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1.2 Fluxing & Preheat Control

Fluxing and preheat are the quiet enablers of reliable soldering, setting the stage before molten metal ever touches the board. Flux chemistry removes oxides and shields copper, while controlled heating activates that chemistry and ensures solder wets cleanly through barrels. When these two steps are tuned and repeatable, soldering becomes less about firefighting and more about stable, predictable flow.

1.2.1 Why flux + preheat are a pair

Flux cleans and protects copper; preheat drives solvent out and activates the chemistry. If coverage is patchy or preheat is wrong, you get bridges, icicles, blowholes, poor top-side fill, and sticky residues. Nail both and wave/selective soldering becomes… calm.



1.2.2 Flux types (pick what your line can actually run)

Flux family

What it is

Preheat need

Clean/no-clean

Notes

No-clean, low-solids (alcohol)

Rosin/resin + activators in alcohol

Moderate

Usually no clean

Flexible window; common on mixed-tech

VOC-free (water-based)

Water + organic acids

Higher (more energy to dry)

Often no clean

Needs stronger preheat and tighter spray control

Water-wash (OA)

Organic acids, high activity

Moderate

Must wash

Great wetting on tough holes; plan wash capacity

Rule of thumb: VOC-free needs more uniform preheat; OAs demand washing discipline; low-solids no-clean is the easiest to live with if design is friendly.



1.2.3 Coverage: how much, how even, and where

What “good” looks like

  • Even, thin film on the underside copper/holes you intend to solder—no dry islands, no puddles.
  • Just enough to wet through barrels; “more” rarely helps and often spits/balls.

How to get there

  • Spray fluxer (inline):
    • Set pressure/speed to coat only the windowed areas.
    • Use UV tracer (if available) and a blacklight: you should see a consistent “glow carpet”—no zebra stripes.
    • Verify no overspray onto masked SMT (unless pallet shields it).
  • Foam fluxer (wave):
    • Keep specific gravity/°Bé in the vendor band; log it each shift.
    • Stone height and air rate stable = repeatable bubbles = repeatable coat.
  • Selective (mini-wave):
    • Micro-spray just the THT cluster; program two light passes over dense pins instead of one heavy blast.
    • Mask tall SMT or angle the nozzle to avoid flux shadowing.

Measuring dose
Pick one method and do it at least daily: weight gain on a coupon, vendor’s acid number/titration, or test coupons that show solder spread. The goal is consistency, not chasing a magic number.



1.2.4 Preheat: temperature bands that work

Two jobs: dry the flux (no boil at the wave) and activate it (chemistry on). Track top-side copper temperature right before the solder contact.

Typical targets at wave entry (guide, confirm with your flux datasheet):

  • No-clean (alcohol): 90–120 °C top-side, ramp about 1–3 °C/s
  • VOC-free (water-based): 110–140 °C top-side, ramp 1–3 °C/s
  • OA (water-wash): 100–130 °C top-side, ramp 1–3 °C/s

Signals you’re right

  • Flux no longer glossy at the last preheat, no active bubbling at wave entry.
  • Light, even smoke (rosin) right as solder hits; none before.
  • Top-side wetting appears quickly on test holes (windowed header or coupon).

Signals you’re wrong

  • Too cold: spitting/solder balls, poor top-side fill, dull joints.
  • Too hot: burnt/brown residue, tacky boards, increased oxidation, lifted mask around pads.



1.2.5 Fast tuning by symptom (smallest fix first)

Symptom

Likely cause

First move

Poor top-side barrel fill

Not enough flux in holes; preheat too low/uneven

+1 light spray pass or slow conveyor slightly; raise mid-zone preheat 5–10 °C

Bridging across fine rows

Flooded flux film; cold entry

Trim flux dose; add 1–2 s dwell in last preheat; check wave angle

Solder balls/spitting

Solvent not driven off

Lower early-zone temps (dry, don’t boil); add short dwell; verify spray isn’t puddling

Brown/white residue

Over-baked flux; VOC-free baked too long

Reduce late-zone temp 5–10 °C; even the ramp; confirm conveyor speed

Blowholes/outgassing

Moist boards; flux trapped in holes

Bake boards or add longer, gentler preheat; relieve mask near pads

Sticky boards

High solids or cold exit

Trim dose; ensure top-side hits band; confirm pot temp isn't compensating with extra heat



1.2.6 Instruments that make this repeatable

  • Top-side thermocouples taped/soldered to copper near THT fields; run a profile per product and after oven/line changes.
  • IR pyrometer aimed just before wave contact for spot checks between profiles.
  • Flux monitor: UV lamp snapshot, specific gravity/°Bé log, and a daily coupon test.
  • Conveyor tach: speed readout tied into the recipe—no “about 1.2 m/min” guessing.

1.2.7 Housekeeping: small rituals, big stability

  • Filter flux and purge lines weekly; clean spray nozzles/foam stones (clogs = stripes).
  • Keep flux tank lids closed; alcohol evaporates and changes solids fast.
  • Wipe preheat reflectors/IR panels; a dusty heater is a random heater.
  • Pallet seals clean and flat—flux doesn’t fix leaks.
  • Park profile plots and flux logs with the product’s Golden Recipe.



1.2.8 Pocket checklists

Setup (per product)

  • Flux family & dose method chosen (UV/weight/titration)
  • Spray/foam settings saved; overspray check done
  • Top-side preheat target set (per flux); ramp ≈ 1–3 °C/s
  • TCs placed near worst-case THT clusters; profile saved

Start of shift

  • °Bé / density logged; UV pattern looks even
  • Preheat zones at setpoint; conveyor speed verified
  • Quick IR spot: top-side temp in band at wave entry

If defects rise

  • Adjust preheat first (dry & activate), then dose
  • Re-run 1–2 panels, inspect top-side fill/bridging
  • Save “before/after” note in the recipe comments



By keeping flux coverage uniform, preheat in band, and adjustments minimal, assembly avoids spitting, bridging, and residues. The payoff is smooth soldering with fewer surprises—clean joints, stable yield, and a process that runs calmly day after day.


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