1.5 Common defects & corrections

When you encounter defects like solder balls, non-fills, or bridging, it’s important to systematically review the entire process before concluding the issue is with the solder wave itself. Often, these defects are strongly linked to upstream preparation steps. By carefully managing flux activation and the thermal ramp rate, you can ensure volatile solvents fully evaporate, stabilize the temperature differential (ΔT), and maximize throughput by preventing the need for rework.

The troubleshooting protocol

Most through-hole (THT) soldering defects can be traced back to flux application, preheating, or wave dynamics. Effective troubleshooting requires a systematic, disciplined approach: always verify preparation first, adjust only one variable at a time, and objectively validate the result on a single test panel.

Adjusting multiple machine settings at once makes it impossible to identify the true root cause and invalidates any potential solution.

Preparation Check: Before making any adjustments to the wave settings, confirm that flux coverage is adequate. Also, verify that the top-side board temperature is within the target specification just before it enters the wave.
Mechanical Check: Ensure the pallet is seated securely and all clearances look correct. Check that the conveyor fingers are clean and the conveyor angle is properly configured.
Process Adjustment: Once preparation and mechanics are validated, adjust one specific parameter on the solder pot—such as conveyor speed/dwell, wave height, or chip wave balance. Then, run a single sample board to isolate and observe the effect of that change.

Defect triage: symptoms and supportive actions

The following table lists common THT defects, their probable causes, and recommended corrective actions to help resolve the issue while minimizing disruption to the production 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 create adequate scrubbing action.	Raise the chip wave by 0.2–0.5 mm to increase scrubbing action, or increase dwell time by +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.	Ensure a steady exit from the wave, sometimes aided by a slight reduction in conveyor speed at the exit point; a solder thief pad should be included in the next design revision.
Icicles or Trailing Solder	Excessive dwell time leading to a cold exit; component leads may be cut too long.	Increase conveyor speed to reduce dwell time and facilitate a quicker exit; utilize an air or N₂ knife to trim trails; or increase pot temperature 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 connected to a large copper plane (thermal mass) which acts as a heat sink.	Increase the top-side temperature target by 5–10 °C; reduce conveyor speed slightly to allow for longer contact time and better heat transfer.
Skips or Poor Wetting	Oxidized component or board finish; insufficient flux; contact time might be a bit too short.	Apply a second, very light flux spray pass; decrease conveyor speed; or use a nitrogen (N₂) inert atmosphere over the wave to reduce oxidation.
Solder Balls or Spatter	Incomplete evaporation of flux solvents before contacting the wave; the solder fountain might be excessively turbulent.	Increase preheat duration (a longer, slower ramp is better than a higher peak); lower the fountain height 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 too tight, restricting outgassing.	Enforce a pre-assembly bake cycle for bare boards; apply a longer, gentler preheat profile; request a slightly larger mask clearance 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.	Recalibrate the nozzle Z-height; decrease path speed to 5–8 mm/second; for difficult pins, a two-pass approach (quick pre-wet followed by slower final pass) often works well.

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 typically need to be resolved in the next PCB design revision.

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

Process adjustment guidelines

Effective process optimization involves adjusting the most appropriate parameter for the specific defect you’re trying to correct.

Addressing Solder Starvation or Poor Top-Fill: The primary control is Conveyor Speed / Dwell. A longer contact time increases heat transfer. The secondary control is Pot Temperature, which can be increased by +5 °C if needed.
Addressing Bridging or Solder Spatter: The primary focus should be on 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 its scrubbing action.
Addressing Icicles or Trailing Solder: The primary controls are the peel-off angle, air knife pressure, or exit timing. Note that increasing the pot temperature generally does not resolve icicle formation.

Selective vs. Wave Soldering Adjustment

The approach to defect resolution differs significantly between selective soldering and bulk wave soldering.

Selective Soldering Adjustments: Process adjustments here are entirely localized to a specific joint or nozzle. You modify parameters like nozzle diameter, Z-height, path speed, or spot dwell time. For challenging joints, programming a two-stage pass is often more effective than simply 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 should never be used for scrubbing; applying aggressive pressure there introduces turbulence and can cause widespread bridging.

Recap: Wave Soldering Defect Resolution

Defect	Probable Cause	Corrective Action	Key Parameter Adjustment
Bridging (Fine Pitch)	Excessive flux; flat conveyor angle; low chip wave.	Increase chip wave scrubbing; improve drainage.	Raise chip wave 0.2–0.5 mm; increase dwell +0.3 s.
Bridging (Row End)	Poor flow separation; missing solder thief pad.	Ensure steady exit; add solder thief in design.	Apply slight deceleration at wave exit.
Icicles / Trailing Solder	Excessive dwell; long component leads.	Accelerate exit; trim trails; prolong liquid state.	Increase conveyor speed; increase pot temp +5 °C.
Poor Top-Side Fill	Insufficient preheat/flux; narrow hole; thermal mass.	Increase top-side temp; allow longer contact.	Increase top-side temp 5–10 °C; reduce conveyor speed.
Skips / Poor Wetting	Oxidation; insufficient flux; short contact time.	Apply light second flux; reduce speed; use N₂ blanket.	Decrease conveyor speed.
Solder Balls / Spatter	Incomplete solvent evaporation; turbulent fountain.	Extend preheat; reduce fountain turbulence.	Increase preheat duration; lower fountain 0.5–1.0 mm.
Blowholes / Voids	Moisture in PCB; trapped volatiles; tight mask ring.	Pre-bake boards; extend gentle preheat; increase mask clearance.	Enforce pre-assembly bake; apply longer preheat.
Selective Soldering Non-Fills	Nozzle Z-height too high; path speed too fast.	Recalibrate Z-height; slow path; use two-pass approach.	Decrease path speed to 5–8 mm/s.