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Woven Glass Standard Updated
July 31, 2013 |Estimated reading time: 3 minutes
Circuit board performance and quality are becoming more critical as semiconductor frequencies continue to rise and line spacings shrink. That means dielectric constants now have a greater impact on designs, and minor variations in material quality can play a role in long-term reliability.
These trends prompted the IPC 3-12d Woven Glass Reinforcement Task Group to revise IPC-4412B, Specification for Finished Fabric Woven from ''E'' Glass for Printed Boards. Changes in the standard which forms the basis for many printed boards focus on three technical areas: The dielectric constant, a spread glass definition, and a hollow glass definition. IPC-4412B also adds a position clarification on borate/diboron trioxide that was created in response to proposed regulatory changes.
One of the key technical changes is a revision in the dielectric constant (Dk) or relative permittivity) value. It is now 7.1 for E-glass @1 GHz, down from the 7.3 value in the previous version of the standard. The update reflects the latest industry measurements. It is an important reference point when it comes to the base materials used to construct printed boards.
"Circuit designers want higher speed and less transmission loss, which are directly related to Dk," said Doug Eng, business development manager at PPG Industries Inc. and 3-12d task group vice chair. "We updated the reference values for the Dk, which will help designers when they pick up the values for reference designs."
Because the distribution of glass within a fabric determines the homogeneity of the glass and resin distribution, committee members added a definition for spread glass. This definition ties back to the effective localized Dk properties.
"As fabrics get more homogenous, the Dk values get more homogenous," said Eng. "If the material isn't homogenous, changes in material quality can alter signal transmissions and change the loss parameters."
IPC-4412B also defines a hollow filament, which is a longitudinal void within a glass fiber. A hollow filament is a potential latent defect via conductive filament formation. Because risks and allowable specification levels for these voids vary widely, acceptable levels of hollow filament are to be negotiated between user and supplier. These variables include the end-use application design and operating conditions such as via spacing, bias voltage, operating temperature, and humidity.
"Hollow filaments are hard to detect at the outset, but they can turn into defects depending on operating conditions," Eng said.
A clarification of material properties is the fourth major change in the standard. It addresses the regulations that are playing an increasingly significant impact on the electronics industry. The European Union began discussing potential placement of diboron trioxide and other borates on the Substances of Very High Concern candidate list. Committee members decided it was important for the glass and electronics industry to clarify the inert nature of borates within a glass network, so they updated the glass composition section of IPC-4412B.
"We wanted to explain that glass is a form of material, it's not in a form composed of raw materials that can be pulled apart," said Tom Newton, IPC's director of PCB programs, standards and technology. "Silicon dioxide and borates like diboron trioxide don't exist in glass once they're melted together with the other components. These raw materials are completely consumed in the manufacturing process; they're no longer present as raw materials in the laminate and the printed boards."
Now that the standard's been published, it's likely to see increased usage. That's because designers and manufacturers need to know more about the intricacies of printed boards.
"People throughout the printed board supply chain are constantly asking about base materials to better understand infrastructure components like circuit boards," Eng said. "As the industry goes to higher performance, laminate and printed board parameters and performance become much more important."