ESD Control Program

     

A control program must be designed for each application, the type of material or devices being worked on and their susceptibility to ESD events (ESDS) will indicate which of the available ESD control elements need to be implemented and how to achieve this in the most cost effective way, in the end it must be a compromise in between being proactive (avoidance of the ESD event) and reactive (fixing or diminishing de effect of an ESD event). By implementing a sound ESD control program there will be yields in many ways, for example, a reduction in the number of failures at final testing and hence reduction of testings' time, money savings by reducing scraping and having to do rework, improving customer's image and satisfaction of your company by not experiencing early failures, perhaps during the warranty period. There is a standard (S20.20-1999) produced by the ANSI/ESD Association which sets down guidelines for the Human Body Model (HBM) aiming to control ESD/ESC on objects sensitive to potentials in excess of 100V. Here below are some of the most common control guidelines and some indication of their application reasoning behind.

 

Control Program Management


Any ESD control program should take into consideration some basic critical elements. Many companies today use what is known as JIT/QC (Just In Time / Quality Control) because of their tight time table to move devices out. A company must have full management commitment to implementation of the ESD control program and subsequently to its continuous maintenance, without it, a reasonable program cannot be implemented.
An ESD/ESC company co-ordinator must be appointed, which could work fulltime or partially depending of the size of the company. This person must be empowered by the management and be provided with resources to achieve the required control program goals, auditing, enforcement, etc... This person need to be trained as to be kept abreast of new ESD/ESC developments in new technology applicable to the program, this person will also continually look out for improvement of the applicable ESD/ESC techniques as technology and processes evolve.
A planning for the different parts of the implementation of a control program must be designed and custom applied to the particulars of a given organisation, for example according to the sensitivity of the devices or materials handled by the organisation. It is envisage that many individual sub-steps have to be outlined eg training, testing for compliance, earthing, flooring, warning mechanisms, antistatic wrist and foot wear gear, etc...
Following of the current national and international ESD standards, in particular standards produced by the ESD Association, must be an important part of any control program. Also for larger companies the writing of specific internal mandatory procedures, practices and their up-keeping maintenance is crucial.
 

 

Common Techniques Available:

Earthing is a most important part of any ESD/ESC control program as it is the primary path whereby electrons on bodies and objects move to/from earth. All conductive or dissipative elements which are interconnected together through electrical paths must be equi potentially bonded together as to achieve similar/same potential, this can be achieved through a sound grounding system, where the potential in between objects may be zero. Where the bodies are of the insulating or non conductive type there are other methods of control eg ionisers and by controlling relative humidity and temperature. In the case of isolated bodies like aircraft or ships the interconnecting or bonding of all grounding conductors should satisfy the requirements of a good equipotential equilibrium. 

  • To avoid the accumulation of static charges or the mounting of the ESC (ElectroStatic Charging) process of staff while doing work at a bench and also for protecting equipment and staff if a charged object is placed over the workbench, is recommended to provide a work mat with static dissipating qualities instead of one being fully conductive. Persons working on equipment with voltages over +/- 50V could be placed in danger if their working mats were fully conductive.
  • Use ESD protecting materials for the transport of your PCB, any use of varieties of styrofoam must be avoided. The single-use shielding antistatic bags is recommended (after a return trip they should be discarded). Note that protection from an outside discharge is as important as taking care of handling a transported module which may have been bouncing around inside a bag as it could be charged on its own due to the effect of triboelectricity.
  • ESD and ESC awareness/prevention training program. Management as well as staff likely to handle electronic sensitive device must fully understand the consequences behind an ESD discharge and the charges accumulation process (ESC) and they must be trained to use and apply appropriate techniques for handling the ESDS.
  • Use of antistatic wrist-straps while handling devices with electronic components is mandatory. If the flooring provided is of the antistatic or conductive type then antistatic shoes or foot-straps (heel and toe straps) are a must to use. 
  • Use of balanced ionisers, where economically viable, are recommended for controlling ESC prone bench areas and on some ESD critical parts of a fabrication line.
  • People having to handle ESD sensitive equipment must avoid wearing clothing made of synthetic fibres as they are likely to contribute to the accumulation of static charges in the body, particularly where these are used loosely. It is recommended that people wear materials of 100% cotton or a blend of cotton/polyester blend at some 50% ratio. Bad ESD materials would be nylon, wool, polyvinyl compounds, etc…