When a Ground Is Not a Ground
by Jan C. Hoigaard, SpectraScan
The discussion over what a correct ground for static control should
be has been raging since the days when Benjamin Franklin experimented
with a kite.
The self proclaimed "experts" have always argued in favor of the
electrical ground. First because it was always there and, therefore,
convenient to use. More recently, it has been argued that electrical
ground is best because "everything" is at the same potential.
An electrical ground originating from a copper stake driven into
the ground is required by electrical code. This copper stake normally
is located as close as possible to the main electrical service box.
The neutral incoming line and (independently) the green ground wire of
all electrical wiring, equipment and outlets throughout a facility
served by the main box are connected to the electrical ground.
The real world is not that simple. The recommendation for using the
electrical ground as a reference for static-control systems could only
have originated from people who never worked with frequencies higher
than 60 Hz.
The electrical ground is shared by all electrical apparatus and
equipment in the facility and almost always is contaminated with 60-Hz
AC potentials as well as high-voltage transients. Here are the
important facts about electrical ground:
· There is no guarantee that an electrical ground really is a hard,
voltage-free ground. In fact, based on actual measurements, electrical
ground can be anything but ground.
It is common to have a 60-Hz AC voltage on the green electrical
ground wire. The theory_that even if there is a voltage on the
electrical ground, everything will be at the same voltage and no harm
can be done_simply does not hold water.
· Voltages on the electrical ground certainly can damage highly
static-sensitive ICs and even pose a danger to operators.
But the real IC killer on electrical grounds is transient voltage
spikes, the very-short-duration voltage peaks that ride on top of the
nominal line voltage. These transient spikes rise from zero to
hundreds or even thousands of volts and back to zero again in a very
short time. Spike duration may vary from a few seconds to less than 1
µs.
Any such narrow spike is basically an RF signal that consists of a
fundamental frequency and a very large number of harmonics or
multiples of the fundamental frequency. These harmonics may stretch
from a few kilohertz to several gigahertz.
Since these transient spikes are RF signals, they can travel very
long distances through electrical wiring conduits without losing much
power. The electrical conduits simply act as RF transmission lines.
The transient spikes are primarily generated on the hot and neutral
wires. But since the hot, the neutral and the ground wires travel side
by side through the electrical conduits, any voltage spike riding on
the hot and the neutral wires will induce an almost equal voltage
spike on the electrical ground wire.
High-voltage transient spikes can and do travel on the ground wires
throughout the facility and can reach the static-control grounding
system with very little attenuation. As a result, a workstation
surface or any electronics on the workstation can rapidly rise to a
high voltage. The operator, grounded through several megohms, cannot
rise from zero to a high voltage very quickly.
A PCB loaded with static-sensitive ICs and handled by an operator
at the workstation may be exposed to a voltage difference of up to
several thousand volts for the duration of the spike. Instant
destruction of one or more ICs may result.
So where do these high voltage transient spikes originate? Some may
come from sources outside the facility, such as generators and
transformers being switched on and off at the power station or local
thunderstorms causing lightning strikes near power lines.
Most of these spikes normally are generated within the facility or
from nearby facilities. The culprits usually are large electrical
motors and contactors used for air-conditioning, air compressors,
machine-shop equipment and environmental test laboratory equipment.
All such motors and contactors have high-inductance coils
surrounded by high-intensity magnetic fields when power is applied to
them. Each time a motor or any other equipment using coils is turned
off, the strong magnetic field surrounding the coil collapses to zero
very quickly. This rapidly collapsing field induces a very high, but
short-lasting, voltage across the coil. This induced voltage is the
source of transient spikes.
Transient spikes on the electrical ground can only be measured with
a wideband differential oscilloscope where one input is connected to
electrical ground and the other input to true earth ground. More and
more static-control managers now use and recommend true earth ground
for static-control systems and the discharge of static electricity. A
true earth ground is a direct, dedicated connection to an 8' to 12'
copper stake driven into reasonably moist ground or a connection to a
metal water pipe.
Such a dedicated earth ground is guaranteed to be at zero potential
and typically has a resistance into ground of a small fraction of 1 W.
There are many good reasons for a true earth ground. The most
important ones are:
· Only a dedicated true earth ground from a metal water pipe or a
copper stake is free from any voltages, spikes or transients.
· A true earth ground typically provides a discharge path into ground
of much less than 1 W.
· The only potential failure mode for a true earth ground is a broken
wire or a ground stake driven into very dry soil, such as desert sand.
In a real-world situation, I was contracted to discover why one
assembly line produced nearly 100% good PCB assemblies while an
identical assembly line located in another part of the building
produced nearly 100% defective PCB assemblies. I found that the
electrical ground for the assembly line with the failure problems had
frequent voltage spikes reaching 1,800 V.
The culprit turned out to be an environmental test laboratory
located next door to the assembly line. This laboratory was testing
electronic equipment for vibration and shock impact in several axes,
and the test apparatus used many large electrical motors programmed to
turn on and off at frequent intervals. For this somewhat extreme case,
using a true earth ground for the workstations and the soldering
stations solved the problem.
About the Author
Jan C. Hoigaard has served as president of SpectraScan since 1980
when he founded the company. Previously, he was a program manager on
NASA and DoD space and satellite programs, and was affiliated with
TRW, Singer, Varil and Watkins-Johnson. Hoigaard received an E.E.
degree from O.T.S. in Oslo, Norway. SpectraScan International, 2812 E.
Bijou St., Colorado Springs, CO 80909, (719) 447
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]> *
* with <subject: subscribe/unsubscribe> and no text in the body. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
|
