Laminated materials are designated with FR (flame resistant) and G grades. FR–1 is the least flame resistant, and FR–5 is the most. G10 and G11 have special characteristics as described in Table 17–1.Do not use FR–1. There are many examples of boards with burned spots, where high wattage components have heated a section of the board for a period of time. This grade of PCB material has more in common with cardboard than anything else.FR–4 is commonly used in industrial-quality equipment, while FR–2 is used in high-volume consumer applications. These two board materials appear to be industry standards.Deviating from these standards can limit the number of raw board material suppliers and PCB houses that can fabricate the board because their tooling is already set up for these materials. Nevertheless, there are applications in which one of the other grades may make sense. For very high frequency applications, it may even be necessary to consider Teflon or even ceramic board substrate. One thing can be counted on, however: the more exotic the board substrate, the more expensive it will be.In selecting a board material, pay careful attention to the moisture absorption. Just about every desirable performance characteristic of the board will be negatively impacted by moisture. This includes surface resistance of the board, dielectric leakage, high-voltage breakdown and arcing, and mechanical stability. Also, pay attention to the operating temperature.High operating temperatures can occur in unexpected places, such as in proximity to large digital ICs that are switching at high speeds. Be aware that heat rises, so if one of those 500-pin monster ICs is located directly under a sensitive analog circuit, both the PCB and circuit characteristics may vary with the temperature.After the board substrate material has been selected, the next decision is how thick to make the copper foil laminated to it. For most applications, 1-ounce copper is sufficient.If the circuit consumes a lot of power, 2-ounce may be better. Avoid -ounce copper, because it tends to break between the trace and the pad.
薄板材料被指定以FR(阻燃性)和G等级FR-1阻燃性最低,FR-5最高.G10和G11属于特殊用途等级.FR-1板子上的布高压元器件的位置因高压元件发热易出现烧伤点,这个阻燃等级的PCB板的性能合普通纸板有很大的共性,所以使用需慎重.FR-4被广泛用在工业级质量要求的设备上,FR-2 高电性，在普通家电中使用较多.----, 对于一些高频应用,可以考虑聚四氟乙
烯(TEFLON)或者陶瓷做底层,尽管如此,板材材料越特殊,成本也越高,选材时要考虑潮湿的影响,因为潮湿会降低板子的各种性能.包括板子表面阻抗的,电介质泄漏,高压损耗合或电弧,合机械稳定性.同样,需要关注工作温度.在一些意想不到的地方会出现高的工作温度,比如靠近处于高速运算中的大的数字集成芯片的地方.请注意发热的问题,如果一个500个pin脚的超级芯片被放置在一个敏感的模拟电路下方时,PCB和电路的特性都会因为温度而变化.当板子基材选定后,下面就是决定覆铜层的厚度.在大多数情况下,1-盎司铜就足够了.如果电路功耗较大,那么2-盎司也许会更好一些.避免用0.5盎司铜,因为这会导致铜皮联通不可靠.
