Wireless Test Fixture Considerations

New developments are gaining attention for their potential in accommodating advanced, loaded-board layout.

Test fixtures continue to play a very important role in both the fabrication and assembly of printed circuit boards. Bare-board fixtures check the quality of unpopulated boards before chips are added, while loaded-board fixtures are used to test circuit boards with microprocessors and other components mounted on them. In either application, the fixture is customized by the user (or outside test resource) for specific production runs.

As technology advances, PCBs are becoming more densely populated. Solid-state circuitry and discrete components grow in complexity as they shrink in size, and increasing signal speeds are on the horizon. To address the challenges associated with "high-tech" boards, production managers and test system suppliers are taking a new look at the "wireless" fixture. Wireless versions of loaded-board test fixtures have been in limited use for over a decade. Manufacturers are multiplying, creating more demand for flexible, new test solutions, and the wireless product concept is gaining attention for its greater potential in accommodating advanced, loaded-board layouts.

The conventional wireless fixture (Figure 1) makes use of double-ended probes and substitutes a PCB for the fixture's wiring. This type of wireless fixture has both advantages and disadvantages over the standard wired fixture.

Figure 1: Conventional wireless fixture

Advantages
The major performance advantage of the conventional wireless fixture is the controlled characteristic impedance and shortened electrical signal path gained by replacing the wires with a PCB. This improved characteristic impedance allows for increased bandwidths for analog tests and increased rates for digital test vectors. Another important advantage is the consistent electrical repeatability from fixture to fixture that is achieved when fixtures are replicated. Users who require multiple fixtures for production lines have noted substantial cost and time saved through the use of wireless fixtures. The unit's slim profile also handles easily and requires less storage.

In summary, the advantages of conventional wireless fixture design include:

electrical signal quality and speed
streamlined profile
duplicate fixtures without debugging.

Disadvantages
The most significant disadvantage of conventional wireless fixture design pertains to the wireless PCB itself. The concept of using a PCB to replace the wires seems logical, but implementation is difficult and expensive. The internal PCB must be designed, fabricated and thoroughly tested. The integrity of this test is dependent upon the test equipment used by the PCB supplier. Users that have been successful with wireless fixtures typically design and fabricate their own fixture PCB.

The additional time required to perform any type of engineering change associated with the fixture's test points is another disadvantage. These types of changes may include adding or relocating test points and moving or removing connections between the pin electronics of the tester and the test point.

The assembly and disassembly process for wireless fixtures can be difficult and time-consuming. After the double-ended receptacles are installed, the PCB must be attached by tightening a series of screws into standoffs, which will gradually pull the internal board into contact with the double-ended probes. Every probe must be pre-loaded properly to achieve optimum electrical contact.

High initial cost is yet another disadvantage of the conventional wireless fixture design. The internal PCB is an added expense; double-ended probes are vulnerable to excess pressure and more costly to deploy.

Summarized, the disadvantages of conventional wireless fixture design include:

Internal PCB--design and fabrication expense; wait period for production and testing
Double-ended probes-high cost per test point; less robust contact design
Assembly/disassembly-time-consuming and difficult
Internal stress-required pre-loading for all double-ended probes
Electrical-extra set of probes in the electrical path
ECNs (engineering change notices)-time-consuming and difficult to perform.

New Design Concept
A substantially different wireless design has been introduced recently. It resolves the disadvantages associated with double-ended probing and appears to further push the performance envelope in testing small, fine-pitch contact points. Obtaining a required, internal PCB at reasonable cost in a timely manner remains a challenge to be solved.

The new design incorporates an interlayer material and techniques first used successfully in bare-board test environments. Figure 2 shows a cross section of the new design. At first glance, this profile appears to be much more streamlined than the traditional double-ended probe design. In this new design, double-ended probes have been replaced by single-ended probes that "float" within the fixture. The spring within the probe is used to attain contact force against the unit under test (UUT) and is also used to achieve contact force against the internal fixture PCB.

Figure 2: New approach to wireless fixturing

The new concept enables more robust probe deployment. Since probe receptacles are not used, traditional 100-mil probes can be used to make contact with 70-mil centers. This process represents a reduction in both operating time and cost, since smaller probes are likely to be replaced more often. The same benefit applies to 50-mil test points, now addressed by 75-mil probes, and 40-mil centers, achieved with 50-mil probes.

The new probe that floats can also be applied to conventional wired fixtures and offers a migration path between these two distinct fixture models. Previous wired and wireless fixtures were unique configurations with few common parts. Using this new design, a fixture in a wired application can be readily converted for wireless use (Figure 3).

