ESD Systems - Questions And Answers

Question 1: What precautions are being taken to eliminate corrositivity and contamination of ESD-sensitive goods through the transport of control bags? Question 2: Is air ionization an effective way to neutralize static charges on the surfaces of insulators by means of corona discharge? Does time play a factor in the amount of air volume that can be ionized? If so, what procedures can be taken to speed up the movement of the ions? - Fred Franklin, The Electrostatic Discharge Magazine,

A1.ESD Sensitive (ESDS) devices, say populated onto a printed circuit board (PCB), may have components that are also humidity sensitive. Only by using completely sealed ESD protective packaging can you properly protect the PCB from ESD. By using a shielded ESD protective bag and heat sealing the opening will you protect the ESDS contents from all types of ESD. If the metal layer in the shielding bad is thick enough, as it is in moisture vapor barrier (MVB) shielding bags, then you can also protect the contents (again assuming that you properly heat seal the opening) from excessive moisture content, e.g., corrosion of metal contacts and other moisture sensitive parts. Contamination is another issue and may be related to materials that must maintain some level of cleanliness or minimized exposure to airborne particulates. Another issue are older technology pink poly bags which may have topical treatments that can outgas within the bad causing contamination. Most of today's materials in packaging are more stable but should be evaluated if outgassing or other forms of contamination are of concern. A2. Air ionization is one solution to minimize charge accumulation on insulators (since you can't ground or remove charges on non-conductive materials). Corona discharge is one of the most common mechanisms to initiate air ionization. Depending on the technology used (AC, DC, pulsing DC) in corona discharge you can achieve various levels of efficiency of ionization to energy input. There are advantages to each type of technology, AC typically yields better balancing and DC better feedback controls. Another big factor in output is the carrier or air fan and its air flow (cubic feet per minute (CFM). To increase the ion flux, you would want to create a large density of ions in balance (both + and - air ions) by minimizing recombination and collection from the circuit. Air movement helps tremendously in carrying the air ions to the critical areas (ESD protective areas). To get the most out of your air ionizer, remember to keep the air flow and work area unobstructed, minimize the distance between the ionizer output and the work area, employ high output fans, and keep the emitters clean. Depending on the manufacturers unit and how it is setup, you can achieve decay times of less than 3 seconds within the protected coverage areas.

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