3.12 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 choice between cleaning and 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.

3.12.1 Core Cleaning Technologies: Aqueous vs. Solvent

Cleaning technologies are divided into water-based (Aqueous) and chemical-based (Solvent/Vapor). The choice depends on the flux chemistry and the required throughput.

Mandate: The cleaning medium must be specifically matched to the solder paste or flux used to ensure solubility. Cleaning an unmatchable residue is impossible.

3.12.2 Cleaning Process Types

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

Automated Cleaning Systems

• Inline (Spray-in-Air): Used for high-volume production. PCBs 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 different 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).

Manual and Benchtop Methods

• Manual Cleaning: Used only for low-volume, prototypes, or rework sites. Involves brushing or wiping with high-purity Isopropyl Alcohol (IPA) or a specific solvent blend. Limited effectiveness for cleaning under low-standoff components (BTCs/BGAs).

3.12.3 Fixture and Tooling Design

Fixturing is critical to ensure cleaning effectiveness and prevent board damage. The fixture protects the assembly while exposing all necessary surfaces to the chemical and mechanical action.

• Jigs and Carriers: Boards are typically held in custom carriers or jigs to stabilize them during the high-pressure wash/rinse stages.
• BTC/BGA Stand-Off: Fixtures must be designed to promote the flow of cleaning agent under dense components, allowing residues to be flushed out from the low-standoff gaps.
• Tooling Holes: Fixtures use the existing tooling holes on the PCB for alignment and stable mounting.

3.12.4 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).
• Drying: Thorough drying 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 post-wash bake to ensure all trapped moisture is removed.

4.5 Board Washing & Cleanliness Testing:

• Defining "Ionic Contamination" limits (Rose Test) to ensure reliability.

Final Checklist: Cleaning Process Validation