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54 SMT007 MAGAZINE I JANUARY 2022 right printing and slump properties at higher relative humidity levels. is may impact paste printing rework processes such as leadless de- vice or BGA component site printing. Typical relative humidity ranges for the PCB rework and repair area are between 30-60%. With a 40% relative humidity level, surface resistance is lowered on floors, carpets, table mats, and other susceptible areas (Figure 1). Increased humidity levels can be accom- plished through humidification systems. Hu- midifiers add water vapor to the air, which forms a thin protective film on surfaces and serves as a natural conductor to dissipate electrostat- ic charges. When humidity levels drop below 40% RH, this protection disappears, thereby increasing the possibility of damage or defects within electronic components and devices. ere are numerous risks to operating the PCB rework and repair area when the air is at low relative humidity. If any existing static moni- toring or control systems fail (e.g., a ground connection becomes disconnected, opera- tors missing a wrist strap, foot grounders, or grounding mats will have had coating spilled on them making them insulating surfaces), there is no backup to controlling static charg- ing impacts. Secondly, any reworked board which is not touching an ESD-safe surface or handled properly by an ESD-protected rework technician can be damaged. In many cases, controlling the humidity levels is easier than ensuring that non-charge-generating material does not enter the workspace. ese are some of the risks to operating the rework area in a low humidity environment. Adding humidity to the rework area can- not replace a robust ESD control system just by employing humidification, rather it may be used in conjunction with such a control pro- gram. Such a program should contain the fol- lowing elements: • A training/retraining program of PCB rework/repair technicians • Operators wearing wrist straps when handling ESD product • A static dissipative floor • All ESD benches identified as such and properly connected, each with its own connection to ground • Operators wearing dissipative footwear • Operators wearing ESD-safe smocks correctly • Charting/monitoring of the relative humidity along with monitoring alarms • Keeping insulators out of the rework/ repair operating zone per EOS/ESD 2020 requirements Figure 1: Example of relative humidity vs. surface resistivity for bond paper. 1 Figure 2: Typical humidification nozzles. (Source: Condair Corporation)

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