每一份层压板的数据一览表都总结了该电路板材的各项参数。他们描述的是PCB板材的电气与机械性能，包括相对介电常数、介质损耗、热膨胀系数，以及热导率。电路设计工程师认为这些数值都是精确的，因为他们的设计完全是依赖于这些参数的。但事实上这些参数的精确程度往往是取决于所采用的测试方法。不同的实验室即使是在进行同一种材料的同一种测试时也会产生不同的测试结果。本文将简单介绍一下在衡量一种印刷电路板基材的特性时会进行的一些测试；在接下来的几篇博文中，针对一些特定的测试方法，我们将给出更加详细的介绍，另外我们还会解释这些测试结果是如何影响用现代CAE软件工具来进行PCB材料的模拟的。

在选用PCB 层压板来进行设计的时候，相对介电常数一般是要考虑的第一个参数。它可以通过不同的的技术来测量，可以用带铜箔的或者不带铜箔的绝缘材料。相对介电常数，或者Dk，是一个复杂的参数，一般来说，在电路板材的三根轴线上，它的值都是不一样的。它随着频率的变化而变化，但是Dk的测试结果也会根据测试的方法以及测试的效果不同而有所变化。

可惜，目前还没有理想的PCB板材Dk的测试方法，所以甚至连一个层压板材供应商及PCB板材使用者可以用来比较他们的测试结果的标准测试方法都没有。事实上，国际电子工业联接协会，IPC（www.ipc.org）共有13种不同的Dk测试方法。美国测试与材料协会，或ASTM(www.astm.org)也有一组测试Dk的方法，而且很多层压板供应商甚至层压板用户也会有他们自己的一套测试方法。

由于Dk测试方法的不一致，所以各方要想在测试数据上达成一致的话是非常困难的，比如说材料供应商和他们的客户。当把材料的Dk用于CAE软件设计工具来设计高频电路的时候这个问题就变得十分严峻了。高频微带线或者带状线电路的阻抗取决于PCB材料的Dk值。举个例子，如果为滤波器设计一个谐振电路，在计算滤波器谐振电路的阻抗时和CAE程序一样会用到材料的Dk值，从而得出它的中心频率。如果材料实际的Dk值和测试所得的Dk值是不一样的，CAE程序给出的电路性能频段预测将不同于最终加工成PCB后实际的工作表现。为了能够理解为什么不同的测试方法会得出不一致的测试结果，我们将简单的回顾一些主要的用于测量PCB相对介电常数的测试方法。

Dk测试是发展出来测试PCB原材料的，当电路已经蚀刻到层压板上时，就把这个电路作为测试设置的一部分。由于多种原因，就像我们会在之后的两篇博文中看到的那样，这些测试得出的结果会有略微的差异，有些公司会进行多次的测试，把这些结果的平均值作为一个具有代表性的可以用来做设计以及建模的Dk值。一般来说，最精确的Dk测试也是这些测试方法中最需要工程设计的。但是这些需要如此高度关注细节的测试方式并不适合用于产线的测试，因为在处理大量材料的时候速度是非常关键的。

简而言之，测试PCB原材料Dk的最常用的方法（上面还没有形成电路）有如下几种：

·         夹具式的带状线谐振测试，用于测试不覆铜箔的绝缘材料，参考IPC标准IPC-TM-650 2.5.5.5c

·         全板谐振测试（FSR），用于覆铜箔的没有蚀刻电路的层压板，参考IPC标准IPC-TM-650 2.5.5.6

·         分离介质谐振器测试(SPDR)，用于测试不带铜箔的绝缘材料

·         波导微扰测试，用于测试不带铜箔的PCB原材料

下一篇博文将提供关于以上各个Dk测试技巧的更多细节。

有的测试方法涉及到在层压板上加工特定的电路，当我们知道一个电性能参数，比如相位角或者频率，根据介质层的厚度，铜箔的厚度，以及微带线的尺寸，我们可以用微带线的设计公式来计算绝缘材料的介电常数。这些测试方法包括：

·         相差长度方法，在层压板上加工已知长度以及阻抗的微带传输线，当我们知道精确的频率的情况下就可以测量它的相位了；

·         微带环谐振器测试方法，在层压板上蚀刻形成微带环谐振电路，然后测量得到这个电路的精确频率；

·         边缘耦合微带谐振电路测试，在层压板上加工形成边缘耦合微带谐振器，然后测试它的精确频率；

·         通过在层压板上加工带通滤波器来进行测试，找到他们精确的中心频率。

我们将在下一篇博文中提供更多有关以上Dk测试方法的细节。

所有这些测试方法的不同在于如何准备待测试的材料。有一些要求蚀刻掉所有表面的铜箔，而其他测试方法则要求在层压板上加工形成一个特定的有精确尺寸的测试电路，然后在特定的频率上进行测试，典型的频率大小是10GHz。有些测试方法只能在一个特定频率上测试Dk，而其他测试方法则可以在一个频段上进行Dk的测试。下一篇博文将进一步讨论前四种Dk的测试方法，再下一篇博文将探索那些用特定电路来进行测试的测试方法，我们也会探讨一些在比较不同电路测试得到的结果时会遇到的一些潜在问题。获取精确的测试结果的目标是为了能够给出一个有用的设计Dk值，或者能够得到一个有信心用于商用CAE软件仿真的，在不同PCB板材上设计高频电路时使用的相对介电常数。

