7. Pointing Accuracy and Wobble

A hot topic in discussions about the use of spring contact probes is the pointing accuracy or wobble. The subject is easily understood: A certain air gap or play is required between plunger and barrel to ensure the plunger’s axial movability. Depending on the extension height between contact tip and base plate a certain axial displacement from the ideal centre point is possible. The longer the distance, the larger is the possible displacement. For longer contact designs (e.g. for dual-level fixtures) this factor is more critical than for shorter standard series.

In the overall consideration some additional variables should be taken into account::

    • As is between plunger and barrel, there is an additional air gap between spring contact probe and receptacle.
    • The drilled hole where the receptacle will be mounted adds its own tolerances (position, deflection).
    • Positioning of the board assembly to be tested adds its own tolerances.
    • The board assembly itself adds its production tolerances.
    • The fixture’s mechanical guide which aligns board and test probes adds its own tolerances.

            Considered as a whole, it is the sum of many individual sources of potential error which finally decides whether the test tip will hit its target – or not.  It is advisable to keep each source of tolerance from the list at its minimum. The instant of hitting the test pad is of decisive importance for the spring contact probe, as subsequent displacements are not to be expected, once the tip has hit its target. The structural design is the crucial factor in determining how precisely the tip is aligned with its central axis.

            A possible solution: The spring presses the plunger against the conical closing flange. The plunger is automatically centrically aligned. After release of a previously applied lateral force, the plunger or probe tip will re-align.

            How we meet the goal is invisible for the user’s naked eye. But you can rest assured, that all our experience from nearly 30 years in the business will be used to the full. By the way, we don’t consider graphically presented diagrams on pointing accuracy particularly useful, as they only show laboratory results which have been achieved far away from practical use.

            7.1 Recommendations for board layout (design rules) | Minimum size for test pads

            Derived from the brief outlines above, we recommend to design test pads as filled circles with a diameter of ≥ 0.8 mm. This value is built upon today’s typical tolerances in fixture construction and receptacle installation, provided system and workmanship are delivered by experienced expert companies.

            7.2 Optional auxiliaries like additional guide plates

            There are additional means for very small test pads and/or pitches, to increase pointing accuracy. Let’s look for example at probing a flat flex cable (FFC) with a centre-pitch of 0.5 mm. For such applications we advise to use either very short contact series (see our finepitch test probes), or to guide the plungers of longer types using an additional guide plate. That way even pad sizes of only 0.3 mm in diameter can be safely hit.

            7.3 The rigid-pin fixture and its advantages/disadvantages

            Some fixture assemblers prefer so-called “rigid-pin fixtures” for pitches of 0.8 mm and smaller. As the naming suggests, the contact to the device under test is not established by spring contact probes, but by rigid, slim needles – basically wire sections, made of spring steel or copper alloys. These rigid pins are guided through a sandwich-like stack of guide plates. By means of inclination, the small fine-pitch grid of the device under test is expanded in several steps into a larger grid-field where normal spring contact probes of standard size are assembled. The “art” of creating such a fine-pitch adapter lies mainly in the software-aided planning of the different guide layers.

            The advantage of high spring forces despite the smallish pitch is emphasized, as the source of spring force are standard spring contact probes for 2.54 mm pitch.

            But there are clear disadvantages too:

            • The entire design of the fixture becomes complex and costly through the multi-level stack of guide plates.
            • Overall height of the fixture is relatively large.
            • From an electrical point of view, there are additional interconnections added to the circuit.
            • The overall wiring length is considerably increased.

            The availability of a comprehensive range of fine-pitch spring probes often makes the costly design of rigid-pin fixtures superfluous. In most cases, the test points for fine-pitch contacting tasks are optimally implemented as gold-plated and clean contact surfaces. High spring forces are not required. Quite the contrary: Often they are undesirable when contact areas are used as bonding pads in a later process step (and bonding pads must not show any indentations from test probing). When used in a climate chamber – irrespective of high or low temperature – the rigid-pin fixture may impose disadvantages due to its bulky construction. But, for sure, there are areas of application, where this contacting technology justifies its existence. Concerning pointing accuracy, the rigid-pin fixture is clearly in the green range, as a precisely drilled guide plate directs the contact probe perfectly onto the test pad – similar to the descriptions of chapter 7.2.