Batch vs. In-Line Cleaning of No-Clean Assemblies
We are often asked, “If I’m using a no-clean process, should I (or can I) clean?” This is a good question. When using a no-clean paste, residues left behind can inhibit adhesion of conformal coating or underfill material and interfere with probe contact during product testing. Residues can also absorb environmental moisture, which can trigger a variety of problems. Additionally, if no-clean flux is not fully heat activated, it can pose a great risk to product reliability.
The Two Most Common Cleaning Options
A variety of cleaning options exist. The two most common are batch cleaning and in-line cleaning systems. With this cleaning equipment, often the benefit of one is the limitation of the other. For example, batch systems are less costly and require a smaller footprint on the assembly floor, whereas in-line cleaners typically require a larger capital expenditure and three times the floor space.
Comparing Your Cleaning Options
The attributes listed above should not be the only deciding factors in equipment selection. The more important question is, “What are the critical parameters of our electronics assembly?”
When choosing a cleaning system, consideration should be given to:
Component size, lead pitch and presence of low-standoff components
Density of components
Component placement; for example, do large components “shadow” smaller components and restrict/prevent cleaning solution impingement
When using a cleaning solution/saponifier, it is crucial that the chemistry have an opportunity to penetrate critical areas. Areas that are difficult to clean, such as tightly spaced or low stand-off components, are prone to entrapping contaminants and need to be addressed.
Batch Cleaning Considerations
When boards are loaded into a batch cleaner, they are typically oriented vertically. Therefore, the cleaning solution and rinse water run directly off the board, not allowing for dwell and penetration between and underneath components. Batch cleaners are also especially prone to problems with larger components that will shield or “shadow” smaller components. As the solution makes contact with the larger components and runs down the board, it leaves areas of the board that are not effectively cleaned. The effectiveness and consistency of batch cleaners is also heavily dependent on how assemblies are loaded into the system (i.e. spacing, orientation); care must be taken to ensure cleaning solution can reach all areas of all assemblies in each batch.
In-Line Cleaning Considerations
With in-line systems, boards are placed horizontally on a belt and fed through the cleaner. This orientation allows for the cleaning solution to dwell and penetrate critical areas. The belt speed can be adjusted; reducing the speed allows for greater contact time with the cleaning chemistry. In-line cleaners generally allow greater flexibility with adjustment of nozzle angles as well, giving the process engineer flexibility to match the process to the needs of a particular assembly and ensuring the acute impingement angles needed to ensure penetration of cleaning solution under small and low-standoff components.