Measuring Performance

Of Microwave Substrates

 Circuit-board material parameters are printed on every laminate data sheet. They describe the electrical and mechanical characteristics of a PCB material, including such parameters as relative dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), and thermal conductivity. Design engineers count on these values to be accurate, since their circuits depend on them. But the accuracy often depends on the test method used to measure a material parameter. Even when different laboratories perform the same test on the same material, they can obtain different results. This blog will provide a brief overview of the different tests used to evaluate a printed-circuit material’s characteristics; the next several blogs will go into greater details on specific tests, and will explain how various test results impact the way PCB materials are modeled with modern computer-aided-engineering (CAE) software tools.

 Relative dielectric constant is typically the first parameter consulted when selecting a PCB laminate for a design. It is measured by many different techniques, using dielectric materials with and without copper cladding. Relative dielectric constant, or Dk for short, is a complex parameter which typically has a different value for all three axes of a circuit-board material. It normally varies with frequency, but the results of Dk measurements can also vary according to the type of test used and how each measurement is performed.

 Unfortunately, there is no “ideal” method for measuring PCB material Dk, so there is not even a “standard” test technique that laminate suppliers and PCB material users can agree upon when comparing the results of their measurements. In fact, the industry trade association for interconnecting electronics, IPC (www.ipc.org), has 13 different test methods to determine Dk. The American Society for Testing and Materials, or ASTM (www.astm.org) also has a set of measurements for Dk, and many laminate suppliers as well as users have their own test methods.

 Because of the differences in the Dk test methods, it is often difficult to achieve good agreement between measured values from different groups, such as materials suppliers and their customers. This can be critical when a Dk value for a particular material must be used in a commercial CAE software design tool when creating high-frequency circuit designs. The impedance of a high-frequency microstrip or stripline circuit depends on the Dk value of the PCB material. If designing a resonant circuit for a filter, for example, the Dk value that is used by the CAE program is what will be used in calculating the impedance of the filter’s resonant circuit, and thus its center frequency. If the actual Dk value of the material is different than what has been measured for that material and entered into the CAE program, the software will deliver predictions of circuit performance that are at different frequencies than those achieved when the filter is finally fabricated on that material. In understanding why there are differences in the results from different test methods, it may helpful to briefly review some of the main measurement approaches used to determine PCB relative dielectric constant.

 Dk measurements have been developed to test the raw PCB material and when a circuit has been fabricated on the material, using the circuit as part of the test setup. For various reasons, as we will see in the next two blogs, these measurements provide somewhat different results, and some companies may perform several of the measurements and use an average value of the results to arrive at a representative Dk value for design and modeling purposes. In general, the most accurate Dk measurements are also the most engineering-intensive of the test methods. But test approaches that require such attention to detail may not be ideally suited for production-line measurements, where speed is important for handling large material volumes.

 In brief, the measurements most commonly used for evaluating the Dk of raw PCB materials (without circuits on them) are

• the clamped stripline resonator test, per IPC standard IPC-TM-650 2.5.5.5c, for dielectric material without copper cladding;

• the full sheet resonator (FSR) test, per IPC standard IPC-TM-650.2.5.5.6, for copper-clad laminates without circuits;

• the split-post-dielectric-resonator (SPDR) test, for dielectric material with no copper cladding; and

• the waveguide perturbation test, for raw material without copper cladding.

More details on each of these Dk test techniques will be provided in the next blog.

 Tests that involve fabricating specific circuits on a laminate under test make use of microstrip design equations to calculate the Dk of a dielectric material, based on the dielectric thickness, the copper thickness, and the dimensions of the micostrip, when an electrical parameter, such as phase or frequency, is known. These tests include:

• the differential phase length method, in which microstrip transmission lines of known length and impedance are fabricated on a laminate and tested for phase at precise frequencies;

• the microstrip ring resonator method, in which microstrip ring resonator circuits are etched onto a laminate and measurements performed to determine the precise frequency of the circuit;

• the edge-coupled microstrip resonator circuit test, in which an edge-coupled microstrip resonator is fabricated on a laminate and measurements are made of its precise frequency; and

• tests based on fabricating microstrip bandpass filters on a laminate, and measurements performed to find their precise center frequencies.

More details on these Dk measurement measurements will also be provided in a future blog.

 All of these measurement approaches differ in how they prepare a material for testing. Some require full removal of copper cladding, while others require fabrication of a specific test circuit with precise dimensions for evaluation at a specific frequency, typically 10 GHz. Some measurements are designed to determine Dk at one frequency, while others will measure Dk across a swept range of frequencies. The next blog will take a closer look at the first four Dk test methods, with the blog after that exploring the circuit-based measurement methods and some of the potential problems when comparing Dk measurement results determined with different types of circuits. The goal of achieving accurate results is to establish a valid “design Dk” or a value of relative dielectric constant that can be used with confidence in commercial CAE software tools when designing high-frequency circuits on different PCB laminates.