Designers are making good use of advanced machinable plastics that stay stable at temperature extremes while dissipating electrostatic charge.
Machined Plastics pass the test
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 1
3 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."
|ESd 420||2.0 X 105 in./in./°F|
|Semitron ESd 420V||1.5 X 105 in./in./°F|
|Semitron ESd 480||1.7 X 105 in./in./°F|
|Semitron ESd 520HR||1.5 X 105 in./in./°F|
|Vespel SP-1||3.0 X 105 in./in./°F|
TYPICAL PROPERTY COMPARISONS
Semitron ESd 420V
Semitron ESd 480
|Ultimate tensile strength, kpsi|| |
|Tensile modulus, kpsi|| |
|Elongation, at break, %|| |
|Flexural strength, kpsi|| |
|Compressive strength, kpsi @ 10% deformation|| |
|Hardness, Rockwell|| |
|Notched Izod Impact, ( 1 /8 in.), ft-lb|| |
|Surface resistivity, Ω/sq|| |
1 X 109
1 X 106
1 X 109
Static decay, Mil-B-81705C, sec, maximum
|Volume resistivity, Ω/sq|| |
1 X 109
1 X 106
1 X 109
THERMAL Deflection temperature, °F @ 264 psi
Continuous use temperature, °F
|Water absorption, % 24 hr @ 73°F|| |
|Water absorption, % at saturation @ 73°F|| |
Quadrant Engineering Plastic Products,
(800) 366-0300, www.quadrantepp.com