How static damages
Most of us were introduced to ESD (electrostatic discharge) by the classic
example of crossing the shag carpet to the doorknob. When working with
microelectronics and other ESD-sensitive items however, the static-gremlin
becomes more subtle and dubious.
A doorknob-jolt takes hundreds of volts
before it is perceptible, but many microelectronic components can be
destroyed or suffer latent damage by a dozen volts or fewer. And
unfortunately the component doesn't have to be touched to be damaged or
destroyed; ESD damage can occur by induction, or by the electrical field
given off by any object or body with a charge imbalance brought near the
product.
While training, auditing and process analysis to isolate ESD-related can be
costly and time-consuming at first, the time and effort spent there is
realized in fewer product rejects, fewer customer returns and ultimately a
brand's reputation for high quality. Learning the sensitivity of your
product is a very good place to begin. Contacting the
National ESD Association and reading
ANSI ESD S20.20-1999: Standard for the development of an
ESD Control Program is another good idea.
Why grounding the body isn't enough
Wrist straps were a popular solution for years--they are inexpensive and do
ground the operator's body. But as electronics became smaller, the wrist
strap became less and less effective. The reason for this is natural fibers
like cotton and hair are insulative and unfortunately quite adept at
tribocharging
How an ESD strap or garment works
Static Shielding
Grounding
Ground Monitoring
Choosing the Proper ESD Garment is key to your
quality control.
Static control is a crucial element to successful and quality driven
electronics manufacturing. The high costs of not controlling static
electricity in terms of damaged components are increasingly recognized. In
fact, device damage often will not show up immediately, but instead later
under normal operation of the product. The costs to manufacturers in terms
of component failures, returned products, warranty costs, etc. can be
tremendous. Articles published by the ESD Association state that average
product losses due to static can range from 8% to 33%. Translated, the
actual cost of ESD damage to the electronics industry can easily run into
the billions of dollars annually. ESD garments are a critical element of an
overall system designed to minimize damage caused by electrostatic
discharge.
What does an ESD garment do?
ESD garments can be classified into two broad categories based on their
performance; groundable ESD garments and non-groundable ESD garments. A
groundable ESD garment has electrical continuity across all fabric panels of
the garment through the sleeves, collar, pockets and main body parts. This
electrical continuity makes it possible to ground the entire garment by
connecting a ground cord to a single grounding point anywhere on the
garment (we put a 4mm snap at the hip because it's the most secure point on
the garment, the most convenient from the operator's standpoint and because
the snap we use is the most commonly used among wrist straps and other
personnel grounding equipment. This ground connection provides a safe path
to ground for any static charge that is produced on the clothing worn under
the ESD garment.
A non-groundable ESD garment does not necessarily have electrical continuity
between the different parts of the garment, but has a conductive grid in the
fabric that acts as a shield. This protective grid draws or wicks static
charges from a static generator (the clothing worn beneath the garment, in
this case) and evenly discharge the static into the environment. This is
called corona discharge, a steady dissipation of the static into the
surrounding atmosphere. The speed at which this discharge happens is
dependent upon temperature, relative humidity and the surface size of the
charged object..
When a garment is not grounded, caution should be exercised to make sure
that any static charge present on the garment is not discharged onto a
static-sensitive device.
Due to increased competition, six sigma quality, ISO9000 and most electronic
industry company requirements for ESD protection, a grounded ESD garment is
the preferred choice in a majority of electronic manufacturing environments.
