Laminate testing Electrical properties

The electrical properties of both general purpose glass fibers are comparable (see Table III). The refractive index is a property of considerable importance with regard to the appearance of a glass fiber in a laminate or composite. Electrical properties and the coefficient of linear expansion of borosilicate and boron-free E-glass were measured on bulk annealed samples. The differences in test results between both fibers are considered to be insignificant [4]. 

BS 4618, Recommendations for the Presentation of Plastics Design Data. Part 2. Electrical Properties, Section 2. 3 Volume Resistivity Section 2.4 Surface Resistivity (London, 1975) BS 5102, Phenolic Resin Bonded Paper Laminated Sheets for Electrical Applications (London, 1974) BS 5762, Methods for Crack Opening Displacement Testing (British Standards Institution, London, 1979)... 

Electrical. The dielectric strength of laminates will decrease with increasing thickness and is highly dependent upon the direction of the electric field stress. This property will show higher when tested across the sample s thickness whereas end-to-end testing will show lower values. Laminates with higher resin content will show better electrical properties but poorer physical properties than laminates with lower resin content. 

It was decided to febricate these parts from epoxy-glass laminates, as their properties are more than adequate for the intended application. In order to select between NEflA grade G-10 and G-11 types of epoxy-glass laminates, thermal shock tests were performed. Samples of the two types of laminates 2.5cm thick were obtained from the Micarta Division of Westinghouse Electric Corporation. 

Laminates based on Rhone Poulenc s Keramid 601 polyimide resin are fabricated in a conventional laminating press, and processed in a manner similar to that used for epoxies but with an extended cure cycle or post-cure. The room-temperature mechanical and electrical properties are similar to epoxy laminates, as shown in Table 9.4. At elevated temperatures, the polyimides exhibit exceptional stability. In particular, the thermal coefficient of expansion in the Z axis does not change significantly up to approximately 240°C, as shown in Fig. 9.11. Exhaustive tests have shown that polyimide-based multilayer boards can withstand repetitive thermal cycling at elevated temperatures (>150°C) without cracking of plated through holes. Similar excellent results were also obtained in solder shock tests (10 s at 288°C in molten solder). The thermal stability of these materials is retained at temperatures of approximately 200°C for continuous exposure in air, which has qualified them for military applications. 

The third problem associated with water based varnishes is poor electrical insulation properties of the laminate after moisture conditioning. This problem is probably the most critical problem because insulation failures of the laminate can lead to electrical failure of the finished printed circuit board. This property is measured by conditioning the finished laminate in a high moisture environment and then testing the dielectric breakdown strength. ED24574 has excellent insulation resistance. This was achieved by a proprietary resin composition. 

The National Electrical Manufacmrers Association (NEMA) has organized and maintained standards on the manufacture, testing, and performance of laminated thermosetting products in the form of sheets, rods and tubes. NEMA material descriptions and the properties of NEMA-type laminates are provided in Appendix E. 

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