3.2 Cleaning Methods & Fixtures

Aqueous/solvent/vapor, spray vs ultrasonics, and fixturing that protects parts while actually getting them clean.

Cleaning circuit boards is as much about how you hold and protect them as it is about the chemistry you choose. Aqueous sprays, semi-aqueous systems, vapor degreasing, and even targeted ultrasonics each have their sweet spots—sprays for most mixed-technology boards, vapor for tight gaps or water-sensitive builds, and semi-aqueous for stubborn flux loads. The key is controlling the “4T+M” variables: time in each stage, wash and rinse temperatures, turbulence from nozzles or rotation, titrated chemistry strength, and the megohm rating of final DI water. Good fixturing tilts boards for drainage, shields vulnerable parts, and leaves no pockets where fluid can hide. Strong rinses, clean baths, and the right drying method—whether convection, IR assist, vacuum bake, or solvent displacement—finish the job without residue or corrosion. By pairing method selection with proof tools like UV tracers, ionic testing, and adhesion checks, cleanliness becomes consistent, documented, and process-driven rather than trial and error.

3.2.1 Methods at a glance (what each is good for)

Method	How it works	Best for	Pros	Watch-outs
Aqueous (spray-in-air)	Hot DI + chemistry, high-energy jets, then DI rinses	Mixed tech, most no-clean/OA residues	Scalable (inline), low solvent risk, strong rinsing	Rinse quality & drying under BTCs; chemistry control needed
Aqueous immersion	Soak + spray bars/rotation	Dense racks, shadowed areas	Contact from all sides	Can trap chemistry unless rinses are strong
Semi-aqueous	Solvent loosens → water rinse removes	Heavy/wave residues, flux-rich	Powerful on stubborn soils	Two chemistries to manage; good emulsification required
Vapor degrease (modern solvents)	Hot solvent vapor condenses, dissolves residues; gravity drains	Tight gaps, hydrophobic soils, water-spot-sensitive parts	Drying is inherent; minimal water	Solvent selection, capture & EHS; plastics compatibility
Ultrasonic (targeted)	Cavitation scrubs in a bath	Burnt flux on robust parts, fixtures	Gets into crevices	Risk to MEMS/relays/crystals; use cautiously or avoid

Inline vs batch: Inline is repeatable for volume; batch is flexible for NPI and diverse product sizes.

3.2.2 How to choose (simple matrix)

Geometry / Risk	Residue load	Coating/HV/Hi-Z?	Recommendation
Roomy standoffs, light no-clean	Low	No	Aqueous spray (short cycle) or vapor if water is hard to manage
Many BTCs/QFNs, tight gaps	Medium–High	Maybe	Aqueous spray + hot DI cascade with strong drying, or vapor
Flux-rich wave/selective	High	Sometimes	Semi-aqueous → hot DI rinse or robust aqueous with higher impingement
Water-sensitive labels/parts	Any	Yes/No	Vapor or aqueous with fixturing & shields; verify compatibility
MEMS/relays present	Any	Yes/No	Avoid ultrasonics; use spray/vapor and protect components

3.2.3 The 4T+M controls (what you actually set)

Time: wash + rinse + dry durations; long enough to saturate crevices, not cook labels.
Temperature: wash 45–65 °C typical; rinse 25–60 °C; vapor solvents per spec.
Turbulence (impingement): nozzle pressure/angle, spray pattern, part rotation.
Titration (chemistry strength): maintain by titration or refractometer; adjust for soil load.
M—Megohm DI: final rinse ≥ 10–15 MΩ·cm (process target), stable and logged.

3.2.4 Spray-in-air setup (what “good” looks like)

Nozzles: mix of fan + cone; aim across and into component fields; stagger heights to break shadowing.
Pressure/flow: enough to flip beads from under BTC edges (don’t sandblast mask). Start around 1.5–3.0 bar and tune by coupon.
Conveyor/rotation: slow enough for full wet-out; rotate or oscillate baskets for complex builds.
Rinse train: at least 2–3 stages (counter-flow/cascade). Last stage is hot DI to cut spots.
Breaks & drains: brief dwell after each stage so chemistry doesn’t drag forward.

Proof tools: glass test coupon with fiducials, hydrophobic marker tests, and UV tracer to check spray reach.

3.2.5 Ultrasonics (if you must)

Limit to fixtures, shields, pallets, or very robust boards.
Keep frequency ≥ 40 kHz and power low; short bursts.
Do not use near MEMS mics/gyros, reed relays, quartz cans, or partially sealed components.
Always rinse and dry thoroughly—ultrasonic loosened soils redeposit if not carried off.

3.2.6 Vapor degrease basics (modern, compliant units)

Solvent choices: modern fluorinated / modified alcohol blends—check plastics, labels, and EHS.
Zones: boil sump → rinse sump → freeboard; keep freeboard chiller cold to prevent loss.
Process: pre-clean dip (optional) → vapor zone dwell until drip runs clear → slow lift to avoid streaks.
Maintenance: water/sebum load skimming, solvent purity testing, leak checks, operator PPE.

