2.2 Fluxes, Alloys & Aids

FluxesHand soldering is a controlled process, but its chemical core relies on the quality and solderpurity alloys formof the chemicalmaterials used. This chapter details the critical selection of wire solder, flux core chemistry, and metallurgicalsupplementary foundationaids. Using the wrong alloy, excessive flux, or contaminants drastically compromises joint integrity and accelerates rework bench corrosion.

2.2.1 Solder Wire Mandates

Solder wire is a composite material consisting of everythe solder joint, quietly determining whether a connection wets instantly or fights back againstalloy, 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,diameter, and the rightflux supportcore. toolsEach parameter must be specified to ensure efficiency and joint quality.

A) Wire Diameter and Alloy

• Diameter: The wire diameter must be sized to match the volume of the joint. 0.3 – 0.5 mm wire is standard for control,SMT manufacturers avoid chasing defectspads and insteadfine-pitch buildrework. 0.8 – 1.0 mm wire is preferred for high-mass THT pins and lugs. Using wire that is too thick leads to excessive solder deposition and bridging.
• Alloy Purity: The alloy must be sourced with a stable,documented predictableCertificate process.of

  Analysis (CoA) to confirm metal percentages and minimize impurities that cause 2.2.1dull, granular joints or premature oxidation.

B) Flux familiesCore (whatSelection

they

The areflux andcore whycleans you’dthe pickpad them)during the initial wetting phase. The core chemistry must be compatible with the assembly's overall cleaning requirements.

FamilyFlux Core Type	What’s insideRequirement	Clean after?	Where it shines	Watch-outsRationale
No-cleanClean (rosin/resin, low-solids)NC)	Halide-freeMild activatorsformulation in alcoholrequired.	UsuallyLeaves no	Everydaya SMD/THT, rework	If overheated or puddled,benign residue can turn sticky/insulating—use just enough
VOC-free no-clean (water-based)	Organic acids in water	Usually no	Sites that limitdoes solvents	Needsnot morerequire preheatcleaning, tomaximizing dry;throughput. avoidHalides floodsintroduce (spitting)corrosion risk.
Water-washSoluble (OA)WS)	StrongMandates organicmandatory acids	Yescleaning (mustimmediately wash)after soldering.	OxidizedProvides leads,superior toughoxide barrels, big copper	Requires real wash process; don’t leaveremoval on boards
RMAstubborn (rosinor mildlyaged activated)	Classiccomponents rosin	Usuallybut no	Legacy/service	Canleaves brownhighly corrosive residue if overheated;left stillon gothe lightboard.

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

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2.2.2 GelSolder vsAlloy liquidChoice

flux

The selected alloy must match the Bill of Materials (BOM) and wherethe eachmanufacturing wins)thermal requirements.

• GelSn63/Pb37 /(Tin-Lead): tackyOffers a single, low eutectic melting point (183°C), providing a wide, forgiving thermal process window. It achieves superior wetting at lower temperatures. Use is restricted by RoHS compliance mandates.
• SAC305 (SnAg3.0Cu0.5): The standard lead-free alloy. Requires a higher operating temperature (liquifies at 217°C). It offers superior long-term reliability against thermal cycling fatigue compared to tin-lead.
• Low-Temp Bismuth-Based: Used exclusively for highly heat-sensitive components (e.g., plastics, displays) or specialized step-soldering processes. These joints are mechanically weaker and require strict qualification for shock/vibration.

2.2.3 Supplementary Fluxes

Extra flux

 is mandatory for rework, as the core flux is typically exhausted by the time the technician begins work.

• Stays putPurpose:: perfectSupplementary forflux chemically reactivates the surface, removes residual oxides, and improves the solder's surface tension, aiding drag soldering QFPs/QFNs,and fine-pitchwick touch-ups.usage.
• Form:Creates aFlux smoothis soldertypically “river”applied with avia bevel/hoofpen, tip.
• Pickliquid no-cleandropper, or gel. The pen is preferred for generalprecise rework,SMT application; OA gel onlyis ifoften you’llused wash.for large rework areas like BGA reballing.

LiquidMandate: The chemical compatibility of the supplementary flux must match the flux core. Do not mix flux chemistries (e.g., water-soluble pen or bottle)

• Wicks into holes and under leads—best for THT and wicking cleanup.
• Good pretreat on oxidizeda padsno-clean before adding wire.
• Use pens for control; bottles love to over-applyassembly).

2.2.4 Essential Rework Aids and Tools

UseRework justrequires enough:specialized atools thin,to shinyensure filmsafe beatscomponent puddles.removal Ifand itsite smokes hard before solder touches, you’re too hot or too much.preparation.

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

AlloyAid / Tool	Melts atFunction	Hand-feel	Typical setpoint (tip)	NotesRationale
Sn63/Pb37	183Solder °CWick (Braid)	FastRemoves wetting,solder forgivingfrom pads and through-holes via capillary action.	The wick must be 315–350flux-coated °C	Easiestand wider than the pad to hand-solder;achieve notefficient RoHSthermal transfer and removal.
SAC305Desoldering Pump (Sn-Ag-Cu)	217–221 °CVacuum)	SlightlyRemoves slowerbulk wettingsolder from THT barrels.	340–380 °C	RoHS; useRequires a biggerclean, grounded tip beforeand crankingnozzle tempto prevent thermal damage to the hole plating.
Sn42Bi58Bottom-Side Preheater	Raises the local board temperature to 80 – 120°C (low-temp).	138Mandatory °C	Quickfor melt,soldering brittle	260–300heavy °C	Savescopper plastics;layers fragileor joints;large avoidground mixingplanes; withdramatically SnPb/SACreduces the iron's required dwell time and minimizes thermal shock.
Micro-alloyed SACTinner/Cleaner	~217–221Used °Cto revive oxidized, black tips.	Chemical maintenance for the tip, allowing rapid recovery without excessive sanding or scraping.

Final Checklist: Materials and Aids Selection

Parameter	Mandate	Rationale
Solder Alloy	Must match BOM specification and thermal profile requirements (SnPb vs. SAC).	LikeEnsures SAC,correct ajoint bitintegrity steadierand melting point control.
Wire Diameter	0.3 – 0.5 mm for SMT; 0.8 – 1.0 mm for THT.	Prevents bridging and ensures controlled solder volume deposition.
340–380Flux °CCompatibility	BetterCore thermalflux fatigue;and samesupplementary techniqueliquid asflux SACmust be chemically compatible (e.g., both No-Clean and Halide-Free).	Prevents corrosive residues and residue interference with electrical testing.
Preheater	Mandatory use on all heavy copper or multi-layer ground plane rework.	Reduces required iron dwell time to less than 6 seconds, protecting the PCB laminate.
Tip Maintenance	Tinner/Cleaner used for immediate tip care; brass wool used for quick cleaning.	Maintains optimal heat transfer efficiency and extends tip life.

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.

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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.
  

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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.
  

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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.

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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.

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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.
  

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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.

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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
  

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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.