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3.2 Cleaning Methods & Fixtures

Residues left behind in assembly are often invisible, yet they can dictate whether a circuit survives years in the field or fails within months. The effectivenesschoice of boardbetween cleaning hingesand no-clean is a risk decision that ties together product reliability, regulatory compliance, and manufacturing economics. This chapter details the methods and fixturing required to achieve validated cleanliness when the clean mandate is in effect.

3.2.1 Core Cleaning Technologies: Aqueous vs. Solvent

Cleaning technologies are divided into water-based (Aqueous) and chemical-based (Solvent/Vapor). The choice depends on two elements working in harmony: the flux chemistry that dissolves residues and the fixturingrequired that ensures fluids reach every hidden surface. Each method—aqueous spray, semi-aqueous, vapor degrease, or ultrasonic—has strengths tailored to specific geometries, soils, and sensitivities. Yet even the best chemistry fails without proper drainage angles, shielding for delicate parts, and drying that eliminates trapped moisture. Mastery comes from controlling measurable variables—time, temperature, turbulence, chemistry strength, and rinse purity—so that cleaning is no longer trial and error but a predictable, verifiable process.

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

throughput.

MethodTechnology

HowCleaning it worksMedium

BestRationale for

Pros

Watch-outsand Cost Profile

Aqueous (spray-in-air)

HotDeionized DIWater +(DI) chemistry,± high-energy jets, then DI rinsesSaponifier/Detergent

OpEx Focus:Mixed tech,Best most no-clean/OA residues

Scalable (inline), low solvent risk, strong rinsing

Rinse quality &for dryingWater Soluble (OA) underflux BTCs;and chemistrylight controlNo-Clean needed

Aqueousresidues. immersion

SoakRequires +continuous spray bars/rotation

Dense racks, shadowed areas

Contact from all sides

Can trap chemistry unless rinses are strong

Semi-aqueous

Solvent loosens →DI water rinsemonitoring removesand complex

wastewater treatmentHeavy/wave residues, flux-rich

Powerful on stubborn soils

Two chemistries to manage; good emulsification required.

Vapor degrease (modern solvents)Degreasing

HotSpecialized solventNon-Aqueous vaporSolvent condenses, dissolves residues; gravity drainsVapors

Cycle Time Focus:Tight gaps,Best hydrophobicfor soils,rosin-based water-spot-sensitiveand partshighly specialized No-Clean fluxes.

Fastest cycle timeDrying; solvent is inherent;recycled minimalvia water

Solventdistillation, selection,minimizing capture & EHS; plastics compatibilitywaste.

Ultrasonic (targeted)Semi-Aqueous

CavitationSolvent scrubsfollowed inby awater bathrinse.

BurntHybrid approach used for heavy flux onloads robustthat parts,water fixtures

Getsalone intocannot crevices

Riskpenetrate. toRequires MEMS/relays/crystals;a usesubsequent cautiouslythorough or avoiddrying.

Inline vs batch:Mandate: InlineThe cleaning medium must be specifically matched to the solder paste or flux used to ensure solubility. Cleaning an unmatchable residue is repeatableimpossible.

3.2.2 Cleaning Process Types

The mechanical method chosen depends on board density, volume, and component sensitivity.

A) Automated Cleaning Systems

  • Inline (Spray-in-Air): Used for volume;high-volume batchproduction. isPCBs move continuously through wash, rinse, and dry zones via high-pressure spray jets. Requires tight control over spray pressure to prevent damage to fragile components.
  • Batch (Offline): Used for high-mix, low-volume production. Boards are loaded into racks, and the entire chamber runs through the cycle (wash, rinse, dry). More flexible for NPIdifferent board sizes.
  • Ultrasonic: Uses high-frequency sound waves to generate microscopic cavitation bubbles that scrub dense areas. Mandate: Exercise caution; the intense mechanical energy can damage sensitive components (e.g., MEMS, relays, large ceramic capacitors).

B) Manual and diverseBenchtop productMethods

sizes.

  • Manual



    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 sprayCleaning: (shortUsed cycle)only for low-volume, prototypes, or vaporrework sites. ifInvolves waterbrushing isor hard to manage

    Many BTCs/QFNs, tight gaps

    Medium–High

    Maybe

    Aqueous spray + hot DI cascadewiping with strong drying, orhigh-purity vapor

    Flux-richIsopropyl wave/selective

    High

    Sometimes

    Semi-aqueousAlcohol → hot DI rinse(IPA) or robusta aqueousspecific withsolvent higherblend. impingement

    Water-sensitiveLimited labels/parts

    Any

    Yes/No

    Vaporeffectiveness orfor aqueouscleaning withunder fixturing & shields; verify compatibility

    MEMS/relays present

    Any

    Yes/No

    Avoid ultrasonics; use spray/vapor and protectlow-standoff components (BTCs/BGAs).



