对于移动和便携式电子终端，更加纤薄的印刷电路板（PCB）材料的机械特性在保持轻薄设计方面，拥有一些显而易见的优势。但是，使用薄层压板，需要考虑诸多事项，因为介质材料的厚度会影响诸如50欧姆线等高频应用中常用的阻抗控制线的导体厚度。在选择薄型PCB层压板材料，以实现更加轻薄的电路设计之前，对目前市场上的各类薄型层压板材料及其关键特性，以及这些层压板材料在实际的制造和应用环境中的性能，作一番审视和比较，将大有裨益。

除重量更轻巧，外形更纤薄之外，薄型层压板的另一个极具吸引力的优点是，在典型情况下，可支持30 GHz及更高的毫米波频率。薄型层压板材料有助于防止毫米波频率上常见的多余信号传播模式。当然，诸如材料的介电常数和耗散因子（损耗）等其他因素，也会影响其在高频应用中的使用方式。

随着频率不断提高，波长越来越短，电路特性也更加精细。通常，会要求使用较低介电常数（Dk）的PCB层压板材料，因为在特定频率下特定阻抗的导线，增大介电常数会缩短导线的长度。虽然这有利于设计和制造面向低频应用的电路，但是，这会在毫米波频率上造成极小的电路容差，对设计人员提出严峻的挑战。此外，薄型PCB层压板有助于实现更加小的电路尺寸，因此，应当仔细地挑选薄型层压板，以在毫米波电路应用中发挥其最大优势。

下面略举数例，以表明层压板厚度和Dk值对电路尺寸的影响。采用不同厚度和Dk值的层压板材料，来制造50欧姆微带线导体，所得到的导体厚度可能大相径庭。每一种介质材料上都压合了一层0.5盎司的铜箔。例如，采用Dk值为2.2的5mil厚层压板，制成的50欧姆微带线的导体宽度相当宽，达到14.8mil。而采用具备同样厚度，但Dk值为4.5的层压板制成的导体宽度，则缩小至8.9mil。

如果采用薄型层压板材料，那么，导体宽度将显著缩小。例如，采用Dk值为2.2的2mil厚层压板，制成的50欧姆微带线的导体宽度仅为5.6mil。同样地，如果采用Dk值为4.5的2mil厚层压板，那么，所制成的50欧姆微带线的导体宽度将缩小至3.3mil。一般而言，导体的宽度越大，意味着加工良率越高、损耗越低，尽管损耗也与层压板材料的耗散因子有关。

典型情况下，专为毫米波频率应用而开发的薄型层压板材料，均采用某种树脂材料，不论是否使用填充物。树脂材料包括，含有玻璃纤维编织布、陶瓷填充物以及玻璃纤维编织布和陶瓷混合填充物的碳氢化合物、液晶聚合物（LCP）和聚四氟乙烯（PTFE）等材料。相比于其他层压板材料，PTFE的层压板具备较低的Dk值和极低的损耗，因此在高频电路设计领域广受称道。PTFE层压板材料使用了多种不同的填充物来增强机械性能，但却以提高介电常数和耗散因子（损耗）为代价。

薄型材料的吸湿性非常低，可在复杂多变的环境条件下保持始终如一的电气性能。PTFE层压板的铜箔与介质材料表面之间的粘合非常牢固，抗撕强度很高，对于制造高频电路的宽度较窄的导体，这一点至关重要。尽管PTFE层压板材料通常在z轴（厚度）上具备较高的热膨胀系数（CTE），薄型PTFE层压板的优点之一便是最大限度地降低了z轴上的CTE值。此外，PTFE层压板还能在广阔的频率范围内实现稳定的Dk值和耗散因子（损耗），因而是宽带电路设计的理想材料 

当然，基于PTFE的材料也以其特殊的制造要求而闻名，例如，这种材料与生俱来的热塑性，要求在镀铜之前进行特殊的孔壁处理。较之于采用随机玻璃纤维填充的PTFE层压板，玻璃纤维编织布的PTFE层压板具备更高的机械稳定性。但是，因PTFE介质材料中的玻璃纤维密度不均匀，而导致的介电常数的轻微偏差，使得一些设计者在较高频率上使用玻璃纤维编织布的PTFE层压板时，遇到了被称为“交织效应”的现象。由于材料越薄，波长越短，这个效应越显著，因此，设计者在选择材料时应当考虑其应用可能受到的影响。

当CTE是一个至关重要的要求时，那么，采用陶瓷填充物的PTFE层压板将能降低z轴上的CTE值，例如，要求利用稳定的镀通孔（PTH），将不同电路层互连起来的多层电路，就需要降低CTE值。当然，层压板中的填充物类型，也会影响面向毫米波应用的电路的制造。填充物的颗粒大小，将限制为实现信号和接地连接而钻的PTH之间的间距，对于毫米波频率应用，这一点极其重要。

大体上，高频PCB发生的损耗，可归咎于介质损耗、导体损耗和辐射损耗。介质损耗取决于材料，可根据耗散因子，来比较不同材料的介质损耗。典型情况下，辐射损耗是设计选择的结果，例如，使用带状线、微带线或共面波导（CPW）技术，尽管使用具备较高Dk值的层压板，有助于降低微带线电路中的辐射损耗。

在30 GHz及更高频率下，导体的表面粗糙度和导体与介质材料之间的粘合度，会加剧导体损耗。在波长极短的毫米波频率上，相比于光滑的表面，粗糙不平的导体表面，实质上就是延长了毫米波信号所要传输的距离。例如，通过使用低粗糙度的铜箔，或者经反转处理的铜箔，可以改善毫米波频率上的插入损耗性能。此外，在毫米波频率应用中使用薄层压板材料时，诸如银等低损耗表面处理，也有助于降低插入损耗。

 

Thinner printed-circuit-board (PCB) materials have some obvious mechanical advantages in maintaining low profiles and light weight in mobile and portable electronic designs. But using thinner laminates requires a number of considerations, since the thickness of the dielectric material will impact the conductor thickness of a controlled-impedance line such as the 50-Ohm lines commonly used at high frequencies. Before selecting a thinner PCB laminate material to save circuit height or weight, it may help to review the types of thinner laminate materials currently available, their key characteristics, and how each compares when used in practical manufacturing and application environments.

