Carts
and Shelves don't really have to be grounded...do they?
It depends:
They do NOT
need to be grounded if:
-
The
product placed on the shelves is NOT susceptible to damage
from ESD
-
The
product is ESD Sensitive but will ALWAYS be stored in a
sealed ESD Bag, ESD Tote, or some other acceptable method to
protect it from from risks from ESD damage, and as long as
the ESD protective container will NOT be opened unless it is
moved to an ESD-safe area before removing it from the
protective packaging
Explanation:
ESD
Floor, does it really matter if a few areas fail?
The ESD Floor (or
conductive floor) is typically the cornerstone of an ESD program.
A good conductive floor provides a dependable ESD ground, allowing
people, carts, and product to move around an area, increasing
mobility and productivity with low risk of damage from Electro
Static Discharge.
If your customers
had a choice of having their product built on a part of the ESD
Floor that passed or failed, which would they choose? How
would you choose if your job or perhaps your life depended on the
quality of the product?
How should I ground my ESD carts? Should I use a grounding chain,
grounding cable, or conductive wheel?
The nice thing
about grounding chains is that they are cheap, but the down side is
that they have a higher failure rate. Grounding chains and
grounding cables are typically lightweight and tend to lose
connection with the floor when they run over the smallest "dust
bunny".
Another problem
that I see with both the chains and cables is that they work well
when they are clean and are used on type of floor such as ESD wax or
other homogonous material, but when they are used on conductive
floors with discrete conductive elements such as conductive vinyl
floors that use vertical carbon
strands. The floor is conductive wherever these vertical
conductive elements come to the surface, but the rest of the surface
is typically insulative. If a link of a chain (or ball of a
cable) happens to stop on a conductive element, the chain or ball
may make contact if the link is clean and heavy enough to make
contact.
In many cases a cart seems to be grounded when it is moving but fails about 50%
or more of the times when it stops. If you are relying on the
cart's ground to protect your product, then you typically have less
than a 50% possibility of the ground working.
Conductive wheels
are consistently conductive while moving or stopped (assuming the
floor is conductive and the wheel is relatively clean)
and provide a much more consistent ground than cables or chains.
Will a current
limiting resistor keep components safe from quick discharges?,
A current
limiting resistor is often built into wrist strap cords, foot
straps, and other grounding wires used for ESD purposes. As
usual, the answer is not that simple. It is often a good idea
to have the 1 or 2 mega-ohm resistor for electrical safety.
For example, if you were working on a high voltage device, such as
120 Volts AC, and you got between the 120 Volts AC and the grounded
metal work surface, you might very grateful (and alive) if you had a
1 mega-ohm resistor in-between you and the electrical ground.
The same is true with floors, and it may be against the fire
code to have a floor that is too conductive due to the increased
potential danger from electrical shock. Floors typically have
a direct connection to the electrical or building ground, but the
resistance is built into the floor material or the backing material.
That is fine for high voltages, but what about ESD sensitive
components?
An ESD sensitive
component can be damaged by placing it on a metal surface regardless
of whether it has a current limiting resistor. We will use a
metal wire cart as an example.
Example
of How a Metal Cart can Damage an ESD Sensitive Part
A cart is
grounded to earth or electrical ground (with or without a current
limiting resistor): If a component is charged and then placed
on the cart, the part will cause an ESD event because the part will
discharge into the cart. The reason for this is that the metal
cart will act like a huge capacitor and the discharge from the part
will essentially try to charge this big capacitor. Since the
resistor (if any) is between the cart and ground rather than between
the component and the cart, it will not slow the discharge, so the
discharge will be quick.
How quickly will
the discharge happen? Here are the calculations:
Using a typical two shelf
36" x 18" wire cart as an example, the capacitance between the
cart and the conductive floor was measured at 150 nanofarads.
Assuming that the part you placed on the cart is a circuit board and
it has a component with metal parts and maybe 0.5 ohm of resistance
between the metal and wherever majority of the charge is stored on
the circuit board.
Calculate the
time constant: R x C = Time Constant
0.5 Ohms * 0.000000150 farads = .000000075 seconds, or 75
nanoseconds
Since it takes 5
time constants to charge a capacitor,
it would take 5 * 75 nanoseconds = 375 nanoseconds to fully
discharge 150 nanofarads
How to Avoid ESD Damage from a discharge to a Metal
Cart
To avoid ESD Damage from placing an ESDS part on a metal cart (or
other metal surface)
-
Enclose the component in an shielded ESD bag or enclosed ESD
Tote
-
Install an ESD mat on the surface of the cart, to avoid
direct contact between the ESDS component and the metal surface
of the cart. Make sure the mat is grounded to the cart
More to come
......
