1. Appearance and electrical performance requirements
The most intuitive effect of pollutants 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-BGA, chip-level package (CSP) and 0201 components in components, the distance between components and the board is shrinking, the size of the board is getting smaller, and the assembly density is increasing. In fact, if the halide is hidden beneath the component or cannot be cleaned at all, local cleaning can lead to disastrous consequences due to the release of the halide. This can also cause dendrite growth, which can lead to short circuits. Improper cleaning of ion contaminants will lead to many problems: low surface resistance, corrosion, and conductive surface residues will form dendritic distribution (dendrites) on the surface of the circuit board, resulting in local short circuit, as shown in the figure.
The main threats to the reliability of military electronic equipment are tin whiskers and metal intercompounds. The problem persists. The whiskers and metal intercompounds will eventually cause a short circuit. In humid environments and with electricity, if there is too much ion contamination on the components, it can cause problems. For example, due to the growth of electrolytic tin whiskers, corrosion of conductors, or reduction of insulation resistance, the wiring on the circuit board will short circuit, as shown in the figure
Improper cleaning of non-ionic pollutants can also cause a series of problems. May result in poor adhesion of the board mask, poor pin contact 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 it, and may encapsulate and carry other residues and other harmful substances. These are issues that cannot be ignored.
2, Three anti-paint coating needs
To make the coating reliable, the surface cleanliness of PCBA must meet the requirements of IPC-A-610E-2010 level 3 standard. Resin residue that is not cleaned off before surface coating can cause the protective layer to delaminate, or the protective layer to crack; The activator residue may cause electrochemical migration under the coating, resulting in failure of the coating rupture protection. Studies have shown that the coating bonding rate can be increased by 50% by cleaning.
3, No cleaning also need to be cleaned
According to current standards, the term “no-clean” means that residues on the board are chemically safe, will not have any effect on the board, and can remain on the board. Special testing methods such as corrosion detection, surface insulation resistance (SIR), electromigration, etc. are primarily used to determine the halogen/halide content and thus the safety of non-clean components after assembly. However, even if a no-clean flux with low solid content is used, there will still be more or less residue. For products with high reliability requirements, no residues or other contaminants are allowed on the circuit board. For military applications, even clean no-clean electronic components are required.
Post time: Feb-26-2024