IPC-A-610 Class 3 First Article Inspection and Line Qualification

Tolerance

First article inspection for high reliability electronics requires total compliance with IPC-A-610 Class 3 criteria before full production release. Class 3 covers hardware where continuous operation or immediate response is critical, downtime is unacceptable, and operating conditions are unusually severe. Mission-critical aerospace, life-support medical gear, subsea energy equipment, and automotive safety systems all operate at this standard.

On the factory floor, the gap between Class 2 dedicated service and Class 3 high reliability comes down to tight dimensional windows, zero-defect structural limits, and explicit solder fillet minimums. A surface mount pad or plated through-hole that easily passes Class 2 visual checks often fails outright under Class 3. Engineers qualifying a Class 3 line view the first article panel as direct proof of line accuracy, stencil deposit geometry, thermal balance, and placement drift.

Class 3 rules leave almost no room for process drift in SMT solder joint geometry. On leadless chip parts like 0402s, 0201s, and bottom-terminated packages, Class 3 specifies a minimum end fillet height equal to the solder thickness plus fifty percent of the termination height; Class 2 allows thickness plus twenty-five percent. Side overhang on rectangular chip components cannot exceed twenty-five percent of the termination width, down from the fifty percent allowed on commercial boards.

Pushing past twenty-five percent side overhang shortens electrical creepage paths and shifts stress distribution during thermal cycles. When thermal shock testing swings between minus fifty-five degrees Celsius and one hundred twenty-five degrees Celsius, micro-cracks in the intermetallic layer begin at the narrowest point of the fillet perimeter. Engineers track placement drift against pad centerlines using high-magnification AOI systems calibrated to physical target grids, checking that pick nozzles apply no mechanical bias across larger board layouts.

Under Class 3 rules, plated through-hole joints must reach at least seventy-five percent vertical solder fill inside the barrel, even across internal ground planes and thermal heat sinks. Class 2 sets that mark at fifty percent, which often hides cold joints on heavy internal copper layers. Solder must form a complete three-hundred-sixty-degree fillet on both primary and secondary lands, with positive wetting angles on every lead surface.

Circumferential wetting on the secondary side must reach one hundred percent. If preheat temperatures drop during wave or selective soldering, high-mass components and thick multilayer boards draw off heat quickly, causing barrel fill to plummet. Inspectors check the line before the first panel enters the reflow zone.

Qualification protocols call for embedding thermocouples inside the internal layers of a test coupon to map actual thermal gradients across the board, avoiding pinholes and blowholes from outgassing in damp laminate.

Bottom-terminated parts like Quad Flat No-Lead packages and Micro-Leadframe packages complicate Class 3 line release inspection. Since non-plated copper lead edges leave QFN side fillets un-wetted, acceptance relies on solder coverage beneath the center thermal ground pad. Class 3 limits voiding under QFN thermal pads to twenty to twenty-five percent total area, compared to thirty-five percent under Class 2.

Any single void covering over five percent of the ground pad creates a localized hot spot that shortens IC junction life. To manage this, stencil apertures for thermal pads use matrix windowpane patterns instead of large open cutouts. Matrix printing stabilizes paste release, gives trapped gas an escape path, and keeps components from floating during reflow.

Choosing stencil thickness means balancing paste volumes between fine-pitch 0.4 millimeter QFN leads and larger discrete passives, which usually calls for step-down stencils with laser-milled relief pockets.

Qualified Class 3 lines verify solder paste volume before placing a single part. Inline 3D Solder Paste Inspection machines check deposit height, area, and volume per pad right after printing. Transfer efficiency ~ the ratio of deposited paste volume to nominal aperture volume ~ must hold between eighty percent and one hundred twenty percent across the panel.

Stencil aperture area ratios (aperture surface area divided by sidewall area) must stay above 0.66 for clean paste release. Below that limit, paste clings to aperture walls, leaving thin deposits, height variations, and open joints. Volume variations over fifteen percent across a pad array create uneven surface tension during reflow, causing passive chips to skew or tombstone.

Solder paste deposit height variance exceeding fifteen percent across a two-terminal passive pad array generates uneven surface tension that pulls chip components off pad center during reflow.

First article inspection for Class 3 builds requires full physical verification before releasing automated runs. Quality inspectors work through a fixed sequence during first-article sign-off.

