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'stepping up' to meet thermal challenges in wave soldering: when higher preheat temps and longer contact time don't improve hole fill, what's next?

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.

KARL SEELIG is vice president of technology at AIM Solder; kseelig@aimsolder.com.

CARLOS TAFOYA is technical applications manager at AIM.
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 The process uses SAC 305 solder and a
equipment 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[degrees]-108[degrees]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[degrees] 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[degrees]F per zone.
 * Topside PCS temperature up to 110DC
 * Some improvement in hole fill, but not
 quite yet acceptable
 Run 4: Increase preheat settings by
 another 50[degrees] F per zone. Increase
 flux loading
 by changing valve factor parameter from
 50% to 70%.
 * Topside PCB temperature up to
 120[degrees]C
 * Topside temp >100[degrees]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[degrees]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.


[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]
Key Process Parameters

Conveyor Speed: 1.25 ft/min

Total Contact Time: 8.3 sec

Solder Temperature: 525 [degrees]F

Peak Topside Temperature: 108[degrees]C

Duration (amblent to wave): 5:18

Preheater Location Zone a3 Set Zone a2 Zone a1
& Tips Set Point Set Point Set Point
 ([degrees]F) ([degrees]F) ([degrees]F)

Top: Calrod 250 300 300

Bottom: Forced Air 200 250 250

FIGURE 2. Original process could only reach topside temperatures
of 90[degrees]-108[degrees]C before burning out flux activators,
with preheat time of over 5 mm.


[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[ILLUSTRATION OMITTED]
Key Process Parameters

Conveyer Speed: 1.25 ft/min

Total Contact Time: 11.5 sec

Solder Temperature: 525[degrees]F

Peak Topside Temperature: 133[degrees]C

Duration (ambient to wave): 5:18

Preheater Location & Type Zone #3 Zone #2 Zone #1
 Set Point Set Point Set Point
 ([degrees]F) ([degrees]F) ([degrees]F)

Top: Calrod 380 470 470

Bottom: Forced Air 330 380 380

FIGURE 4. The new process achieves topside temperatures of
120[degrees]-130[degrees]C, with temperatures above
100[degrees]C for over 2 min. High thermal endurance flux
sustains its activity throughout the entire soldering cycle,
enabling better hole fill.
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Title Annotation:MATERIALS WORLD
Author:Seelig, Karl; Tafoya, Carlos
Publication:Printed Circuit Design & Fab
Date:Aug 1, 2013
Words:978
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