Which are the ESD sources for IC on PCB?
The situation becomes quite complex when ICs are mounted on the PCBs. It has been found with computer simulation that the ESD threat to ICs mounted on PCBs can vary almost by the factor of 100. The reasons behind the variations can be the type of ESD source, location or point of discharge, design and structure of the PCB as well. The results of simulation also showed that the level becomes higher on un-mounted individual ICs.

The source of the ESD threat

ESD threats for the ICs mounted on PCB can come from a charge personnel, charged PCB assemblies, or a combination of both. A person can gather charges from walking or any other motion or movement. When the person subsequently touches the metallic parts of the PCB assembly, an ESD may take place. The ESD damage can be more serious if it takes place through any metallic object like a tool, ring, and metallic watchband. This hand or metal ESD can cause current peaks with high discharge. However, ESD taking place because of charged personnel can be prevented with wrist straps or any other preventive measures.

The second source for ESD can be PCB assemblies that are charged. A PCB assembly can also get charged when they are carried on a conveyor belt, while shipping, or if it is handled by a person who is already charged. If this charged PCB comes in contact with some conductive surfaces and/or if it is plugged to some conductive contact that has source of charge, the assembly will be discharged because of ESD. This is also known as charged board model or CBM.

ESD may also discharge the charged PCB and a charged person in conductive contact. This phenomenon is described as personnel charged board model (PCBM).

Other Points of Discharge

Discharge points for ESD can be the IC pins, PCB traces amid ICs, connector pins of the PCBs. However, discharge points can be located physically anywhere in the PCB assembly leaving the connector pins. As the IC pins extend on top of the surface of the PCB, ESD arc may also terminate on the IC pins. ESD, in this case, does not reach the device through the PCB trace.

PCB traces commonly takes place near the metallic point where an ESD threat may occur. ESD arc in this case terminates on the PCB trace among IC pins and not on the IC pin. Other than PCB connector pins, the assembly can also face ESD when it is installed in some equipment or in high-level assembly.

PCB Structure

PCB structure is another important factor that is subjected to ESD threats. If the ESD receptor is PCB assembly, structure of the ground reference point and the discharge path to an IC and from an IC to the specific ground reference are equally important. The local ground structure and ground structure of the external area, both plays an important role.