Physical and chemical bonds combine to form a wide variety of PCBA contaminants.
Physical bond binding
The contaminants form physical tension with the surface of the PCBA, and are combined by the surface tension between the molecules. Since physical bonding is a physical effect, its physical bond energy is relatively low, generally between 0.8 × 103 to 2.1 × 104 J/mol. Turnkey PCBA companies use flux during assembly. Even if we clean it, the rosin, resin and glue residue attached to the PCBA will form a physical bond.
Chemical bond combination
The pollutants chemically react with the surface of the PCBA, and the atoms of the two combine, and after the chemical reaction, an ionic compound or a covalent compound will be formed. The strength of the “chemical bond” is relatively large between (4.2 ~ 8.4) × 105J/mol. The use of rosin acid by a turnkey PCBA assembly company has the potential to produce this effect. Rosin acid corrodes the surface of the PCBA board, and then the water in the air is used as a medium to cause the circuit board to conduct electricity. After the circuit board conducts electricity, a chemical reaction occurs to form ions.
Three types of PCBA pollutants
polar pollutants
FS Technology divides polar pollutants into inorganic pollutants, ionic residues and ionic pollutants. After PCB assembly work, this contaminant on the PCBA will ionize under specific conditions to produce positively or negatively charged ionic compounds, commonly halides, acids and their salts. These different ionic compounds have different properties, and therefore different reactions occur in different solutions. Some turnkey PCB assembly companies do not follow the storage specifications and place the PCBA board in a humid environment, which will be devastating. Due to the action of the water, the electronic components generate electricity, and the ions of the polar contaminants migrate to the opposite polarity on the conductor. Solder feet will be considered conductors of the PCB board. These conductive phenomena will lead to a decrease in insulation resistance and increase the leakage current between solder joints or wires. If we don’t take care of it, we could short circuit or just burn the board.
The main types of ionic pollutants are as follows:
- FluxActivators
- Perspiration
- IonicSurfactants
- Ethanolamines
- OrganicAcids
- Plating Chemistries
non-polar contaminants
Non-polar pollutants mainly include organic pollutants and non-ionic pollutants. This non-polar contaminant on the PCBA cannot conduct electricity by itself, and FS Technology regards it as a resistor or an insulator on the circuit board. These non-polar pollutants will reduce the current of the running PCBA board. If the amount of such substances is too large, it will even lead to blocking the current, which is what we often call power outages.
Pay attention to PCBA error analysis
Through the PCBA error, you can know the specific cause, of course, you can also find more problems. The most common ones are the resinous residue of rosin itself, the oxide protective film caused by redox reaction in wave soldering, and the grease or wax of SMT placement machine or DIP insertion machine. Of course, the most difficult thing to avoid is the grease on the workers during manual welding. These non-polar contaminants are super viscous and adsorbent. It picks up dust from the air. When the PCB is combined with machine residues, an insulating film is created. These insulating films prevent electrical contact between the surfaces of PCB components such as connectors, switches, relays, etc.
Maybe you think you don’t need to pay too much attention to these issues, but FS Technology recommends that you need to pay attention to it. These effects are very serious. It may be just a brief period of poor contact at first, but it will intensify over time. The resistance value of the circuit board will gradually increase, and even cause an open circuit in severe cases. Rosin sometimes covering solder joints can also hinder PCBA testing, especially the combination of non-polar contaminants and polar contaminants can increase the degree of contamination.
The main types of ionic pollutants are as follows:
- Rosin
- Oils
- Greases
- HandLotion
- Silicone
- Adhesive
particulate pollutants
Particulate pollutants refer to dust, smoke, fiberglass filaments and lint that are visible to the naked eye. It also includes solder balls or tin bead dross caused by improper operation during soldering. These large particles of contaminants also cause poor current flow, which can cause short circuits that can be harmful to electronic assemblies.
Particulate pollutants can be removed by mechanical methods such as high-pressure gas injection, manual peeling, and PCB cleaning.
So far, the whole content of FS Technology on PCBA pollutants is over. If you need more professional PCBA knowledge, you can go to FS Technology’s blog to view it.