Figure 3: New approach on a wired fixture

Most fixtures go through some wiring or probe placement revisions as the design of the PCB under test becomes finalized. These revisions can create havoc with a wireless fixture. The new concept allows the initial fixture to be built as a wired unit, used for program debug, then readily modified as ECNs are received. Once the wiring and probe locations become stable, the internal PCB for the wireless fixture can be designed without time pressures and with some assurance that the production PCB is now a stable design. The wired fixture then readily converts to a wireless fixture. This conversion is achieved by removing the spacer plate and frame shown in Figure 3, and replacing it with an internal PCB and the spacer plate shown in Figure 2. If the internal PCB must be reworked or replaced, the fixture can be reconfigured as a wired fixture in the interim.

Fixture Accuracy
The fixture design has been found to offer notable pointing accuracy in loaded-board fixtures. This accuracy is achieved by precisely controlling the tips of the probes only where they make contact with the PCB under test (Figure 4).

Figure 4: Fixture probe pointing accuracy

Fixture designers have attempted a variety of techniques to guide the tips of the probes and improve upon pointing accuracy. Each attempt has achieved limited success due to the unavoidable misalignment that occurs between the moving top plate and the vacuum well. Wherever probes are rigidly mounted in receptacles, and the probe tips are guided by tight holes in the top plate, even a slight misalignment between the two plates will cause the probes to bind.

In the new design, the probes are allowed to float in the vacuum well, which is accomplished by drilling a hole in the vacuum well slightly larger than the probe barrel. This slight clearance gives the probe the freedom to move, prevents binding and allows a single, tight-fitting hole to be used in the top plate. This approach avoids the constraints of a rigid infrastructure and still achieves good pointing accuracy.

Assembly and Disassembly
The major assembly/disassembly limitations using double-ended probes are eliminated with the new concept. On previous wireless designs, the double-ended probes were all pre-loaded to achieve contact with the internal PCB, which is gradually pulled into place by tightening a series of screws. This time-consuming and delicate process is now eliminated. Since no internal forces are present, the unit can be assembled with a limited number of screws. The vacuum used to actuate the fixture compensates for any forces applied to the internal PCB during operation.

Electrical
A wireless fixture using double-ended probes has a much longer electrical path: two probes and a receptacle are required to make the connection from the UUT to the wireless PCB. In the new design, only one probe is used to connect the UUT to the wireless PCB, which reduces the electrical path and increases the reliability of the connection.

ECNs

Throughout the UUT design and manufacturing process, engineering changes are sometimes required. Although special techniques have been developed, facilitating ECNs is still more difficult than modifying a wired fixture. With a migration path from wired-to-wireless, the new design can be used to minimize ECNs.

Advantages
Summarized, the advantages of a wireless fixture design with a probe that floats include:

robust design -
- 100 mil probes; 70 mil centers
- 75 mil probes; 50 mil centers
- 50 mil probes; 40 mil centers
migration possible-wired to wireless; wireless to wired
most accurate fixture-tips captured by top plate; probes are loose in holes; no binding
assembly/disassembly--easy; no internal force
electrical-just one probe.

Disadvantages
Some of the major drawbacks of wireless fixturing remain with the new concept. Disadvantages of a wireless fixture design with a probe that floats include:

PCB-high cost; time to obtain a tested board.
ECNs-time-consuming and difficult to perform.

Summary
The need for wireless fixtures in today's rapidly changing test environment is increasing, due to new demand for flexible solutions and the growing complexity of loaded boards under test. In some cases, the electrical characteristics required to produce a stable and repeatable test are achievable only with the wireless fixture concept.

The new design can be used in a wide range of test applications that demand an alternative approach. The unique probe installation technique allows for more robust probes, placed on a tighter center spacing than standard probe/receptacle technology will permit.

Wireless fixtures certainly are not for everyone. Experience has shown that most successful integration has occurred when the customer designed and fabricated the required internal PCB. When possible, wireless test is best adopted by users with existing PCB fabrication resources. The new design does give test engineers the freedom to move from wired to wireless formats, debugging and incorporating changes to ensure a stable and accurate design for the wireless PCB. As more test professionals gain experience with wireless technology, new techniques will continue to evolve. The new concept is one advance that positions fixture designs for twenty-first century requirements.

Gary St. Onge is vice president, text fixture division and Jeff Sendzicki is operations manager with Everett Charles Technologies, Pomona, CA, (909) 625-5551

Parts of this paper were presented in the 1998 Nepcon West Proceedings, Anaheim, CA and are reprinted with permission.

