1.2 Fluxing & Preheat Control

FluxingThe perfect solder joint begins with chemistry and heat. Through-hole soldering is a high-risk thermal process, and fluxing and preheat are the quietmandatory enablerscontrols ofrequired 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

Fluxto cleansguarantee andclean protects copper; preheat drives solvent outsurfaces and prevent component micro-cracking. This stage ensures that the component lead is chemically ready for the solder's molten embrace and that the assembly is thermally prepared to withstand the dramatic temperature change without failure.

1.3.1 The Selective Solder Mechanism

Selective soldering is the high-precision alternative to wave soldering, necessary for boards with activatesmixed technology where sensitive SMT components are on the chemistry.bottom Ifside. coverageProgramming this process is patchya direct translation of the board's thermal and geometric constraints into machine motion. Success is measured by the quality of the smallest joint, achieved by optimizing nozzle size, path sequence, and solder dwell time.

Key Components

1. Flux Applicator: A micro-jet or preheatspray nozzle applies flux only to the target pads.
2. Preheat Zone: Heaters (usually IR or convection) bring the joint area to the target top-side temperature (Chapter 1.2).
3. Mini-Wave Nozzle: A small, localized fountain of molten solder (often 2-15 mm in diameter) contacts the joint.

1.3.2 Programming Mandates: Motion and Thermal Management

The selective program must define the trajectory and the required contact time (dwell) for every joint.

A) Dwell Time and Penetration

Dwell time is wrong,the youamount getof time the joint remains in contact with the molten solder. This is the primary control for ensuring bridges,barrel icicles,fill.

blowholes,

• Standard poorJoint: 1.5 – 3.0 seconds is typical for standard leads and plated through-holes (PTHs).
• Heavy Thermal Load: Joints connected to large copper planes or thick boards require increased dwell time (greater than 3.0 seconds) to overcome the heat sink effect (Chapter 1.1).
• Verification: Dwell time is confirmed by inspecting the top-side fillfillet, and stickymeasuring residues.barrel Nailfill bothpercentage on a microsection.

B) Nozzle Choice and wave/selectivePin Clearance

The nozzle size must be matched to the component pitch and the component-to-component spacing.

• Nozzle Size: Choose the smallest possible nozzle that covers the entire pad cluster. A typical range is 4 mm to 8 mm for standard connectors.
• Pin Clearance: The program must ensure the nozzle tip maintains a 3 – 4 mm keepout clearance from all surrounding SMT components, lead bodies, and nearby THT pins to prevent thermal damage or accidental solder contact.

1.3.3 Sequence and Path Optimization

The order in which joints are soldered is critical for managing heat distribution and preventing mechanical stress.

A) Soldering Sequence

The path of the mini-wave must be strategically planned:

1. Solder Least Thermally Demanding Joints First: Start with smaller pins or those on thin traces. This allows the system to build up heat in the local area.
2. Solder Most Thermally Demanding Joints Last: Address large planes, chassis lugs, and high-mass components at the end of the sequence. This ensures the maximum thermal energy is available without overheating nearby sensitive components.
3. Process Clusters Logically: Solder all pins of a single connector in one continuous motion (drag soldering) whenever possible to maximize throughput and minimize repositioning time.

B) Drag vs. Dip Methods

Selective soldering becomes…utilizes calm.two primary motion patterns:

• Drag

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  (Most Common): The nozzle maintains contact and moves along the pin row. This is fast and achieves high throughput, but requires perfect pin alignment.
• Dip (Spot Soldering): The nozzle contacts a single pin or small cluster, pauses for the required dwell time, and retracts. This is mandatory for single pins or where nearby SMT components restrict drag motion.

1.3.4 Tooling and Process Control Checkpoints

Selective soldering relies heavily on precise tooling and consistent process metrics.

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

Flux familyCheckpoint	WhatProcess itControl isRequirement	Preheat need	Clean/no-clean	NotesRationale
No-clean,Nozzle low-solids (alcohol)Height	Rosin/resinControlled +to activators0.5 in– alcohol	Moderate1.0 mm	Usually nocontact cleanheight relative to the pad/pallet.	FlexibleEnsures window;the commonmini-wave ontop mixed-techremains stable and minimizes solder spatter.
VOC-freeFlux (water-based)Jet Calibration	Water + organic acids	Higher (more energyConfirmed to dry)apply flux only to the target pad cluster.	OftenPrevents noflux residue from contaminating adjacent SMT components or clean	Needs stronger preheat and tighter spray controlareas.
Water-washNitrogen (OA)Purity	100% nitrogen blanket or inert atmosphere over the mini-wave.	Mandatory for lead-free soldering; minimizes dross formation and improves wetting performance by preventing oxidation.
Solder Pot Purity	OrganicDross acids,removed highon activitya scheduled basis; alloy verified for copper contamination (Chapter 1.4).	Moderate	MustEnsures wash	Greatconsistent wettingviscosity onand toughprevents holes;defects plancaused washby capacityimpurities.

Rule

Final ofChecklist: 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.

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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-sprayProgram 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.

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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.
  

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1.2.5 Fast tuning by symptom (smallest fix first)Optimization

SymptomParameter	LikelyOptimization causeMandate	FirstImpact moveon Yield
PoorDwell top-side barrel fillTime	NotSet enoughto flux1.5 in– holes;3.0 preheatseconds toobaseline; low/unevenincreased for heavy planes/thick boards.	+1Guarantees lightbarrel sprayfill passand ortop slowfillet conveyor slightly; raise mid-zone preheat 5–10 °Cformation.
BridgingNozzle across fine rowsPath	FloodedSequence fluxmoves film;from coldlow-mass entryto high-mass joints.	TrimManages fluxheat dose;distribution; addprevents 1–2thermal sstress dwellon insensitive last preheat; check wave angleparts.
Solder balls/spittingMotion	SolventUse notdrag drivenfor offlinear pin rows; dip for isolated pins.	LowerMaximizes early-zonethroughput tempsand (dry,ensures don’tprecise boil);contact add short dwell; verify spray isn’t puddlinggeometry.
Brown/white residueKeepout	3 – 4 mmOver-baked flux;clearance VOC-freemaintained bakedfrom tooSMT longcomponents and plastic bodies.	ReducePrevents late-zonethermal tempdamage 5–10and °C;solder even the ramp; confirm conveyor speedsplash.
Blowholes/outgassingAtmosphere	Nitrogen (N2)Moist boards;flow fluxverified trappedat inthe holesnozzle.	BakeCritical boardsfor orreducing adddross and improving lead-free longer, gentlerwetting 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.

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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.
  

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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.
  

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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
  

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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.