Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-06-02
2002-10-22
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S758010
Reexamination Certificate
active
06469531
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of test equipment for testing printed circuit boards, and more particularly to board test fixtures and other mechanical interfaces for electrically interconnecting electronic circuit cards having electronic components and the like to the interface probes of a loaded-board tester.
BACKGROUND OF THE INVENTION
Loaded-Board Test Fixtures
After printed circuit boards (PCB's) have been manufactured and loaded with components, and before they can be used or placed into assembled products, they should be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation. Other reasons for testing printed circuit boards are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components and electro-mechanical components also may require adjustment after installation.
Loaded-board testing has complex multiplexed tester resources and is capable of probing soldered leads, vias and testpads on loaded boards with topside and bottom side components. Loaded-board testing includes analog and digital tests, such as tests for electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector testing, vectorless testing, and circuit functional testing. Loaded-board testing requires very low contact resistance between the test targets and the fixture components.
Advances in circuit board and electronic component packaging technology have escalated the probe spacing demands placed on loaded-board test equipment. Existing state-of-the-art technology requires loaded-board test equipment capable of accessing test targets which are spaced apart by 50 mils (center to center) or less, where test targets are physical features on a PCB or electronic component which may be probed during testing. One of the greatest challenges faced by loaded-board test equipment manufacturers now and in the future is a high false failure and test malfunction rate caused by physical and electrical contact problems. These problems are exacerbated by existing fixture limitations in probing accuracy, probing pitch (center to center spacing), and surface contamination.
As component and board geometries shrink and become denser, loaded-board testing becomes more difficult using standard fixtures. Existing shortwire, loaded-board fixtures can consistently hit test targets equal to or greater than 35 mils in diameter with equal to or greater than 75-mil pitch. Targets which are smaller or more closely spaced cannot be probed with consistency due to prohibitive component and system tolerance stack-ups.
A variety of test fixtures have heretofore been available for testing loaded boards on test equipment. A device under test (DUT) typically embodies a PCB loaded with electronic components and electronic hardware.
FIG. 1
shows a conventional shortwire, loaded-board fixture, which consists of a DUT
108
with outer-layer artwork, a standard
106
or variable
118
tooling pin for alignment, a probe protection plate
104
, standard spring probes
120
whose tips
116
exactly correspond to test target locations
110
and
112
, spacers
114
to limit the deflection of the DUT under vacuum loading, a probemounting plate
102
in which the spring probes
120
are installed, personality pins
100
which are wired to the spring probes
120
, and an alignment plate
122
which aligns the wirewrap tails of the personality pins
100
into a regularly spaced pattern so that they can line up with interface probes
124
mounted in the tester (not shown). Note: a spring probe is a standard device, commonly used by the test community, which conducts electrical signals and contains a compression spring and plunger that move relative to the barrel and/or socket when actuated. A solid probe also conducts electrical signals but has no additional parts which move relative to each other during actuation.
During test, the DUT
108
is pulled down by vacuum or other known mechanical means to contact the tips
116
of the spring probes
120
. The sockets of the standard spring probes
120
are wired to personality pins
100
, and an alignment plate
122
funnels the long, flexible personality pin tips
126
into a regularly spaced pattern. The tips
126
of personality pins
100
contact the interface probes
124
located in the tester (not shown). Once electrical contact between the DUT
108
and the tester is established, in-circuit or functional testing may commence. Hewlett-Packard Company Application Note 340-1 titled “Reducing Fixture-Induced Test Failures,” (printed December 1990 and can be obtained from Hewlett-Packard Company in Palo Alto, California), discloses shortwire fixturing and is incorporated herein for all that it teaches. U.S. Pat. No. 4,771,234 titled “Vacuum-Actuated Test Fixture” by Cook et al. discloses a longwire fixture and is incorporated herein for all that it teaches.
FIG. 2
shows one conventional fixture that attempts to address limited-access problems during testing. The term “limited-access” refers to something that cannot easily be reached, or accessed, due to physical restrictions or constraints. For example, a limited-access PCB may contain many targets that are too closely spaced to accurately probe using existing fixture technology. The term “standard-access” refers to that which can be reached, or accessed, using existing fixture technology. The fixture of
FIG. 2
consists of a DUT
206
with testpads
208
and
210
, a tooling pin
204
, a probe protection plate
202
, standard spring probes
214
and
216
installed in a probe-mounting plate
200
, and short probes
212
and
220
commonly referred to as “ULTRALIGN” probes (Ultralign is a registered trademark of TTI Testron, Inc.) installed directly in the probe protection plate
202
. Upon actuation, standard spring probes
216
and
214
located in the probe-mounting plate
200
push against the floating plungers of “ULTRALIGN” probes
212
and
220
. These short plungers are forced upward to contact test targets
208
and
210
, while the sockets
218
and
222
remain fixed within the probe protection plate
202
. An “ULTRALIGN” fixture may contain a mixture of spring probes for probing standard-access targets and “ULTRALIGN” probes for probing limitedaccess targets.
Despite its potential benefits, the “ULTRALIGN” fixture can be expensive and does not probe targets with a pitch of less than 50 mils. An “ULTRALIGN” fixture only permits limited probe travel which may result in poor connectivity between the probes
212
and
220
and the test targets
208
and
210
. Also, these probes are costly and require expensive maintenance to replace worn or broken “ULTRALIGN” probes. An example of this type of fixture is disclosed in U.S. Pat. No. 5,510,772 entitled “Test Fixture for Printed Circuit Boards” to Seavey, which is incorporated herein for all that it teaches.
FIG. 3
shows a conventional guided-probe protection plate fixture. Guided-probe protection plates are used in standard loaded-board test fixtures to improve the pointing accuracy of spring probes. These plates contain cone-shaped through-holes which guide, or funnel, the tips of spring probes toward test targets. Such a fixture consists of a probe-mounting plate
300
with standard spring probes
312
and
314
, a guided-probe protection plate
302
with spacers
310
and cone-shaped holes
316
for guiding the spring probes to the test targets
306
and
308
on the DUT
304
. Additional manufacturing steps and increased fixture maintenance are required due to increased wear on the probes and the probe protection plate, and generally only narrow probe tip styles can be used. Although probing accuracy is slightly enhanced with
Kanack Kris J.
Sayre Tracy L.
Slutz Robert A.
Agilent Technologie,s Inc.
Kobert Russell M.
Mitchell Cynthia S.
Sherry Michael
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