With some forethought during the design and layout process, circuit boards can be easily, economically, and reliably tested in a bed-of-nails environment. To reach this goal, three general areas are important. First, determine what connections are needed from an electrical standpoint. Then, decide how to provide contact access for the probes and how to align the board. Finally, provide the data to the fixture fabricators and test programmers in an efficient format.
Sometimes, all of the design goals cannot be met due to physical or electrical constraints of the UUT. However, by meeting as many of these goals as possible, you will minimize fixture costs and maximize fixture reliability.
Electrical Connections
To obtain full in-circuit test coverage, in-circuit test systems and manufacturing defects analyzers (MDAs) need electrical access to each node on the circuit board. A node (or net) is one electrical connection, although it can go to multiple places and components. Examples are GND, VCC, and DATA0. Dedicated functional test fixtures generally use a lot less probes since they only need access to circuits at the block level.
Mechanical Considerations
Probe Spacing
Ideally, boards should be tested with industry-standard 0.100″ spring probes. These low-cost and reliable probes are designed to contact circuit points that are 100-mils or more apart from center to center.
If you do not have the luxury of 0.100″ spacing, standard probes are designed for 0.075″ and 0.050″ access. Even closer spacing is available for the brave-of-heart. As the probe size decreases, the cost increases, and obtaining high reliability becomes more difficult.
Probe Access
Usually, all modern test fixtures accommodate bottom access to the UUT. Whenever possible, provide access at the bottom of the UUT for every net on the board.
Fixtures also can be designed to have access to the tops and sides of the UUT. However, due to the added complexity, non-bottom access increases the cost and decreases the reliability of the fixture.
Probe Targets
For reliable probing, the probes must have good targets. Each probe should contact a target 0.035″ dia or larger. The targets should be as large as possible, ideally 0.050″ to 0.060″. When top probing an assembly, targets on the top should be 0.045″ or larger.
The target can consist of a test pad, round or square; a through-hole with a soldered lead; an open through-hole, or a non-masked via. For probing on soldered leads, make sure that the lead trim length is consistent within ±0.030″.
Open through-holes must have a relatively small diameter so that the probe can contact the hole edge. For standard probing, the hole should be less than 0.050″.
Avoid probing on leads of surface-mount (SM) devices. Because of the variability of the placement and edge geometry, probing can be unreliable on SM leads. Also, the probe can press the SM lead to the pad, causing a bad connection to test good.
Target Plating and Solder Mask
Solder mask or conformal coating prevents the probes from making electrical contact, so do not cover the probe contact areas with either of these materials. Ideally, the solder resist should be at least 0.020″ radially from the probing target. This avoids the problem of the probe touching the solder mask first, preventing it from contacting the target.
Target surfaces should be gold or solder-coated for best probing. Generally, harder materials are more difficult to obtain a good contact and have a tendency to dull the probes prematurely.
Alignment Holes
UUTs are accurately positioned on the test fixture with the use of guide pins that mate into tooling holes in the UUT. The UUT should have at least two tooling holes for positioning. These should be as far apart as practical, ideally on opposite corners.
Tooling hole sizes that work well fall into the range of 0.125″ to 0.250″ dia. Diameters of 0.125″, 0.156″, 0.187″, and 0.250″ are standard. These sizes are easy to accommodate with off-the-shelf guide pins and are strong enough for use without damage or premature wear.
If the UUT is symmetrical, use a third nonsymmetrical hole to prevent installing the UUT onto the fixture incorrectly.
Tolerances between the tooling holes and targets should not exceed ±0.002″, and probe targets should be at least 0.175″ from the center of the tooling hole.
Overall UUT Size
There are benefits to keeping the UUT small:
Most fixture systems accommodate UUTs up to about 12″ × 16″ with standard fixture kits. If your overall board size is larger, special, more expensive, kits become necessary.
The overall cost of the test system is reduced (less test points are required).
The cost of replacing fixtures in the event of ECOs is reduced.
Related large-board problems, such as board-flex during vacuum fixturing, are reduced.
