This article discusses the major steps in the PCB design flow, from basic terminology to the primary steps required to move an example design through the schematic, layout, and manufacturing stages.
Understanding the Terminology
Schematic capture and simulation tools – A schematic capture program allows the user to draw a document representing the electrical component symbols and the interconnections between them in a graphical way. Before generating a PCB, the symbols are mapped to component footprints and the symbol interconnections are converted to a netlist that specifies the connections between the component footprints in the layout process. A schematic tool that allows the user to do interactive circuit simulation with the same schematic circuit representation used for layout is advantageous. Circuit simulation can be useful for both initial design analysis and testing the design (i.e. verification testing and troubleshooting) once complete.
PCB layout tools – A PCB layout program generates the mechanical and wiring connection structure of the PCB from the netlist. The layout program allows the wiring connection structure to be placed on multiple layers and, once complete, allows the user to generate the computer aided design (CAD) files needed to manufacture a PCB.
Gerber files – The CAD files that need to be sent to a PCB manufacturer so it can build the PCB layer structure are called Gerber files. The RS-274X is the most commonly supported Gerber file format.
NC drill files – The numerically controlled (NC) drill files indicate the size and position of holes used for unplated holes, plated through-holes, or holes for vias. Some quick-turn PCB manufacturers have only select hole sizes available.
Printed circuit board (PCB) – A wafer board defining the mechanical and copper wire structure of the circuit. (It is sometimes called a PWB for printed wiring board).
PCB structure and details
A PCB can be considered a layered structure, usually with multiple copper and insulating layers. The main portion is a non-conductive (insulative) material (substrate) usually made from fiber glass, and epoxy. The substrate material used to separate layers comes in different thicknesses, from 0.005” to 0.038”. Conducting layers consist of copper (Cu) foils that are etched away in specific areas where the user does NOT want connections to occur. A single-layer PCB has the substrate with one layer of copper foil on the top (see Figure 1).
A double layer PCB (see Figure 2) has two layers of copper foil (one on the top and one on the bottom).
If more than two layers are required due to increased complexity of the PCB, other layers of copper can be built-up or added to the ones shown above (usually in pairs). For example, a 4-layer PCB can be made up of two double-layered PCBs laminated (sandwiched) together with a core material in between. Made out of epoxy/fiber, this core layer is called a prepreg (pre-impregnated), and it insulates and supports the other layer structures. It is common for modestly complex boards to have 6, 8, or 10 layers (with increased manufacturing cost). Some highly complex PCBs have up to 32 layers or more of traces and copper planes (see Figure 3).
The height of the substrate is usually the thickness of one or multiple sheets of laminate material and is usually much smaller than the height of the core prepreg material layer.
Multilayer PCB – A PCB with more than one copper foil layer. The layers are preferably renamed in the design tool to unique names (such as power or ground) as desired by the user.
Layer Stack Up – The copper organization of multiple layer PCBs with the intent of having specific signal and ground planes on certain layers for routing convenience and electromagnetic shielding purpose. A four-layer board will typically have the following layer structure, where the top and bottom layers are reserved for signal routing and the inner layers are reserved for ground and power planes:
• Copper Top
• Inner 1
• Inner 2
• Copper Bottom
Finished PCB Height - Standard finished PCB thicknesses are commonly found as shown – this thickness includes all copper, substrate and prepeg layers:
• .031” (also .039" is common)
• .062" (most commonly used size)
• .093"
• .125"
Shown in Figure 4 is a more realistic layer stackup of a four-layer PCB showing the various thicknesses of the layer structures from a typical PCB manufacturer yielding the common 0.062” finished PCB height.
Schematic drawing and footprint selection
The first step in successfully creating a PCB is the proper creation of an appropriate schematic drawing of the circuit. All of the components selected in the schematic must have correctly assigned packages associated with them so that the generated part list and netlist representing required connections will be correct. Figure 1 is a Multisim SPICE simulation schematic that helps illustrate the basic steps from schematic to final PCB generation:
Notice here that power and ground symbols are considered virtual parts that connect into a single net, and these symbols are treated specially in the schematic environment. Since these are net connections only and do not connect to a footprint for layout purposes, it is appropriate to have them appear in a black color.
Once satisfied with the schematic (and functionality or performance through the use of simulation), it is time to prepare the schematic for transfer to the PCB layout environment.
