Why does PCBA need to be cleaned?
1.Appearance and electrical performance requirements
The most intuitive impact of contaminants on PCBA is the appearance of PCBA. If placed or used in a high temperature and humid environment, there may be moisture absorption and residue whitening. Due to the widespread use of leadless chips, micro BGAs, chip-scale packages (CSPs), and 0201 components in components, the distance between components and circuit boards is shrinking, the size of circuit boards is getting smaller, and the assembly density is getting higher. In fact, if the halide is hidden under the component or cannot be cleaned at all, local cleaning may lead to catastrophic consequences due to the release of halides. This can also cause dendrite growth, which can lead to short circuits. Improper cleaning of ionic contaminants can cause many problems: low surface resistance, corrosion, conductive surface residues will form dendrites (dendrites) on the surface of the circuit board, causing local short circuits in the circuit, as shown in the figure.
The main threat to the reliability of military electronic equipment is tin whiskers and intermetallic compounds. This problem has always existed. Tin whiskers and intermetallic compounds will eventually lead to short circuits. In a humid environment and with electricity, if there is too much ionic contamination on the component, it may cause problems. For example, due to the growth of electrolytic tin whiskers, corrosion of conductors, or reduction in insulation resistance, the wiring on the circuit board will short-circuit, as shown in the figure
Improper cleaning of non-ionic contaminants can also cause a series of problems. It may cause poor adhesion of the circuit board mask, poor contact of the pins of the connector, poor physical interference, and poor adhesion of the conformal coating to moving parts and plugs. At the same time, non-ionic contaminants may also encapsulate the ionic contaminants in them, and may encapsulate and bring in other residues and other harmful substances. These are issues that cannot be ignored.
2.Conformal coating needs
To make the conformal coating reliable, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 Level 3 standard. Resin residues that are not cleaned off before surface coating can cause delamination of the protective layer or cracks in the protective layer; activator residues may cause electrochemical migration under the coating, resulting in coating cracking and protection failure. Studies have shown that cleaning can increase the coating adhesion rate by 50%.
4.No-cleaning also needs cleaning
According to current standards, the term “no-cleaning” means that the residue on the circuit board is chemically safe, will not have any effect on the circuit board, and can be left on the circuit board. Special detection methods such as corrosion detection, surface insulation resistance (SIR), and electromigration are mainly used to determine the halogen/halide content, thereby determining the safety of non-cleaned components after assembly. However, even with low-solids no-clean flux, there will still be more or less residue. For products with high reliability requirements, no residue or other contaminants are allowed on the circuit board. For military applications, even no-clean electronic components are required to be cleaned.