1. Verify bare board lot traceability, date codes, glass transition temperature specification, laminate material documentation, and surface finish thickness plating certificates.
2. Inspect bare printed circuit board coupons for surface contamination, solder mask registration offset, and pad oxidation using 40x optical magnification.
3. Measure solder paste volume deposits on a minimum of five sample boards using calibrated 3D inline solder paste inspection equipment to confirm Cpk values above 1.67.
4. Confirm pick-and-place feeder tape alignment, component orientation, part number marking, reel lot codes, and polarities against the bill of materials master file.
5. Execute a single-board placement run and inspect pre-reflow alignment under optical magnification, checking for lead off-center placement exceeding ten percent of lead width.
6. Pass the instrumented first article panel through the reflow oven with attached thermocouples, recording liquidus time, ramp rates, peak temperatures, and cooling rates across heavy ground planes.
7. Perform full 3D Automated Optical Inspection and X-ray Computed Tomography on the reflowed panel to verify fillet geometry, barrel fill, component registration, and internal voiding percentages.
8. Cross-section destructive test coupons attached to the panel break-away tabs to confirm internal intermetallic layer thickness and plated hole wall integrity.

Board surface finishes directly dictate wetting behavior and intermetallic bonding in Class 3 reflow profiles. Electroless Nickel Immersion Gold offers the flat co-planar surface needed for fine-pitch placement, but gold plating thickness must stay between 0.05 and 0.15 micrometers. Anything over 0.2 micrometers risks gold embrittlement, leading to brittle fractures along the nickel-tin interface under shock loads.

Organic Solderability Preservatives are cheap and flat, but thermal breakdown over multiple reflow passes oxidizes pads on the secondary side. Hot Air Solder Leveling promotes solid solderability, but its uneven surface profile degrades fine-pitch placement accuracy. Engineers review finish reports and run wetting balance tests to verify surface energy before releasing Class 3 parts to the floor.

Checking component solderability prevents latent joint failures caused by oxidized leads or compromised termination metals. Passives held past shelf life or exposed to humidity develop tin whiskers and surface oxides. Dip-and-look testing per IPC/ECA J-STD-002 evaluates lead solderability by dipping terminations into a temperature-controlled solder bath with specified flux.

Parts need ninety-five percent minimum continuous coverage before release to feeder banks. Lead co-planarity on QFPs and BGAs cannot exceed 0.08 millimeters. A single lead bent up by just 0.1 millimeter produces an open circuit or cold joint that top-down optical inspection will miss.

IPC-A-610 Class 3 cleanliness rules set strict limits on post-assembly ionic contamination. Residues from no-clean flux, skin oils, and airborne dust create conductive bridges, driving electrochemical migration and dendrite growth under voltage bias and humidity. Class 3 caps ionic contamination at 1.56 micrograms of sodium chloride equivalent per square centimeter, measured by Resistivity of Solvent Extract testing.

High-density boards with clearances under 100 micrometers require inline aqueous washers with modified solvents to scour flux out from under low-standoff BGAs and QFNs. Leaving active flux behind leads to leakage currents, board corrosion, and dielectric breakdown in the field.

First article process documentation locks in machine setups, material lots, operator IDs, and environmental telemetry. Cleanroom relative humidity must hold between thirty and sixty percent to keep paste from slumping or drying out too fast. Ambient temperature must stay at twenty-two degrees Celsius plus or minus two degrees.

Tweaking placement force, feeder indexing speeds, stencil wiping intervals, or oven setpoints mid-run immediately voids first article qualification. The line stops until a new first-article sign-off is completed.

Under IPC-A-610 clause 1.4.1, Class 3 criteria supersede customer documentation whenever purchase orders require high-reliability compliance, tying the assembler to specified process limits regardless of internal defaults.

Fixture

Qualifying an SMT line for Class 3 builds requires systematic checks on every mechanical, optical, and thermal subsystem on the floor. Placement accuracy underpins overall hardware reliability. Pick-and-place equipment specs cite high placement speeds under ideal kinematics, but Class 3 mode throttles axis acceleration to guarantee placement repeatability within twenty-five micrometers at three sigma.

Board clamps, support pins, and vacuum fixtures keep panels from flexing during fast placement. A flex of only 0.5 millimeters when dropping a component squeezes paste outward, creating micro-bridges under fine-pitch leads that visual checks easily miss. Custom vacuum plates milled to match bottom-side board contours provide flat support across double-sided runs, removing board bounce and resonance.

