1.3 Selective Solder Programming

Selective soldering transforms a complex manual skill into a programmable, repeatable process, but only if the recipe balances geometry, heat, and flow. Every choice—the nozzle type, fountain height, entry angle, and drag path—directly shapes how fillets form and whether defects appear. With the right setup, the miniwave becomes a precise tool that delivers neat joints without masking struggles or operator intervention.

1.3.1 The machine in one glance (what you actually program)

A selective solder cell is a jet/spray fluxer → preheat → solder pot with one or more miniwaves. Your program turns three knobs into repeatable joints:

Where the nozzle goes (XY path and Z heights),
How it meets each pin group (angle, speed, dwell),
What the fountain looks like (height, nitrogen, pump).

Everything else—bridges, icicles, top-side fill—is a consequence of those three.

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).

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.

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) Engulf

Drag speed: start 5–12 mm/s for 1.0–1.6 mm holes; slower for thick boards/heavy planes.
Contact length: keep 1–2 pad diameters of wave “under” the pin as you move—if the footprint outruns the cup, you’ll starve the barrel.
Angle: 5–10° in the direction of travel helps gas escape and reduces back-side bridges.

C) Exit

Dwell on last 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 even and bridges drop.
Mixed mass: light pass first (quick drag) for wetting, then a second, slower pass only on stubborn pins.

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 → tweak

Symptom	First move	If still there…
Poor top-side fill	+0.2–0.5 s dwell or −2 mm/s speed	Raise preheat 5–10 °C; bump pot +5 °C; add second light pass
Bridging at row end	Add 0.3–0.5 s end dwell + quick kick-out	Reduce fountain 0.2–0.5 mm; steer a thief tail
Icicles / spears	Slightly faster exit; +5 °C pot	Trim dwell; confirm lead protrusion (12.1)
Solder balls/splash	Lower fountain 0.5–1.0 mm	Check flux dryness (12.2); reduce speed oscillation
De-wet / dull joints	+5 °C pot or +0.2 s dwell	Confirm finish age; flux coverage; try N₂ increase

Nitrogen: keep O₂ low enough for clean wetting; too much flow roughens the wave—find the minimum that keeps oxides down.

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.

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.

1.3.8 Common headaches → smallest reliable fix

Pain	Why it happens	Smallest fix that sticks
Bridges in tight 2.00 mm rows	Cup overfills the trench; exit too lazy	Smaller nozzle; add end-dwell+kick; 5–10° angle
Random non-fill on ground pins	Heat sink → flux exhausted	Raise preheat; add second light pass; slow 2 mm/s on those pins only
Splash marks near shields	Fountain too tall / early turbulence	Lower fountain; soften entry on bare board
Icicles on tabs	Excess dwell with cold exit	Faster lift; +5 °C pot; tighten protrusion spec
Blowholes	Moist board / trapped volatiles	Bake 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.

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.

1.3.10 First Article script (10 minutes that pays)

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

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

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.