Classification of PCBA contaminants

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Aug 31, 2022  ( 1 post )  

There are many types of contaminants on PCBA boards, which are mainly formed by the combination of physical bonds and chemical bonds.

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. If it is cleaned even if it is cleaned, the rosin, resin and residual glue attached to the PCBA will form a physical bond.

"Chemical bond" combination: refers to the chemical reaction between the pollutant and the surface of the PCBA, the atoms of the two are combined, and after the chemical reaction occurs, 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

Polar pollutants can be divided 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. When the assembly company does not follow the specifications, when the PCBA board is placed in a humid environment, due to the action of water, the electronic components generate electricity, and the ions of the polar pollutants will migrate toward the conductor with the opposite polarity. Soldered pins will be regarded as conductors. These conductive phenomena will lead to a decrease in insulation resistance and increase the leakage current between solder joints or wires. If not handled in time, a short circuit may occur or the circuit board may be directly burned.

Contaminated PCBA 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. The specific reason needs to be analyzed by PCBA error to know. 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 will absorb dust in the air, and when combined with machine residues, it will produce an insulating film, thereby preventing electrical contact on the surface of PCB components such as connectors, switches, and relays. Maybe you don't think you need to pay too much attention to these problems, but FS Technology can tell you responsibly that these effects are very serious. At the beginning, it may only be a short-term poor contact, but as time goes on, this situation will intensify, and the resistance value of the circuit board will gradually increase, and even cause an open circuit in severe cases. Sometimes the rosin covering the solder joints also hinders the PCBA test, especially the non-polar contaminants will aggravate the degree of contamination with the combination of polar contaminants.

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.

Large particle contaminants cause PCBA board bubbles

Particulate pollutants can be removed by mechanical methods such as high-pressure gas injection, manual peeling, and 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.

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