Feeder maintenance and tape indexing calibration maintain placement stability across long runs. Pneumatic and electronic feeders wear down over time, introducing pitch errors and side-to-side play that throw off part pickup. Vision cameras center components on the fly, but large mechanical offsets force nozzles to twist parts past nominal limits, adding placement torque.

Worn nozzle tips, leaking vacuum seals, and paste buildup on nozzle faces let components slip during fast gantry transfers. Automated inspection stations clean nozzle tips in ultrasonic baths and test vacuum seal integrity before allowing placement on Class 3 boards.

Which Feeders Cause Class 3 Lead Skew?

Feeder wear hits hardest in paper-tape indexing mechanisms running 0402 and 0601 passives, where ratchet tolerances loosen after roughly two million cycles. Worn ratchet teeth vary tape indexing by up to sixty micrometers per step. The camera still finds the component centroid, but an off-center pick causes nozzle edges to grip the metallic termination instead of the ceramic body.

The nozzle then twists the part onto the pad, causing heavy side overhang that breaches the Class 3 twenty-five percent limit. Switching to electronic smart feeders with optical encoders removes ratchet backlash, maintaining alignment within fifteen micrometers over extended runs.

AOI programming for Class 3 requires fine tuning to catch real defects without flooding operators with false alarms. Systems rely on multi-angle LED ring lights, overhead cameras, and side optics to build 3D models of solder joints. Inspection algorithms apply statistical spatial models to evaluate fillet height, heel distance, toe curvature, and wetting angle.

Setting limits too tight pushes false-call rates above five percent, fatiguing operators and causing manual verification escapes. Qualifying the AOI system involves running a challenge panel with intentional placement offsets, bridges, tombstoned parts, and missing chips. Approval requires one hundred percent defect detection with zero escapes across ten consecutive passes.

Automated X-ray Inspection is mandatory for verifying hidden joints under Class 3 guidelines. Ball Grid Arrays, QFNs, and Column Grid Arrays conceal their solder connections under opaque packages. Standard 2D X-ray projects vertically through the board, producing flat shadow images to evaluate solder ball diameter, bridging, and gross voids.

But 2D systems stack top and bottom features on double-sided boards, cluttering the view. 3D X-ray Computed Tomography slice reconstruction isolates individual vertical planes, letting engineers inspect top-side BGA balls independently from bottom-side passives. CT scans calculate void percentages inside individual solder spheres, spotting micro-voids, head-in-pillow faults, and ball deformation at sub-micron resolution.

Thermal profiling on dense panels requires thorough mapping of reflow oven zones. Ovens running Class 3 profiles rely on forced convection with independent top and bottom closed-loop controls. Nitrogen inerting inside the heating chamber keeps oxygen levels below one hundred parts per million.

Reduced oxygen lowers liquid surface tension, speeds solder wetting, and prevents bare copper pads from oxidizing during thermal cycles. Thermocouples attached to heavy components, small passives, board corners, and internal planes track real-time ramp rates, soak windows, liquidus duration, and peak temperatures. Delta T across the panel during reflow must stay under five degrees Celsius to avoid board warping and uneven joint solidification.

Section 2.3 of IPC-J-STD-001 requires automated line halting whenever reflow oven oxygen concentration exceeds one hundred parts per million during Class 3 active production passes.

Line qualification relies on process FMEA checks to uncover factory floor vulnerabilities. Operators audit board conveyors, edge-clamp transfer belts, and wave soldering pallets to ensure zero mechanical shock reaches boards carrying un-reflowed paste. Vibration on edge rails causes paste to slump, leading to solder balls and bridging.

ESD protocols require grounded rails, ionizing blowers at feeder banks, and continuous wrist-strap monitoring to protect sensitive semiconductor gates during handling.

SMT line failures show up in specific subsystems, requiring clear diagnostic routines before releasing full production runs.