Vacuum Fixtures
To accommodate vacuum sealing, there are some special considerations in designing UUTs for vacuum fixtures, including:
Leads and component bodies should be at least 0.125″ from the edge of the UUT for gasket sealing integrity.
Probe loading should not exceed 25 probes per square inch; 40 is the theoretical maximum for vacuum fixturing at sea level.
Routed openings should be minimized so that less gasketing is required.
Components on the bottom of the UUT should be less than 0.125″ high to prevent special fixture machining. Heights beyond 0.375″ can require very significant fixture machining.
There should be no open holes, such as unmasked and unsoldered vias. If it is necessary to have openings, minimize the number.
Rectangular board outline shapes are more economical to fixture.
Mechanical and Pneumatic Fixturing
Mechanical and pneumatic fixturing have some considerations during the design process:
Bottom components should be less than 0.300″ high to eliminate special machining.
Top components should be less than 1.150″ (mechanical) or 3.325″ (pneumatic) to eliminate special machining. These dimensions can vary from one type of fixture system to another.
Except for very dense UUTs, pressure-rod placement normally is accommodated without special design. There should be some spaces not occupied by components on the top of the UUT for pressure-rod placement. The number of pressure-rod spaces should be approximately the number of probes divided by 25, with a minimum of four. These spaces should be somewhat evenly distributed and at least 0.1″ dia.
Panelized UUTs
Panelized UUTs—multiple UUTs in a single panel—can be tested using the same guidelines as unpanelized UUTs. However, consider the case of testing once the individual UUT is broken away. You must provide tooling holes for alignment in the individual UUTs and have one fixture position with both guide pins for individual UUTs.
Vectorless Test
Vectorless test uses special sensor plates over UUT components to detect open connections and polarity of capacitors. Open connections on bused devices generally are a concern with SM UUTs.
These sensor plates can be mounted on the top, bottom, or sides (for example, for access to the ends of connector pins or sockets) of the UUT. If you are using this technology in your fixtures, make sure that the test pads are 0.100″ or more from the edge of the component. This gives the fixture vendor mechanical room to accommodate both test probes and sensor plates without compromising reliability.
Generally, it is best to have all of the components to be tested with vectorless test technology on the opposite side from the standard test probes.
Data Considerations
The fixture fabricator must be able to determine the physical location of each probe. Minimally, this can be determined with a schematic, bare-board, and drill file for the UUT.
In the best case, the fixture fabricator is provided with direct data that specifies the electrical and physical probe locations. An optimal way of doing this during the design process follows these guidelines:
Define test points as 0.035″ (or larger) dia bottom-access pads with a 0.1″ dia keep-away. They can be vias, through-holes, or pads.
Place one of these test points on each network of the schematic.
If you follow this process, the benefits will be many:
Annotated schematics that are useful for UUT troubleshooting.
Minimal fixture cost since the fixture fabricator can easily use the resultant pick-and-place data and netlist.
A very reliable fixture since it will use standard probes with efficient targets. Bottom access to each net is guaranteed.
This technique works well for both SM and through-hole UUTs.
Summary
Building test fixtures for almost anything is possible given enough time and money. Using the guidelines presented here will help you design boards that are economical and reliable to test. In many cases, you may not be able to fully follow the guidelines. But if you deviate as little as possible, you will achieve the best results.
About the Authors
Brian Laine is the sales manager at CheckSum. He previously held engineering and engineering management positions at Fluke and Summation. Mr. Laine received a B.S. degree in computer science from Oregon State University and an M.B.A. degree from Seattle University.
Ken Hallmen is the marketing manager at CheckSum. Before joining the company, he held engineering and engineering management positions at Tektronix and Fluke. Mr. Hallmen earned a B.S. and an M.S. degree in electrical engineering from the University of Washington and an M.B.A. degree from Seattle University.
CheckSum, P.O. Box 3279, Arlington, WA 98223, (360) 435-5510.
www.checksum.com.
Copyright 1998 Nelson Publishing Inc.
August 1998
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