The performance of your fixture depends upon the quality of the music wire pins used in the fixture. Fixtures without false opens are fixtures that work - and the least expensive you can build. The important element when targeting SMD pads is accurately placing the hole in the fixture plate so that the point of the music wire pin will be directed to the center of the pad area available for contact. With a narrow pad width (.006), along with the clearance for the music wire pin, the “concentricity” of the point becomes the difference between “false opens” and a good test. The worst case is that of the "hit or miss pin" - where the point rotates around the axis of the music wire pin and produces the intermittent open.

How big of a target do I have to hit?

(Pad width may well be half the music wire pin diameter)

Tensility

For applications where smaller diameter pins are required, the tensile strength becomes crucial in pin selection. The higher the tensile factor, the less support structure is required in the test fixture itself - which equates to a lower drilling cost. For these instances, Chirish offers tungsten and gold-plated tungsten pins. Tungsten has the highest ultimate tensile strength of all metals, a high electrical conductivity, and retains a superior point after extensive use. As the metal naturally oxidizes, the high conductivity of the material is not compromised, thus making plating unnecessary unless desired.

This chart illustrates the unsupported length without buckling of the different materials under the typical load of a universal grid tester. The distance represents the distance allowed between support plates in the test fixture for the different diameters of the varied materials. Tungsten with its higher tensile strength is used for smaller diameter pins to limit buckling and the number of support plates required.