Machined plastics pass the test

Machined Plastics pass the test

Associate Editor

Test sockets made from Semitron ESd 420V, a new PEI-based ESD material from Quadrant EPP, offer high-temperature resistance and tight dimensional control for fine-pitch applications.

Designers of back-end test devices generally target materials with different surface resistivity ranges for specific applications. The new materials from Quadrant EPP were developed to flatten the percolation curve. Flattening the curve ensures consistency in the typical resistivity target range of 1 X 10⁶and 1 X 10⁹Ω/sq, something previously unavailable to designers and users of high-precision test sockets.

The first use for these materials: Ensuring super-dense ICs pass final QA.

Designers of semiconductor consumables are relying more and more on advanced machinable plastics to put 0.5-mm and finer-pitch ICs through their paces during back-end testing. One of the key properties for test sockets devices, says Dr. Richard Campbell, director of product development, Quadrant Engineering Plastic Products, Reading, Pa., is ESD protection. Also important is mechanical strength, plus dimensional stability over the full range of temperature and environmental conditions. These properties let materials withstand significant insert loads over 65 to 311°F, a typical requirement for test sockets.

"The first generation of material used to build test sockets was based on Vespel SP-1 (polyimide). This was replaced by unfilled and glass-fiber-filled Torlon PAI (polyamide-imide) thanks to the latter's lower coefficient of linear-thermal expansion (CLTE). Lower CLTE gave the sockets better dimensional stability, longer wear, and lower cost," says Campbell.

Third-generation test sockets added protection from static discharge by incorporating new static-dissipative (ESD) materials. From a material standpoint, ESD protection is usually discussed in terms of surface resistivity and/or discharge rates, adds Campbell. Surface resistivity (for electric current flowing across a surface) is the ratio of dc voltage drop per unit length to the surface current per unit width. It in effect is the resistance between two opposite sides of a square and is independent of the size of a square or its dimensional units. Surface resistivity is expressed in Ω/square.

"Although the initial targets were higher, most fabs now want the surface resistivity to be between 1 X 10⁶and 1 X 10⁹Ω/sq to provide decay rates of <0.1 sec. This helps ensure that the device will not be damaged by stray discharges during movement and testing in a socket," says Campbell.

Using the CLTE, engineers can predict how much expansion and contraction to expect during testing in test sockets used to evaluate 0.5-mm-pitch ICs. The extremely tight tolerances needed for the 0.5-mm-pitch ICs dictate socket materials have a CLTE close to that of the silicon chip. That's because the closer the CLTE match, the more accurately holes in the socket stay lined up during testing at temperature extremes.

"In some environments where humidity is high and poorly controlled, some polymers see a lot of dimensional change due to moisture absorption. It's too much for small, high precision parts such as sockets. This is particularly true for test sockets used in testing next-generation fine-pitch chips," says Campbell.

Quadrant EPP has been working closely with socket designers to develop two new grades of low-moisture-absorbing materials. These materials offer ESD protection at least equivalent to current ESD materials and are dimensionally stable over the entire temperature range (65 to 311°F).

The first material, Semitron ESd 420V, is based on PEI (polyetherimide). Its proprietary reinforcement technology provides high strength and stiffness to withstand high chip-insertion forces with no deflection. It also improves upon the overall stability of Quadrant's current PEI-based ESD materials and offers surface resistivity of 1 X 10⁶and 1 X 10⁹Ω/sq. With a heat-deflection temperature of 420°F, the material provides a more cost-effective, high-strength alternative to other ultra-high-temperature resistant materials.

Furthermore, unlike crystalline materials in which the CLTE rises two to threefold at the glass-transition temperature, Semitron ESd 420V maintains its low CLTE to over 400°F. This is a significant advantage in maintaining dimensional stability and mechanical strength of a test socket throughout the full test temperature range.

In addition, the company has developed a new reinforced PEEK (polyetheretherketone) called Semitron ESd 480. The material also has a surface resistivity of 13 10⁶and 1 X 10⁹Ω/sq, but its heat-deflection temperature is 480°F. Its chemical resistance makes it suitable for wafer handling and other structural applications in wet process tools where static dissipation is important.

"A major advantage of Semitron ESd 420V and 480 is that they maintain their dielectric performance even after repeated exposures to high voltages," says Campbell. "In contrast, other typical carbon-fiber-enhanced products suffer dielectric breakdown and become irreversibly more conductive when exposed to moderate voltage. Thus they can't en-sure continued ESD protection to the wafer or device," he adds. The new materials are also nonsloughing, and thus minimize contamination, he notes. "This makes them ideal for machined nests, sockets, and contactors for test equipment and other electronic device handling and testing components."

COMPARATIVE CLTEs
MATERIAL	AVERAGE VALUE
ESd 420	2.0 X 10⁵in./in./°F
Semitron ESd 420V	1.5 X 10⁵in./in./°F
Semitron ESd 480	1.7 X 10⁵in./in./°F
Semitron ESd 520HR	1.5 X 10⁵in./in./°F
Vespel SP-1	3.0 X 10⁵in./in./°F