For small and medium-sized industrial, automation, and electronics manufacturers, the reliability of PCBA (Printed Circuit Board Assembly) directly impacts product quality, production efficiency, and customer trust. Reflow soldering, a critical step in PCBA manufacturing, often faces various defects that can compromise your assemblies. In this guide, we’ll break down the 12 most common reflow soldering defects, their root causes, and actionable prevention strategies—so you can avoid costly reworks and get consistent, high-quality PCBA.
1. Non-wetting & Poor Wetting
Non-wetting occurs when solder fails to spread evenly across pads or component leads, resulting in weak joints with poor electrical conductivity. This defect is particularly common with lead-free solders, which have higher melting points and lower wettability than traditional SnPb alloys.
Causes: Oxidized pad/lead surfaces, insufficient plating thickness, low soldering temperature, inadequate preheating, mismatched plating-solder combinations, or contaminated solder/flux.
Prevention: Store PCBs and components properly, use PCBs with ≥5μm plating, adjust preheat/welding temperatures, and consider nitrogen atmosphere soldering for better wettability.
2. Black Pad
A “black pad” defect affects ENIG (Electroless Nickel Immersion Gold) plated PCBs. While the gold layer appears intact, the underlying nickel layer oxidizes into brittle black nickel oxide, risking joint failure.
Causes: Nickel oxidation outpacing gold deposition during plating, loose gold layer structure, or high phosphorus content in nickel baths.
Prevention: Control nickel bath phosphorus content (~7%), ensure nickel thickness ≥4μm and gold thickness ≤0.1μm, and avoid over-baking PCBs before soldering.
表 1Black Pad
3. Bridging
Bridging is the unwanted solder connection between adjacent pads or leads, often leading to short circuits. It’s a major issue for densely packed PCBs with fine-pitch components.
Causes: Dense circuit design, residual contaminants, insufficient preheating, or misprinted/smothered solder paste.
Prevention: Optimize pad design to avoid overcrowding, increase preheat temperature, and use nitrogen atmosphere soldering. Defective joints can be reworked with a soldering iron.
表 2Bridging
4. Noncoplanarity & De-soldering
Noncoplanar components (with uneven leads) cause poor solder contact, leading to bridging, misalignment, or de-soldering. This is common with surface-mount devices (SMDs) handled improperly.
Causes: Non-compliant component lead dimensions, lead warping during handling, insufficient solder paste, or low placement pressure.
Prevention: Source qualified components, avoid lead damage during assembly, and ensure uniform solder paste printing.
表 3Noncoplanarity
5. Tombstoning
Tombstoning (component “standing on end”) occurs when one end of a chip component lifts off the pad, rendering the PCBA non-functional. It’s more frequent with miniaturized components like 0201 or 01005.
Causes: Uneven solder paste printing, inaccurate component placement, temperature imbalance, or mismatched substrate thermal properties.
Prevention: Improve printing/placement precision, use substrates with better thermal stability (e.g., ceramic over epoxy), and optimize component layout.
6. Flux Residues
Excessive flux residues mar PCB appearance and pose electrical reliability risks (e.g., corrosion or leakage current). This defect stems from improper flux selection or process control.
Causes: Wrong flux type (e.g., rosin-based instead of no-clean), high resin content, inadequate cleaning, or mismatched process parameters.
Prevention: Select appropriate flux (no-clean for low-residue needs) and implement proper cleaning protocols for residue-sensitive applications.
7. Solder Balls
Solder balls (≥0.13mm diameter, within 0.13mm of conductors) violate electrical clearance rules. IPC standards reject PCBs with >5 solder balls per 600mm².
Causes: Moisture in PCBs, poor solder mask treatment, excessive flux, insufficient preheating, or loose solder paste particles.
Prevention: Bake PCBs before soldering, use ≥25μm through-hole copper layers, adjust preheat temperature, and select suitable solder masks.
表 4Solder Balls
8. Wick
Wicking happens when solder climbs component leads instead of forming a joint between leads and pads, resulting in weak connections.
Causes: Faster lead heating than pads, poor pad solderability, improper via design, or expired pad plating.
Prevention: Slow heating rates to reduce pad-lead temperature differences, use high-quality pad plating, and optimize via design.
9. Solder Voids
Voids (air pockets) in solder joints reduce mechanical strength and electrical conductivity. While minor voids are harmless, excessive voids compromise reliability.
Causes: Excessive flux in solder paste, low preheat temperature, short soldering time, or organic contamination.
Prevention: Adjust preheat/soldering parameters, control flux ratio in solder paste, and avoid assembly contamination.
10. BGA Voids
Ball Grid Array (BGA) voids are specific to BGA components, caused by trapped flux or moisture under the package.
Causes: Improper temperature profiling, lead-free/lead混用 (mixed lead-free and leaded materials), or moisture in PCBs.
Prevention: Optimize temperature curves, avoid material mixing, and use non-expired PCBs.
11. Component Excursion
Component excursion (misalignment) occurs when SMDs shift during reflow. Shifts >50% of pad width are unacceptable; the ideal limit is <25%.
Causes: Low placement machine precision, non-compliant component tolerances, insufficient solder paste viscosity/placement pressure, excessive flux content, solder paste slump, expired solder paste, program rotation errors, PCB processing issues, MELF component design, or excessive airflow.
Prevention: Calibrate placement coordinates, use high-viscosity solder paste, increase placement pressure, select suitable solder paste, and adjust motor speed.
12. Component Crack
Component cracks compromise functionality and reliability, often caused by thermal stress or mishandling during assembly.
Causes: Pre-assembly damage, thermal mismatch between PCB and components, improper placement handling, high soldering temperature, moisture absorption from poor storage, or excessive cooling rates.
Prevention: Use optimized thermal profiles, store components per specifications, and invest in quality placement/welding equipment.At our PCBA manufacturing facility, we prioritize defect prevention through strict process control, advanced equipment, and expert quality management. Whether you’re in industrial automation, electronics, or OEM/ODM sectors, we deliver consistent, high-reliability PCBA solutions tailored to your needs. Let’s partner to eliminate soldering defects and boost your product performance.
