficient flux wetting ability.
• Insufficient amount of flux applied.
• Uneven application of flux.
• Inability to apply flux in certain areas of the PCB.
• Some areas of the PCBwere not tinned properly.
• Severe oxidation on certain solder pads or legs.
• Unreasonable PCB routing (improper distribution of components).
• Incorrect board orientation.
• Insufficient tin content or excessive copper content; impurities causing an increase in the solder liquid's melting point (liquidus line).
• Blockage in the foaming tube, uneven foaming, resulting in uneven flux application on the PCB.
• Improper air knife settings (flux not evenly blown).
• Poor coordination between board speed and preheating.
• Improper hand soldering technique.
• Unreasonable tilt angle of the chain.
• Uneven wave peak.

Solder joint too bright or not bright enough

• Issue with FLUX:

2. Can be changed by modifying its additives (FLUX selection issue).
4. FLUX might have slight corrosion.

• Poor solder quality (e.g., low tin content).

Short Circuit

• Short circuit caused by solder:

2. Cold solder occurred but was not detected.
4. Solder did not reach the normal operating temperature, resulting in "solder bridging" between solder joints.
6. Fine solder balls bridged between solder joints.
8. Cold soldering occurred, leading to bridging.

• Issues with FLUX:

2. Low activity and poor wetting of FLUX lead to solder bridging between solder joints.
4. Insufficient insulation resistance of FLUX causes short circuits between solder joints.

• PCB issues:

For example, a short circuit caused by the detachment of the PCB's solder mask.

Large smoke, strong taste

• Issues with FLUX itself:

2. Resin: If ordinary resin is used, the smoke will be more significant.
4. Solvent: Here, it refers to the possibility of the odor or irritating smell of the solvent used in FLUX.
6. Activator: The smoke is significant and has an irritating odor.

• Inadequate ventilation system.

Splashing, Solder Balls

• Flux

2. High water content (or exceeding the standard) in FLUX.
4. High boiling point components in FLUX (not fully evaporated after preheating).

• Process

2. Low preheating temperature (FLUX solvent not completely evaporated).
4. Fast conveyor speed without achieving preheating effect.
6. Poor chain angle, resulting in air bubbles between solder and PCB, which burst and produce solder balls.
8. Excessive FLUX coating (no air knife or malfunctioning air knife).
10. Improper operation during hand soldering.
12. Humid working environment.

• PCB issues

2. Moist PCB surface, inadequate preheating, or water generation.
4. Poorly designed PCB venting holes, causing entrapment of gas between PCB and solder.
6. Unreasonable PCB design, with components' pins too close together, leading to gas entrapment.
8. Poorly drilled PCB through-holes.

Bad soldering leads to incomplete solder joints

• Insufficient wetting during soldering, resulting in incomplete solder joints.
• Poor wetting properties of the FLUX.
• Weak reactivity of the FLUX.
• Low wetting or activation temperature, and a narrow process window.
• Using a dualwave soldering process, causing the active components of the FLUX to completely evaporate during the first solder pass.
• Excessive preheating temperature, leading to premature activation of the flux, resulting in little or no activity during soldering or very weak activity.
• Slow board traversing speed, causing excessive preheating temperature.
• Uneven application of FLUX.
• Severe oxidation of solder pads and components leads to poor solder wettability.
• Insufficient FLUX application, resulting in incomplete wetting of PCB solder pads and component leads.
• Unreasonable PCB design, causing improper arrangement of components on the PCB, affecting the soldering of certain components.

Poor Foaming of FLUX

• Incorrect selection of FLUX type.
• The foaming pipe holes are too large (the foaming pipe holes of non-clean FLUX are smaller, while those of resin-based FLUX are larger).
• The foaming area of the foaming tank is too large.
• The air pressure of the air pump is too low.
• The foaming pipe has leaky or blocked air holes, causing uneven foaming.
• Excessive addition of diluent.

Excessive Foaming

• Excessive pressure.
• The foaming area is too small.
• Excessive FLUX was added to the soldering groove.
• Failure to add diluent in a timely manner, resulting in excessively high FLUX concentration.

Color Change in FLUX

Some non-transparent FLUX contains a small number of photosensitive additives. These additives change color when exposed to light, but they do not affect the soldering effect and performance of the FLUX.

PCB solder resist film delamination, peeling, or bubbling

• More than 80% of the reasons are issues during the PCB manufacturing process.

2. Insufficient cleaning
4. Poor-quality solder resist film
6. Incompatibility between PCB material and solder resist film
8. Contamination entering the solder resist film during drilling
10. Excessive solder leveling during hot air leveling

• Some additives in the flux can damage the solder resist film.
• Excessive tin liquid temperature or preheating temperature.
• Excessive frequency of soldering.
• Prolonged PCB immersion time in the tin liquid during hand soldering.

Changes in high-frequency downlink signals

• Low insulation resistance and poor insulation properties of FLUX.
• Non-uniform residue and uneven distribution of insulation resistance, which can form capacitance or resistance in the circuit.
• FLUX's water extraction rate does not meet the requirements.
• The above issues may not occur during the cleaning process (or can be resolved through cleaning).

Poor soldering on PCBs is an eternal challenge for the electronics manufacturing industry. Whether it's emerging technologies or traditional processes, we need to maintain constant attention and improvement. By gaining a deep understanding of the summarized types and causes of defects in this article, we can better prevent and address these issues, further enhancing production efficiency and reducing scrap rates, thereby delivering more reliable and high-quality products. It is worth emphasizing that teamwork and continuous learning are the keys to overcoming these challenges. Only through sharing knowledge and experience, and continuously improving our processes and technologies, can we adapt to rapidly changing market demands. Let us collaborate hand in hand, make joint efforts, and strive for excellence to achieve the sustainable development of the electronics manufacturing industry.

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