Bendable pin board test fixture

An apparatus is described for interfacing an automatic board test system to an electronic circuit card. This interface permits short and reliable connections between the test system and electronic components located on the circuit card for the electronic signals which test these components. This apparatus works equally well with vacuum actuated and mechanically actuated systems.

BACKGROUND 
This invention relates to the field of art pertaining to board test 
fixtures and other mechanical interfaces for electrically interconnecting 
electronic circuit cards and the like to electrical switching systems. 
A board test system consists of numerous electronic sources and detectors 
which are connected through an electric switch, or scanner, to a plurality 
of contact points referred to as scanner pins. A board test fixture then 
provides an interface between these scanner pins and the electronic 
components located on an electronic circuit card. Since the electronic 
signals which are used to determine whether the electronic component is 
operating properly must pass through the board test fixture both on their 
way to and from the electronic component, the board test fixture must 
maintain the signal quality of these signals to ensure that the electronic 
component is not incorrectly diagnosed as operating properly or 
improperly. 
In order to insure maximum signal quality, the length of the signal path 
between the scanner and the electronic circuit card should be kept as 
short as possible. This normally dictates a vertical configuration with 
the board test fixture sitting directly on top of the scanner and the 
electronic card directly on the fixture. However, any board test fixture 
must be easy to assemble and maintain in order to be cost effective and 
many prior art vertical configuration test fixtures have sacrificed 
assembly and maintainability to obtain short lead length The ability to 
automate assembly of the fixture is also an important feature. 
Various prior art solutions have attempted to address these requirements. A 
first prior art solution uses a stiff probe pin to conduct the electronic 
signal directly from a spring loaded scanner pin to the electronic 
component under test. The probe pin passes through a first plate having a 
hole for each scanner pin and through a second plate have holes drilled 
according to the location of the electronic components on the card. This 
probe pin may be vertical if the component is located directly over the 
scanner pin, or the probe pin may be at an angle. At an angle, the probe 
pin may miss the component which it is trying to contact, or may make a 
high resistance contact, both being undesirable. 
A second prior art solution replaces the stiff probe pin with a flexible 
probe pin. This permits the holes in both the first and second plate to be 
at right angles to the scanner pins and electronic components assuring a 
low resistance contact with the electronic component. 
Both the first and second prior art solutions are impractical because they 
require a large number of scanner pins in order for a scanner pin to be 
located sufficiently close to the component to be able to use a straight 
or slightly bent pin. More scanner pins mean additional expensive pin 
electronics. In order to reduce the number of scanner pins, the fixturing 
method should permit signals from the scanner pins to be routed, or 
translated, to probe pins located at a different x and y axis positions 
from the scanner pins with respect to the plane of the probe plate. For 
incircuit and functional testing, the fixturing method must permit 
translation between the component location and the scanner pin location. 
A third prior art solution permits translation at the expense of long 
connecting wires. The fixture again has two plates, the first plate 
drilled according to the locations of the electronic components on the 
card and the second plate with holes positioned above the scanner pins. A 
spring loaded probe with a wire wrap post is mounted in the first plate 
making electrical contact with the electronic components on the card. An 
interconnect pin with a wire wrap post is mounted in the second plate 
making electrical contact with the scanner pin. A wire is then wire 
wrapped between the probe pin and the interconnect pin to complete the 
electrical connection. This wire is typically quite long to enable the 
fixture to be opened for easy building and maintenance. Because the wire 
is long, the electrical performance of the third solution is inferior. A 
fourth prior art solution is similar to the third prior art solution but 
uses short wires between the probe pin and the interconnect pin. However, 
in order to use short wires, the second plate of the fourth solution is 
divided into interconnect strips. The interconnect strips are arranged in 
rows below the probe plate. Electrical connections are made one row at a 
time between the probe pins and the interconnect pins located in these 
strips. Starting at one end and after each row of connections is made, 
each strip is sequentially mounted across the bottom of the fixture. The 
fourth prior art solution, although offering good electrical performance, 
is difficult to wire, especially if there is a large number of x and y 
axis translations. This solution is impossible to automate and service is 
very difficult. 
The fifth and final prior art solution is referred to as the "basic 
matrix". This solution uses a printed circuit board to perform the x and y 
axis translation. The probes which contact the electronic components are 
mounted directly to the top of the printed circuit board and the scanner 
pins contact the bottom of the printed circuit board to complete the 
connection. This solution is expensive requiring a custom printed circuit 
board for each fixture and special probes. Furthermore, this solution 
causes scanner pins located directly under probes to be made unusable, a 
very undesirable feature. 
