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PCB board cleaning plays a crucial role in PCB production, processing, board modification design, and other manufacturing processes. Electronic processing plants and circuit board engineers adhere to specific standards when determining the method and evaluation criteria for PCB cleaning effectiveness. The following outlines the correct detection method and scientific evaluation standards for PCB cleaning:

1. Raw Material Quality Requirements:

1) Tin-lead solder used for pressure processing must conform to the requirements of GB/T 31311 regarding chemical composition. Cast tin-lead solder should meet the standards of GB/T 8012.

2) Flux quality assessment should consider the flux’s appearance, physical stability, color, non-volatile content, viscosity, density, water extractability, halogen content, solid content, weldability, dryness, copper mirror corrosion, insulation resistance, ion pollution, and other relevant factors.

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2. Printed circuit board cleaning quality requirements

Currently, China’s electronic industry lacks a unified standard for the cleaning quality of printed circuit boards (PCBs). Developed countries commonly adhere to industry standards that define specific requirements for PCB cleaning quality, including:

1) J-STD-001B specifies:

A. Ionic contaminant content: < 1.56 μg NaCl/cm2
B. Flux residue: < 200 μg NaCl/cm2 (secondary < 100 μg NaCl/cm2, tertiary < 40 μg NaCl/cm2)
C. Average insulation resistance > 1 × 10^8 Ω, with a standard deviation (log10) < 3.
2) IPC-SA-61 provides process-specific values.

3) Mil-STD-2000A mandates ionic contaminant content < 1.56 μg NaCl/cm2. Additionally, Mil-P-28809 specifies that the resistivity of the cleaning solution can serve as a criterion for cleanliness, requiring a resistivity greater than 2 × 10^6 Ω·cm to qualify as clean. This method is suitable for monitoring cleaning processes.
Due to the proliferation of commercial ion contamination testers, results can vary among different testing systems, albeit generally higher than manual tests. Thus, the concept of an equivalence coefficient has been proposed to enable comparability across diverse systems.

4) Process-specific requirements:

– Process A: Bare PCB testing

– Process C: Bare PCB → SMT → Reflow soldering → Cleaning → Testing

– Process D: PCB → SMT → Reflow soldering → Cleaning → Wave soldering → Cleaning → Testing

3. Detection methods

3.1 Visual inspection: Without magnification, inspect PCB surfaces directly for any visible residues.

3.2 Surface ion contamination test methods:

1) Resistivity of Solvent Extract (ROSE) Test Method:

This method employs a 75% isopropyl alcohol and 25% deionized water solution to dissolve pollutants from the PCB surface. The ions in these pollutants reduce the solution’s resistivity. By measuring the resistivity before and after washing and calculating the amount of ions per unit area (expressed as μg NaCl/cm2), cleanliness standards can be assessed. Manual testing can follow GB/T 4677.22 or IPC-TM-650 2.3.25, MIL-STD-2000A, using 1.5 mL of solution per cm2 of PCB area, with a minimum washing time of 1 minute.

The resistivity is measured using a conductance bridge or equivalent instrument. The NaCl equivalent per unit area is calculated using the formula (5-2), where Wr is the NaCl equivalent per square centimeter, 2 is the resistivity of the solution containing 1.56 μg NaCl/cm2, and p is the resistivity of the collecting liquid.

Instrumental methods can follow IPC-TM-650-2.3.26 or IPC-TM-650-2.3.26.1, adjusting the isopropyl alcohol content to 75% and purifying the solution to at least 20 MΩ·cm resistivity before testing.

2) Ion Chromatography Test Method:

This method, as per IPC-TM-650 2.3.28, involves an ion chromatograph and follows specific equipment requirements. After placing the PCB and extraction solution (75% isopropyl alcohol and 25% deionized water) in a sealed bag, the sample undergoes extraction at 80±5°C for 1 hour. The ion content per square centimeter (expressed as μg NaCl/cm2) is then determined based on the standard sample.

These methods ensure rigorous assessment of PCB cleaning quality, crucial for maintaining electronic manufacturing standards.