Probe Choice Can Make the Difference
in Effective Testing
by, Robert Poirier, Senior Product Manager
and diagnosis of components and systems in computer systems
presents quite a few special challenges. Sometimes it can
be very difficult to make contact with the exact point that
you need. High density, multi-layer circuit boards and closely
spaced components with fine pitch leads makes probing with
a voltmeter difficult and sometimes even dangerous. One slip
while the system is running can mean a catastrophic short.
The goal is to find and fix the problem, without causing any
are many possible solutions to this dilemma. Let's take a
look at some of the options that are available for probing
on board mounted devices and components.
The single point probe is the most common and obvious probing
option. Most digital multi-meters (DMMs) are supplied with a
set of test leads that are quite useful for a wide variety of
testing situations. This type of probe will do quite well for
checking the voltage of a power supply or checking a back-up
battery. However, most of these probes are going to be too large
to be used with confidence on a high-density board. They can
easily slip and damage other components or cause a short. The
tips on most of these DMM probes are usually about .080"
diameter and have a fairly blunt point. Combine that with the
rounding off of the tip that results from normal wear and tear,
and you have a nearly impossible situation. These tips can be
sharpened, but without the outer plating, the underlying base
metal tends to tarnish quickly.
The best way to get at small contact points on high-density
boards is by using a very sharp probe tip. While sharp tips
do have a tendency to be damaged easily, the tip can usually
be re-sharpened since there is no plating. Several manufactures
make probes with sharp pointed tips that can be easily replaced.
Sharp pointed stainless steel points start out at .080"
diameter and taper to a very sharp tip. Another type of probe
tip starts out with
.040" diameter stainless steel wire and is gradually tapered
to a long and very sharp tip. This type of tip is also very
handy for getting into small places, and with the sharp point,
it will be less likely to slip off.
probe with a tip that can be extended and retracted is another
handy multi-purpose tool. These probes have a fine diameter
(.040") stainless steel wire and usually have an insulator
covering over all but the very end of the probe to prevent
accidental contact. With a tip like this, you can extend the
reach of your probe up to 3 inches. In addition to being able
to get at those hard-to-reach test points, the long slender
tip improves visibility of the point that you are trying to
steel is a fine material for making sharp points, but is a
poor choice for an electrical conductor. In addition, after
repeated sharpening, the tip may be worn away to such an extent
that it may become too short to probe into small places. Of
course it can be replaced with a new tip, but that still doesn't
address the problem of the inherent resistance of stainless
is a solution for this problem. The Precision Electronic Probe
(See fig. 1) uses standard Automated Test Equipment (ATE)
probes commonly referred to as "pogo pins," for
its probe tip. This probe is available from Pomona Electronics
and Hewlett Packard. These pogo pins come in a wide variety
of configurations and are readily available. In fact, many
users might find that their own production test areas use
these pins. Not only does this probe feature the replaceable
tip, it is also small enough to get into much tighter places
than a full-size probe.
In some cases, the requirement is for a connection that can
be left in place. If the test point is accessible, your best
option might be to use a hook or pincer clip. These clips
are available from a variety of sources in several different
sizes. Some of the earlier clips were designed to fit onto
bus wires or hook-up wire. These hooks now seem quite large
compared to the tiny components found in today's computer
companies now offer IC test clips with miniature hooks and
pincer wires that are fine enough to fit onto some of the
tiniest component leads. These hook test clips can attach
to leads on a QFP package with .3mm lead pitch. They can even
be stacked side-by-side with as many as five or six clips
on adjacent leads of a .5mm chip leads. As long as you do
not need to attach to more than a few leads at a time, these
clips may be the perfect answer to your probing needs.
IC test clips are the ideal solution for probing dozens of
leads on an IC chip. They also work very well for finding
that one chip lead among many. When there are so many you
can easily lose count part way down the side of the chip.
With an IC test clip, you can reliably connect your probes
to as many chip leads that you need to access without damaging
the chip. IC clips are available for most chip formats from
Dual In Line package (DIP) through Quad FlatPack (QFP).
clip solves a number of other problems as well. What if you
need to connect to all of the leads coming out of a QFP chip?
Of course, you could solder a short pigtail to each chip lead
and then connect to these with the small hook clips discussed
above. That might work, but soldering those tiny wires to
the chip would take most of your day. Furthermore, how do
you explain the new growth of hair on microprocessor? So,
how do you connect to the chip? The answer is an IC test clip.
There is an IC clip for most popular IC chip formats. These
test clips fit over the top of the chip and there is a contact
point for each chip lead. The clip also has a set of numbered
contact pins that allow mass connection to a logic analyzer
or an oscilloscope.
Truth About Test Leads
Most of us pay little attention to the leads that we use until
it's too late. So what do we need to know about test leads
and probes? The most important elements are safety issues,
materials, and the level of accuracy required. For example,
safety is often overlooked. If your work is limited to electronic
circuits below 30 Volts, then the design of the leads is really
not a safety issue. But if you need to use the leads at higher
voltages or close to distribution circuits, you should use
leads that have the proper markings and ratings for situation.
used in the construction of the leads can affect your readings.
If you are trying to measure a very low voltage, say in the
microVolt range, or very low resistance, dissimilar metals
that are used in the manufacture of the leads can introduce
errors that will be very difficult to trace. The contact between
dissimilar metals can act just like a thermocouple and generate
their own voltage. There are specially designed test leads
for low voltage measurements that will minimize the errors
induced by minor temperature differences along the measurement
path. These leads are a little more expensive, but they will
give you accurate voltage and resistance measurements into
six decimal places.
common circumstances, the physical condition of the leads
that you are using is probably the most important issue that
will affect your measurements. Loose connections and corroded
plugs or probe tips can cause intermittent or faulty readings.
The easiest cure is to replace the leads as soon as it looks
like there may be a problem. You can never tell when you will
need to make a critical measurement. The price of a set of
leads is small compared to the cost of inaccurate data.
the best job we can in the shortest amount of time possible
is the optimum goal for almost anyone. Choosing the best probe
to perform the test job at hand can make a big difference
in the quality of the test and the time it takes to complete