High Performance Test Sockets, Aries Electonics RF Test sockets Larsen Associates

Reliability
A socket's reliability is crucial to help lower the cost for each device tested. Ideally, a socket should require no repair, and operate reliably during its full life cycle. In reality, it will need cleaning, repair or replacement. Cleaning can require a complete breakdown of the test board. Repairing a socket almost always requires test board breakdown, followed by another setup. Socket adjustments, although performed infrequently, use up valuable test time. As a result, throughput is lowered and cost per device tested is increased.

Temperature can affect socket life and reliability. Each device must be tested under the same conditions at which it will be used, and test temperature extremes can range from -55?C to +150?C. Naturally, sockets that can withstand these temperatures through hundreds of thousands of cycles are more expensive than conventional test or burn in sockets, but they will pay for themselves through long service life.

Thermal considerations involve continuous service temperature and Coefficient of Thermal Expansion (CTE). Different materials expand at different rates under the same temperature conditions, so the housing material must be flexible enough to accommodate the se variations, and tough enough to withstand them.

Occasionally, a socket contact, or the socket itself, must be replaced. This can result from simple wear-and-tear, or from using a socket that's inadequate for the purpose. In both cases, the situation incurs the downtime cost and time needed to effect the repair, and cost of replacement parts.

Performance
Performance is the second major concern for production of wireless IC's. The ideal test socket should be electrically "transparent"; that is, its presence will have no effect on the test results. This characteristic can be achieved with a low self-inductance and capacitance between contacts, and the contact's ability to maintain controlled impedance.

A socket's high frequency capability is inversely proportional to its contact inductance. High lead inductance values can lead to unwanted oscillations and signal degradation. That's why package engineers concentrate on developing packages with low lead inductance.

Inductance and capacitance influence the overall impedance value of the socket contacts. Therefore, the test socket's contact impedance should closely match that of the device package. Impedance mismatches can lead to signal reflections, which can cause signal loss and oscillations.

Impedance is a complex term that defines the resistive and reactive elements of an electrical signal medium, whether it's a wire or printed circuit board (PCB) microstrip trace. The resistive element is determined by the wire's resistance (Ohms, ?) and the reactive element is determined by the inductance (Henry, H) and capacitance (Farad, F) of the PCB microstrip trace. As an electrical signal travels through a trace, its fundamental characteristics change as the impedance changes. And as the signal's frequency increases, so does its reaction to the change in impedance.

Although it's relatively easy for board designers to match lead impedances, it's often difficult for socket manufacturers to do likewise. Most sockets simply do not have any means of impedance control.

Mutual inductance and capacitance characteristics pose other problems. As a signal's frequency increases, so does its electromagnetic field. If the socket's design and materials of construction contribute to coupling characteristics, then unwanted electric al "cross-talk" between these fields will promote the development of oscillations, reduce gain level and increase signal degradation, with a subsequent detrimental effect on performance.

Theory and Reality
How does this translate into actual practice? Let's examine two key components of a high performance test socket - the housing and the contacts.

Polyetherimide is a typical housing material. It is a thermoplastic that displays excellent mechanical, thermal and electrical properties. In production settings, automated device handlers are widely used. It's important that the housing's mechanical specifications are close to those of the handler's plunger mechanism (which is usually made of polyamideimide) because these two parts can come in contact with each insertion of an IC package. Flexural strength is also critical, because of the repeated cycle of pressure and release that occurs when the housing is used as a hard stop to eliminate damage to the device under test.

The continuous service temperature rating for polyetherimide is -60?C to +170?C; that exceeds even the strict temperature testing range established for military applications. Ultem's CTE is very low (3.1 x 10-5 in/in/?F), which helps maintain close dimensional stability of the housing for IC package alignment, and good co-planarity between the socket and test board.

The dielectric constant and dissipation factor of polyetherimide are 3.15 and .0015 respectively, at 1 MHZ - extremely respectable numbers for RF applications. These two specifications minimize the socket's mutual conductance, thus reducing "cross-talk" and leakage currents at high frequencies.

Contacts
The heart of any test socket is its contacts, because they provide signal transmission paths between the device and the test board. Proper signal transmission depends upon proper contact design and implementation.
The Microstrip Contact ™ (patent owned by Aries Electronics) provides many of the characteristics required by high performance testing. It provides minimal signal loss in high bandpass applications, and delivers long cycle life (over 500,000 cycles) in auto mated handler testing. Additionally, it provides a virtually transparent connection between the device-under-test and the printed circuit board.