Selecting the correct pcb cleaning chemistry is the single most critical decision for preventing electrochemical migration and ensuring long-term reliability in harsh environments. Buyers must move beyond generic "wash" instructions and specify exact chemical compatibility, concentration windows, and cleanliness thresholds to avoid latent failures in the field. This guide provides the technical specifications, risk mitigation strategies, and validation protocols necessary to procure high-reliability assemblies.

Key Takeaways

• Cleanliness Thresholds: The industry baseline for ionic cleanliness is < 1.56 µg/cm² NaCl equivalent (per IPC-J-STD-001), but high-reliability Class 3 electronics often require stricter limits, typically < 0.75 µg/cm².
• Surface Energy Matters: For subsequent conformal coating or potting, the PCB surface energy must typically exceed 40 dynes/cm to ensure proper wetting and adhesion.
• Chemistry Types: Aqueous chemistries (alkaline saponifiers) are standard for water-soluble fluxes, while semi-aqueous or solvent-based chemistries are required for difficult "no-clean" residues or low-standoff components.
• Rinse Quality: The final rinse water resistivity must be monitored; a minimum of 2 MΩ·cm is required, though 10 MΩ·cm is preferred for sensitive circuits.
• Validation Tip: Do not rely solely on ROSE testing for modern no-clean fluxes; it often yields false passes. Ion Chromatography (IC) is the gold standard for identifying specific corrosive species.
• Material Compatibility: Highly alkaline chemistries (pH > 11) can attack aluminum heat sinks, anodized surfaces, and certain component markings if not inhibited properly.
• Low Standoff Risk: Components with standoffs below 50 µm (2 mils) require chemistry with low surface tension (typically < 30 dynes/cm) to penetrate and flush out residues.

Contents

• Scope, Decision Context, and Success Criteria
• Specifications to Define Upfront (Before You Commit)
• Key Risks (Root Causes, Early Detection, Prevention)
• Validation & Acceptance (Tests and Pass Criteria)
• Supplier qualification checklist (RFQ, audit, traceability)
• How to Choose (Trade-Offs and Decision Rules)
• FAQ (cost, lead time, DFM files, materials, testing)

Scope, Decision Context, and Success Criteria

The scope of pcb cleaning chemistry selection extends beyond simply removing visible flux. It encompasses the entire tribological interaction between the flux residue, the cleaning agent, the wash process (spray/immersion), and the PCB materials. The goal is to remove ionic and non-ionic contaminants without damaging the hardware.

Measurable Success Metrics

1. Ionic Cleanliness (ROSE): The assembly must pass the Resistivity of Solvent Extract test with a value below 1.56 µg/cm² NaCl equivalent.
2. Surface Insulation Resistance (SIR): Under high humidity (85°C/85% RH), the insulation resistance between traces must remain above 100 MΩ (10^8 Ω) for 168 hours.
3. Visual Cleanliness: No visible residue at 10x to 40x magnification, specifically checking around fine-pitch leads and under component bodies (using tilt inspection).

Boundary Cases (Out of Scope / Failure Modes)

• Trapped Chemistry: If the cleaning chemistry itself is not rinsed out, it becomes a contaminant. A pH check of the rinse water that deviates > 1.0 pH from neutral indicates failure.
• Component Damage: Any degradation of component markings, label adhesion, or elastomeric seals constitutes a process failure, even if the board is "clean."

Specifications to Define Upfront (Before You Commit)

When issuing an RFQ or engineering drawing, vague terms like "clean thoroughly" are insufficient. You must define the process window and chemical constraints.

Critical Process Parameters

• Chemistry Type: Specify if the process must be Aqueous (water + saponifier), Semi-Aqueous (solvent wash + water rinse), or Vapor Degreasing (solvent only).
• Concentration Window: Define the allowable concentration range for the cleaning agent, typically 10% to 25% by volume for aqueous systems.
• Wash Temperature: Specify the thermal profile. Most saponifiers are active between 140°F (60°C) and 160°F (71°C). Exceeding this can damage components; going lower reduces solubility.
• Wash Time / Belt Speed: For inline systems, define the exposure time, typically 3 to 5 minutes in the wash section.
• Spray Pressure: High-pressure sprays (up to 80 PSI) are needed for low-standoff components but can damage delicate wire bonds or unsealed switches.
• Rinse Water Quality: Specify Deionized (DI) water with a resistivity > 2 MΩ·cm (standard) or > 10 MΩ·cm (high reliability).
• Drying: Forced hot air or air knives must ensure zero trapped moisture. Drying temperatures usually range from 80°C to 110°C.
• Flux Compatibility: The chemistry must be explicitly matched to the flux type (e.g., OA, RMA, No-Clean).
• Material Compatibility: List all sensitive materials (e.g., Aluminum, Polycarbonate, Acrylic adhesives) that the chemistry must not attack.
• Environmental Compliance: The chemistry must meet local VOC (Volatile Organic Compound) regulations and REACH/RoHS standards.
• Surface Tension: For HDI boards, specify a chemistry with reduced surface tension (often achieved via surfactants) to ensure under-component cleaning.
• Bath Life: Define the criteria for dumping and refilling the bath (e.g., based on NVR - Non-Volatile Residue accumulation or pH shift).

