Contanimation of PCBA

Definition of PCBA contanimation: Any surface deposits, impurities, slag inclusions and adsorbed substances that reduce the chemical, physical or electrical properties of PCBA to unqualified levels.

PCBA contanimation mainly includes the following aspects:

1. The components that make up the PCBA, the pollution or oxidation of the PCB itself, etc. will cause the PCBA board surface pollution;

2. Residues generated by the flux during the manufacturing process are also the main pollutants;

3. Hand marks, chain claws and jig marks produced during welding, and other types of contanimation, such as plugging glue, high temperature tape and flying dust, etc.;

4. Dust, water and solvent vapors, smoke, tiny organic matter in the workplace, and the pollution caused by static electricity to attach to the PCBA.

The Harm of PCBA Contanimation

Contanimation may directly or indirectly cause PCBA potential risks, such as:

1. The organic acid in the residue may cause corrosion to PCBA;

2. During the electrification process, the electric ions in the residue cause electromigration due to the potential difference between the solder joints, which makes the product short-circuit and fail;

3. Residues affect the coating effect;

4. After time and environmental temperature changes, cracks and peeling of the coating appear, which causes reliability problems.

Typical Problems of PCBA Failure Caused by Contanimation

1. Corrosion. PCBA assembly uses iron substrate. Due to the lack of solder bottom coverage on the iron substrate, Fe3+ is quickly generated under the corrosion of halogen ions and moisture, which makes the board surface red. In addition, in a humid environment, acidic ionic pollutants can also directly corrode copper leads, solder joints and components, causing circuit failure.

2. Electromigration. If there is ion contamination on the PCBA surface, electromigration is very easy to occur. The ionized metal moves between the opposite electrodes, and is reduced to the original metal at the reverse end to appear dendritic distribution, which may cause a local short circuit of the circuit.

3. Poor electrical contact. In the PCBA assembly process, some resins such as rosin residues often contaminate gold fingers or other connectors. When PCBA is working in a hot climate, the residue will become sticky, which is easy to absorb dust or impurities, causing the contact resistance to increase or even open circuit failure.

The Necessity of PCBA Cleanup

1. Appearance and electrical performance requirements. The most intuitive impact of contaminants on PCBA is appearance. If it is placed or used in a high temperature and high humidity environment, the residue may absorb moisture and whiten. Due to the extensive use of leadless chips, micro-BGAs, chip-scale packaging (CSP) and 01005 in components, the distance between the components and the circuit board is shrinking, and the assembly density is also increasing. If the halide is hidden under the component where it cannot be cleaned, local cleaning may cause disastrous consequences due to the release of halide.

2. The conformal coating needs of the three-proof paint. Before surface coating, resin residues that have not been cleaned can cause delamination or cracks in the protective layer. The active agent residue may cause electrochemical migration under the coating, leading to failure of the coating rupture protection. Studies have shown that cleaning can increase the coating adhesion rate by 50%.

3. No-cleaning also needs to be cleaned. According to the current standard, the term “no-clean” means that the residues of the circuit board are safe from a chemical point of view, will not have any impact on the circuit board production line, and can be left on the circuit board. However, even if a no-clean flux with a low solid content is used, there will still be more or less residues. For products with high reliability requirements, no residues or contaminants are allowed on the circuit board. For military applications, even no-clean electronic assemblies are required to be cleaned.