In our PCB construction article, we outlined the fabrication process and the basic building block from which custom PCBs are most commonly created – 2 layers of conductor separated by a fixed-width substrate. In this article, we’ll dig deeper into the properties of the constituent parts of this building block.
Unless you’re making an exotic PCB, your conducting material will be copper. When fabricating a copper conductor PCB, the only real flexibility you get is in the thickness of the copper which is often referred to as copper weight. Most fabricators offer a range of options from 0.5 to 4 ounces. For reference, 1 ounce (oz) copper is approximately 1.4 mils thick. Single ounce copper also happens to be the most frequently utilized thickness for outer PCB layers and 0.5 oz copper is the most common option for internal layers.
So why would you ever select thicker copper? There are several possibilities, but the most common is to increase current carrying capability. If you’re running high current through a trace it is often desirable to minimize the resistance of this path, which is accomplished by increasing cross-sectional area (height and width) of the conductor. When increasing copper weight in your design, no matter your rationale for doing so, it is important to keep this effect on trace resistance in mind. If, for example, you have other traces in your design that require a controlled 50 Ohm impedance, increasing the copper weight will require a reduction in trace width to maintain 50 Ohms.
Substrate (also commonly referred to as dielectric material) is the insulating material sandwiched between two conducting layers on a PCB. As previously mentioned, this sandwich is the basic building block used in the PCB fabrication process. When PCBs of greater than 2 layers are constructed, a resin impregnated cloth material commonly referred to as prepreg is used as a substrate between adjacent building blocks (the figure below depicts this configuration).
Unlike the conductor, there is a much wider range of options available for use as substrate material, though the vast majority of PCBs manufactured today utilize a glass reinforced substrate known as FR4. This material is inexpensive, flame resistant, resists the absorption of water and generally functions very well for many applications. With all of these great qualities, why would you ever use a different substrate? Well the properties of all substrate materials are actually a function of many variables including frequency of operation and temperature. For certain applications, the performance breakdown of FR4 at higher frequencies and / or extreme temperatures requires the use of these alternate materials.
Substrate Application Examples
Examples of high frequency applications where other substrates are commonly utilized are: wireless, antennas, radar and high speed digital and radio frequency design. So, at what frequency does the performance of FR4 become unacceptable? Well, unfortunately the answer to that is, it depends. As a general rule of thumb, once you exceed 1 GHz, it’s time to more carefully consider your options. With that said, there are designs with signaling exceeding 3 GHz on FR4, though these designs typically require more sophisticated modeling to ensure the substrate will not adversely affect performance. For high performance, high frequency designs the industry leading provider of PCB materials is Rogers Corporation. They have a large array of products catering to various needs of the designer.
For those working in aerospace and / or cryogenics, materials that can withstand wide temperature fluxtuations are often necessary. For these applications, polyimide is frequently used. Polyimide materials are capable of functioning in temperature environments that range from cryogenic to over 500 degrees Fahrenheit. Similar to FR4, this material is also highly resistant to fire (a very important property in aerospace applications). However, polyimide has one important drawback that should be considered when using the material, it is hygroscopic (absorbs water). So the designer must have a strong understanding of the full set of environmental conditions (humidity, due point, etc.) their hardware will be exposed to when operating.
The thickness of all substrates can be varied just like that of our conductor, though substrate thickness is measured in units of mils or millimeters (not ounces like our copper conductor). The thickness of the substrate affects the rigidity of your PCB and also influences electrical properties such as characteristic impedance of copper traces. It is possible to specify the thickness of a specific layer in a PCB stackup, but the more common scenario is specifying the thickness of your entire PCB and the manufacturer selects the individual substrate thicknesses to arrive at your overall thickness requirement.
Though there are wide-ranging options for materials when designing and manufacturing printed circuit boards, the vast majority of designs utilize the same basic materials: copper conductor and FR4 substrate. If your design doesn’t operate at high frequencies, carry large amounts of current or need to function over large temperature ranges, you’re most likely fine in accepting these default materials.