Key Parameter Table

Parameter	Typical Range	Tolerance / Limit	Why it Matters
Wash Concentration	10% – 25%	± 2%	Too low = dirty boards; Too high = foaming/residue.
Wash Temperature	60°C – 70°C	± 5°C	Heat activates the saponification reaction.
Rinse Resistivity	2 – 18 MΩ·cm	Min 2 MΩ·cm	Ensures no conductive ions are left by the water.
Spray Pressure	40 – 80 PSI	Max 80 PSI	Mechanical energy dislodges solid residues.
Conveyor Speed	0.5 – 1.5 m/min	± 10%	Determines dwell time in the chemical zone.
Drying Temp	90°C – 110°C	Max 120°C	Must evaporate water from vias without delamination.
pH Level	9.5 – 11.5	± 0.5 pH	Critical for saponification; too high attacks aluminum.
Surface Tension	20 – 30 dynes/cm	Max 30	Required to penetrate < 2 mil standoffs.

Key Risks (Root Causes, Early Detection, Prevention)

Failure in pcb cleaning chemistry usually manifests as latent defects—boards that pass initial testing but fail in the field due to corrosion or leakage currents.

1. White Residue Formation

• Root Cause: The cleaning agent reacts with the flux but isn't fully rinsed, or the chemistry is exhausted (loaded with flux salts) and re-deposits residue.
• Early Detection: Visual inspection under UV light (flux residues often fluoresce).
• Prevention: Monitor bath loading (NVR) and change chemistry frequently. Ensure rinse water temperature is high enough (> 50°C) to prevent thermal shock precipitation.

2. Dendritic Growth (Electrochemical Migration)

• Root Cause: Ionic residues (halides) left between traces combine with moisture and bias voltage to grow metal filaments.
• Early Detection: Ion Chromatography (IC) testing during qualification.
• Prevention: Enforce strict ionic cleanliness limits (< 0.75 µg/cm² for Class 3). Use SIR testing for validation.

3. Component Entrapment (Chemical Traps)

• Root Cause: High surface tension prevents chemistry from exiting the space under QFNs, BGAs, or RF shields. The trapped fluid becomes a corrosive battery.
• Early Detection: "Pop-off" tests (prying off a component) to inspect the underside for liquid or corrosion.
• Prevention: Use low surface tension chemistry. Implement "air knives" at correct angles. Avoid placing vias under low-standoff components without tenting.

4. Material Attack (Etching/swelling)

• Root Cause: pH is too high for amphoteric metals (Aluminum) or solvents are incompatible with plastics (Polycarbonate).
• Early Detection: Visual inspection for dulling of solder joints or pitting on heat sinks.
• Prevention: Verify material compatibility charts. Use inhibited alkaline cleaners for aluminum assemblies.

5. Solder Joint Degradation

• Root Cause: Excessive ultrasonic energy or extremely aggressive acidic/alkaline exposure weakening the intermetallic layer.
• Early Detection: Shear testing or cross-section analysis.
• Prevention: Limit ultrasonic power density. Keep pH within the 4–10 range if possible, or minimize exposure time.

6. Foaming in the Wash Tank

• Root Cause: Saponification of rosin flux creates soap. If anti-foaming agents are depleted, foam blocks spray nozzles.
• Early Detection: Pressure sensors on spray manifolds drop; visual observation of foam.
• Prevention: Use chemistry with integral defoamers. Monitor spray pressure.

7. Incomplete Drying

• Root Cause: Insufficient air flow or temperature in the drying zone.
• Early Detection: Water spots visible on the board; failure in subsequent conformal coating (delamination).
• Prevention: Ensure drying modules are calibrated. Use air knives to shear water off flat surfaces.

