Sunday, April 4, 2010

Software Bug or Electrostatic Discharge? Maybe the humidity was to high for the Cat for a good test?

Have you ever had a field report of a problem with a product that makes no sense, and is impossible to reproduce?

This type of problem falls into the class of problems known as Single Event Upset. SEU incidents can be caused by energetic Cosmic Rays, which I've seen people use as a excuse for their buggy software unfortunately. If you are designing equipment for operation *in* outer-space or near other ionizing radiation sources this is a real problems that needs dealt with, here on Earth not so much. However today's shrinking geometries are increasing the levels of susceptibility.

What is a real everyday problem that does not get addressed enough is "Static Electricity". The chain of custody from raw components to a finished product in the customers hands must consider the issue of Electrostatic Discharge (ESD) for the entire life cycle of a product.

If at all possible visit the contractor assembler or supplier of your hardware, with a critical eye to their handling and storage of boards during the entire assembly process.

What is important to understand is that ESD mitigation during building of a product is about culture and training, it is not about the parts and the assemblies.

What you should see:

  • Any person that has contact with components or assemblies is wearing static dissipative smocks, and heal straps.
  • Any person working at a bench working with an assembly is wearing a wrist strap. The exception being working with High Voltage or non-isolated line power supplies for personnel safety.
  • Wrist-strap testers and a daily log of them being tested. Test at each shift change.
  • All board assemblies are being held in appropriate static dissipative containers.
  • Board assemblies are not touching each other. This can cause ESD as well as mechanical damage.
  • Static dissipative chairs and floor mats.
  • Static dissipative coatings on the floor, and logs of application of the coatings.
  • In a design lab, a Electrostatic Gun, to create repeatable ESD events.

What you should not see:

  • Inappropriate clothing like Wool Sweaters.
  • Inappropriate furniture.
  • Boards piled on top of each other.
  • Boards being transported in cardboard trays.
  • Boards being transported on Teflon coated cookie sheets, in the assumption that is an improvement over cardboard.
  • Large number of random failures or reports from the floor of "bad parts".
  • Large number of random failures in warranty repairs; indicative of latent ESD damage.
  • Few wrist straps. No sign of wrist strap replacements.
  • No wrist strap testers.
  • Cats. Rubbing a Cat to make static is not a replacement for a Electrostatic Gun.

ESD really does effect the bottom line, so make a case from the bottom line point of view. If you'd like something that you can inflict on Management to try to get them to change the ESD culture, start with chapter seven from the US Air Force Directive General Shop Practice Requirements for the Repair, Maintenance, and Test of Electrical Equipment.

For more in-depth treatise of the subject start with the six part series from the Electrostatic Discharge Association:

ESD Fundamentals:

  • Part One--An Introduction to ESD.
  • Part Two--Principles of ESD Control.
  • Part Three--Basic ESD Control Procedures and Materials.
  • Part Four--Training and Auditing.
  • Part Five--Device Sensitivity and Testing.
  • Part Six--ESD Standards.

Keep in mind that a 25 Volt ESD event to our nano-sized components might be the equivalent of a large Oak Tree getting hit by Lightning at our Human level scale...