Using C3/CI as a Process Control Tool
A Guide to Cleanliness in PCBA Manufacturing
In PCBA (Printed Circuit Board Assembly) manufacturing, cleanliness is essential. Residues left over from processes can compromise circuit performance, potentially leading to failures. This guide explores the use of the C3/CI (Cleanliness Index) method as a process control tool, offering insights into maintaining and assessing cleanliness at every stage.
What is Cleanliness, and Why is it Important?
Cleanliness in PCBAs refers to the accumulation of residues from materials, chemicals, and processes during manufacturing. These residues impact circuit performance based on their composition—whether they are insulating and non-corrosive or conductive and corrosive. Insulative residues support uninterrupted circuit performance, while corrosive or conductive residues can cause parasitic leakage and even short circuits.
Maintaining cleanliness in PCBA manufacturing is vital as residues between leads, vias, and pads can affect reliability. Critical components, such as QFNs and FETs, tend to trap flux, creating potentially conductive, harmful residues.
Traditional Cleanliness Testing and Its Limitations
Traditional cleanliness tests, like ROSE and SIR, were once industry standards. But as time goes on, we have observed their limitations as well.
For instance, the ROSE test uses a mix of IPA and DI water to measure residue across an entire board. However, it struggles to detect residues under low-standoff components. Similarly, SIR testing evaluates material cleanliness but doesn’t account for process residues from assembly stages, meaning it’s less effective as a comprehensive process control tool.
C3/CI Operations: How It Works
The C3/CI process isolates a 0.1 square inch test area on a board, soaking it with heated DI water in a cycle to extract residues into a 2.2 mL solution. By normalizing the test area and volume, C3/CI enables consistency across different boards and specific locations on the same board.
A copper electrode with a 50-mil gap and 10-volt bias measures the conductivity of extracted residues. A current flow above 50-60 µA indicates residue presence, while a jump to 250 µA signifies potential parasitic leakage or shorting due to corrosive residue. This method provides real-time feedback on the cleanliness of specific areas.
Limitations of C3/CI
While C3/CI is effective, it has some limitations:
Under Component Testing: It doesn’t fully assess residues beneath large components, such as QFNs or BGAs, without removal.
Surface Limitations: Testing above a QFN only reflects residue on the component’s top and sides, not the leads and ground plane underneath.
Interpreting C3/CI Data
The C3 graph reveals µA readings that indicate cleanliness levels. For example, readings from 50 µA to 157 µA suggest minimal conductivity, while values reaching 250 µA denote high corrosion potential. These indicators allow manufacturers to differentiate between clean and dirty areas effectively.
Practical Applications of C3/CI in Process Control
C3/CI can be strategically used to monitor cleanliness at critical points:
Bare Board Areas: Assessing solder mask porosity and via cleanliness.
SMT Areas: Inspecting low-standoff areas where residues are likely to accumulate.
Selective Soldering Zones: Evaluating residue on both the top and bottom sides, especially around contact points.
Rework and Cleaning: Ensuring that reworked areas and solderball removal meet cleanliness standards.
In conclusion, the C3/CI process control tool empowers PCBA manufacturers to make informed decisions about cleanliness, enabling predictive maintenance and quality assurance. Through focused residue assessment, manufacturers can ensure reliable performance, enhancing product quality and longevity.
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