Esd Control And Handling Procedure : Places Where Esd Controls Required & Summary
5. Assembly areas:
Assembly areas are considered most critical because of the amount of handling of susceptible items. This area, being the most visible, is usually the best controlled. Some problems that may need to be checked are;
Presence of unnecessary static generating materials
Personal items such as plastic cups, thermos bottles, purses, sweaters, and other items of clothing should not be on work benches
Care should be taken in handling of boards after mounting the components, and it may be noted that static damage may still result by improper handling of the stuffed boards.
At the soldering area, the operator is required to wear wrist strap well grounded through a 1MW. It is better to have a common ground for the test equipment and for the static-free work stations. Even if we provide a separate grounding for static-free workstations, it is highly likely that the ground wires short each other at some point or the other along their length. Therefore, it is recommended, if in doubt, to check the equipment grounding thoroughly instead of going-in for a separate grounding.
Insulative brushes are used for flux application and cleaning. This may result in generation of static leading to component damage. Use of cotton is recommended while cleaning or for flux application.
Use of static generating rubber gloves is to be dispensed with. Boards are to be transported in lidded conductive trays or anti-static bins. When removing components from these bins, the personnel should take necessary anti-static precautions.
6. Test and trouble shooting:
Boards are to be tested at anti-static work stations to avoid any static failures. When placing or removing the board, operator has to wear wrist strap to avoid any problem. Utilization of freeze sprays based on aerosol freon to troubleshoot suspected cold temperature failures may result in static build-up. Use of special anti-static sprays is recommended.
7. Storage, Packing and shipping:
Boards are to stored in anti-static or conductive bags and arranged in such a way that no physical damage takes place. Use of bubble-sheet or other materials for cushioning of PCBs is to be avoided if it is found to generate static. Similarly, items returned to vendors for repairs/ replacement are to be shipped in protective packages.
7.Summary
Static control program can be condensed to four primary control measures or good ESD practices:
1. Personnel grounding
2. Grounded work surfaces and equipment
3. Purge of static hazardous materials (static generators)
4. Protective packaging for susceptible items when transported or stored away from the work area.
The above are essential for the prevention of ESD failures. The first three items define a controlled workstation, whereas the fourth, protective packaging, provides for transport and storage.
A protective packing with internal layer of anti-static material and an outer layer of conductive film is recommended. The inner layer will prevent any charge build-up by rubbing as the material is anti-static. The outer conductive layer will help in discharging any static that may accumulate to the ground. It should be noted that when conductive tins are used, they should be lidded to prevent any static damage. Use of anti-static bags should be limited to workplace where static generation is absent or significantly low. This is because that anti-static material is static preventive and not static protective. They are ideal as an intimate wrap, that is, the bag that touches the sensitive items and avoids static generation by itself during movement.
Electrostatic shielding is the basis for much of the protective packaging used within the electronics industry. Conductive containers totally enclosing susceptible items provide a blockage of external fields and any charge accumulation rests on the outside surface only. Such containers are ideal for storage and transportation of susceptible items when away from static controlled areas.