3.2.7 Fixtures & shields (cleaning is 50% fixturing)

Goals: expose residues, prevent water traps, and protect “do-not-wet” parts.

Angle for drainage: tilt 10–20° so trapped pockets empty; avoid perfectly flat panels.
Open architecture: wire frames or perforated trays; no solid pans under boards.
Hold-downs: minimal contact points; avoid masking solder joints; use PEEK/Delrin grips that survive chemistry/heat.
Component covers: snap-on caps for relays, MEMS, trimmers, speakers, buzzers, displays, open frame coils, and any user-lubed mechanisms.
Label guards: mask or shield labels/ink that aren’t wash-proof; spec wash-resistant labels at design time.
Connector attitude: orient so housings don’t cup water; leave ports facing downstream.
Board spacing: if racking multiples, keep ≥25–40 mm gaps for spray reach and airflow.
Drain time: post-rinse tilt for 10–20 s before dryers.

3.2.8 Chemistry & rinse control (keep numbers, not guesses)

Wash bath: titrate or refractive index each shift; top up deliberately—don’t chase foam.
Filters & oil skimmers: change per delta-P or hours; soil-loaded baths redeposit grime.
DI plant: log resistivity, TOC if available; sanitize on schedule.
Carry-over watch: conductivity of intermediate rinses trending up = drag-out; add a stage or increase overflow.

3.2.9 Drying that really dries

Method	Where it shines	Notes
Convection (hot air)	General purpose	Plenty of airflow; don’t just heat—move air
IR assist	Heavy boards	Mind plastics; combine with airflow
Vacuum bake	BTCs/tight gaps	60–90 °C under vacuum for 10–30 min pulls moisture from crevices
Solvent displacement	Water-spot sensitive	Use approved displacement fluids; verify material compatibility

Proof: weight before/after, IR camera for cold wet spots, or a quick “blot & UV” check on suspect areas.

3.2.10 Symptom → smallest reliable fix

Symptom	Likely cause	First fix
White residue / streaks	Incomplete rinse; chemistry dried on	Add hot DI stage; slow conveyor; increase overflow; verify titration
Water spots under BTCs	Drying air can’t reach; no drain angle	Add tilt + drain dwell; vacuum dry; raise airflow, not just temp
Sticky flux patches	Low impingement; chemistry weak/cold	Raise spray energy; +5–10 °C wash; titrate & adjust concentration
Re-deposited soil film	Dirty bath/filters	Change filters; refresh wash bath; increase pre-rinse
Label curl/ink bleed	Label not wash-proof; temp/chem too harsh	Mask labels; lower temp; spec wash-resistant labels next rev
Corrosion after days	Ionic residues left; poor DI	Add DI polish stage; check DI ≥ 10–15 MΩ·cm; consider vapor on sensitive builds
Buzzers/relays fail	Ultrasonic/mechanical intrusion	Shield components; avoid ultrasonics; change orientation

3.2.11 Qualification & monitoring (keep it honest)

At NPI / chemistry change

Run worst-case boards with witness coupons.
Verify: IC (ion chromatography) species within limits, SIR on comb patterns after humidity bias, coat-wet if coating follows.
Lock recipe IDs (temps, nozzle set, speeds, titration band, DI spec, dryer setpoints).

Routine

Daily: titration/refract, DI resistivity, spray pattern check, filter ΔP, drain dwell verified.
Weekly: nozzle inspection, bath skimming, DI log review.
Monthly/Quarterly: IC on sentinel product, dryer audit (vacuum bake efficacy), EHS checks (solvent levels, emissions logs).

3.2.12 Pocket checklists

Setup (per product)

Method chosen (aqueous / semi-aq / vapor); recipe ID loaded
Fixtures angled; shields on sensitive parts; label guards in place
Wash temp/chem in spec (titration logged); DI ≥ 10–15 MΩ·cm
Nozzle pattern verified; conveyor/rotation set; drain dwell enabled
Dryer mode set (vacuum if BTC-dense)

First article

UV/marker coupon shows spray reach; no shadow stripes
Boards emerge spot-free, dry to touch; no label/ink damage
IC/SIR planned for first lot (if risk product)

If issues appear

Fix rinse & drying before blaming chemistry
Change one knob (impingement/time/temp/tilt) and re-inspect
Capture before/after photos; note recipe rev comment

Bottom line: pick the cleaning method by geometry and risk, then win with fixtures that drain, rinses that actually rinse, and dryers that really dry. Control the 4T+M (time, temp, turbulence, titration, megohm DI), protect sensitive parts, and prove cleanliness with quick screens plus periodic deep tests. Do that, and “clean” stops being a guess and becomes a boring, repeatable step.