PRINTED CIRCUIT BOARD MATERIALS
Printed circuit board materials are mostly constructed from a resin reinforced by a fabric cloth. Epoxy (FR-4), polyimide and cyanate ester are the more common resin systems in use today. Fiberglass is the most popular fabric.
Temperature range is most often expressed as the glass transition temperature (Tg) of the material. The material's Tg is the point above which the mechanical properties of the material begin to rapidly deteriorate. Printed circuit board materials change from hard, brittle substances to soft, rubber like substances after they reach their glass transition temperature.
Typical Tg ratings for the more common material systems are as follows: Printed circuit board materials change from hard, brittle substances to soft, rubber like substances after they reach their glass transition temperature.
Typical Tg ratings for the more common material systems are as follows:
|
Tg
|
|
| Polyimides | 260 C-270 C |
| Modified Polyimides | 240 C-260 C |
| Cyanate Esters | 240 C-250 C |
| BT Epoxies | 225 C-240 C |
| MultiFunctional Epoxies | 160 C-190 C |
| TetraFunctional Epoxies | 140 C-160 C |
| Modified FR-4's | 120 C-130 C |
| Standard FR-4's | 115 C-125 C |
The dielectric constant, or relative permittivity, of a material determines the speed of an electrical signal traveling through the material. The propagation speed is inversely proportional to the square root of the dielectric constant. The dielectric constant of a printed circuit board material is dependent on the test frequency used and tends to drift as the temperature changes. Printed circuit board materials are generally composite structures of a low dielectric constant resin reinforced with a high dielectric fabric. The dielectric constant of the material is proportional to the volume ratio of the resin to fabric.
Typical dielectric constants of materials measured at 1 MHz are as follows:
|
Er
|
|
| Vacuum | 1.0 |
| Pure Teflon | 2.1 |
| Polyimide-Glass | 4.0-4.6 |
| Cyanate Ester-Glass | 3.2-3.8 |
| Epoxy-glass | 4.4-5.2 |
| Water | 70.0 |
Recognizing the challenge of developing an interface that was not the weakest link in the chain, ESH created a problem solving team. This team examined the component parts of an ATE performance board - namely, the engineering, design, material selection and manufacturing components.
ESH 2.7®
The ESH engineering team formulated a printed circuit board material that addresses the special requirements of ATE performance boards. The goal was to provide a low dielectric constant, high temperature material that is dimensionally stable enough to construct very large, very thick multi-layered printed circuit boards.
This material exhibits a dielectric constant of 2.7 measured at TDR frequencies (input pulse rise times of 60 picoseconds or less). This compares favorably with cyanate ester or specially reinforced polyimides and approaches the performance of pure Teflon.
ESH 2.7's glass transition temperature is approximately 250 degrees Centigrade. This is considerably higher than the epoxy systems used for the majority of boards and comes within a few degrees of pure polyimide systems.
As the operator of a printed circuit board facility devoted to performance boards, ESH emphasized manufacturability. ESH 2.7 is dimensionally more stable through processing than Teflon based materials, and the capability exists to produce high layer count boards up to 16" x 16" with overall thickness exceeding 0.150". ESH 2.7 is less brittle than polyimides. This means that machining operations such as routing and countersink/counterbore can be carried out with less chance of fracturing the material.