Cleanliness Requirements for Assembled PCBs

Before a PCB can go into assembly and be deemed fit for processing, it should go through some basic cleaning steps. Similarly, after passing through automated or manual assembly, a PCB will need to have any leftover residues and contaminants removed with a comprehensive cleaning process. This is about more than just applying a spray-on product as there are industry standards to comply with when devising cleaning processes for electronics.

This article will look briefly at the PCB cleaning requirements and industry standards on PCBA cleanliness. We’ll look at some of the common contaminants, as well as the optimal cleaning processes for various contaminants.

PCBA Cleaning Standards

The cleanliness requirements for PCBAs depend on various factors, such as the deployment environment and the type of components used in the device. The potential reliability problems that can arise in PCBAs has driven development of industry standards on PCBA cleanliness. PCBAs are cleaned before and after assembly to meet requirements of industry acceptability standards. The main standards defining PCBA cleaning requirements were originally developed by IPC:

• IPC-5704 - This standard defines cleanliness requirements on bare PCBs, such as cleaning processes, selection of cleaning materials, and equipment.
• IPC-610 - This specifies general acceptability standards for PCBAs, including cleanliness requirements that list acceptable levels of residues and other contaminants.
• IPC-J-STD-001 - This standard applies to soldered assemblies and includes requirements for PCB cleanliness and acceptable levels of residues on PCBAs.

Together, these standards cover most general-purpose electronic devices, including Class 2 and Class 3 assemblies. In addition to these industry standards, various manufacturers may have their own cleanliness requirements as part of their quality control programs. There are other industry-specific standards that define more stringent cleanliness requirements, such as in the automotive and medical device industries.

Cleaning Goals and Processes

Cleaning processes can be targeted for specific contaminants and materials; there is no single process or material set that is universal to all contaminants. In an ultra-clean production environment, the major material to be cleaned is flux residue, including no-clean flux residues.

PCB/PCBA cleaning is generally performed for the following reasons:

Removal of flux residue: Flux is used during soldering to ensure solder wetting and strong solder joints. Excess flux residues need to be cleaned, particularly if no-clean flux is used and remains uncured on the board (see below).

Removal of contaminants: Contaminants such as dust, dirt, oils, and fingerprints can slowly contribute to corrosion spots over time. These are typically cleaned with an aqueous solution.

Aesthetics: Clean PCBAs look better and more professional, which is essential for applications where the PCBA is visible or where the appearance matters.

Long-term reliability: A clean PCBA is less likely to experience electrical shorts, corrosion, or ECM across the board surface once deployed in its intended environment.

Pre-Assembly

First, there is typically a pre-cleaning process required before assembly to remove any contaminants leftover from fabrication or storage. Obvious contaminants on the bare PCB are identified visually, including dust, fingerprints, debris, or other particulate matter. The cleaning process could be manual or involve ultrasonic cleaning for tough films or thick particulate matter. After drying and a quick final inspection, the PCBs are generally ready for assembly. These cleaning processes can generally be light as long as the boards were stored properly.

Post-Assembly

There is also a post-assembly inspection and cleaning process that should be performed. Typically during assembly, the assembly could encounter multiple chemicals, including:

• Solder balls and flux residues
• Encapsulants
• Thermal compounds
• Shielding compounds
• Debris or dust in the production environment

A post-assembly inspection will identify any visible contaminants leftover from processing. A cleaning method appropriate for the assembled PCB material, components, and contaminants. Common cleaning methods include solvent cleaning, aqueous cleaning, ultrasonic cleaning, and vapor phase cleaning, each with different equipment requirements and throughput. After cleaning, the boards are inspected and select pieces are tested for quality control.

Cleaning Leaded vs. Lead-Free Assemblies

Cleaning processes can change when cleaning lead-free PCBAs compared to leaded assemblies. The flux used in lead-free soldering can be more difficult to remove, so alternative cleaning agents may be needed. Residues from no-clean flux used in lead-free soldering leaves a residue which, it is claimed, is not harmful to the assembly as long as it is fully cured. However, residues should still be removed in products requiring maximum long-term reliability.

Some of the cleaning agent options for working with lead-free assemblies are summarized below.

Once any cleaning process has completed, the residual cleaning agents and chemicals need to be properly handled and disposed of. IPC-5704 provides guidelines on handling and disposal of these chemicals, as well as any residues they may contain.

When you need to specify cleanliness requirements in your fabrication and assembly documentation, use the complete set of CAD tools in OrCAD from Cadence to build your circuit board and create outputs. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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