PCBA Tech - PCBA board cleaning process and requirements

In the PCBA processing process, solder paste and flux will produce residual substances. The residue contains organic acid and decomposable electric ions. Among them, organic acid has a corrosive effect. Residual electric ions on the pad can also cause short circuits. The residue on the PCBA board is relatively dirty, and does not meet the customer's requirements for product cleanliness. Therefore, it is very necessary to clean the PCBA board.

Generally, customers' cleaning requirements for PCBA boards are as follows:

1. Appearance and electrical performance requirements

The most intuitive effect of contaminants on PCBA is the appearance of PCBA. If it is placed or used in a high temperature and humid environment, the residue may absorb moisture and whiten. Due to the extensive use of leadless chips, miniature BGAs, chip scale packaging (CSP) and 0201 components in components, the distance between components and circuit boards has been shrinking, the size of the board has become smaller, and the assembly density has increased. In fact, if the halide is hidden under the component or in a place that cannot be cleaned at all under the component, local cleaning may cause catastrophic consequences due to the release of the halide. This can also cause dendrite growth, which can result in short circuits. Improper cleaning of ionic contaminants can cause many problems: low surface resistance, corrosion, conductive surface residues will form dendritic distribution (dendrites) on the surface of the circuit board, causing local short circuits in the circuit.

For the reliability of military electronic devices, a major threat is tin whisker and intermetallic compounds. This problem has always existed. Tin whiskers and intermetallic compounds will eventually cause a short circuit. In a humid environment and when there is electricity, excessive ion contamination on the assembly may cause problems. For example, due to the growth of electrolytic tin whiskers, the corrosion of conductors, or the reduction of insulation resistance, it will cause short circuits on the circuit board.

Improper cleaning of non-ionic pollutants can also cause a series of problems. It may cause poor adhesion of circuit board masks, poor contact of connectors, physical interference to moving parts and plugs, and poor adhesion of conformal coatings. At the same time, non-ionic pollutants may also wrap ionic pollutants in them, and may cause other Some residues and other harmful substances were wrapped and brought in. These are issues that cannot be ignored.

2. Three anti-paint coating needs

In order to make the coating of the three-proof paint reliable, the surface cleanliness of the PCBA must meet the requirements of the IPC-A-610E-2010 three-level standard. Resin residues that are not cleaned 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 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 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. Corrosion, surface insulation resistance (SIR), electromigration, and other special detection methods are mainly used to determine the halogen/halide content, and then to determine the safety of the no-clean assembly after the assembly is completed. 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 other contaminants are allowed on the PCB. For military applications, even no-clean electronic assemblies are required to be cleaned.

Common PCBA cleaning process

1, fully automated online cleaning machine

The physical picture of a fully automated online cleaning machine is shown in Figure 7. The cleaning machine thoroughly and effectively cleans organic and inorganic contaminants such as rosin flux, water-soluble flux, and no-clean flux/soldering on the surface of SMT/THT PCBA after soldering. It is suitable for large-scale PCBA cleaning. It uses safe and automated cleaning equipment to be placed on the Denso production line, and completes the chemical cleaning (or water-based cleaning), water-based rinsing, and drying processes online through different cavities. During the cleaning process, the PCBA is cleaned in different solvents through the conveyor belt of the cleaning machine. The cleaning fluid must be compatible with components, PCB surface, metal coating, aluminum coating, labels, writing and other materials. Special parts need to consider whether they can withstand cleaning.

The cleaning process is as follows: in-chemical pre-washing---chemical cleaning---chemical compartment-pre-rinsing---rinsing---final spraying---air cutting drying---drying .

2, semi-automated offline cleaning machine

This cleaning machine thoroughly and effectively cleans the residual rosin flux, water-soluble flux, no-clean flux/solder paste and other organic and inorganic contaminants on the surface of SMT/THT PCBA after soldering. It is suitable for small batch and multi-variety PCBA cleaning. It can be set anywhere on the production line through manual handling. It can complete chemical cleaning (or water-based cleaning), water-based rinsing, and drying in a cavity offline. During the cleaning process, PCBA usually needs to be fixed by a fixture or placed in a basket. The cleaning solution must be compatible with components, PCB surface, metal coating, aluminum coating, labels, writing and other materials. Special parts need to be considered whether they can withstand Clean. The placement density and placement angle of PCBA in the cleaning basket have certain requirements, and these two factors have a direct impact on the cleaning effect.

3, hand red washing machine

The manual cleaning machine thoroughly and effectively cleans the residual rosin flux, water-soluble flux, rosin flux, no-clean flux/solder and other organic and inorganic contaminants on the surface after SMT/THT PCBA soldering. It is suitable for the cleaning of small batch samples of PCBA. Through temperature control, it is adapted to the manual cleaning process of MPC micro-phase cleaning agent and completes the chemical cleaning in a constant temperature bath.