PCMCIA is only one more type of PC card–as long as you ignore the many characteristics that make it as unique as its difficult-to-remember moniker. These fragile, very small, densely populated and tightly registered cards provide some interesting challenges when it comes to practical test-fixture solutions.
For the record, PCMCIA is the acronym for Personal Computer Memory Card International Association. While PCMCIA cards have many benefits for designers, they also are a source of frustration for test engineers. In an effort to help solve PCMCIA test problems, we explored a variety of techniques to find better approaches to test fixturing.
The physical dimensions of the PC card are 2.126″ x 3.37″. One end of the card has a standard 68-pin female connector; the other end may have various types of connectors depending on the functionality of the card.
There are currently three types of PC cards:
Type I–3.3 mm thick; commonly used for memory expansion applications.
Type II–5.0 mm thick; typically used as a fax/modem card.
Type III–10.5 mm thick; most commonly used for storage applications.
The PC cards inside the PCMCIA housings are obviously short on real estate to mount all of the components and still leave enough room for those ever-so-sought-after test pads. So how do you access enough points on the card to allow for an accurate and comprehensive test of the UUT?
The most common solution for obtaining access to the UUT has been through the 68-pin connector. Most PCMCIA manufacturers adopt multiple techniques to test the card. Boundary scan, cluster testing, go, no-go testing, and inductive or capacitive coupling to test for opens are all techniques being used for in-circuit and functional testing. All of these applications require some type of physical interface to the UUT.
When fixturing the card, it is important that it remain as flat as possible. With all of the forces that are being applied to the UUT, we felt that the best solution is a mechanical-type actuation for probing the UUT. This allows the card to rest on stops or supports mounted on the top plate while the probe field is lowered (for top access) or raised (for bottom probing). By using mechanical means of probe actuation, we eliminate the use of vacuum on the UUT.
For standard boards (0.060″ in thickness), vacuum fixtures are ideal because they do not overflex the UUT when being pulled down to pre-seal. However, when trying to use vacuum on a PCMCIA assembly to pre-seal, over-flexing of the card may occur.
Vacuum may be used to assist the probe actuation by sealing off the probe field between the top and bottom plates. Since vacuum is not being used to pull the card down or pre-seal the UUT, some type of mechanical device has to hold the UUT in place so the probes won’t lift the card off the fixture.
The force of the probes is an important issue. Now that the no-clean process in board assembly has become commonplace, higher-force test probes can penetrate the flux. Because the UUT is less than 0.020″ thick, a lower-force probe may have to be used to avoid puncturing the UUT.
The technique for accessing the 68-pin connector is relatively simple. The best method we found mounts a sacrificial mating connector on a small extender or breakout card (Figure 1). Using this technique, you can easily replace the breakout card with minimal downtime on the fixture.
The mating connector self-aligns to the UUT. The breakout card can be mounted on a standard off-the-shelf side access unit (SAU). The SAU can be a manual actuation or automated with pneumatics or vacuum.
To avoid wiring directly to the breakout card from the system, interface transfer pads are located on the bottom of the breakout card. The breakout is then probed from the bottom, similar to the UUT.
The other common technique for gaining access to the primary connector uses a spring probe designed to probe the 68-pin connector. The probes and sockets can be mounted into a probe block. The probe block can then be mounted to the SAU (Figure 2).
A ribbon cable or some type of flexible connector can be pushed onto the wire-wrap tail of the sockets. The other end of the ribbon cable can be plugged onto a row of transfer pins leading to the fixture interface.
The alignment of the probe block to the UUT must be very precise. For the probe block to align to the connector, there has to be some type of self-alignment involved in the actuation. Without it, the probes could bend or break as the probe block is moved toward the connector.
The best way to accomplish self-alignment is to use an initial rough alignment to contact the connector, followed by a precise alignment once the probes start to contact the connector. This can be done with a conical probe block with the probes recessed into the block.
If necessary, a secondary SAU can be added to obtain access to connectors located on the side opposite to the 68-pin connector. Due to the very small size of the secondary mating connector and lack of SMD or through-hole mating connectors, the actual mating cable assembly may have to be mounted to the SAU. The other end of the mating cable assembly can be terminated to a header for easy replacement of the cable assembly. Multiple cable assemblies can be substituted, depending on the connector configuration of the UUT.
With the use of replaceable SAUs, we discovered that it would be possible to apply the same concept to the top and bottom probe plates. Because the size of the PC card is a constant and the only thing that would change from card to card is the location of the test pads, we found that a replaceable top and bottom probe plate inserted into guides in the fixture could actually make the fixture into a universal PCMCIA PC card test fixture.
The top and bottom probe plates become replaceable modules that can be configured to match specific PCMCIA PC cards (Figure 3). This application would be practical in low-volume applications where multiple board types are present.
If the volume is high, it makes more sense to dedicate one fixture to each card type. In either application, the replaceable probe plate modules allow easy access for maintenance or engineering changes to the UUT.
The final consideration is to provide an interconnection between the PC and the PCMCIA card. This is best accomplished with an extender card that plugs into the PCMCIA slot on the PC motherboard. The extender card allows total access to the signals needed to test the port and the PC can generate the required test stimuli.
The extender card can be inserted manually with the use of an SAU or automatically with the use of pneumatics or solenoids. Test pads can be located on the breakout card just as they were on the breakout card used for testing the PCMCIA card.
About the Author
James R. O’Leary, an Account Manager in Austin, TX, has been affiliated with TTI Testron for the last four years. Previously, he worked as a Test Engineer at Motorola, American Laser Systems and Storage Technology. TTI Testron Inc., 41 Century Dr., Woonsocket, RI 02895, (800) 262-4894.
ATE Fixturing
Copyright 1995 Nelson Publishing Inc.
August 1995