• Pick Nozzle Vacuum Collapse occurs when worn synthetic nozzle tips lose seal integrity on small passive components, causing partial component drop, rotational skew, or complete part loss during high-speed gantry movement.
• Solder Paste Slump results from improper paste viscosity control, excessive ambient cleanroom humidity, or aggressive stencil squeegee pressure that shears paste below its yield stress point, creating solder bridges between fine-pitch pads.
• Reflow Delta T Drift emerges when heating elements degrade or blower motors lose velocity inside central oven zones, introducing thermal imbalances that cause cold solder joints on massive connectors while overheating adjacent passive chips.
• Inert Gas Stratification takes place when nitrogen injection ports clog inside the peak reflow zone, driving oxygen levels above two hundred parts per million and causing severe surface oxidation on leadless component terminations.
• Edge Conveyance Rail Chatter happens when mechanical drive chains wear, introducing physical vibration into panels traveling between placement and reflow, which disrupts pre-reflow solder paste geometry.

Selective soldering units for Class 3 through-hole connectors need dedicated nozzle flow calibration and flux volume checks. Ultrasonic atomizers apply controlled flux droplets to bottom-side pin arrays without spreading into surrounding areas. Nitrogen shrouds over the solder wave stop dross build-up, maintaining clean alloy contact with lead pins.

Bi-weekly pot analysis by atomic absorption spectroscopy tracks contamination, keeping copper in lead-free SAC305 pots under 0.30 percent by weight and gold under 0.20 percent. Impurities raise the alloy melting point, slow wetting speed, and bridge high-density connector pins.

Cross-sectional analysis revealed barrel cracking beneath QFN corner ground pads, resulting in the rejection of eighty-four assemblies.

When first-article inspection turns up Class 3 non-conformances, component solderability and board fabrication defects are common suspects. A thorough audit anchors process settings in direct physical measurement instead of assumed component flaws. When placement offset exceeds five percent of pad width, engineers perform laser alignment on the gantry linear motors instead of tweaking software offsets in the vision engine.

Fixing underlying mechanical errors protects process capability against drift and wear over time.

High false-call rates on fine-pitch quad flat packages are sometimes attributed to board warping, but inspection showed that missing support pins under the stencil printer caused squeegee flexure and uneven paste height across the panel center.

Microsection

Precision microsectioning provides the definitive test for Class 3 solder joint compliance. Where optical and X-ray checks evaluate surface features and density shifts, microsectioning reveals internal crystal structure, intermetallic layer growth, barrel copper wall thickness, and micro-void distribution. Test coupons on breakaway panel tabs go through the exact printing, placement, reflow, and cleaning cycles used for production circuits.

Qualification rules require sectioning, potting, grinding, polishing, and etching these coupons to evaluate joint integrity under high-magnification optical and scanning electron microscopes.

Plated through-hole microsections verify copper wall thickness, inner-layer interconnects, and total vertical solder fill against IPC-A-610 Class 3 rules. Standards require a continuous minimum barrel plating thickness of twenty-five micrometers, with isolated thin spots staying above twenty micrometers. Before sectioning, coupons undergo solder float testing at two hundred eighty-eight degrees Celsius for ten seconds.

This thermal shock simulates wave soldering and extreme field exposures. Microsection checks look for barrel cracks, foil detachment, resin recession, and laminate delamination. Inner-layer foil separation ~ hole wall pull-away or post separation ~ is a critical Class 3 failure that rejects the whole batch.

Intermetallic compound growth between solder and the underlying copper pad dictates long-term joint strength. In reflow, molten tin reacts with copper to create a two-part intermetallic structure: a Cu6Sn5 eta-phase layer against the solder and a Cu3Sn epsilon-phase layer next to the copper pad. Class 3 profiles must build a continuous Cu6Sn5 layer between 1.0 and 3.0 micrometers thick.

Anything under 0.5 micrometers signals low heat input during reflow, yielding a weak bond and incomplete wetting. An intermetallic layer over 4.0 micrometers, brought on by excessive peak temperatures or extended time above liquidus, forms a brittle interface prone to cleavage fractures under shock or vibration.

Take a qualification run for a 12-layer avionics control board weighing four hundred fifty grams, carrying two 256-pin fine-pitch BGAs, four QFNs, and scores of 0402 passives. The reflow profile used a lead-free SAC305 alloy with a liquidus temperature of two hundred seventeen degrees Celsius. Initial microsectioning on a coupon showed an intermetallic layer of just 0.4 micrometers beneath the QFN thermal pad and barrel fill of sixty percent on internal ground planes.

