1.3 Selective Solder Programming

SelectiveThe perfect solder joint begins with chemistry and heat. Through-hole soldering transformsis a complexhigh-risk manual skill into a programmable, repeatablethermal process, butand onlyfluxing ifand preheat are the recipemandatory balancescontrols geometry,required heat,to guarantee clean surfaces and flow.prevent Everycomponent choice—micro-cracking. This stage ensures that the nozzlecomponent type,lead fountainis height,chemically entryready angle,for the solder's molten embrace and drag path—directly shapes how fillets form and whether defects appear. Withthat the rightassembly setup,is thermally prepared to withstand the miniwavedramatic becomestemperature a precise tool that delivers neat jointschange without masking struggles or operator intervention.failure.

1.3.1 The Selective Solder Mechanism

Selective soldering is the high-precision alternative to wave soldering, necessary for boards with mixed technology where sensitive SMT components are on the bottom side. Programming this process is a 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 spray 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 the amount of time the joint remains in contact with the molten solder. This is the primary control for ensuring barrel fill.

• Standard Joint: 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 fillet and measuring barrel fill percentage on a microsection.

B) Nozzle Choice and Pin 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 glancecontinuous motion (what you actually program)

   A selective solder cell is a jet/spraydrag fluxersoldering) →whenever preheatpossible →to soldermaximize pot with one or more miniwaves. Your program turns three knobs into repeatable joints:

   • Where the nozzle goes (XY paththroughput and Zminimize heights),repositioning time.
   • How

it

B) meetsDrag eachvs. pinDip group (angle, speed, dwell),

What the fountain looks like (height, nitrogen, pump).

Methods

EverythingSelective else—bridges,soldering icicles,utilizes top-sidetwo fill—isprimary amotion consequence of those three.

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1.3.2 Nozzle types & when to use them

Nozzle	What it is	Shines at	Watch-outs
Laminar wettable (titanium, coated)	A small, smooth “cup” that wets with solder	General purpose headers, 2.54 mm pitch, medium mass	Needs clean coating; too tall a fountain → splash
Non-wettable	Edges don’t wet, sharper jet	Fine pitch, close SMT keepouts, minimal side creep	Slightly narrower process window
Turbulent/chip wave	Vigorous flow, rougher surface	Stubborn holes, heavy copper/ground lugs	Easy to overdo → bridging/splatter
Multi-nozzle gang	Several small cups at once	Parallel rows with identical geometry	Less flexible around odd parts
Mini-dip (selective dip pot)	Pocketed bath (no XY drag)	Press-fit pins, big tabs, shields	Needs precise pallets/masks

Sizing tip: start with nozzle Ø ≈ pad diameter + 1–2 mm (enough to engulf pad, not the neighborhood).

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1.3.3 Keep-out shields & masking (protect the innocent SMT)

• Pallet windows: machine the composite so only THT pads are exposed; give ≥ 2.5–3.0 mm dam to seal (13.1).
• Clip-on fences: thin titanium shields (magnet/fixture posts) around tight SMT—great for NPI before you cut a new pallet.
• Angles beat shields: often a 5–10° approach with a smaller nozzle clears nearby parts so you don’t need heroic masking.
  

Design echo: keep 3–4 mm component keepout around THT groups you plan to solder with a miniwave.

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1.3.4 Paths that don’t make bridges (drag logic that works)

Think entry → engulf → exit.

A) Entry

• Approach slightly up-stream of the first pin; lead in from bare FR-4 so solder is stable before you touch copper.
• Z-down until the board just kisses the fountain (1–2 mm cup rise visible). Program a soft Z to avoid slamming the pallet.

B) Engulfpatterns:

• Drag speed(Most Common): start 5–12 mm/s forThe 1.0–1.6nozzle mmmaintains holes;contact slowerand formoves thick boards/heavy planes.
• Contact length: keep 1–2 pad diameters of wave “under”along the pin asrow. youThis move—ifis 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 footprintrequired outrunsdwell the cup, you’ll starve the barrel.
• Angle: 5–10° in the direction of travel helps gas escapetime, and reducesretracts. back-sideThis bridges.is mandatory for single pins or where nearby SMT components restrict drag motion.

1.3.4 Tooling and Process Control Checkpoints

C)Selective Exit

soldering

• Dwellrelies heavily on lastprecise pad: 0.3–0.8 s dwell at row end lets the meniscus finish; then a quick kick-out to bare board.
• Add a “thief tail” path beyond the last pin if the layout didn’t include robber pads.

Patterning

• Long headers: program segmented passes (e.g., 6–8 pins per pass) with tiny step-backs; heat stays eventooling and bridgesconsistent drop.
• Mixedprocess mass: light pass first (quick drag) for wetting, then a second, slower pass only on stubborn pins.
  

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1.3.5 Dwell & fountain tuning (hit top-side fill without icicles)

Your dwell is set by thermal mass × hole geometry × preheat.

Starting bands (tune by build)

• Pot temperature: SAC 260–275 °C; SnPb 240–255 °C.
• Fountain height: visible rise 1–2 mm above board; more is splashy, less starves.
• Drag speed: 5–12 mm/s (thick/heavy → slower).
• Spot dwell (single pins, tabs): 0.8–2.0 s on target.
  

Fast symptom → tweakmetrics.

