pcba

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.

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.

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.

The clock crystal and associated circuits should be placed in the center of the PCB and should have a good ground plane, not close to the I/O interface. Clock generation circuits are not allowed in the form of daughter cards or daughter boards, and must be fabricated on separate clock boards or load-bearing boards. Only the equipment related to the clock circuit is installed in the PCB clock circuit area of ​​FS TECH, avoid installing other circuits. Other signal lines should not be routed near or below the crystal: Clock circuits and ground planes are used under the crystal if other signals pass through the plane violating the image plane function. If the signal passes through the ground plane, there will be a small ground loop and affect the continuity of the ground plane.

Ground loops will have problems at high frequencies

Shielding measures can be used to shield the clock crystal and clock circuit;

If the clock case is metal metal, the PCB design must have copper under the crystal, and this part has a good electrical connection to the complete ground (through a porous ground). Advantages of laying ground under the FS Tech clock: The circuits inside the crystal oscillator generate RF currents. If the crystal is encased in a metal case, the DC power source discharges the transient current generated by the case through RF radiation through the ground plane, referenced to the DC voltage and the RF current circuit within the crystal.

In short, the metal casing is a single-ended antenna, and the nearest image layer, the ground plane layer, sometimes has two or more radiation coupling effects, as the RF current to the ground.

The bottom of the PCB crystal is also conducive to heat dissipation

The clock circuit and crystal ground will provide an image plane that reduces the common mode currents of the associated crystal and clock circuit, thereby reducing RF emissions. Ground can also absorb differential mode RF currents. This plane must be connected to the complete ground plane through multiple points and through multiple holes, which provides low impedance. To improve the effect of the ground plane, the clock circuit of FS Technology should be close to the ground plane. Since most of the surface mount crystals are packaged in plastic, the SMT packaged crystal has more RF energy radiation than the metal shell crystal, and the RF current in the crystal will radiate into the space and be coupled with other devices.

FS Technology-Only improve high-quality PCB assembly services for customers