1. Appearance and electrical performance requirements
The most intuitional impact of contaminants on PCBA is the appearance of PCBA. If it is placed or used in a high temperature and humid environment, it may absorb moisture and the residue may turn white. Due to the widespread use of leadless chips, micro BGA, chip-scale packaging (CSP) and 0201 components in components, the distance between components and circuit boards is decreasing, the size of circuit boards is getting smaller and smaller, and the assembly density is increasing. In fact, if the halide is hidden under the component or cannot be cleaned at all, locally cleaning may lead to catastrophic consequences due to the release of the halide. This can also cause dendrites to grow, which can lead to short circuits. Improper cleaning of ionic contaminants can cause many problems: low surface resistance, corrosion, and conductive surface residues will form dendritic distributions (dendrites) on the surface of the circuit board, causing partial short circuits in the circuit.
The main threats to the reliability of military electronic equipment are tin whiskers and intermetallic compounds. This problem has always existed. Tin whiskers and intermetallic compounds will eventually cause short circuits. In a humid environment with electricity, if there is too much ion contamination on the components, it may cause problems. For example, due to the growth of electrolytic tin whiskers, the corrosion of the conductor, or the reduction of insulation resistance, the lines on the circuit board will be short-circuited.
Improper cleaning of non-ionic contaminants can also cause a series of problems. It may lead to poor adhesion of the circuit board soldermask, poor contact of the pins of the connector, poor resist of physical interference, and poor adhesion of the conformal coating to moving components and plugs. At the same time, non-ionic pollutants may also envelop the ionic pollutants in them, and may envelop and bring in other residues and other harmful substances. These are issues that cannot be ignored.
2. Conformal coating needs
To make the coating of the Conformal coating reliable, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 level III 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, leading to the coating crack and failure of protection. Studies have shown that cleaning of PCBA can increase the coating adhesion rate by 50%.
3. Non-cleaning also needs to be cleaned
According to the current standard, the term "non-clean" means that the residue on the circuit board is chemically safe, will not have any effect on circuit board, and can be left on the circuit board. Corrosion testing, surface insulation resistance (SIR), electro-migration and other special testing methods are mainly used to determine the halogen/halide content, so as to determine the safety of non-clean solder paste after assembly. However, even using low solids ratio non-clean fluxes, there will still be more or less residues. For products with high reliability requirements, residues or other contaminants are not allowed to be on the circuit board. For military applications, even require cleaning of non-clean assembled PCBAs.