A need exists for a low cost, easy to build test fixture for which assembly 
is automatable. The test fixture should minimize the loss of scanner pins 
and provide for translation in the x and y axis directions between probe 
and scanner pin. 
SUMMARY 
In accordance with the preferred embodiment of the present invention, an 
apparatus is described for interfacing an automatic board test system to 
an electronic circuit card. This interface permits reliable electrical 
connections between the test system and electronic components located on 
the circuit card. The apparatus differs from the prior art in that the 
connections maximize the quality of the connection to the test system 
while permitting easy and automatic assembly of the test fixture. This 
apparatus works equally well with both vacuum actuated and mechanically 
actuated systems. 
The present invention is superior to the prior art in several ways. First, 
the apparatus is a vertical fixturing scheme which minimizes the 
interconnection lead length. Second, the apparatus is easy to assemble and 
the assembly may be automated to further reduce the cost. Third, the 
apparatus avoids where possible making the interconnection points to the 
test system unusable. Finally, the apparatus does not require the 
electronic components to be located on a grid of any sort; the electronic 
components may be located anywhere on the card.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 is a fragmentary elevational view of the preferred embodiment of the 
present invention in a first position with an electronic circuit card. The 
test fixture comprises an alignment plate 22, probe plate 23, two probe 
assemblies 50 and 51, two interconnecting pins 53 and 54, interconnect 
wire 55 and 56, walls 41 and 42, alignment pins 84 and 85, and vacuum seal 
80. The electronic circuit card to be tested consists of a printed circuit 
board 90, component 92, and printed circuit trace 91. The printed circuit 
board 90 has two tooling holes which may be used for alignment. A vacuum 
chamber is formed by the board 90, vacuum seal 80, and probe plate 23. A 
vacuum manifold, not shown, is attached over a hole in the probe plate to 
draw the air from the vacuum chamber. Vacuum is used in the preferred 
embodiment to force the electronic circuit card against the probe 
assembly, although mechanical means may also be used. The test fixture is 
mounted on the test system scanner. The scanner consists of a plate 21 
with scanner pins 70. The construction of scanners is well known in the 
prior art as are various mounting and locking mechanism for holding the 
test fixture to the scanner. The present invention is capable of operating 
with a wide variety of scanners and mounting and locking mechanisms. 
The probe assembly typically consists of a probe socket which is mounted 
into the hole in the probe plate and a probe mechanism having probe tip 94 
which is inserted into the probe socket. The probe mechanism may be 
replaced as the probe tip 94 wears out. The probe socket consists of a 
tube which accepts the probe mechanism and a square wire wrap post. The 
probe mechanism consists of a probe tip 94 and a spring to bias the probe 
tip 94 to its maximum extended position in a housing which fits in the 
probe socket. Both probe sockets and probe mechanisms, including a variety 
of probe tips, are well known in the art. 
The interconnecting pin 53 or 54 consists of insulated body about the same 
length as the probe assembly and a wire wrap post 58 or 59 which is 
considerably longer than the wire wrap post of the probe assembly 50 or 
51. In the preferred embodiment of the present invention, the wire wrap 
post 58 or 59 of the interconnecting pin is approximately one inch long. 
The probe plate 23 is constructed from a strong insulating material. In the 
preferred embodiment of the present invention, a sheet of plastic 
approximately one-half of an inch thick is used as the probe plate. In the 
preferred embodiment of the present invention, the plate 23 has a manifold 
hole which is not shown. The plate 23 is then drilled for the probe 
assemblies 50 and 51, interconnecting pins 53 and 54 and alignment pins 84 
and 85. The locations of the probe assemblies 50 and 51 and the alignment 
pins 84 and 85 are determined by the locations of the components and 
traces on the electronic circuit card 90. These locations are often 
available from CAD/CAM systems as points. This permits a fixture builder 
to select the points from the CAD/CAM files for the electronic circuit 
card to be tested and to have the probe plate automatically drilled at 
these points. Automatic drilling is considerably less expensive than 
manual drilling. The location of the interconnecting pins is determined, 
with one important exception, from the location of the scanner pins. The 
scanner pins are on a fixed grid which may also be automatically drilled. 