8. "No-Clean" Interaction Issues

• Root Cause: Partially cleaning a "no-clean" flux is worse than not cleaning it at all. It exposes the active activators that were encapsulated in resin.
• Early Detection: White powdery residue appearing days after wash.
• Prevention: Commit to either fully cleaning (removing 100% residue) or not cleaning at all. Do not "touch up" clean.

Validation & Acceptance (Tests and Pass Criteria)

Validation proves that the pcb cleaning chemistry and process settings actually work for your specific assembly density.

Acceptance Criteria Table

Test Method	Standard	Pass Criteria	Frequency
Visual Inspection	IPC-A-610	No visible residue at 10x-40x.	100% of lots
ROSE Testing	IPC-TM-650 2.3.25	< 1.56 µg/cm² NaCl eq.	Per shift / Per lot
Ion Chromatography	IPC-TM-650 2.3.28	Chloride < 0.5 µg/cm²; Bromide < 0.5 µg/cm².	Quarterly / New NPI
SIR Testing	IPC-TM-650 2.6.3.7	> 100 MΩ at 85°C/85% RH.	Process Qualification
Dyne Pen Test	ASTM D2578	> 40 dynes/cm (for coating).	Spot check
Rinse Water Resistivity	Internal	> 2 MΩ·cm (Inline monitor).	Continuous

Sampling and Coverage Hints

• Dummy Components: For validation, use dummy components (glass slides or clear plastic components) mounted on the PCB to visually verify under-component cleaning.
• Worst-Case Locations: Always sample the area with the highest component density or the lowest standoff (e.g., the center of a large BGA field).
• Extraction Efficiency: For ROSE testing, ensure the extraction time is sufficient (often > 10 minutes) and the solution is heated if allowed, to fully solubilize residues.

Supplier Qualification Checklist (RFQ, Audit, Traceability)

Use this checklist to audit a Contract Manufacturer (CM) or cleaning service provider.

• Chemistry Control: Does the supplier have an automated dosing system to maintain concentration, or is it done manually? (Automated is preferred).
• Bath Monitoring: Is the wash bath monitored for NVR (Non-Volatile Residue) or specific gravity to determine when to dump/refill?
• Rinse Water Quality: Is there a closed-loop DI water system? What is the resistivity setpoint?
• Process Traceability: Can they link a specific board serial number to the wash batch, temperature, and speed data?
• Spray Shadowing: Has the supplier performed a "spray shadow" analysis for your specific board layout (tall components blocking short ones)?
• Drying Capability: Does the machine have sufficient air knives and heating zones to dry complex RF shields?
• ESD Control: Is the cleaning machine grounded, and does the air flow generate static charges? (Ionizers may be needed).
• Material Compatibility: Has the supplier verified that the chemistry will not degrade your specific labels, inks, or adhesives?
• Waste Treatment: Does the supplier have a compliant wastewater treatment process for the spent chemistry?
• Laboratory Access: Does the supplier have in-house ROSE or IC testing, or do they outsource it? (In-house allows faster feedback).
• Maintenance Schedule: Is there a documented schedule for cleaning nozzles, replacing filters, and calibrating sensors?
• Change Control: Will the supplier notify you before changing the chemistry brand or concentration parameters?

How to Choose (Trade-Offs and Decision Rules)

Selecting the right chemistry involves balancing cleaning power against material compatibility and cost.

1. If you are using Water-Soluble (OA) flux, choose an Aqueous chemistry (saponifier) with a high-quality DI water rinse.
2. If you are using Rosin (RMA) or No-Clean flux and require cleaning, choose a Semi-Aqueous or engineered Solvent-based chemistry, as water alone will not dissolve the resin.
3. If your assembly has components with < 2 mil standoff (e.g., CSPs, flip chips), choose a chemistry with low surface tension (< 30 dynes/cm) to ensure penetration.
4. If you have aluminum heat sinks or brackets, choose a pH-neutral or inhibited alkaline chemistry to prevent oxidation and pitting.
5. If you require conformal coating, choose a chemistry that is verified to leave zero surfactant residue, as surfactants can cause coating delamination (fish-eyes).
6. If you are manufacturing High-Frequency (RF) boards, choose a process that guarantees the removal of all flux residues, as residues act as parasitic capacitance.
7. If environmental regulations are strict (e.g., Europe), choose a VOC-free or low-VOC aqueous chemistry rather than solvent vapor degreasing.
8. If you have a high-mix, low-volume production, choose a batch cleaner (dishwasher style) over an inline cleaner to save on chemistry usage and setup time.
9. If you are using lead-free solder (higher reflow temps), choose a chemistry designed for "burnt-in" flux residues, which are harder to clean.
10. If cost is the primary driver and reliability requirements are low (Class 1), choose a standard No-Clean process and skip the cleaning step entirely (if aesthetics allow).

