Learning objectives
- Run the wave soldering sequence of fluxing, preheat, wave contact and cooling
- Set and check the main wave parameters and match them to hole fill and bridging faults
- Explain how selective soldering works and when it is the better choice than wave
- Maintain nozzles, pallets and the solder pot so results stay repeatable
Lesson
### Lesson 1: Wave soldering
Wave soldering solders every through hole joint on a board in one pass over a river of molten solder. The board, often carried in a pallet that protects surface mount parts on its underside, moves through four actions. A fluxer, foam or spray, applies flux to clean the surfaces and help the solder climb into the holes. Preheaters warm the board, with topside temperatures in published guides typically around 100 to 150 degrees Celsius, driving off flux solvents and reducing thermal shock. The board then passes over one or two molten waves, a turbulent chip wave that reaches into tight places and a smoother main wave that completes the joints as the board peels away. For lead free SAC solder the pot runs around 250 to 260 degrees Celsius, and the contact or dwell time over the wave is about 2 to 4 seconds, set by conveyor speed. Dross is managed at the pot and the solder composition is analysed on schedule, because a drifting alloy makes drifting joints. These published windows are starting points, always validated against the flux, the machine and the product.
### Lesson 2: Selective soldering, and choosing between the two
Selective soldering solders only the joints the program names. A programmable nozzle, in single or multi nozzle or mini wave form, works point by point. Each site receives its own local fluxing, its own local preheat, and its own solder contact, with the program setting flux volume, nozzle size, solder temperature, contact time and the speed and angle at which the nozzle peels away. Nitrogen shrouding is fitted on many systems. Because only programmed sites see solder, nearby surface mount parts and heat sensitive components stay safe, and many boards need no pallet at all. Repeatability lives in the nozzle: a nozzle that wets poorly or is neglected will produce joints that wander from shift to shift, so nozzle care is a scheduled duty, not an afterthought.
The choice between the processes is engineering judgement. Many joints, a simple layout and high volume favour the wave. Sparse joints crowded near sensitive parts favour selective. James Onsomu teaches in his PCB quality book that wave soldering has quality parameters, including temperature profiles and solder flow characteristics, that must be monitored closely for reliable connections. The defect patterns teach the parameters. Bridging follows excess solder, poor peel off or weak flux. Poor hole fill follows low preheat, poor flux penetration, or heavy copper planes drinking the heat out of the joint. Icicles, blowholes from moisture and outgassing, solder balls and lifted pads each name their own cause for the operator willing to read them.
Key numbers
- Topside preheat at the wave: roughly 100 to 150 degrees Celsius in published guides, validated per flux and machine
- Solder pot for lead free SAC: around 250 to 260 degrees Celsius
- Wave contact or dwell time: about 2 to 4 seconds, set by conveyor speed
- Selective programs: set per joint, covering flux volume, nozzle size, temperature, contact time and peel off behaviour
Defects to spot
- Bridging and shorts: excess solder, poor peel off, or too little flux activity
- Insufficient hole fill: low preheat, weak flux penetration, or heavy thermal mass to ground planes
- Icicles: peel off too fast or solder too cool as the board leaves the wave
- Blowholes and pinholes: moisture or flux outgassing through the joint as it solidifies
- Solder balls: flux spatter or excess flux at the wave or at selective sites
- Lifted pads and lifted components: excess heat or mechanical stress at the wave
- Cold joints at selective sites: too little flux or too little heat in the per joint program
- Thermal damage at selective sites: dwell held too long on one joint
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. What is the typical solder pot window for lead free SAC at a wave machine, per published guides?
A. 180 to 190 degrees Celsius
B. 250 to 260 degrees Celsius
C. 320 to 340 degrees Celsius
D. Room temperature
Show answer
Answer: B
2. Boards show poor hole fill on joints connected to large ground planes. What is the most likely family of causes?
A. Too much preheat
B. Labels printed too dark
C. Flux that is too active
D. Low preheat, weak flux penetration, or heavy thermal mass drinking heat from the joint
Show answer
Answer: D
3. What sets the dwell time of a board over the wave?
A. Conveyor speed
B. The height of the factory ceiling
C. The colour of the pallet
D. The number of operators on shift
Show answer
Answer: A
4. When is selective soldering usually the better choice than wave?
A. When the board has hundreds of identical joints and a simple layout
B. When a few through hole joints sit close to surface mount or heat sensitive parts that must be protected
C. When the flux has expired
D. When the pallet is missing
Show answer
Answer: B
5. Selective joints begin to vary from shift to shift with no program change. What should be checked first?
A. The packing bench
B. The humidity in the office
C. The nozzle condition and its scheduled wetting and maintenance
D. The colour of the solder mask
Show answer
Answer: C
Previous: Stage 9: Through hole insertion
Next: Stage 11: Cleaning