The exception occurs when a probe assembly is located directly over a 
scanner pin as shown by probe assembly 51 in FIG. 1. For these cases, the 
hole for interconnecting pin is drilled off to one side. The maximum 
distance to offset the hole for the interconnecting pin is determined by 
the length of the interconnecting pin and the tolerances of the scanner 
pins and fixture For the preferred embodiment of the present invention, a 
0.100 of an inch is used for the offset. The holes for the interconnecting 
pins do not need to pass completely through the probe plate, although in 
the preferred embodiment they do for convenience of drilling. Drilling the 
holes for the interconnecting pins may also be computerized by the CAD/CAM 
system since the system knows the location of both the components and the 
scanner grid. 
After the probe plate is drilled, the alignment pins, the probe assemblies 
and the interconnecting pins are installed by pressing the pin or assembly 
into the probe plate, a process which lends itself to automation. The 
probe plate must then be wired. The x and y axis translation features, as 
well as the multiple connection capabilities of the fixture are provided 
by wiring each probe assembly to one or more interconnecting pins. In the 
preferred embodiment of the present invention, the method of wiring is by 
wire wrapping. Wire wrapping is fast and lends itself to automation. FIG. 
1 illustrates the interconnect wires 55 and 56 used for connecting the 
interconnecting pin to the probe assembly in the preferred embodiment of 
the present invention. 
After the probe plate has been wired, the fixture is assembled by 
connecting the probe plate 23 to the alignment plate 22. The primary 
requirement is to position the alignment plate 22 at a constant fixed 
distance, from the probe plate 23. In the preferred embodiment of the 
present invention, the alignment plate is located approximately an one and 
one-eighth inches plus or minus one sixteenth of an inch, from the probe 
plate. The preferred embodiment of the present invention uses a wall 41 
and 42 to position the alignment plate with respect to the probe plate. 
Walls offer the added advantage of keeping dirt out, reducing the 
possibility of accidental damage to the fixture, and adding stiffness to 
the probe plate to reduce deflection. Alternate methods, including posts, 
may be used. 
The alignment plate 22 has holes 26 on a grid which corresponds to the 
location of the scanner pins 70. The wire wrap posts of the 
interconnecting pins 58 pass through these holes in the alignment plate to 
make contact with the scanner pins. The holes 26 in the alignment plate 22 
are bored out to a tapered shape to assist in assembly. These tapered 
holes 26 are especially important in the special case described above 
where a probe assembly lies directly above the scanner pin. In this case 
as shown in FIG. 1, the wire wrap post 59 of the interconnecting pin 54 
which has been offset to on side of probe pin 51 is bent as it is passes 
through the the hole 26 in the alignment plate 22. The tapered bore of the 
hole 26 makes assembly of the offset interconnecting pins possible and 
permits easy disassembly for maintenance and repair. 
Accurate alignment of the test fixture is essential for reliable operation. 
Alignment for the printed circuit board 90 to probe plate 23 is maintained 
by the alignment pins 84 and 85. Two or more alignment pins may be used 
depending on the size of the board. Alignment between the probe plate 23 
and the alignment plate 22 is not critical since the wire wrap posts of 
the interconnecting pins are free to bend although it does determine the 
required length of the wire wrap post of the interconnecting pin. The 
alignment between the alignment plate 22 and the scanner 21 is controlled 
through the mounting and locking hardware well known in the prior art. 
The method of operation of the test fixture is as follows. The printed 
circuit board is placed on the fixture with the alignment pins 84 and 85 
of the test fixture passing through the tooling holes in the printed 
circuit board 90. The component 92 and trace 91 of the electronic circuit 
card must now be brought into contact with the probe tips 93 and 94. This 
may be achieved in several ways including vacuum and mechanical actuating 
means. The Preferred embodiment of the present invention uses a vacuum 
actuating means. Air is removed from the vacuum chamber created between 
the electronic circuit card and the probe plate 23. A seal 80 is used to 
maximize the force and reduce noise. As the electronic circuit card is 
drawn toward the probe plate, the probe tips 93 and 94 of the spring 
loaded probe assemblies 50 and 51 push against the ends of the component 
92 and the trace 91 making a good low resistance contact. 
FIG. 2 is a fragmentary elevational view of the preferred embodiment of the 
present invention in a second position with the electronic circuit card. 
FIG. 2 illustrates the printed circuit board with the vacuum applied in 
position for testing. When the vacuum is released the card returns to the 
position in FIG. 1 and may be easily removed and the next card to test 
installed. 
Mechanical means may also be used for driving the electronic circuit card 
against the probe tips. This may be accomplished, for example, by placing 
a weight on top of the board. The present invention works well with any 
method of forcing the electronic circuit card against the probe tips.