介电常数（Dk）是印刷电路板材最重要的参数之一。电路设计者依靠它来确定微带电路的阻抗以及物理尺寸。但是在层压板的data sheet上我们经常可以看到同种材料有不同的Dk值，比如一个过程Dk以及一个规格Dk。有的材料供应商甚至可能还会提供一个用于计算机辅助设计CAE软件仿真器的Dk值。同一个参数为什么要有这么多不同的值，有没有一个在设计电路时可以信任的Dk值？

正如在前几篇博文中详细介绍的那样，有很多不同的测量微波层压板Dk值的方法，而这些不同的测量方法对于同种材料经常会给出不同的结果。有一些测量方法使用的是“原始”的PCB材料—没有加工形成电路—而有的方法则使用一个可预测表现的具有熟知性能的电路，从而确定该材料的Dk值。材料供应商可能会用类似于“生产Dk”的说法来特指材料在生产过程中的目标值，“规格Dk”则是由前两篇博文中提到的一种或者几种测试方法来确定的。通常，对于给定的层压板来说，过程以及规格Dk值是一样的。

一个更加有意义的Dk版本是目前公布在罗杰斯产品选择指导书上的“设计Dk”，而且它也是用于MWI-2010阻抗计算器的Dk值，该应用可以从罗杰斯官网上免费下载www.rogerscorp.com/acm。设计Dk值在用于电路设计的目的时可以提供最为准确的也是最具重复性的结果，尤其在商用CAE电路和系统模拟程序中。

对于某些材料，过程或者规格Dk可能和设计Dk的值相同。举个例子来说，受欢迎的罗杰斯RT/duroid® 6002微波层压板，它的过程Dk以及设计Dk在Z-轴方向都是2.94。唯一的区别在于，在data sheet上过程Dk只针对于特定的10GHz，而设计Dk则适用于8到40GHz的频率范围。这两个值是用两种不同的测试方法得出的结果。

同时，Dk值也可能有些差别。罗杰斯RO3010™层压板在10GHz Z-轴的过程Dk是10.2，但是为了达到更加精确模拟的目的，在使用商用CAE软件仿真的时候我们推荐的设计Dk是11.2。

既然我们已经测量得到了过程以及规格Dk的值，那为什么还需要一个“设计Dk值”？正如前两篇博文中提到的那样，有很多测试方法来测试层压板的Dk值。举个例子，全球贸易组织IPC(www.ipc.org)列举了13种不同的确定材料Dk值的测试方法。材料供应商可以使用这些测试方法中的任何几种来确定他们自己的Dk值。同样，层压板的使用者可能会在用它来进行设计之前用他们自己不一样的方法来确定层压板的Dk值。在之前作为例子提到的两种材料，每个例子中都用了不同的测试方法来确定过程/规格Dk值以及设计Dk值：夹具式的带状线方法用来确定过程/规格Dk值而差相长度方法用于测量设计Dk值。

比如在罗杰斯内部，虽然全板共振（FSR）测试方法也可以用于验证性测试，但是X-波段夹具式的带状线谐振器测试用于规格或者过程Dk的标准的验证性测试。在罗杰斯内部分离介质谐振器测试（SPDR）方法也可以用来描述材料的特性。为了确定设计Dk，所有材料都会用微带线差相长度方法来进行测试。

虽然没有任何一种测试方法是完美的，但是差向长度方法因为它的简便性而比较受欢迎。它根据在同一种层压板材上加工形成两个长度差别很大的微带电路，用同一种连接器或者测试制具来确定对于一个给定的测试频率不同电路之间的相位差。Dk值可以根据物理长度以及相位差通过简单的计算来确定。在实际过程中，这个过程将在尽可能多的频率上进行重复。这并不是一个快速的方法，但是它的确提供了Z-轴方向精确的结果，且非均质性材料（x和y轴上的Dk值）几乎对测试没有什么影响。

这个测试方法使用经常用在实际应用中的微带电路。为了把铜箔效应考虑在内，这个方法还应用在高频测试上，一片带表面粗糙的铜箔的层压板会比带表面光滑的层压板所测得的显性Dk值更高。用低频率测试方法可能就不会显示铜箔表面粗糙度对于测得的Dk值的影响。

罗杰斯所有的高频层压板的设计Dk都已经确定了，而且已经报告给所有主要的CAE模拟软件工具的开发者。另外，这些值已经包含在了MWI-2010微波阻抗计算器、产品选择指导，以及罗杰斯的计算尺中。

