Microprobing with the Fine-Pitch Active Probe

Transcription

1 Microprobing with the Fine-Pitch Active Probe A guide to using the Fine-Pitch Active Probe and the Fine-Pitch Dual Positioner for applications where handheld probes are not suitable. This application note will help you set up and successfully probe fragile structures. Hewlett-Packard Application Note 1241

2 Microprobing with the Fine-Pitch Active Probe Introduction The current trends in surface mount, module, and interconnect technologies predict that standard surface mount pitches will soon become as small as 10 mils. Present technologies already use 20 or 25 mil pitches, which present significant probing difficulties even for the experienced user. New problems brought on by increased speed/performance and by decreased pitches make traditional handheld oscilloscope probes unwieldy. Cascade Microtech provides a solution to these problems with the introduction of the Fine-Pitch Active Probe (Figure 1). This application note provides information for successfully probing high-speed fine pitch modules with the Cascade Microtech FPA-200. First, the FPA-200 probe and micropositioning system are described. The next section helps you set up the probe and dual positioner, including how to use the probe with a probe power supply. Then, you ll find helpful tips for probing successfully on the first try. Finally, you ll see an example measurement application and its setup. Description The FPA-200 consists of two major components: Fine-Pitch Active Probe The Fine-Pitch Active Probe is a probe solution for high-frequency applications. It features Hewlett-Packard s bipolar/hybrid circuit technology optimized for probing high-density modules. The 2.5 GHz bandwidth probe is micropositioned for probing contacts as small as 25 microns (1 mil) wide. A low-inductance ground contact system preserves probe and system bandwidth. Compatibility with the MTS-2200 Module Test System provides a complete solution for multichip module (MCM) or fine-pitch surface mount technology (SMT) board characterization and troubleshooting. The probe is powered from a connector at the front of the HP oscilloscope, or with the HP 1143A Probe Offset Control and Power Module. You can use it with any measurement instrument with a 50-ohm input. Here are the probe s main features: bandwidth of 2.5 GHz input resistance of 100 input capacitance of approximately 0.60 pf dynamic range of ± 5 V peak ac and ± 50 V dc variable dc offset of ± 50 V excellent immunity to ESD and over-voltages Figure 1. The Fine-Pitch Active Probe Fine-Pitch Dual Positioner (FPD-100) The FPD-100 dual positioner is a probe holder for the active probe and its ground contact. With its flexible articulating arms, it can reach over and around daughter boards, power supplies, and disk drives. The FPD-100 provides three-axis position adjustments, is electrically-isolated from the base, and includes vacuum hold-down with vacuum release for each positioner (Figure 2).

3 2 Microprobing with the Fine-Pitch Active Probe Setup for probing The FPA-200 User and Service Guide provides a more comprehensive list of setup and steps. However, a few pointers will help you make successful measurements, and avoid some of the typical mistakes new users make. For safety s sake, be sure to keep your DUT (device under test) away from the area where you are configuring the dual positioners and the probe. Once the probe is mounted and the contacts are connected, then mount the DUT to your probing surface. Configuring the Dual Positioners The articulated arms are easily mounted to the vacuum base. Changing the reach or angle of the arms is done by loosening the toggle screws 1/2 turn. ferrite bead Figure 3. Connections from the ground wire to the active probe signal and ground contacts. Making contact with the device Make sure the tilt thumbscrew is lowered fully and that the vacuum is securing the base to the probing surface. Position the probe over the DUT, fine-position the signal and ground contacts with the x and y-axis thumbscrews, and then lower with z-axis thumbscrews. vacuum base Figure 2. The FPD-100 Fine-Pitch Dual Positioner Mounting the probe Before connecting to the oscilloscope, you need to mount the active probe and its ground needle to the FPD-100 dual positioner arms. Loosen the toggle screw on the end of the positioner arm, attach the active probe and ground needle block, adjust the angle, and tighten the toggle screws. Once attached to the FPD-100, connect the ground and signal together with the low-inductance ground wire, as shown in Figure 3. Restrain the cable using the cable clamp on the positioner. This prevents the probe from scratching across the DUT if a cable is bumped or pulled. Disengaging from the device When the measurement is complete, use the z-axis thumbscrews to disengage the contacts. Then, use the tilt thumbscrew to raise the probes off the DUT. Press the vacuum release switch to break the vacuum seal from the probing surface. Using the probe with an oscilloscope If using an oscilloscope with integrated probe power (like the HP 54720), be sure to plug the probe and power connector into the same channel, so that the oscilloscope interprets voltages correctly. Using the probe with a power supply If the probe connects to the HP 1143A power module for probe power, set the Offset controls to Local and Zero while calibrating or measuring. If your instrument does not provide a 50 input, connect a type-n(f) to BNC(m) adapter and a 50 feedthrough (such as the HP 10100C) to the probe output. Then, connect the probe to your instrument s input.