Thermocouple logs confirmed that peak temperatures on inner ground planes reached only two hundred twenty-four degrees Celsius, staying above liquidus for forty-two seconds. Heat absorption by heavy internal copper planes starved the joints of thermal energy, stalling the metallurgical reaction.

Engineers adjusted the convection profile to resolve the microstructural failure. Extending preheat soak by twenty-five seconds balanced heat distribution across heavy board sections before entering the peak reflow zone. Raising setpoints in zones six and seven by seven degrees Celsius brought peak temperatures on internal ground planes up to two hundred thirty-two degrees Celsius, while keeping light passives below two hundred forty-eight degrees Celsius.

Time above liquidus on thermal planes reached sixty-eight seconds. Re-testing a new coupon verified a continuous Cu6Sn5 intermetallic layer measuring 1.8 micrometers thick, while X-ray checks confirmed barrel fill reached ninety-two percent, meeting Class 3 requirements.

Micro-voiding inside solder joints is evaluated through both microsectioning and high-resolution X-ray. Voids in BGA spheres break down into macro-voids, micro-voids, and planar micro-voids along the pad interface. Macro-voids come from trapped flux solvents during reflow.

Micro-voids under twenty-five micrometers in diameter ~ Champagne Voiding ~ typically stem from outgassing copper impurities. Planar micro-voids along the intermetallic line severely degrade shock resistance, acting as crack triggers during drop impacts. Class 3 limits total voiding in any BGA sphere to fifteen percent of ball area, with interfacial micro-voids capped at five percent.

Thermal profiles trace directly back to thermocouple attachments on the qualification vehicle.

Reading microsections requires chemical etching to reveal phase boundaries. Reagents like two percent ammonium persulfate or mild nital attack tin-lead and lead-free structures, bringing out grain boundaries, intermetallic layers, and micro-cracks. Un-etched samples mask fine cracks, giving a false sense of solid bonding.

Technicians mount coupons in epoxy, grinding sequentially with silicon carbide paper from 240 down to 1200 grit before polishing with diamond suspension to 0.05 micrometers. Poor polishing technique scratches the surface or smears resin over cracks, hiding defects from the metallurgist.

Plated barrel copper wall thickness below twenty micrometers causes high-stress cracking along the hole knee during thermal shock testing from minus fifty-five to one hundred twenty-five degrees Celsius.

Choosing test protocols during Class 3 qualification means balancing non-destructive screening, destructive testing, and actual field risk.

1. Define structural acceptance thresholds based on final deployment environment stress profiles, matching IPC-A-610 Class 3 specifications.
2. Mandate dedicated breakaway tab test coupons on every production panel geometry to permit destructive microsectioning without destroying active product circuitry.
3. Establish daily metallurgical cross-sectioning schedules for high-volume runs, auditing barrel fill, copper wall thickness, and intermetallic growth rates.
4. Correlate non-destructive 3D X-ray tomography density maps against physical destructive microsection measurements to calibrate inline inspection algorithm accuracy.
5. Reject entire manufacturing lots when microsection analysis reveals inner-layer post separation, knee cracks, or intermetallic layer thicknesses below 1.0 micrometer.

Laminate breakdown during reflow is a subtle failure mode in high-reliability boards. High-density multilayer builds use high-Tg FR-4 rated above one hundred seventy degrees Celsius or polyimide rated over two hundred fifty degrees Celsius. If reflow heat exceeds the decomposition point, the resin degrades.

Microsections reveal z-axis thermal expansion damage where glass fibers pull away from the epoxy matrix. Resin recession ~ where resin shrinks off the copper barrel wall during heating ~ is allowed under Class 3 only up to twenty-five micrometers, provided minimum copper thickness holds. Any delamination between laminate plies rejects the assembly immediately.

Thermal cycling tests during qualification subject populated panels to hundreds of temperature swings. Real-time continuity monitoring during testing catches intermittent opens caused by barrel expansion or micro-cracking. A joint with solid resistance at room temperature can expand and separate at one hundred twenty-five degrees Celsius, then close back up as it cools.

Qualification setups use fast daisy-chain resistance monitoring to catch microsecond spikes during thermal shifts, making sure latent interconnect breaks do not pass first-article approval.

What long-term intermetallic structural degradation occurs when Class 3 assemblies undergo field operation near maximum rated ambient temperatures for extended service lives?

