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2.2 Fluxes, Alloys & Aids

Fluxes and solder alloys form the chemical and metallurgical foundation of every solder joint, quietly determining whether a connection wets instantly or fights back against the iron. Their interaction with heat defines process windows, while aids like preheaters, wick, and tip cleaners turn difficult work into something repeatable and safe. By choosing the right chemistry for the board, the right alloy for compliance and reliability, and the right support tools for control, manufacturers avoid chasing defects and instead build a stable, predictable process.

2.2.1 Flux families (what they are and why you’d pick them)

Family

What’s inside

Clean after?

Where it shines

Watch-outs

No-clean (rosin/resin, low-solids)

Mild activators in alcohol

Usually no

Everyday SMD/THT, rework

If overheated or puddled, residue can turn sticky/insulating—use just enough

VOC-free no-clean (water-based)

Organic acids in water

Usually no

Sites that limit solvents

Needs more preheat to dry; avoid floods (spitting)

Water-wash (OA)

Strong organic acids

Yes (must wash)

Oxidized leads, tough barrels, big copper

Requires real wash process; don’t leave on boards

RMA (rosin mildly activated)

Classic rosin

Usually no

Legacy/service

Can brown if overheated; still go light

Rule of thumb: start with no-clean for hand work; go OA when wetting is genuinely hard and you can wash reliably.



2.2.2 Gel vs liquid flux (and where each wins)

  • Gel / tacky flux
    • Stays put: perfect for drag soldering QFPs/QFNs, fine-pitch touch-ups.
    • Creates a smooth solder “river” with a bevel/hoof tip.
    • Pick no-clean gel for general rework, OA gel only if you’ll wash.
  • Liquid flux (pen or bottle)
    • Wicks into holes and under leads—best for THT and wicking cleanup.
    • Good pretreat on oxidized pads before adding wire.
    • Use pens for control; bottles love to over-apply.

Use just enough: a thin, shiny film beats puddles. If it smokes hard before solder touches, you’re too hot or too much.



2.2.3 Picking the solder alloy (feel, temp, reliability)

Alloy

Melts at

Hand-feel

Typical setpoint (tip)

Notes

Sn63/Pb37

183 °C

Fast wetting, forgiving

315–350 °C

Easiest to hand-solder; not RoHS

SAC305 (Sn-Ag-Cu)

217–221 °C

Slightly slower wetting

340–380 °C

RoHS; use a bigger tip before cranking temp

Sn42Bi58 (low-temp)

138 °C

Quick melt, brittle

260–300 °C

Saves plastics; fragile joints; avoid mixing with SnPb/SAC

Micro-alloyed SAC

~217–221 °C

Like SAC, a bit steadier

340–380 °C

Better thermal fatigue; same technique as SAC

Don’t mix alloys on one joint (e.g., SnPb wire on SAC joints). Pick one, label the bench, and color-code spools/tips to avoid cross-contam.



2.2.4 Wire solder specs that actually matter

  • Diameter:
    • 0.3–0.5 mm → SMD and fine work (control).
    • 0.8–1.0 mm → THT and tabs (feed rate).
  • Flux core: no-clean, halide-free is a safe default.
  • Spool handling: cap when parked; store sealed & cool. Old wire can get crusty—if it won’t wet, don’t fight it.



2.2.5 Aids & helpers (small tools that change the day)

  • Preheaters (plate, hot air, IR): bring the work to 80–120 °C surface temp; planes and big pins go from stubborn to easy.
  • Solder wick (fluxed braid): sizes #2–#4 cover most jobs. Press the iron on the braid, not the pad. Lift while molten.
  • Solder pump: for big through-holes; follow with a quick wick polish.
  • Tip tinner/activator: revives dull tips—dip, wipe, re-tin.
  • Heat shunts / clips: protect plastics while you work a nearby joint.
  • Good light + 5–10×: see wetting and bridges in time to fix them.



2.2.6 Preheater basics (when and how to use)

Use a preheater when:

  • Pads tie to big copper/planes,
  • Thick or heavy boards,
  • Large through-hole pins or shield tabs,
  • You’re lifting pads because you’re lingering.

How to run it:

  • Target board surface 80–120 °C (warm to touch, not scorching).
  • Let mass soak for 30–90 s; then solder at normal tip temps.
  • Keep plastics/leads comfortable—move hot air, don’t cook one spot.

Outcome: same joint, lower tip temp and shorter dwell → less risk.


2.2.7 Quick playbooks (flux + alloy + aid)

Drag-solder a 0.5 mm QFP (RoHS)

  • Alloy: SAC · Flux: no-clean gel · Tip: bevel/hoof
  • Pre-tin two corner pads → place → flood a thin gel film → 340–360 °C, light solder feed, glide the hoof.
  • Clean bridges with flux + clean chisel or a kiss of wick.

Stubborn THT ground pin

  • Alloy: SAC (or SnPb in legacy) · Flux: liquid pen
  • Preheat pad area to 90–110 °C → larger chisel tip → touch lead+pad, feed from the opposite side.
  • If still starved: add a spot of gel, +10 °C on tip, try again.

Delicate connector near plastic

  • Alloy: keep normal · Flux: pen (no floods)
  • Mask/clip the plastic · Preheat gently · Short, repeatable touches.


2.2.8 Cleaning choices (only when you should)

  • No-clean: wipe only if sticky or customer requires; use approved solvent and lint-free swabs. Don’t smear flux across pads.
  • OA: always wash promptly; ionic residues corrode.
  • After wick work, a tiny solvent touch can remove flux glass and improve cosmetics.



2.2.9 Bench discipline (the boring parts that save boards)

  • Label each bench: alloy, tip temp band, flux type.
  • One alloy per bench (and per tip set).
  • Cap flux pens; purge gel syringes; log opened dates.
  • Replace tips when pitted; keep a spare chisel & bevel ready.


2.2.10 Pocket checklist (fluxes, alloys & aids)

Before work

  • Bench alloy matches traveler; spool/date OK
  • Tip size suits pad; tip freshly tinned
  • Flux chosen (gel for fine pitch, pen for THT); caps on hand
  • Preheater ready for planes/tabs; fume extraction on

During work

  • Thin film of flux—no puddles
  • Lowest tip temp that wets in 2–4 s (SMD) / 3–6 s (THT)
  • For tough joints: add preheat or bigger tip, not +50 °C

After

  • Clean only if OA or sticky
  • Re-tin tip; cap flux; note any alloy/flux changes on traveler



Conclusion: Standardizing flux selection, alloy usage, and support aids ensures joints flow cleanly and consistently without excess heat or rework. This disciplined approach reduces defects, protects sensitive parts, and keeps soldering reliable across operators and builds.

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