When higher preheat temps and longer contact time don’t improve hole fill, what’s next?

Problem

• Wave soldering process cannot achieve topside fillets on thermally challenging assembly.
• Operators manually touch up 100% of solder joints on specific components.

PCB Description

• 3"x12" power management PCB is 150 mils thick with heavy copper planes throughout and ENIG final finish.
• It is densely populated with SMT components on both sides, and contains large PTH rectifiers and electrolytic capacitors.

Process and Equipment

• The process uses SAC 305 solder and a popular, no-clean VOC-free flux designed for Pb-free wave soldering. Local environmental regulations mandate use of VOC-free flux formulations.
• A selective solder pallet that holds two PCB assemblies shields the SMT components and adds additional thermal mass during soldering (Figure 1).
• The PCB is preheated to a topside temperature of 100°-108°C and has 8.3 sec. of wave contact at a conveyor speed of 1.25 ft./min. (Figure 2).
• The wave solder machine is an Electrovert Electra outfitted with a spray fluxer, three bottomside forced air preheaters, three topside Calrod preheaters, and nitrogen-inerted chip and smooth waves.

Diagnosis

• Process engineers have tried improving hole fill by re-profiling to increase preheat temperatures and/or contact time, but cannot get better results. In many cases, the results get worse as more heat or wave contact is added. The process shows classic symptoms of flux burnout.

Improvement strategy

• Switch to a flux that has better thermal endurance and methodically step up the heat in the process, observing changes in PCB temperature and solderability.

Results

Run 1: Change flux; maintain same process parameters and evaluate results.

• No change in topside hole fill
• Indicates flux activity or loading is not a factor

Run 2: Begin increasing preheat temperatures with small step of 30°F per preheat zone.

• Marginal change in topside temperature
• No change in topside hole fill, indicating need for more heat

Run 3: Increase preheat setting by additional 50°F per zone.

• Topside PCB temperature up to 110°C
• Some improvement in hole fill, but not quite yet acceptable

Run 4: Increase preheat settings by another 50°F per zone. Increase flux loading by changing valve factor parameter from 50% to 70%.

• Topside PCB temperature up to 120°C
• Topside temp >100°C for 2 min. to raise PCB core temperature
• Considerable improvement in topside fill, most joints are acceptable

New process:

• The new process maintains the same belt speed of 1.25 ft./min., but now achieves a topside temperature of 120°C and 11.5 sec. of total contact time (Figure 4).
• The VOC-free no-clean flux leaves no visible or palpable residue and provides high post-soldering electrical reliability.
• Quality and throughput are improved; costs and bottlenecks associated with manual touchup are reduced.

Karl Seelig is vice president of technology at AIM Solder; kseelig@aimsolder.com. Carlos Tafoya is technical applications manager at AIM.

What is Flux Burnout?

Flux burnout occurs in wave soldering when the flux’s activators get spent in the preheat portion of the process, before the circuit board reaches the solder wave. It can happen to no-clean and water-washable fluxes, and can present big problems on thermally massive PCB assemblies.

When heated, flux activators begin removing existing oxides from solderable surfaces, and continue removing new ones that form during the heating process. They should remain active throughout the soldering cycle to facilitate wetting, but have a finite lifespan. If the activators are fully expended during the preheat cycle, new oxides build up and hinder joint formation.

PCB assemblies with high thermal mass challenges – design elements like thick copper planes, bulky components or poor thermal relief on ground ties – need extended preheat cycles to warm them to soldering temperature and extended wave contact times to let solder wick up the holes. It is not uncommon for thermally challenging assemblies to experience flux burnout, especially with the slower conveyor speeds of Pb-free wave soldering processes.

Diagnosing flux burnout. Try this simple test: slow the wave solder machine’s conveyor speed and examine hole fill.

• If slowing the conveyor improved hole fill, the flux was still active. The increased preheat and contact time resulted in better hole fill.
• If slowing the conveyor did not improve hole fill, the flux was spent. The flux stopped cleaning the oxides before soldering was completed.

Most wave soldering fluxes are designed to maintain activity and reliability across a wide process window, from fast and cool profiles to slow and hot ones. When high thermal mass PCBs demand extreme time-temperature exposure, typical flux activators may not survive. Specialized activators with enhanced thermal endurance are needed to ensure good solder wetting, acceptable hole fill and reliable mechanical performance.