你有设计或者加工方面的问题吗？John Coonrod 和 Joe Davis可以提供帮助。今天就登陆罗杰斯技术支持中心向工程师提问吧。

Learn To Apply Design Dk

 Dielectric constant (Dk) is one of the most essential of printed-circuit-board (PCB) material parameters. Circuit designers rely on it for determining such things as impedances and the physical dimensions of microstrip circuits. Yet, it is not unusual to see a laminate data sheet with different values of Dk for the same material, such as a process Dk and a specification Dk. A material supplier may even recommend an additional value of Dk, to be used in computer-aided-engineering (CAE) software simulators. Why all the different numbers and is there one value of Dk that is the one to trust when designing a circuit?

 As detailed in the last several blogs, there are more than a few ways to determine the Dk of a microwave laminate, and these different measurement methods often yield different results for the same material. Some of the measurement techniques are based on the use of “raw” PCB materials—without circuits on them—while some of the methods use a well characterized circuit with predictable performance to then determine the Dk for the material. Materials suppliers may use terms like “process Dk” to refer to the target value for the material when it is being processed, and “specification Dk” to mean a value determined by means of one or more of the measurement methods described in the two previous blogs. Often, the process and specification Dk values are the same for a given laminate.

 A more meaningful version of Dk is the “Design Dk” that is currently published in the Rogers’ Product Selector Guide and serves as the values for Dk in the MWI-2010 Impedance Calculator, available for free download from the Rogers’ website at www.rogerscorp.com/acm. The Design Dk is a value that provides the most accurate and repeatable results when used for circuit design purposes, notably in commercial CAE circuit and system simulation programs. 

 For some materials, the process or specification Dk may have the same value as the Design Dk. For Rogers’ popular RT/duroid® 6002 microwave laminate, for example, the process Dk and the Design Dk are both 2.94 in the z-axis. One difference is that the process Dk is specified at 10 GHz on the data sheet, while the Design Dk is given for frequencies from 8 to 40 GHz. The values were determined using two different test methods.

 At the same time, the Dk values may differ appreciably. Rogers RO3010™ laminate has a process Dk of 10.2 in the z-axis at 10 GHz, but a Design Dk value of 11.2 is recommended for use with commercial CAE software simulators for more accurate modeling purposes.

 If process and specification Dk values are determined by measurements, why should there be a need for a “Design Dk” value? As mentioned in the previous two blogs, there are many test methods for determining the Dk of a laminate. As an example, the global trade organization IPC (www.ipc.org) lists 13 different test methods to determine a material’s Dk. Materials suppliers use any number of these measurement methods for their own determinations of Dk, while laminate users may have their own, and different, methods for determining the Dk of a laminate before using it for design purposes. In the two materials mentioned above as examples, a different measurement was used in each case to find the process/specification Dk and the Design Dk: the clamped stripline method was used for the process/specification Dk and the differential phase length method was used for the Design Dk.

 Within Rogers, for example, the X-band clamped stripline resonator test is used for standard quality assurance (QA) testing of specification or process Dk, although the full-sheet-resonator (FSR) measurement method may also be used for QA testing. The split post dielectric resonator (SPDR) method may also be used to characterize materials within Rogers. For determining the Design Dk, the microstrip differential phase-length method will be used for all materials.

 While none of the test methods is ideal, the differential phase-length method is elegant in its simplicity. It relies on fabricating two microstrip circuits of significantly different lengths on the same laminate material, using the same connectors or test fixture to determine the phase angle differences between the circuits for a given test frequency. A value of Dk can be determined from simple calculations based on the differences between physical lengths and phase angles. The process is repeated for as many frequencies as is practical. It is not a fast method, but it does provide accurate results for Dk in the z-axis, with anisotropic material effects (Dk values in the x and y axes) having little impact on the measurements.

 This test method uses microstrip circuits commonly used in actual applications. It is also performed at the high frequencies often used in applications, to account for “copper effects,” in which a laminate with rougher copper surface can test for a higher apparent Dk value than a laminate with smoother copper surface. Test methods using lower frequencies may not reveal the effects of the copper roughness on measured Dk value.

 The Design Dk values have been determined for all of Rogers’ high-frequency laminates and are being reported to all major developers of CAE simulation software tools. In addition, those values are now included in the MWI-2010 Microwave Impedance Calculator, the Product Selector Guide, and in the Rogers’ Slide Rule.

 Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.