1. Preface
In the previous article, we analyzed the sources of pollutants from PCBA circuit boards in the electronic manufacturing process. Next, we will classify these pollutants and analyze their harmfulness, in order to find targeted and effective methods to clean and remove them, and improve the high reliability of BMS new energy automotive electronic products.
2. Types of pollutants in electronic assembly (PCBA circuit board)
There are many ways to classify electronic assembly pollutants, such as inorganic pollutants, organic pollutants, polar pollutants, non polar pollutants, ionic pollutants, and non ionic pollutants. However, in practical applications and communication, polar pollutants and nonpolar pollutants are mainly distinguished.
2.1 Polar pollutants
Polar pollutants, also known as ionic pollutants, mainly come from residual salts in PCB etching and electroplating, welding residual salts, activators and residues of welding materials, (ionic) surfactants and residues of welding materials, fingerprint sweat salts, and environmental soluble dust.
2.2 Non polar pollutants
Non polar pollutants are mostly non ionic pollutants, including natural resins, synthetic resins, welding oils or greases, metal oxides, adhesive residues, fingerprint oil protection supplies, oils or greases, etc.
2.3 Particulate pollutants
Metal and plastic impurities during machining, rosin particles and glass fibers, solder tank scum, tiny solder balls, tin beads, and dust.
3. Hazards of pollutants from electronic assembly (PCBA circuit board)
Due to the miniaturization of PCBA circuit board components, dense spacing, and the existence of electromagnetic forces between wires, the reliability of electronic assembly has received increasing attention. The potential risks of pollutants generated by electronic assembly to electronic equipment have also received sufficient attention and need to be avoided.
In the electronic assembly process, the main hazards are polar (ionic) pollutants. Polar pollutants are prone to absorb water from the same polar molecules to form an acidic local environment, which can ionize positive and negative ions of electric charge, leading to component corrosion and a decrease in surface insulation resistance. Under the action of potential difference, charged metal ions in pollutants will undergo electrochemical migration, electromigration, etc.
The electrochemical migration failure mechanism of PCBA circuit boards has three elements:
·Ion residue
·Potential difference
·Moisture
It refers to the migration of charged ions under the influence of electromagnetic fields through flux residues, bridge conductors, etc. Electrochemical migration can cause dendritic crystal growth, which decreases the surface insulation resistance. When dendritic crystal growth is severe, leakage current or electrical short circuit will occur.
Three factors that cause electromigration on PCBA circuit boards:
·High current
·Moving metal atoms
·High temperature
Under the influence of an electric field, electron migration causes metal ions to move in metal conductors. “The movement of electrons flows from the cathode to the anode. When the momentum of electrons is transferred to nearby active ions, interruptions or gaps are formed in the conductor, preventing the flow of current and even creating open circuit failures.”. When the number of interconnections increases in a limited space, polar contaminants can bridge conductors, which is beneficial for the sustained movement of ions. Electrification or heating can lead to accelerated electromigration. The miniaturization of electronic components will increase the risk of electromigration.
Non polar (non ionic) pollutant molecules have no eccentric electron distribution and do not ionize charged ions in a humid environment, so there is no chemical corrosion or electrical failure. However, it can lead to a decrease in solderability, affecting the appearance and detectability of welded joints. During welding, some resins may undergo high-temperature decomposition, oxidation, or unpredictable polymerization reactions at the welding temperature, forming modified non ionic pollutant residues that are not easily separated even after cleaning, leaving white or brown residues. The white residue tends to be hygroscopic and conductive, potentially causing current leakage and stray voltage failures on sensitive circuits in humid environments. If the active substance of the welding material is still present in the white residue, ionization will occur in a humid environment, leading to electrochemical migration.
When nonpolar pollutants adsorb polar pollutants through dust, having the characteristics of polar pollutants can also lead to electrochemical migration or electrical failures, such as adhesive residues, finger prints, and grease. At the same time, oil and grease can cause a decrease in solderability.https://www.slw-ele.com/
Particulate pollutants mainly lead to a decrease in the firmness and welding quality of PCBA circuit boards’ solder joints, increasing the risk of pinching or bridging during welding, and small solder balls and solder beads may cause electrical short circuits between conductors.
Therefore, in the era of miniaturization, functionalization, and intelligence of electronic products, it is of positive significance to deeply understand the sources, types, and hazards of pollutants in the electronic assembly process, find appropriate cleaning methods for the reduction and removal of final pollutants, and improve the reliability, stability, and service life of BMS new energy automotive electronic products.