Preparation Steps before Transferring
Ground Definition
In the example schematic, both an analog and a digital ground are used. For layout purposes, it is critical to specify whether the analog and digital grounds should be considered one ground signal (i.e. connected together) or be kept separated for the purpose of routing two separate ground connections.
This example requires that the two grounds be connected together (or C2 would not have the same common return path as the rest of the circuit). The digital ground shows a net name of GND, and the analog ground has a net name of 0. When the grounds are connected, the netlist transferred to the PCB environment will use the net name ‘GND’ for both analog and digital ground net names.
Unit Definition
Set the export settings (mils, mm, etc.) that will be used for clearance and trace width units-- mils (0.001 inch) are the most common units in North America. From within either the schematic or the PCB layout environments, trace and clearance constraints can be predefined to specific values. For example, based on the expected peak currents in a circuit, the default power or ground traces can be set to 25 mils.
Setting the number of layers
If the circuit is relatively simple and electromagnetic compatibility is not an issue, a cost-effective single-layer PCB design may be able to be used. If power or ground planes are required or the circuit is slightly more complex, two, four, or more layers can be selected. When planning for the board design, consider cost impacts as layers are added to the board. For this circuit example, four layers will be selected so that the design can use a power and ground plane.
Transferring the design for PCB layout
Once the final preparation steps are complete, the design is ready to be transferred to the PCB layout environment. A window is typically displayed showing all of the components and nets that are being transferred (see Figure 2).
This is particularly important when updating a design layout with new changes in the schematic at a further stage of the design.
Board Outline Selection
As shown in Figure 3, a generic board outline of a rectangular shape is automatically generated in the tool. However, a custom board outline shape needs to be created to match package dimensions or other specifications.
Usability features such as selection filters are essential for fast selection of different attributes of a design. In our example, a selection filter is used to change the board outline without interacting with the remainder of the design (see Figure 4).
The default rectangular board outline can be repositioned or resized, or one of the following methods can be used to customize the shape of the board outline. With any of these techniques, the existing board outline will first need to be selected and deleted and a new shaped created.
1. Using Board Wizard tools.
2. Import a board outline shape via DXF file.
3. Place a polygon on the Board Outline layer to define a custom or complex shape.
Choosing a circuit material for a high-frequency printed-circuit board (PCB) is generally a tradeoff, often between price and performance. But PCB materials are also selected by two key factors: how well they meet the needs of an end-use application and what kind of effort is required to fabricate a desired circuit with a particular material.
These two factors may not mesh: one material may be well suited for a particular application but may pose challenges in terms of circuit fabrication, and vice versa. There is no foolproof, step-by-step procedure for selecting a PCB material. But by relying on some tangible guidelines designed to evaluate a material in terms of its suitability for circuit fabrication and for meeting the requirements of an application, the process of selecting a PCB for a particular application can be simplified. The approach will be demonstrated with some of the more popular high-frequency PCB materials, and where each stands in terms of fabrication qualities and suitability for end-applications.
Commercial high-frequency PCB materials can be categorized as one of seven generic material types, as shown in Table 1. High-performance FR-4 is included in Table 1 because it is often used in combination with other high-frequency materials for certain applications and requirements. However, in terms of electrical performance, FR-4 is not considered a true high-frequency circuit material.
A number of different mechanical processes are required as part of high-frequency PCB fabrication. In general, the most critical of these would be drilling, plated-through-hole (PTH) preparation, multilayer lamination, and assembly. The drilling process is typically concerned with creating clean holes, which will later be metalized to form viaholes for electrical connections from one conductive layer to another.
Some concerns with the drilling process include smear, burring, and fracturing of the material. Smearing can be lethal to PCB fabrication using a PTFE based material, since there is no way to remove the smear. Fracturing can be fatal for some of the nonwoven glass hydrocarbon materials; however, most of the woven glass hydrocarbon materials do not have this concern.
The PTH preparation process is relatively well defined and straightforward for most non-PTFE materials, although special processing is required when forming PTHs for PTFE-based materials. Ceramic-filled PTFE-based materials offer PTH preparation options which are more forgiving. However, non-ceramic-filled PTFE materials require a special process which can limit final circuit yields.
