1.5 Common defects & corrections

When encountering solder balls, non-fills, or bridging, it is extremely helpful to systematically review the process parameters rather than attributing the issue solely to the wave itself. These defects are frequently the result of subtle preparation issues. In this chapter, we outline procedures for managing flux activation and the thermal ramp rate. Thoughtfully controlling these factors is an excellent way to evaporate volatile solvents, stabilize the temperature differential (∆T) across the board, and improve the throughput of the THT process by minimizing rework.

The troubleshooting protocol

A majority of THT soldering defects relate back to flux application, preheating, or wave dynamics. Effective troubleshooting benefits from a calm, systematic approach: verify preparation first, adjust only one variable at a time, and evaluate the result thoughtfully on a single panel.

Adjusting multiple machine settings simultaneously can obscure the root cause and make it harder to identify the truly effective solution.

Preparation Check: Before adjusting any wave settings, take a moment to verify that flux coverage is adequate and the top-side temperature is within the target range just as the board enters the wave.
Mechanical Check: Verify that pallet seating is secure and clearances look correct. Ensure the conveyor fingers are clean and the conveyor angle is properly configured.
Process Adjustment: Once preparation and mechanics look good, gently change one parameter on the pot (e.g. conveyor speed/dwell, wave height, or chip wave balance). Then, observe the result on a single sample board.

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.	Consider raising the chip wave by 0.2–0.5 mm to increase scrubbing action, or increase dwell time slightly (+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.	Apply a slight deceleration followed by a steady exit from the wave; recommend a solder thief in the next design revision.
Icicles or Trailing Solder	Excessive dwell time leading to a cold exit; component leads may be cut too long.	Try increasing conveyor speed to expedite the exit; utilize an air or N₂ knife to trim trails; or bump 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 attached to a large copper layer acting as a heat sink.	Try increasing the top-side temperature target by 5–10 °C; reduce conveyor speed 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.	Consider applying a second, very light flux spray pass; decrease conveyor speed; or utilize an N₂ blanket 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.	Try increasing preheat duration (a longer, slower ramp is better than a higher peak); lower 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 perhaps too tight, restricting outgassing.	Consider 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; try decreasing path speed to 5–8 mm/second; a two-pass approach (quick pre-wet followed by slower final pass) can help 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 issues are best resolved collaboratively in the next PCB design revision.

Hole and Pad Geometry: If holes are too narrow for the component leads or annular rings are insufficient, proper filling is naturally obstructed. This can also lead to pad lifting during any necessary rework.
- Suggestion: Propose a lead diameter clearance of +0.20–0.45 mm and encourage 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 naturally causes the planes to act as aggressive heat sinks, often resulting in cold joints.
- Suggestion: Recommend implementing 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 naturally increases the probability of bridging and solder splash.
- Suggestion: Utilize solder thieves on the trailing edge of fine-pitch rows and try to maintain a comfortable 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). A helpful secondary control is Pot Temperature (trying 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 looks good, a secondary control is gently adjusting the Chip Wave height and dwell to enhance scrubbing action.
Addressing Icicles or Trailing Solder: The primary lever here is adjusting the peel-off angle, the air knife, or the exit timing. Simply increasing the pot temperature is generally not the most effective approach for icicles.

Selective vs. wave soldering adjustment

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

Selective Soldering Adjustments: Process adjustments here are beautifully localized. Parameters such as nozzle diameter, Z-height, path speed, or spot dwell time are modified for a specific joint. For challenging joints, a two-stage pass is often much more effective than turning up 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. It’s usually best to avoid using the Main Wave for scrubbing, as this can introduce excessive turbulence and widespread bridging.

Final Checkout: Common defects & corrections

Verification Point	Immediate Check (Containment)	Preventative Focus (Design Revision)
Preparation Status	Verify Top-Side Temperature lands within the flux manufacturer’s specific range.	Review DFM guidelines for adequate thermal relief on joints that frequently exhibit cold solder.
Bridging Prevention	Gently adjust Chip Wave height/dwell or utilize the Air/N₂ Knife.	Suggest Solder Thief Pads in the design or consider modifying the wave pallet for better flow.
Resolving Non-Fills	Slightly decrease Conveyor Speed or try +5 °C on the Pot Temperature. A second light flux pass may help.	Review hole clearance specifications against component lead diameters in the master design files.
Eliminating Solder Balls	Try increasing Preheat Time to ensure the board is comfortably dry upon wave entry. Check the flux system for any blockages.	Ensure solder mask clearance is at least 0.10 mm larger than the pad to easily allow flux gas venting.
Adjustment Protocol	Change just one process variable at a time and document the adjustments.	Incorporate successful process changes into standard procedures once verifyed across a few production runs.