Frequently Asked Questions - Choosing a Positioner
What are you mounting it on?
Are you mounting your positioner on a Probe Station, a fixture, or something else?
The reason you need to know this is to find out what kind of reach you need to hit your target. You need to know what the distance is going to be from the base of your positioner to the center of your target, also known as the reach. You also have to remember to calculate your needle length in as well. If you are planning on running active probes, like the Pico Probe® 12C or similar or even Co-Planar probes for multi-contact probes, you will need to know the length and drop for them to calculate into your configuration.
Is the mounting point of your positioner level with your target, or higher or lower?
This will determine the drop or rise you need to calculate into you configuration. Probe stations come with the chuck below the level of the platen as well as above the platen, there is no standard.
What is the surface you are mounting to?
Magnetic bases are probably the strongest, but they can be a little harder to move. Vacuum bases are easy to move (by releasing the vacuum via a valve), but sometimes you give up grip strength. Hard mounts are also available where you actually bolt the positioner into place, but if you need more movement than the travel of the positioner, this will not work.
Terms and Definitions
Resolution
This means the mechanical drive of the positioner, usually measured in movement traveled per turn of lead screw, which is measured in threads per inch, or TPI. The higher the TPI, the higher the resolution; but also the slower the travel from end to end. If you have large targets, you do not need a 100 TPI positioner, at 40 TPI one will actually probably work better for you. If you are trying to hit a one micron trace though, you will become very frustrated with a lower resolution positioner as you will overshoot your target most of the time. Submicron targets are best probed with motorized positioners as just the act of touching or letting go of the positioner could result in your needle jumping off the target. A simple rule of thumb for probing geometries is 3 microns and above 40 TPI, 1-5 Microns 100 TPI, smaller than 1 micron use motorized.
Travel
Travel, or stroke, is the maximum distance that the positioner can move from physical stop to physical stop, usually measured in inches or millimeters. Most positioners travel in a linear line, but some do travel in an arc due to their design. The main drawback of the arc is if you are probing down a row, you will eventually have to move another axis on your positioner to stay on the row or line. Whereas a linear traveling positioner will stay on the line, assuming you have the positioner setup square with the row or line.
Micrometer head versus Knob
Many people think that positioners with micrometers are more accurate than positioners with just a knob and screw. This is not actually always true; it depends on the TPI of the Micrometer versus the TPI of the Lead screw in the standard knob models. Some Micrometers come in lower TPI than offered with the standard knob and lead screw. What is true is that Micrometer heads are good if you are measuring the distance you travel. But they also take up more room or space than the standard knobs, so if you are not using your positioner to measure the distance from one pad to another, they tend to be more cumbersome and bulky than the standard lead screw designs.
Manual or Motorized
The majority of micropositioners are manual, mainly due to costs, motorized positioners simply cost more. You’re not only buying the positioner, but a control for it also. For very high resolution or submicron probing though, you may have to go with motorized. The advantages are you do not touch the positioner to move it, thus causing accidental movement. Motorized positioners can also be utilized in dark boxes, environmental chambers, or simply in places where you can not get your hands to. Some motorized positioners can even be programmed to move from target to target, and some controllers can run multiple positioners too.