ECT spring probes are designed for use in most bare board and loaded board test systems. They are generally installed in board test fixtures, interchangeable test heads, grid systems and in test systems interfaces.
Electrical Current Path and Probe ResistanceThe primary current path in a probe is through the contact junction of the plunger with the barrel and the barrel with the receptacle. Secondary paths include the contact junction between the spring and plunger and the spring and the barrel.
There exist a variety of sources of electrical resistance that must be considered. Resistance is dependent on several factors: conductivity of base metals and plating material; resistance at points of contact between components (which is affected by surface condition); area of contact; force applied at contact junctions; and probe design.
ECT probes are self-biasing, resulting in maximum metal-to-metal contact force between components at critical contact junctions.
Resistance can also be caused by such factors as: receptacle wire terminations; fixture wiring, test interface; incorrect probe selection (wrong tip, inadequate spring force); PCB surface contamination; or high-resistance contacts in the test system.
Electrical resistance is included among probe specifications on each data page. See Performance Testing for details on how ECT probes are electrically tested.
"Mixed" Test CentersProbes designed for varying test center dimensions can be mixed in the same test fixture without costly machining of the probe plate. Probes designed for 0.050 or 0.100 inch travel and 0.050, 0.075, and 0.100 inch test centers can be mixed for bare board testing. (See Figure a. and Figure b. )
In loaded board applications, probes designed for use on 0.050, 0.075 and 0.100 inch test centers can be mixed in single or dual-stage fixtures, even though there may be minor variations in plunger travel. When mounted correctly, probe plunger tips should align when plungers are at recommended working travel - generally 2/3. (See Figure c. and Figure d. )
This will ensure contact integrity between the tip and test pad. Minor adjustments may be required to compensate for variations in accessing component leads, flat test pads or through-holes.
High-Current Spring ProbesThe maximum continuous current rating of a spring probes is determined by its design, size and construction. Typical probes are rated from 2 to 5 amps maximum current (non-inductive) at working travel. While this is sufficient for most board test applications, higher current probes are sometimes required.
ECT High-Current Probes (HCP series) are capable of carrying electrical current up to 45 amps.
Low/High Temperature ApplicationsFigure 4 shows how temperature characteristics of materials used for the spring affect performance of the probe in extreme temperature applications. Stainless steel can be used in the widest temperature range.
Operating temperature range is included in the probe specifications on each data page. Please consult the factory information on probes for higher temperature applications.
Spring Force SelectionSpring force is a major factor in tip selection, especially in No-Clean applications. More effective penetration of board contamination is generally acheived with higher spring force. A probe with a pointed tip (one point) will penetrate contamination with four times the force of a crown tip (four points), given the same spring force.
Spring force is a also a consideration in vacuum fixtures. If you can't pull a vacuum on your fixture, the spring force per square inch may exceed atmospheric pressure. In this case, lower force springs are required.
Spring force specifications are included on each data page. Consult factory for special low spring force requirements.
Receptacle SelectionThere are six basic receptacle terminations offered to meet your wiring requirements.
W-- Crimp, push-on terminal, pre-terminated
W-2-- Wire wrap, .025 inch square post
W-3-- Connector round post, .025 inch dia.
W-4-- FASTITETM push-in terminal
Y-- Push-on Terminal
The press ring makes it easy to press fit the receptacle into the probe plate with no need for cement.
Detents allow the probe to be inserted easily, and retain the probe in an interference press fit for excellent mechanical retention and electrical current flow.
Our 50, 75, and 100 mil probes can be installed in the same fixture. This is accomplished by determining the mounting height of the 100 mil probe and using the appropriate mounting height formula to establish the mounting height of the other probes. In the example, the 100 mil probe is mounted .220 inches above the probe plate. Thus the 50 mil probe is mounted .265 inches above the plate (B=.220+.045).
CAUTION: These are reference guidelines only. Actual mounting heights may vary among fixture designs.
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