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1.3 Selective Solder Programming

Nozzle types, keep-out shields, paths, and dwell optimization—so your miniwave makes neat fillets without babysitting.

Selective soldering is all about controlling where the nozzle goes, how it meets the pins, and what the fountain looks like. The right nozzle type, approach angle, and path shape determine whether you get clean fillets or bridges and icicles. Start with a nozzle just big enough to cover the pad, keep fountain height to 1–2 mm above the board, and program a 5–10° entry for gas escape. Use short end-dwells and kick-outs to prevent bridging, and split long rows into segments for even heating. Tune dwell and speed by symptom—small changes in drag rate, pot temp, or preheat solve most defects. Protect SMT with pallet dams or clip-on shields, but often an angled path works better. Keep nozzles clean, skim dross, and check pump flow so recipes stay valid. For first articles, verify flux coverage, preheat profile, path clearance, and top-side fill before locking the program. Done right, selective soldering becomes fast, repeatable, and drama-free.

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)

  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”).


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



Bottom line: pick the right nozzle, protect nearby SMT with geometry and angle first (shields if needed), and program a path that engulfs, dwells, and exits cleanly. Set fountain height, speed, and dwell just high enough to hit top-side fill—then stop. With good fluxing and preheat, tiny programming moves make selective solder boringly reliable.


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