Fabricating multilayer PCBs presents many challenges. One is the fact that dissimilar materials are often being bonded together, and these dissimilar materials can have properties which complicate drilling and PTH preparation processes. Also, a mismatch between certain material properties, such as coefficient of thermal expansion (CTE), can lead to reliability problems when the circuit is thermally stressed during assembly. A goal of the material selection process is to find a good combination of circuit materials for a multilayer PCB which enable practical fabrication processing while also meeting end-use requirements.
Designers and fabricators have many choices of materials used to bond together the copper-clad laminates that ultimately form a multilayer PCB. As Table 2 shows, the materials differ in terms of dielectric constant, dissipation factor, and processing temperatures. In general, lower lamination temperatures are to be preferred. But if a PCB must undergo soldering or some other form of thermal exposure, it will be necessary to use a bonding material with high reflow (re-melt) temperature, one which is thermally robust and does not reflow at the elevated processing temperatures.
The greatest concern during PCB assembly is due to the effects of thermal stress from soldering. Other sources of thermal stress during PCB assembly are from solder rework or exposure to multiple thermal cycles. In terms of circuit materials, effects from thermal stress can typically be projected by comparing the CTE values for different materials, as shown in Table 3.
In general, a circuit material with a lower CTE will be more robust and handle the thermal stress of PCB assembly better than a material with a higher overall CTE. This is one reason why multilayer PCBs typically use more than one type of circuit material. Materials which might provide good electrical performance may have characteristics (such as high CTE) that make them less than robust to handle the thermal stress of PCB assembly.
By using a combination of materials, some with good electrical properties and others with lower overall CTE, a robust multilayer PCB construction can be designed and assembled. Such a construction is known as a hybrid multilayer PCB, which can provide cost as well as performance benefits. More information about hybrid multilayer PCBs can be found on a paper presented at PCB West 2010. [1]
In general, a circuit material with CTE value of 70 ppm/°C or less is considered robust for PCB fabrication and assembly. As Table 3 shows, however, one of the materials with the best electrical performance also has the worst CTE. This is one reason why ceramic-filled PTFE laminates were formulated. They combine excellent electrical performance with very good CTE. Unfortunately, they exhibit poor dimensional stability, since the material is soft and circuit dimensions can be easily distorted. To provide good electrical performance and CTE with improved dimensional stability, ceramic-filled PTFE laminates with woven glass reinforcement were developed.
When making a choice in high-frequency circuit materials based on fabrication issues, the clear-cut favorite would be ceramic-filled hydrocarbon material with woven glass. These materials feature a low dissipation factor typically on the order of 0.003 and are robust in terms of most circuit fabrication processes. If better electrical performance is required, the choice would be ceramic-filled PTFE with woven glass. These materials typically have a dissipation factor in the range of 0.002 and are generally fabrication-process friendly.
The major concerns in fabricating PCBs with these materials relate to drilling and PTH preparation. For the best electrical performance, the choice is micro fiber glass PTFE, although this material can be difficult in terms of fabricating more complex circuit constructions. The material, which is nearly pure PTFE, is often used for simple high-frequency circuitry such as microstrip filters and couplers. Additionally this material is often used in a hybrid multilayer circuit, in which it supports critical functions, while other materials more friendly to circuit fabrication processes are used for the remainder of the multilayer PCB.
Choosing materials based on end-use applications
There are several different concerns for choosing materials for high frequency applications. A good example in chart form is given from the Rogers Corporation Product Selector guide on the website and a portion of this is shown in Table 4.
Table 4 provides a quick comparison of different circuit materials based on key electrical performance parameters, including dielectric constant (Dk), dissipation factor (Df), thermal conductivity, and CTE. Two values of Dk are listed for each material: process and design.
The process Dk refers to the value determined by industry-standard IPC test method, IPC-TM-650 2.5.5.5c at 10 GHz. This value is used as a process control for making the substrate. The test method is reliable and well proven, but the Dk value is specific to that test methodology and that test frequency. The test method uses a clamped stripline resonator and is a fixture mechanism allowing large volumes of materials to be tested, which is necessary for a laminate manufacturer. However, the fixture is not representative of an actual stripline circuit or a microstrip circuit, and the use of process Dk values in computer-aided-design software simulation tools has been known to yield erroneous results.