Reprinted with permission from CIRCUITS ASSEMBLY, June 1998


	Wireless Test Fixture Considerations By Gary St. Onge and Jeff Sendzicki	New developments are gaining attention for their potential in accommodating advanced, loaded-board layout. Test fixtures continue to play a very important role in both the fabrication and assembly of printed circuit boards. Bare-board fixtures check the quality of unpopulated boards before chips are added, while loaded-board fixtures are used to test circuit boards with microprocessors and other components mounted on them. In either application, the fixture is customized by the user (or outside test resource) for specific production runs. As technology advances, PCBs are becoming more densely populated. Solid-state circuitry and discrete components grow in complexity as they shrink in size, and increasing signal speeds are on the horizon. To address the challenges associated with "high-tech" boards, production managers and test system suppliers are taking a new look at the "wireless" fixture. Wireless versions of loaded-board test fixtures have been in limited use for over a decade. Manufacturers are multiplying, creating more demand for flexible, new test solutions, and the wireless product concept is gaining attention for its greater potential in accommodating advanced, loaded-board layouts. The conventional wireless fixture (Figure 1) makes use of double-ended probes and substitutes a PCB for the fixture's wiring. This type of wireless fixture has both advantages and disadvantages over the standard wired fixture. Figure 1: Conventional wireless fixture Advantages The major performance advantage of the conventional wireless fixture is the controlled characteristic impedance and shortened electrical signal path gained by replacing the wires with a PCB. This improved characteristic impedance allows for increased bandwidths for analog tests and increased rates for digital test vectors. Another important advantage is the consistent electrical repeatability from fixture to fixture that is achieved when fixtures are replicated. Users who require multiple fixtures for production lines have noted substantial cost and time saved through the use of wireless fixtures. The unit's slim profile also handles easily and requires less storage. In summary, the advantages of conventional wireless fixture design include: electrical signal quality and speed streamlined profile duplicate fixtures without debugging. Disadvantages The most significant disadvantage of conventional wireless fixture design pertains to the wireless PCB itself. The concept of using a PCB to replace the wires seems logical, but implementation is difficult and expensive. The internal PCB must be designed, fabricated and thoroughly tested. The integrity of this test is dependent upon the test equipment used by the PCB supplier. Users that have been successful with wireless fixtures typically design and fabricate their own fixture PCB. The additional time required to perform any type of engineering change associated with the fixture's test points is another disadvantage. These types of changes may include adding or relocating test points and moving or removing connections between the pin electronics of the tester and the test point. The assembly and disassembly process for wireless fixtures can be difficult and time-consuming. After the double-ended receptacles are installed, the PCB must be attached by tightening a series of screws into standoffs, which will gradually pull the internal board into contact with the double-ended probes. Every probe must be pre-loaded properly to achieve optimum electrical contact. High initial cost is yet another disadvantage of the conventional wireless fixture design. The internal PCB is an added expense; double-ended probes are vulnerable to excess pressure and more costly to deploy. Summarized, the disadvantages of conventional wireless fixture design include: Internal PCB--design and fabrication expense; wait period for production and testing Double-ended probes-high cost per test point; less robust contact design Assembly/disassembly-time-consuming and difficult Internal stress-required pre-loading for all double-ended probes Electrical-extra set of probes in the electrical path ECNs (engineering change notices)-time-consuming and difficult to perform. New Design Concept A substantially different wireless design has been introduced recently. It resolves the disadvantages associated with double-ended probing and appears to further push the performance envelope in testing small, fine-pitch contact points. Obtaining a required, internal PCB at reasonable cost in a timely manner remains a challenge to be solved. The new design incorporates an interlayer material and techniques first used successfully in bare-board test environments. Figure 2 shows a cross section of the new design. At first glance, this profile appears to be much more streamlined than the traditional double-ended probe design. In this new design, double-ended probes have been replaced by single-ended probes that "float" within the fixture. The spring within the probe is used to attain contact force against the unit under test (UUT) and is also used to achieve contact force against the internal fixture PCB. Figure 2: New approach to wireless fixturing The new concept enables more robust probe deployment. Since probe receptacles are not used, traditional 100-mil probes can be used to make contact with 70-mil centers. This process represents a reduction in both operating time and cost, since smaller probes are likely to be replaced more often. The same benefit applies to 50-mil test points, now addressed by 75-mil probes, and 40-mil centers, achieved with 50-mil probes. The new probe that floats can also be applied to conventional wired fixtures and offers a migration path between these