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ANSWERS TO THE  MOST ASKED ESD QUESTIONS

By: Harlan Snyder

Harlan Snyder,  President of Technical Consulting Associates is an independent consultant on reliability, EMC, and ESD to the disk drive and semiconductor industries in California and the Far East.  More than 20 years experience relating to solutions of ESD problems in disk files, printed circuit cards, semiconductors, thin film, and magnetoresistive (MR) heads.

We have been manufacturing printed circuit cards and boards for several years.  We even do flexible assemblies for the disk drive industry from time to time.  We have had ESD protection in the form of wrist straps and conductive mats.  Now our facility is being completely overhauled with new conveyors, process containers, and packaging machinery that is supposed to be better protection for ESD.  We are even getting new wrist straps and chairs.  Why is this?  Our manager says that the newer products are more ESD sensitive and therefore we have to comply.

Your management (manager ) is correct. The product/technology sensitivity to ESD as measured by the Human Body Model (HBM)  has been decreasing (i.e. they are becoming more sensitive to ESD)  for most products and technologies.  This is due to the smaller and therefore more sensitive semiconductor and thin film devices that are in manufacturing today as compared to five years ago.  Because the chips are more ESD sensitive, the manufacturing processes have to be improved to accommodate without sacrificing yield.  This leads to the "nubbin" of your question.

Most devices and technologies are measured by the HBM for static sensitivity.  Other models may be used, such as the Machine Model (MM), or the Charged Device Model (CDM) however, for purposes of this discussion, these two will be excluded.

As you have been producing presumably satisfactory products, with existing ESD protection requirements, the need for the new requirements is due to the sensitivity level of the new products you will be manufacturing.

Since I do not know the present level of your technology ESD sensitivity I am going to assume it is 1kv HBM.  I am also going to assume that you are being required to obtain a level of protection of 100v HBM, or less, for your new products.  This means that the product(s) are more ESD sensitive by a factor of ten.  Your manufacturing engineers will know what the specific requirements are. 

Since you have described the current level of protection (for the assumed 1kv HBM), it is reasonable to expect that the changes you question are to support the newer and lower (more sensitive) level.  It  is not uncommon  to have an existing (satisfactory) assembly area become more stringent in ESD protection practices when a more sensitive product is introduced.        

All of the ESD protection items mentioned in your query are interrelated.  This means that even if you correct your conveyor system you may not have a satisfactory ESD environment for the new product because of the process containers or totes that are presently in use; or the existing furniture has been deemed to be a risk with the existing wrist straps, etc.  Although I don't recommend a "shotgun" approach to upgrading a facility, it is important to address all of the existing ESD protection, correcting and adding improvements as necessary.  In your case it has been determined that all of the changes are necessary to meet your new requirements.  This has probably been done by a specific study of the process, including actual measurements of the work area.

You will also notice that with the newer and more ESD sensitive technologies there is a more concerned and frequent measurement of the area.  This may mean more frequent wrist strap checks, 100% verification of all process containers (i.e. that they do not charge as verified with a field meter),  regular (such as monthly) measurements to ground of all furniture and carts, etc.  Since you do not mention any clean room requirements, your existing garments may be satisfactory from an ESD standpoint, however these should not be overlooked.  You have not mentioned ionizers in your query.  It is not unusual to have ionization added when a facility is being upgraded.  This may be done selectively at certain work stations where the exposure is greatest.   

As to packaging and shipping the ESD risks are always present.  If your product is more sensitive you will find more use of conductive/dissipative containers and wrapping materials.  Your customer(s) will demand it, even if your customer is another part of your own company.  Some may even require a complete "Faraday shield" in the form of a metallic or metallized shipping unit.  These were unheard of only ten years ago except for certain military products.  Today they are commonplace in the semiconductor and thin film industries.  Ionizers may also be added to the packaging process for additional ESD protection.

Your queries are invited.  Address either the editor or the author of this column.  Anonymity of correspondents and their affiliations are always respected.  Specific questions are welcomed at the monthly meetings of the Society.  

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