4 Microprobing with the Fine-Pitch Active Probe 3 Hints for successful probing Positioning and making measurements with the Fine- Pitch Active Probe is easy and intuitive. However, keep in mind the following considerations to avoid damaging the probe or the DUT. Generally, your DUT is more at risk than your probe tip. When probing one-of-a-kind, fragile DUTs, such as TABmounted ICs on MCMs, or chip-on-board, you need to be particularly careful to avoid damaging your DUT. The potential risks are divided into two categories: mechanical and electrical. Potential mechanical risks In general, users encounter more mechanical problems than electrical when microprobing small structures. Some of these problems include: Loss of vacuum Anything that interrupts the vacuum flow may cause the positioner to move, possibly damaging the DUT. Vacuum interruptions are caused by house vacuum failure or by a vacuum hose disconnecting. When probing critical, one-of-a-kind DUTs, consider using a separate vacuum pump dedicated only to your project. Microscope collisions When viewing the DUT through the microscope and positioning a probe, you can inadvertently collide the microscope with the dual positioner arms. These collisions can occur when focusing the microscope because focusing moves the microscope up and down. When moving down, the microscope can contact the positioner arms. Alternatively, when positioning the probes, you can drive the arms into the microscope. To avoid collisions, use the 0.5x objective, which doubles the microscope working distance. Also, when rough-positioning the articulating arms, set the microscope to the correct working distance, then set up the arms. Cable strain Accidentally snagging the probe cable may move the positioner base, thus moving the probe and possibly damaging your DUT. Be sure the cable is physically restrained in the positioner cable clamp. With the cable securely clamped, the lever arm to the probe vacuum base is shorter, making it more difficult to move the base. Then carefully position the rest of the cable out of the way. Coarse positioning Users generally coarseposition the articulating arms first, setting the overall length and spacing between the active probe contact and the ground contact. The articulating arms are then locked into position with the toggle screws. Do not adjust these when the contacts land on your DUT, particularly if your DUT is fragile. When these screws are loosened or tightened, the probes will move slightly with respect to the DUT, potentially damaging the DUT. Cascade Microtech recommends that you first raise the positioner assembly using the tilt thumbscrew before adjusting the positioner arms. Miscellaneous mechanical problems Anything that causes either the DUT or probe assembly to move inadvertently may damage the DUT. Be careful not to bump the positioner handle or the DUT assembly. Potential electrical risks Many DUTs are powered from the 11 O-volt main power, and the DUT ground is often connected to the earth ground. The measurement oscilloscope or other equipment ground is also connected to earth ground. So if you inadvertently place the ground contact on a non-ground trace, your DUT may temporarily malfunction until the ground probe is correctly placed. Even worse, your ground probe could contact the a power supply line, thereby melting the ground needle or power interconnect with the current. This is not just a consideration for the active probe. Handheld probes can have the same problem. If you have only one DUT, use caution when positioning probes with the power on, or turn off the power for major setup changes. Use a ground isolator or isolated power supply to power the DUT. Be sure to meet all applicable safety requirements and codes.

5 Microprobing with the Fine-Pitch Active Probe Example measurement Figure 4 shows a measurement example, where the Fine- Pitch Active Probe is required to obtain the desired waveforms. This device is a MHz system from NCR Corporation, with the cache memory (4 SRAMs) and controller ASIC fabricated on a ceramic module (MCM). Figure 6. Schematic showing placement of active probes on the module. Figure 4. Measurement system setup Figure 5 illustrates the basic layout of the system. The designer wished to accurately measure the clock waveform shape at each of the ICs, plus the rising edge delay between each. The designer s goal was to compare measured waveforms with simulated waveforms, and to optimize transmission line models and clock driver models for a better match between simulated and measured waveforms. Figure 6 shows the test setup. An active probe was positioned on the clock line at the controller ASIC, while another probe moved to each of the SRAMs as well as to another node at PAL1. The HP digitizing oscilloscope was used to make the measurements. The waveforms were saved as.txt files on a 3.5-inch floppy disk and imported into a Microsoft Excel spreadsheet. Figure 7 shows the measured waveforms. Reducing the range in Excel and displaying only the rising edge to allow accurate timing delay measurements, derives the waveforms shown in Figure 8. Table 1 shows the measured timing delays. Note that the HP54720 will measure the timing delays directly (mean and standard deviation), but for this example we have shown a method using Excel. Saving the waveforms in Excel provides accurate waveform documentation and allows the user to overlay measured waveforms with simulated waveforms. This approach is useful for model optimization as well as for presentations to management. Table 1. Measured delay from V(ASIC) to the SRAMs Figure 5. Schematic of the module (Module courtesy of NCR Corporation, MPD Advanced Development, Colorado Springs). Microsoft is a registered trademark of Microsoft Corporation.

6 Microprobing with the Fine-Pitch Active Probe 5 Hewlett-Packard equipment: HP GSa/s Oscilloscope (1.1GHz) The HP provides sufficient bandwidth (1.1 GHz, 320 ps rise time) to accurately measure a 50-MHz system (rise time 1 ns). The also saves waveforms to disk, which is helpful for optimizing models and documenting results. For more information on Cascade Microtech s fine-pitch probing solutions, call (503) Figure 7. Cache module measurements, detailing waveforms at the ASIC, SRAM1, SRAM2, SRAM3, and SRAM4 For more information on Hewlett-Packard oscilloscopes, call your local HP sales or regional office. References Figure 8. Data range limited to only the rising edge Example equipment list The following equipment was used in the example measurement: Cascade Microtech equipment: 2 Fine-Pitch Active Probes, 2 FPD-100 dual positioners, and the MTS-2000 Module Test System. Cascade Microtech also manufactures other products for module and interconnect characterization, including: MTS-Series Module Test Systems FPM-Series Fine-Pitch Microprobes FPR-Series Fine-Pitch Resistive Divider Probes SCK-Series Surrogate Chip Test Substrate Miscellaneous cables and accessories Acknowledgments Art Potter, Hewlett-Packard- equipment and technical support John Kaufman, NCR Corporation s MPD Advanced Development, Colorado Springs- system for example measurements Arthur Fraser- research, writing, editing, graphics Ken Smith- project management Ron Ares- editing, writing, graphics