two distinct fixture models. Previous wired and wireless fixtures were unique configurations with few common parts. Using this new design, a fixture in a wired application can be readily converted for wireless use (Figure 3). Figure 3: New approach on a wired fixture Most fixtures go through some wiring or probe placement revisions as the design of the PCB under test becomes finalized. These revisions can create havoc with a wireless fixture. The new concept allows the initial fixture to be built as a wired unit, used for program debug, then readily modified as ECNs are received. Once the wiring and probe locations become stable, the internal PCB for the wireless fixture can be designed without time pressures and with some assurance that the production PCB is now a stable design. The wired fixture then readily converts to a wireless fixture. This conversion is achieved by removing the spacer plate and frame shown in Figure 3, and replacing it with an internal PCB and the spacer plate shown in Figure 2. If the internal PCB must be reworked or replaced, the fixture can be reconfigured as a wired fixture in the interim. Fixture Accuracy The fixture design has been found to offer notable pointing accuracy in loaded-board fixtures. This accuracy is achieved by precisely controlling the tips of the probes only where they make contact with the PCB under test (Figure 4). Figure 4: Fixture probe pointing accuracy Fixture designers have attempted a variety of techniques to guide the tips of the probes and improve upon pointing accuracy. Each attempt has achieved limited success due to the unavoidable misalignment that occurs between the moving top plate and the vacuum well. Wherever probes are rigidly mounted in receptacles, and the probe tips are guided by tight holes in the top plate, even a slight misalignment between the two plates will cause the probes to bind. In the new design, the probes are allowed to float in the vacuum well, which is accomplished by drilling a hole in the vacuum well slightly larger than the probe barrel. This slight clearance gives the probe the freedom to move, prevents binding and allows a single, tight-fitting hole to be used in the top plate. This approach avoids the constraints of a rigid infrastructure and still achieves good pointing accuracy. Assembly and Disassembly The major assembly/disassembly limitations using double-ended probes are eliminated with the new concept. On previous wireless designs, the double-ended probes were all pre-loaded to achieve contact with the internal PCB, which is gradually pulled into place by tightening a series of screws. This time-consuming and delicate process is now eliminated. Since no internal forces are present, the unit can be assembled with a limited number of screws. The vacuum used to actuate the fixture compensates for any forces applied to the internal PCB during operation. Electrical A wireless fixture using double-ended probes has a much longer electrical path: two probes and a receptacle are required to make the connection from the UUT to the wireless PCB. In the new design, only one probe is used to connect the UUT to the wireless PCB, which reduces the electrical path and increases the reliability of the connection. ECNs Throughout the UUT design and manufacturing process, engineering changes are sometimes required. Although special techniques have been developed, facilitating ECNs is still more difficult than modifying a wired fixture. With a migration path from wired-to-wireless, the new design can be used to minimize ECNs. Advantages Summarized, the advantages of a wireless fixture design with a probe that floats include: robust design - 100 mil probes; 70 mil centers 75 mil probes; 50 mil centers 50 mil probes; 40 mil centers migration possible-wired to wireless; wireless to wired most accurate fixture-tips captured by top plate; probes are loose in holes; no binding assembly/disassembly--easy; no internal force electrical-just one probe. Disadvantages Some of the major drawbacks of wireless fixturing remain with the new concept. Disadvantages of a wireless fixture design with a probe that floats include: PCB-high cost; time to obtain a tested board. ECNs-time-consuming and difficult to perform. Summary The need for wireless fixtures in today's rapidly changing test environment is increasing, due to new demand for flexible solutions and the growing complexity of loaded boards under test. In some cases, the electrical characteristics required to produce a stable and repeatable test are achievable only with the wireless fixture concept. The new design can be used in a wide range of test applications that demand an alternative approach. The unique probe installation technique allows for more robust probes, placed on a tighter center spacing than standard probe/receptacle technology will permit. Wireless fixtures certainly are not for everyone. Experience has shown that most successful integration has occurred when the customer designed and fabricated the required internal PCB. When possible, wireless test is best adopted by users with existing PCB fabrication resources. The new design does give test engineers the freedom to move from wired to wireless formats, debugging and incorporating changes to ensure a stable and accurate design for the wireless PCB. As more test professionals gain experience with wireless technology, new techniques will continue to evolve. The new concept is one advance that positions fixture designs for twenty-first century requirements. Gary St. Onge is vice president, text fixture division and Jeff Sendzicki is operations manager with Everett Charles Technologies, Pomona, CA, (909) 625-5551 Parts of this paper were presented in the 1998 Nepcon West Proceedings, Anaheim, CA and are reprinted with permission. Reprinted with permission from CIRCUITS ASSEMBLY, June 1998