Spring Contact Probe Selection

Once the fundamental features of the socket body have been established, the most critical element of socket specifications may be spring contact probe selection. The impact of this choice on performance of the socket - both in the DC and RF spectra, and in the sense of durability and ease of maintenance - cannot be underestimated.

The first limiter of probe selection is device pitch. In purely DC and mechanical terms, the largest probe that will fit on a given pitch is preferable. Probes that are larger in diameter tend to last longer, be less expensive and provide a better match between crown and ball diameter. RF considerations may drive the selection of a smaller probe in some cases. While decreasing diameter relative to pitch drives inductance up, it also reduces capacitance and provides bandwidth gains that have application to some high-speed digital devices. Shimano Mfg, recently successfully designed and manufactured spring contact probe as low as 0.2 mm pitch specifically for WLCSP package testing. "Although I haven't seen other requests for such a low pitch," says Yukishiro Funaki, Shimano president, "this isn't the first request for an 0.2mm pitch. "Since most semiconductor manufacturers and test socket houses are just making the transition to 0.4mm pitch, we proactively working on 0.1mm pitch for beta test."

Required RF performance usually is the second limiting factor. A high-frquency test may place significant demands on the allowable inductance or minimum bandwidth of the spring contact probe. Primarily, inductance is a function of length and pitch-to-diameter ratio. Spring contact probes now are available that produce self inductance values as low as 0.3 nH, by virtue of being short as 1.6mm long when compressed. Shimano Mfg have recently unveiled two spring contact probes specifically target for RF performance.

High bandwidths typically are accomplished by increasing the probe's DC performance. Better contact of the probe's disparate members reduces capacitance. Probe designs now are available that offer as little as -0.5 dB at 10 GHz.

A careful description of the RF requirements of the probe - expressed as bandwidth, maximum self inductance or test frequency - permit the socket vendor to select the best probe for the customer's application. Other electrical characteristics may be critical: Sockets may have to handle significant amounts of current or voltage; a low ground inductance, accomplished by placing several probes on the ground pad of a leadless package, in some cases necessitating that two probes be populated per device lead to allow four-wire measurement of resistance.

Tip style is the final critical consideration in probe selection. Quad flat packages (QFPs) and other leaded devices require four-point crowns large enough to prevent lead from slipping off, Leadless packages may require penetration of oxides on lead-free coatings, requiring a single point; and in many cases, it is considered desirable for probes used in a BGA socket to have tip diameters that that are coincidental with the ball's diameter. This will ensure that the ball is struck near its equator, preventing ball damage at its soldering point that may give rise to air pockets during the soldering process.

YC Soon, Sales Manager, may be contacted at Shimano Manufacturing Co. Ltd; email: yc-soon@shimano-inc.com