In addition to lighter weight and lower profiles, thinner laminates are attractive at millimeter-wave frequencies of typically 30 GHz and above. Thinner laminate materials help prevent unwanted modes of signal propagation at the smaller wavelengths associated with millimeter-wave frequencies. Of course, other factors, such as the dielectric constant and dissipation factor (loss) of the materials, can impact how they are used at those higher frequencies.

As frequencies increase, wavelengths grow smaller and circuit features grow finer. This usually encourages the use of PCB laminate materials with lower dielectric constant (Dk), since increasing the dielectric constant for given impedance conductor at a given frequency has the effect of also shrinking the circuit dimensions. While this can have benefits when designing and fabricating circuits for lower-frequency use, it can result in extremely small and challenging circuit tolerances at millimeter-wave frequencies. Because using thinner PCB laminates also leads to finer circuit dimensions, thinner laminates should be carefully chosen to gain the best benefits when used for millimeter-wave circuits.

Let’s look at a few examples of the effects of laminate thickness and Dk on circuit dimensions. When 50-Ohm microstrip conductors are fabricated on laminate materials having different thicknesses and Dk values, the thickness of the conductors can vary widely. In all cases, the dielectric materials were laminated with 0.5-oz. copper. For example, with a 5-mil-thick laminate with Dk of 2.2, the conductor width of a 50-Ohm microstrip line is quite wide, at 14.8 mils. For a laminate with the same thickness, but higher Dk of 4.5, the conductor width narrows to 8.9 mils.

With thinner laminate materials, those linewidths can shrink dramatically. For example, with a 2-mil-thick laminate with Dk of 2.2, the conductor width of a 50-Ohm microstrip line is now only 5.6 mils. And if a 2-mil-thick laminate with Dk of 4.5 is used, the conductor width of a 50-Ohm microstrip line is now only 3.3 mils. A wider conductor usually translates into better fabrication yields and lower losses, although loss is also a function of the dissipation factor of the laminate material. 

Thinner laminate materials and those developed for use at millimeter-wave frequencies are typically based on some resin material, used with or without a filler. Resins include hydrocarbon systems, liquid-crystal-polymer (LCP) materials, and polytetrafluoroethylene (PTFE) systems with woven-glass filler, ceramic filler, and woven-glass and ceramic filler. PTFE-based laminates are generally well known among high-frequency circuit designers for their low Dk values and extremely low loss compared to other laminate materials. The various fillers serve to add mechanical integrity to the PTFE materials, but at the cost of increasing the dielectric constant and the dissipation losses.

In thin sheets, the moisture absorption of these materials is quite low, which supports consistent electrical performance under varying environmental conditions. PTFE-based laminates also have good peel strength of the copper laminate from the dielectric surface, which can be critical when fabricating the narrow conductor widths of high-frequency circuits. Although PTFE laminate materials are typically characterized by high values for the coefficient of thermal expansion (CTE) in the z (thickness) axis, one of the benefits of thinner boards of this material is that the effects of the CTE in the z-axis is minimized. PTFE laminates are also characterized by stable Dk and loss factors over broad frequency ranges, making them suitable candidates for broadband circuit designs.

Of course, PTFE-based materials are also known for their special manufacturing requirements, such as special drilled hole wall preparation prior to plating because of the natural thermoplastic properties of the material. The use of woven-glass layers can improve the mechanical stability of PTFE laminates compared to those with random glass fillers. But some designers have experienced what is referred to as a “weave effect” when using woven-glass PTFE at higher frequencies, due to micro-variations in the dielectric constant as a result of uneven density of glass within the PTFE dielectric. Because this effect is more pronounced for thinner materials and at smaller wavelengths, designers should consider the possible impact on their applications.

When CTE is a critical requirement, PTFE laminates with ceramic filler can reduce the CTE in the z-axis to improved levels, as might be required for multilayer circuits in which stable plated through holes (PTHs) are required to interconnect the different circuit layers. Of course, the type of filler in a laminate can also play a role in manufacturing circuits for millimeter-wave applications. The particle size of the filler material will set a limit on how close PTHs can be drilled for signal and ground connections, which can be critical at millimeter-wave frequencies.

Losses in high-frequency PCBs generally can be traced to the dielectric material, to the conductors, or to radiation losses. The dielectric losses are material dependent, and can be compared for different materials by means of the dissipation factor values. Radiation losses are typically a result of design choices, such as the use of stripline, microstrip, or coplanar-waveguide (CPW) technologies, although the use of laminates with high Dk values can help minimize radiation losses in microstrip circuits.

At frequencies of 30 GHz and higher, conductor losses can be exacerbated by the roughness of the conductor and the integrity of the interface between the conductor and the dielectric material. At the extremely small wavelengths of millimeter-wave frequencies, a rough conductor surface is essentially a longer path for the millimeter-wave signals to follow compared to a smooth surface. For example, insertion-loss performance at millimeter-wave frequencies can be improved through the use of low-profile copper conductors or reverse-treated copper conductors. In addition, the use of low-loss plating finishes, such as silver can also help to minimize insertion loss when using thin laminate materials at millimeter-wave frequencies.