1.5 Common defects & corrections

When encountering solder balls, non-fills, or bridging, process parameters must be systematically reviewed before attributing the issue solely to the solder wave. These defects strongly correlate with upstream preparation failures. Strict management of flux activation and the thermal ramp rate fully evaporates volatile solvents, stabilizes the temperature differential (∆T), and maximizes THT throughput by preventing rework.

The troubleshooting protocol

A majority of THT soldering defects relate back to flux application, preheating, or wave dynamics. Effective troubleshooting demands a rigid, systematic protocol: verify preparation first, adjust only one variable at a time, and validate the result objectively on a single panel.

Adjusting multiple machine settings simultaneously obscures the root cause and invalidates the engineering solution.

1. Preparation Check: Before adjusting any wave settings, adequate flux coverage must be verified and the top-side temperature confirmed to remain within the target specification immediately prior to entering the wave.
2. Mechanical Check: Pallet seating must be verified to be secure and clearances to look correct. The conveyor fingers must be ensured to be clean and the conveyor angle properly configured.
3. Process Adjustment: Once preparation and mechanics are validated, one specific parameter on the pot (e.g. conveyor speed/dwell, wave height, or chip wave balance) must be changed. Then, a single sample board must be run to isolate and observe the effect.

Defect triage: symptoms and supportive actions

The following table associates common THT defects with their probable causes and provides some recommended corrective actions to help address the issue while minimizing disruption to the overall line.

Symptom	Probable Cause	Supportive Action
Bridging (Often on Fine Pitch)	Excessive flux; conveyor angle too flat; chip wave is a bit too low to provide adequate scrubbing.	The chip wave must be raised by 0.2–0.5 mm to increase scrubbing action, or dwell time increased (+0.3 seconds) to improve drainage.
Bridging at the End of a Row	Inadequate flow separation at the exit; missing solder thief pad in the board design.	A slight deceleration followed by a steady exit from the wave must be applied; a solder thief must be required in the next design revision.
Icicles or Trailing Solder	Excessive dwell time leading to a cold exit; component leads may be cut too long.	Conveyor speed must be increased to expedite the exit; an air or N₂ knife must be utilized to trim trails; or pot temperature increased by +5 °C to prolong the liquid state during exit.
Poor Top-Side Fill	Insufficient preheat or flux; hole clearance may be too narrow; pin is attached to a large copper layer acting as a heat sink.	Top-side temperature target must be increased by 5–10 °C; conveyor speed reduced a bit to provide longer contact time for heat transfer.
Skips or Poor Wetting	Oxidized component or board finish; insufficient flux; contact time might be a bit too short.	A second, very light flux spray pass must be applied; conveyor speed decreased; or an N₂ blanket utilized over the wave to mitigate oxidation.
Solder Balls or Spatter	Flux solvents were likely not fully evaporated before contacting the wave; the solder fountain might be excessively turbulent.	Preheat duration must be increased (a longer, slower ramp is better than a higher peak); fountain height lowered by 0.5–1.0 mm to gently reduce turbulence.
Blowholes or Voids	Bare PCB may have absorbed ambient moisture; volatile flux trapped inside the hole; solder mask annular ring is perhaps too tight, restricting outgassing.	A pre-assembly bake cycle must be enforced for bare boards; a longer, gentler preheat profile applied; a slightly larger mask clearance requested in future design files.
Selective Soldering: Non-Fills	Nozzle Z-height might be set too high (insufficient pin contact); nozzle path speed may be too fast.	Nozzle Z-height must be recalibrated; path speed decreased to 5–8 mm/second; a two-pass approach (quick pre-wet followed by slower final pass) resolves difficult pins.

Addressing root causes in design (DFM)

When systemic defects persist despite careful process optimization, the root cause is often related to Design for Manufacturing (DFM). These physical constraints must be resolved in the next PCB design revision.

• Hole and Pad Geometry: If holes are too narrow for the component leads or annular rings are insufficient, capillary flow is physically obstructed. This induces pad lifting during any necessary rework.
  • Requirement: Enforce a lead diameter clearance of +0.20–0.45 mm and require annular rings to have at least 0.25 mm of radial copper.
• Thermal Management: Providing THT pins connected to heavy internal planes without thermal reliefs forces the planes to act as aggressive heat sinks, guaranteeing cold joints.
  • Requirement: Implement thermal reliefs (e.g. 4 spokes, 0.25–0.40 mm wide) for connections to internal planes.
• Component Layout: Orienting pin rows parallel to the wave direction or placing SMT components too close to the THT area exponentially increases the probability of bridging and solder splash.
  • Requirement: Utilize solder thieves on the trailing edge of fine-pitch rows and permanently maintain a strict 3–4 mm keepout zone from all SMT components.

Process adjustment guidelines

Effective process optimization comes from adjusting the most appropriate parameter for the specific defect mechanism.

• Addressing Solder Starvation or Poor Top-Fill: The primary lever is Conveyor Speed / Dwell (a longer contact time increases heat transfer). The secondary control is Pot Temperature (executing a +5 °C increase).
• Addressing Bridging or Solder Spatter: The primary lever is Flux / Preheat (ensuring the board is completely dry upon entry). Once preparation is verified, the secondary control is adjusting the Chip Wave height and dwell to enhance scrubbing action.
• Addressing Icicles or Trailing Solder: The primary lever is adjusting the peel-off angle, the air knife pressure, or the exit timing. Increasing pot temperature does not resolve icicle formation.

Selective vs. Wave Soldering Adjustment

The approach to defect resolution diverges completely between selective soldering and bulk wave soldering.

• Selective Soldering Adjustments: Process adjustments here are entirely localized. Parameters such as nozzle diameter, Z-height, path speed, or spot dwell time are modified for a specific joint. For challenging joints, programming a two-stage pass is vastly superior to increasing the overall pot temperature.
• Wave Soldering Adjustments: Process adjustments here are global and require carefully balancing the entire system. The Chip Wave provides the necessary scrubbing action, while the Main Wave provides a gentle, smooth exit flow. The Main Wave must never be used for scrubbing; applying aggressive pressure there introduces turbulence and widespread bridging.

Final Checkout: Common Defects & Corrections

Verification Point	Immediate Check (Containment)	Preventative Focus (Design Revision)
Preparation Status	Top-Side Temperature must be verified to land within the flux manufacturer’s specific range.	DFM guidelines must be reviewed for adequate thermal relief on joints that frequently exhibit cold solder.
Bridging Prevention	Chip Wave height/dwell must be adjusted or the Air/N₂ Knife utilized.	Solder Thief Pads must be suggested in the design or modifying the wave pallet considered for better flow.
Resolving Non-Fills	Conveyor Speed must be decreased or +5 °C added on the Pot Temperature. A second light flux pass resolves skipping.	Hole clearance specifications must be reviewed against component lead diameters in the master design files.
Eliminating Solder Balls	Preheat Time must be increased to ensure the board is completely dry upon wave entry. The flux system must be verified for any blockages.	Solder mask clearance must be ensured to be at least 0.10 mm larger than the pad to easily allow flux gas venting.
Adjustment Protocol	Exactly one process variable must be changed at a time and the adjustments documented.	Successful process changes must be incorporated into standard procedures once physically verified.