September 25, 1997

Modified oscilloscope probe
tests surface-mount assemblies

Robert Buono, Mahwah, NJ

The small sizes and pad areas of surface-mount components make them difficult to probe. Typically, when you use an oscilloscope probe, it is difficult to keep the probe tip and ground bayonet from slipping off the pad areas of the surface-mount devices. The modified oscilloscope probe in Figure 1 makes it easy to probe such assemblies and obtain reliable measurements. The technique works with most oscilloscope probes. Remove the probe cap, exposing the probe tip and ground "collar." Cut a 4-in. piece of 30-AWG, single-strand, Kynar-insulated wire (the type that engineering labs almost universally use to make jumper modifications).

Strip the insulation from the entire length of the wire. Tightly wrap five turns of the bare wire around the ground collar of the oscilloscope probe. Then, tightly twist the two ends of the wire to prevent the turns from loosening. Apply a small amount of solder to the twisted wire, using just enough heat and time to flow the solder onto the twisted-wire terminations. Repeat the same procedure for the probe tip, using a second piece of bare wire. Cut one wire in each of the twisted pairs, leaving a single bare wire approximately 1-in. long. Now, you can easily solder the two wires to the surface-mount-component pads (signal and ground) that you need to probe.

Although it may seem inconvenient to directly solder the lead wires to the probed pads, the technique offers several advantages. Because the 30-AWG wire is thin, it does not require excessive heat or time to solder it to the pads. The potential for damage to surface-mount devices is thus minimal. Also, the wires are flexible, so small movements in positioning the probe do not damage pads or lift them off the pc board. You can also adjust the wire length to yield the best high-frequency performance (lowest inductance) and best waveform fidelity. In particular, the ground wire needs to be only long enough to reach the nearest ground pad. (DI #2085)

| EDN Access | Feedback | Table of Contents |