Warrant

Line qualification and first-article approval for IPC-A-610 Class 3 builds bring distinct legal and financial responsibilities. Hiring a contract manufacturer for Class 3 boards demands total alignment across purchase orders, engineering specs, and quality records. In Class 2 commercial runs, minor non-conformances are often fixed with unrecorded shop-floor touch-ups.

In Class 3, unapproved rework voids compliance, cancels warranties, and exposes the manufacturer to breach-of-contract claims. Class 3 standards tightly control touch-up work. Per IPC-7711/7721, only certified operators using approved thermal tools and documented profiles matching original reflow parameters may replace components or touch up joints.

Process capability metrics quantify line stability and project defect rates over volume runs. Class 3 contracts require a minimum Cpk of 1.67 on critical steps like paste volume, placement accuracy, and peak reflow temperatures. A 1.67 Cpk represents five-sigma capability, targeting no more than 0.57 defective parts per million opportunities.

Lower Cpk figures, such as the 1.33 common in Class 2, allow wider variance that leads to sporadic defect spikes during production. Qualification protocols derive Cpk metrics from at least thirty consecutive panels during first-article testing to prove statistical stability before release.

Quality managers audit line release records against supplier invoice line items before authorizing batch production.

Setup fees, line changeover costs, and first-article billing represent significant up-front investments in high-reliability runs. Changing over an SMT line for a complex Class 3 board involves clearing feeder banks, checking feeder calibration, mounting stencils, performing optical alignment, loading SPI and AOI profiles, and running thermal profiling panels. Setup fees run between five hundred and two thousand dollars per changeover, depending on feeder count and board complexity.

First-article documentation adds labor costs for AOI, 3D X-ray, microsectioning, chemical testing, and engineering sign-offs. Cutting changeover short or rushing first-article verification to save money almost always leads to scrapped production batches.

Class 3 component traceability requires lot tracking from reel receiving through final packaging. Modern systems assign unique barcodes to component reels, tying placement logs directly to panel serial numbers. If a vendor recalls a semiconductor lot, database tracking pinpoints the exact board serial numbers holding those parts.

Traceability records cover raw board certificates of conformance, paste lot numbers, flux analyses, oven telemetry logs, operator IDs, and complete AOI/AXI image archives. Contract manufacturers must archive quality dossiers for ten to twenty years, depending on aerospace or medical mandates.

Scrap allocation clauses in manufacturing agreements define financial liability for boards destroyed during testing or scrapped from process escapes. Class 3 assemblies frequently use high-cost parts, including rad-hard microprocessors, high-density FPGAs, and precision RF modules that run tens of thousands of dollars per panel. Standard CM agreements cap assembler liability at a multiple of assembly fees, leaving the customer exposed to massive component losses if a line failure ruins a board.

Class 3 contracts override standard liability limits, holding the assembler accountable for component costs when defects stem from unapproved parameter changes, poor feeder maintenance, or unauthorized touch-ups.

Quality documentation retention clauses in Class 3 aerospace contracts mandate secure digital archiving of complete 3D X-ray tomography files and thermal profile logs for twenty years post-delivery.

Certificates of Conformance from contract manufacturers must explicitly state compliance with IPC-A-610 Class 3 standards, backed by verified test data in the dossier. A standard certificate claiming basic compliance with drawings will not pass regulatory audits in medical or aerospace sectors. Quality dossiers require test coupons, microsection micrographs, ionic contamination reports, Cpk calculation sheets, and signed first-article inspection records.

Buyers hold the contractual right to pause production and hold payment if dossiers miss data points or show unverified process changes.

Auditor access rights allow product owners to inspect the factory floor during active runs. Qualification terms grant customer quality engineers full access to verify feeder calibration, check cleanroom logs, witness first-article procedures, and audit operator IPC certifications. If an audit finds uncertified operators doing Class 3 manual soldering or uncalibrated inspection machines on the line, the customer can halt production immediately and demand line re-qualification at the CM’s expense.

Line qualification forms the bridge between engineering design and floor performance. Thorough first-article checks, strict process control, and detailed metallurgical testing ensure Class 3 hardware survives its intended service life. Floor discipline translates directly into reliability in the field.

A qualified line runs steady when stencil maintenance, feeder calibration, and profile validation remain completely synchronized.