In some cases, process Dk values may not be ideal for design purposes. For that reason, a second set of Dk values, the design Dk numbers shown for each material in Table 4, were determined using actual microstrip transmission line circuits, across a wide frequency range. These values are more appropriate for circuit design and modeling.
Table 4 also lists tolerance values for Dk for each material. Some high-frequency applications have very tight specifications for impedance control and the Dk tolerance is a good indicator of how well this material may be suited for those applications. In addition, Table 4 shows values for Df for each material, which is related to dielectric losses. For an application that requires low-loss performance, a material with lower Df value would be a logical choice, although this choice should also be weighed against the ease or difficulty of PCB fabrication with that material.
In addition to dielectric losses, conductor losses are important when comparing circuit materials. Especially for thin circuits, conductor losses can be more significant than dielectric losses.
Conductor losses can be impacted by circuit design, circuit configuration, and the thickness of conductive metals, as well as the surface roughness of the copper conductor layers. An excellent paper discussing this issue [2] has shown that conductor losses are higher for materials with higher amounts of copper surface roughness, compared to materials with smoother copper conductor surfaces. When comparing measurements by this parameter, the surface roughness measurement of concern is the root-mean-square (RMS) roughness of the copper surface.
A smooth copper conductor layer such as rolled annealed copper will typically have surface roughness RMS values around 0.3 microns. A low-profile electrodeposited (ED) copper conductor layer will typically have a surface roughness of around 0.8 microns, with standard ED copper at about 1.8 microns and high-profile copper at about 3 microns.
Figure 1 shows how increased copper surface roughness can result in increased loss. The same substrate — RO4350BTM laminate from Rogers Corp., — was used in both cases. This circuit material is a common ceramic-filled hydrocarbon woven glass. The higher-loss performance with frequency is plotted for this material with standard high-profile ED copper, while the lower loss results from using the same material with low-profile copper having a much smoother surface.
Referring back to Table 4, another material property of interest is thermal coefficient of dielectric constant, or TcDk. Often overlooked in material comparisons, this is a measure of how much the dielectric constant (Dk) will change with changes in temperature. Given as changes in relative dielectric constant, ?r, in parts per million (ppm) for changes in temperature (in °C), large values of TcDk can be an indicator that circuits which perform well under ideal laboratory conditions may not fare as well under less controlled conditions, notably with large swings in temperature.
Another important material parameter In Table 4 is thermal conductivity, or the capability to move heat through a circuit material. This parameter is important to consider for high-power applications in which a large amount of heat must be dissipated. A substrate with high thermal conductivity can assist the thermal management issues with these applications.
Many standard PCB materials have thermal conductivities in the range of 0.25 W/m/K. Additionally, some materials with typically good electrical performance, such as micro fiber PTFE materials, also have thermal conductivities around 0.20 to 0.25 W/m/K. By adding a ceramic filler, the thermal conductivity of a circuit material can be improved. Table 4 shows that the ceramic-filled high-frequency materials have significantly better thermal conductivity than most standard PCB materials, generally with two to three times better thermal conductivity. This improvement can help solve many thermal management issues in high-power PCB designs.
Another material parameter listed in Table 4, moisture absorption, can also be important to consider for high-frequency applications. In an environment with high humidity, a circuit material that absorbs a high amount of moisture will exhibit increases in Dk and loss, both impacting PCB performance.
Circuit materials with high moisture absorption may not suffer degraded performance in controlled environments, but performance can be quite variable in more hostile operating environments. Many standard PCB materials have moisture absorption in the range of 1%. As Table 4 shows, however, most materials formulated for high-frequency applications are characterized by moisture absorption that is considerably less than 1%. For most high-frequency applications, laminates with moisture absorption values of less than 0.25% are considered acceptable.
There are many issues to consider when choosing a circuit material for high-frequency PCB applications. Some are related to fabrication issues for producing the most robust PCB possible, and some to achieving the best electrical performance possible for a given application. Because of various tradeoffs, the material for fabricating the most robust PCB may not be the same one for the highest electrical performance for an application.
Multilayer hybrid PCBs represent one way to choose a blend of materials to combine robustness and good electrical performance. By using charts of material properties such as Table 4, it is easier to compare the critical properties of different high-frequency materials and to simplify that choice when striving for the best tradeoff between ease of fabrication and best electrical performance.
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