3.11 Clean vs No-Clean Decisions

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 choice between cleaning and no-clean is more than a process preference — it is a risk decision that ties together product reliability, regulatory compliance, and manufacturing economics. With geometry shrinking, voltages rising, and customer demands tightening, the ability to prove cleanliness becomes just as critical as achieving it.

3.1.11.1 The Risk Decision: Clean vs. No-Clean

The determination to clean or not clean a Printed Circuit Assembly (PCA) is based on four critical factors. Manufacturers are choosing between leaving benign residue (no-clean) and removing residue (clean, mandatory removal).

• Product Risk: High-voltage, high-impedance, safety-critical, or assemblies requiring conformal coating generally mandate cleaning.
• Geometry: Dense, low-standoff packages like Bottom Termination Components (BTCs), such as QFNs and DFNS, lean heavily toward cleaning to prevent trapped residue.
• Process Control: No-clean requires exceptional process discipline — perfect flux dose, full preheat activation, and dry storage — to ensure the residue is truly inert.
• Environment & Compliance: If the final operating environment is harsh (high humidity) or if the customer contract (e.g., medical, aerospace) or local environmental regulations mandate cleaning, the decision is final.

3.1.11.2 When Cleaning is Mandatory

Cleaning is usually mandatory when the residue poses an electrical, chemical, or mechanical threat to the product's long-term function or coating adherence.

• Coating/Potting: Residues compromise adhesion, leading to defects like fisheyes or under-film corrosion.
• High Impedance/High Voltage: Residues can create leakage paths (reducing Surface Insulation Resistance, SIR), leading to signal detuning, current creepage, or dendritic growth (Electro-Chemical Migration, ECM).
• Contamination: Any use of Water-Soluble (OA) flux mandates immediate and complete water washing.
• High-Risk Assemblies: Medical, automotive (Class 3), or dense assemblies with heavy flux usage trapped under components.

3.1.11.3 Cleanliness Proof: Validation Methods

Cleaning cannot be proven by sight alone. Validation requires measurable, scientific testing to ensure reliability goals are met. Use two layers of evidence—a quick screen for process control and a deep analysis for qualification.

Method	Type	Function and Mandate
ROSE / Ionics Test	Quick Screen (Trend Tool)	Measures the total amount of ionic residue (measured as NaCl equivalent). Used for daily process control trending.
Ion Chromatography (IC)	Deep Analysis (Qualification)	Identifies the specific species and quantity of ionic residues (e.g., chlorides, bromides). Mandatory for NPI and regulatory validation.
SIR (Surface Insulation Resistance)	Deep Analysis (Reliability)	Measures the integrity of the PCB surface under electrical bias and high humidity. Mandatory for high-voltage or high-impedance front-ends.
Contact Angle Test	Pre-Coating Check	Measures the uniformity of liquid dispersion on the board surface. Used to verify the surface is ready for conformal coating adhesion.

3.1.11.4 Process Control for No-Clean Assemblies

When a no-clean decision is made, process discipline must be rigorous to ensure the residue is activated and inert.

• Flux Dose: The flux dose must be precisely controlled (UV verification or weight test). Over-application and pooling of flux can still cause electrical leakage.
• Preheat: The reflow or wave preheat profile must guarantee full flux activation and complete solvent evaporation (Chapter 1.2).
• Rework: Even in a no-clean environment, heavy rework often requires spot cleaning of the specific area to remove excess gel or partially cured residues that may trap moisture.

Final Checklist: Cleanliness Decision and Validation

Mandate	Criteria	Required Action
Risk Assessment	Product designated as High Voltage / High Impedance / Coated / Class 3.	Full cleaning is mandatory; document the cleaning process in the Quality Plan.
Flux System	Flux core or liquid flux chosen is Water Soluble (OA).	Mandatory aqueous cleaning immediately post-solder.
No-Clean Process	Flux application volume is thin and uniform; board is fully dried before storage.	UV dose check and preheat profile verification required per job file.
Proof of Cleanliness	Product is validated via SIR testing or Ion Chromatography (IC).	ROSE testing conducted periodically as an ongoing process control trend tool.
Cost & Compliance	Environmental Health & Safety (EHS) permits are secured for wastewater or solvent usage.	The Cost of Poor Quality (CoPQ) avoided must outweigh the OpEx cost of cleaning.