FAQ (Cost, Lead Time, DFM Files, Materials, Testing)

Q: Why clean "No-Clean" flux? A: "No-Clean" is a marketing term, not a guarantee. You must clean it if:

• You are applying conformal coating (residues prevent adhesion).
• The application is high-voltage (residues reduce breakdown voltage).
• Aesthetics matter (visible flux looks like a defect to end-users).

Q: Can I use Isopropyl Alcohol (IPA) for manual cleaning? A: IPA is effective for some fresh residues but often fails on "burnt-in" lead-free flux.

• It can spread residue rather than removing it (the "white haze" effect).
• It is flammable and not suitable for automated spray processes.
• It does not easily dissolve ionic activators in modern fluxes.

Q: How much does automated cleaning add to the PCB assembly cost? A: Typically adds $0.50 to $2.00 per board, depending on size and volume.

• Cost drivers: Chemistry consumption, DI water generation, and energy for drying.
• Validation testing (ROSE/IC) is an additional NRE or per-lot charge.

Q: What DFM checks help with cleaning? A: DFM for stencil design and component placement are critical.

• Avoid placing tall components (capacitors) immediately next to low components (QFNs) to prevent "spray shadowing."
• Orient components so that the "alleys" between them align with the belt direction (for inline cleaners).
• Add drainage holes in large ground pads or RF shields.

Q: What is the difference between Saponifier and Solvent? A:

• Saponifier: Reacts chemically with rosin (acid) to turn it into soap (salt), which washes away with water.
• Solvent: Dissolves the flux residue physically without a chemical reaction.

Q: How do I know if the rinse water is clean enough? A: Monitor the resistivity.

• Standard: > 2 MΩ·cm.
• High Reliability: > 10 MΩ·cm.
• If resistivity drops, the ion exchange beds in the water treatment system are exhausted.

Q: Can cleaning damage MEMS sensors? A: Yes.

• Pressure sensors and microphones often have ports that can ingest fluid.
• Ultrasonic cleaning can shatter the internal structures of MEMS gyroscopes and crystals.
• Always tape over open ports or use "no-wash" processes for these components.

Q: What is "White Scum" and how do I fix it? A: It is usually polymerized rosin or metal salts.

• Fix: Increase wash concentration, increase temperature, or reduce belt speed.
• Check if the chemistry is loaded (saturated) and needs replacement.

Request a Quote / DFM Review for PCB Cleaning Chemistry (What to Send)

When requesting a quote for assembly that requires cleaning, provide the following to your PCB assembly partner:

1. Cleanliness Specification: Explicitly state the standard (e.g., IPC-J-STD-001 Class 3).
2. Flux Type: Specify if you require Water-Soluble or if No-Clean is acceptable (and if it must be washed).
3. Sensitive Components List: Identify parts that cannot be washed or require special care (MEMS, unsealed switches).
4. Testing Requirements: Define if you need per-lot ROSE testing or periodic IC testing.

Glossary (Key Terms)

Term	Definition
Saponifier	An alkaline chemical that reacts with rosin/resin acids to form a water-soluble soap.
ROSE Test	Resistivity of Solvent Extract. A bulk test for ionic cleanliness measuring total conductivity.
SIR	Surface Insulation Resistance. A test measuring electrical resistance between traces under humidity.
Dyne Level	A measure of surface energy. Higher dyne levels (>40) indicate a surface is clean and ready for coating.
OA Flux	Organic Acid (Water Soluble) flux. Highly active, requires water wash, leaves corrosive residue if not cleaned.
RMA Flux	Rosin Mildly Activated. Traditional flux, harder to clean with water alone.
NVR	Non-Volatile Residue. The solid material left behind when the solvent evaporates; used to measure bath loading.
Shadowing	When a large component blocks the spray of cleaning fluid from reaching a smaller component behind it.
Drag-out	The amount of chemistry carried out of the wash tank on the board, leading to consumption and rinse contamination.
Dendrite	A metal filament that grows between conductors due to electromigration (presence of ions +

Conclusion

pcb cleaning chemistry is easiest to get right when you define the specifications and verification plan early, then confirm them through DFM and test coverage. Use the rules, checkpoints, and troubleshooting patterns above to reduce iteration loops and protect yield as volumes increase. If you’re unsure about a constraint, validate it with a small pilot build before locking the production release.

Related links

• DFM for stencil design
• PCB assembly