3.2.3 Fixture and Tooling Design

Fixturing is critical to ensure cleaning effectiveness and prevent board damage. The 4T+Mfixture controlsprotects (whatthe youassembly actuallywhile set)exposing all necessary surfaces to the chemical and mechanical action.

  • Time:Jigs and Carriers: washBoards +are rinsetypically +held dryin durations;custom longcarriers enoughor jigs to saturatestabilize crevices,them notduring cookthe labels.high-pressure wash/rinse stages.
  • Temperature:BTC/BGA Stand-Off: washFixtures 45–65must °Cbe typical;designed rinseto 25–60promote °C;the vaporflow solventsof percleaning spec.agent under dense components, allowing residues to be flushed out from the low-standoff gaps.
  • TurbulenceTooling (impingement):Holes: nozzleFixtures pressure/angle,use spraythe pattern,existing parttooling rotation.holes on the PCB for alignment and stable mounting.

3.2.4 Rinsing and Drying Mandates

Inadequate rinsing and drying are the most common causes of post-cleaning field failures.

  • Rinsing: Rinsing must remove all residual cleaning agents and dissolved contaminants. Deionized (DI) water is mandatory for final rinsing to prevent conductive minerals from causing electrical leakage. Inadequate rinsing leaves ionic residue that attracts moisture, leading to corrosion and reduced insulation resistance (SIR).
  • Titration (chemistry strength):Drying: maintainThorough bydrying is essential to prevent moisture-induced failures (e.g., corrosion, short circuits). Techniques include forced hot air, infrared (IR), and vacuum assist. Boards with highly porous materials (e.g., thick FR4) may require a titrationpost-wash 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 DIbake to cutensure spots.
  • Breaksall &trapped drains:moisture briefis dwell after each stage so chemistry doesn’t drag forward.removed.

Proof

Final tools:Checklist: glassCleaning testProcess 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 driesValidation

MethodMandate

Where it shinesCriteria

NotesVerification Action

ConvectionChemistry (hot air)Match

GeneralCleaning purposeagent must be compatible with the flux chemistry used on the assembly (NC, RMA, WS).

SDSPlenty ofdocumentation airflow;confirms don’tcompatibility justand heat—moverequired air

IRhandling/disposal assist

Heavy boards

Mind plastics; combine with airflowprocedures.

VacuumMethod bakeSelection

BTCs/tightThe gapsmechanical method chosen (Spray, Batch, Vapor) must provide adequate flow under low-standoff BTCs.

60–90Use °Cthe underROSE vacuumtest forto 10–30verify minsuccessful pullsremoval moistureof fromionic crevicesresidue beneath components.

SolventRinse displacementQuality

Water-spotFinal sensitiverinse conducted with DI water; resistivity monitored and logged.

UseEnsures approvedno displacementresidual fluids;ionic verifycontaminants materialremain compatibility

that

Proof:could weightcause before/after, IR camera for cold wet spots,corrosion 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 titrationleakage.

WaterDrying spots under BTCsIntegrity

Drying airparameters can’t(Time, reach;Temperature) noare drainvalidated, angleespecially for thick or multi-layered boards.

AddIf required, perform a tiltmoisture +content drain dwelltest; vacuumor dry;post-wash raisebake airflow,to notensure just100% tempmoisture removal.

StickyComponent flux patchesSafety

LowIf impingement;Ultrasonic chemistrycleaning weak/coldis used, sensitive components (MEMS, Relays) must be qualified or shielded.

RaiseAudit sprayconfirms energy;shock-sensitive +5–10components °C wash; titrate & adjust concentration

Re-deposited soil film

Dirty bath/filters

Change filters; refresh wash bath; increase pre-rinse

Label curl/ink bleed

Labelare not wash-proof;susceptible temp/chemto toocavitation 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 orientationdamage.



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



By matching method, controls, and fixturing to product risk, cleaning becomes a repeatable safeguard rather than a source of mystery defects. The result is boards that are consistently clean, coatings that adhere, and field performance that can be trusted.

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