SymptomCheckpoint	FirstProcess moveControl Requirement	If still there…Rationale
PoorNozzle top-side fillHeight	+0.2–Controlled to 0.5 s– dwell1.0 ormm −2contact mm/sheight speedrelative to the pad/pallet.	RaiseEnsures preheatthe 5–10mini-wave °C;top bumpremains potstable +5and °C;minimizes addsolder second light passspatter.
BridgingFlux atJet row endCalibration	AddConfirmed 0.3–0.5to sapply endflux dwellonly +to quickthe kick-outtarget pad cluster.	ReducePrevents fountainflux 0.2–0.5residue mm;from steercontaminating aadjacent thiefSMT tailcomponents or clean areas.
IciclesNitrogen / spearsPurity	Slightly faster exit100%; +5nitrogen °Cblanket potor inert atmosphere over the mini-wave.	MandatoryTrim dwell;for confirmlead-free leadsoldering; protrusionminimizes (12.1)dross formation and improves wetting performance by preventing oxidation.
Solder balls/splashPot Purity	LowerDross fountainremoved 0.5–on a scheduled basis; alloy verified for copper contamination (Chapter 1.0 mm4).	CheckEnsures fluxconsistent drynessviscosity (12.2);and reduceprevents speeddefects oscillation
De-wetcaused /by dull joints	+5 °C pot or +0.2 s dwell	Confirm finish age; flux coverage; try N₂ increaseimpurities.

Nitrogen:

Final keepChecklist: O₂Selective lowProgram enoughOptimization for clean wetting; too much flow roughens the wave—find the minimum that keeps oxides down.

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1.3.6 Teach points, Z & vision (make programs portable)

• Fiducials & tooling pins: pick two corners; teach once, then let offset handling place the path accurately.
• Z calibration: touch-off routine per pallet/product; store Z-zero with the recipe. A 0.3 mm Z error is the difference between perfect and splashy.
• Keepout polygons: import from CAD (or draw once) so the CAM won’t route paths under tall SMT or labels.
• Variant handling: parameterize pot temp, speed, dwell as named variables (e.g., ROW1_SPEED, TAB_DWELL) so tweaks are one field, not a re-teach.
  

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1.3.7 Cycle time without risk (go faster the right way)

• Parallelize with dual nozzles only on symmetric rows; keep single-nozzle for odd groups.
• Use two-stage passes (quick wet + targeted slow) instead of one long slow drag; time drops with equal or better quality.
• Skip-cool: hop between groups far apart to avoid local overheating; come back for second passes after a few seconds.
  

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1.3.8 Common headaches → smallest reliable fix📝

PainParameter	WhyOptimization it happensMandate	SmallestImpact fixon that sticksYield
Dwell Time	BridgesSet into tight1.5 2.00– mm3.0 rowsseconds baseline; increased for heavy planes/thick boards.	CupGuarantees overfillsbarrel thefill trench;and exittop toofillet lazy	Smaller nozzle; add end-dwell+kick; 5–10° angleformation.
Nozzle Path	RandomSequence non-fillmoves onfrom groundlow-mass pinsto high-mass joints.	HeatManages sinkheat →distribution; fluxprevents exhausted	Raisethermal preheat; add second light pass; slow 2 mm/sstress on thosesensitive pins onlyparts.
Motion	SplashUse marksdrag nearfor shieldslinear pin rows; dip for isolated pins.	FountainMaximizes toothroughput talland /ensures earlyprecise turbulence	Lowercontact fountain; soften entry on bare boardgeometry.
Keepout	3 – 4 mmIcicles onclearance tabsmaintained from SMT components and plastic bodies.	ExcessPrevents dwellthermal withdamage coldand exit	solder splashFaster lift; +5 °C pot; tighten protrusion spec.
Atmosphere	Nitrogen (N2)Blowholes flow verified at the nozzle.	MoistCritical boardfor /reducing trappeddross volatilesand improving lead-free	wettingBake boards; gentler preheat; relieve mask around holes.

If your fixes smell like flux or preheat, revisit 13.2 first—selective can’t outrun bad prep.

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1.3.9 Maintenance that keeps recipes valid

• Nozzle hygiene: wipe oxides; re-lap/replace worn cups—shape drift changes flow.
• Dross control: skim per shift; dirty pots fake “low temp” behavior.
• Pump & seals: constant flow—no surging.
• Nitrogen knife (if fitted): verify angle/flow; too hard a knife destabilizes the wave.
• Golden coupon: weekly run a small header board; compare top-side fill photos to the golden set.
  

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1.3.10 First Article script (10 minutes that pays)

1. Flux UV check on the THT window; adjust spray if zebra-striped.
2. Preheat profile: top-side thermocouple near the densest pins—hit your band.
3. Teach XY/Z on pallet pins; verify clearance over tallest SMT.
4. Run quick-wet pass, then targeted slow pass on one header.
5. Inspect: top-side fill, bridges, icicles.
6. Tweak one knob at a time (speed, dwell, fountain). Save recipe with a note (“+0.3 s end-dwell row A”).
   

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1.3.11 Pocket checklists

Setup

• Nozzle type/Ø chosen; fountain height set (1–2 mm rise)
• Flux dose uniform (UV/weight) on exposed THT areas
• Preheat hits band at wave entry; conveyor speed verified
• XY path clear of SMT; 5–10° approach angle where possible
• Z-zero taught on pallet; clearance check passes
  

During run

• End-dwell + kick programmed on rows; thieves used if needed
• Two-stage passes on mixed-mass groups
• Nitrogen stable; pot skimmed; pump flow steady
  

If defects rise

• Adjust dwell/speed first; then fountain; recheck preheat
• Capture before/after photos; update recipe comments
  

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By standardizing nozzle use, path strategies, and dwell tuning, selective soldering runs consistently across shifts and products. The reward is faster cycles, fewer touch-ups, and joints that pass inspection quietly—proof that careful programming keeps quality predictable.