2.2 Fluxes, Alloys & Aids
When to use gel vs liquid flux, leaded vs lead-free, and preheaters.
Fluxes, solder alloys, and support tools are the hidden levers that turn a tricky soldering job into a smooth, repeatable process. Flux—whether no-clean, water-wash, or rosin-based—does far more than just “help solder stick”; it clears oxidation, improves wetting, and can even protect the joint mid-process, but its chemistry and cleanup needs vary widely. Choosing between gel and liquid flux changes how the solder flows, just as picking the right alloy—SnPb for ease, SAC for RoHS compliance, bismuth blends for low temperature—affects both temperature control and joint reliability. Even the size of your solder wire or the presence of a good preheater can be the difference between fighting a stubborn ground pin and breezing through it. The supporting cast—wick, pumps, tip tinning compounds, and magnification—keeps the process consistent and the results clean. In the end, success comes from matching each job with the right combination of chemistry, metal, and heat strategy, then handling it with steady, deliberate technique.
2.2.1 Flux families (what they are and why you’d pick them)
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)
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
Bottom line: pick a mild, controlled flux (gel for fine pitch, liquid for holes), match the alloy to requirements (SnPb easiest, SAC common, Bi only when you must), and use preheat to reduce dwell instead of cranking temperature. With the right aids—wick, hoof tip, steady light—hand joints get quick, shiny, and repeatable.