Chapter 2: Define the System Requirements
Arguably the most critical part of designing a new printed circuit board is the initial process of establishing a system architecture to be used as a road map for later steps in the PCB design process. At the outset, your proposed layout is an educated estimate of how the design will evolve into a finished product. By focusing on several key design constraints early in the development cycle, you can mitigate problems that might occur later on as your plans change.
The greatest constraint to your design will be the cost of designing and building each board. This will dictate the board laminate type and the number of wiring layers that will be needed. Equally important is choosing the copper trace width and the technology to be used for wiring channels. A board's dimensions will be constrained by the enclosure that the PCB will be fitted to. Obviously, the size and number of components that can be used will be dictated by the available board area. Nearly every board design constraint will fall into one of seven basic categories. These are:
Environmental and reliability constraints
Proper board size
Selecting board laminate materials
Board types and board thickness
Wiring channel widths and proper thickness
Additional wiring considerations
Thru-hole and pad technology
A detailed evaluation of the design constraint types listed above will allow a PCB designer to establish firm initial guidelines that will serve as a starting point for the layout of each new prototype board. We'll cover all of these bullets in more detail in the following paragraphs.
Environmental and Reliability Constraints
Before board design can begin, it's important to consider the environment that the PCB will operate in. Operating parameters are usually determined by the operating document, which is usually referred to as an application specification. The application specification defines the minimum and maximum temperature, humidity, vibration, noise, voltage and current tolerances that a device must operate under without failure. A good example of this is mil-spec or military grade components, which must meet more stringent operating conditions than commercial grade boards. Most commercial grade boards must operate in a temperature range of 0 to 85 degrees Fahrenheit, whereas military specifications require a board that will work reliably between -40 to 140 degrees.
At higher temperatures, the impedance of the wiring traces increase, which can ultimately degrade circuit performance and speed. To mitigate these issues, a designer can increase the width of the wiring channels. High humidity environments are also detrimental, since they can result in corrosion and shorting. To address these concerns, a designer can add additional conformal coatings and solder masks that seal off the board below. The spec also defines the voltage and current ranges that can't be exceeded if the board is to meet or exceed JEDEC standards. These parameters will also affect the types of materials that must be used to meet the requirements.
Another design consideration that should be taken into account at this time is EMF and RF interference. Crosstalk between adjacent, high-frequency line traces can create electromagnetic fields that will introduce noise into other nodes. These effects can also degrade signal integrity and performance. At this point, a designer is still in the initial stages of the process. However, you'll need to keep these effects in mind as you proceed with the tentative PCB design. Ultimately, all of these operating conditions need to be addressed before you proceed with finalizing the board layout.
Board Size and Surface Mounting
Generally speaking, board size is determined by the amount of space and mounting area that's available inside the intended device's enclosure. Oftentimes, one PCB is integrated with other boards into a discrete package assembly. This can pose a further constraint on the maximum size of the board. The designer must then address the problem of fitting all of the components and wiring channels onto the available board area while not violating any design rules or processing constraints. Therefore, it's critical that the designer work closely with the board layout engineer to resolve any conflicts.
Once you've established the amount of board real estate that's available, the next consideration is the size and number of components that will be placed on the board. This may sound trivial but in many cases, it can be a challenge to integrate all of the proper functions onto each board. If space is tight, you may need to use SMT components rather then traditional thru-hole devices. Or you might determine that you need to go with a multilayer board to accommodate all of the different wiring channels. In extreme cases, you may have to mount components on both sides of the board. The worst case scenario would be to have to increase the overall board size. All of these considerations can significantly impact the cost of assembling each board. A designer needs to evaluate all of these trade-offs and arrive at a solution that does not compromise the integrity of the board.
Selecting Board Laminates
Once the designer has defined the operating conditions and board size constraints, he or she must decide on which board laminate material to use. There are a number of physical and electric parameters that must be analyzed before an appropriate choice can be made. We'll explore these considerations in more detail in the paragraphs below.
The CTE or Thermal Coefficient of Expansion is important and will vary depending on what laminate is chosen. Ideally, the designer will limit the expansion or contraction of the board since severe distortions can lead to cracking, which can expose the board to contamination or corrosion. The MOT is the Maximum Operating Temperature under which each board can safely function. It's a temperature rating that the Underwriters Laboratory will give each board depending on which pre-preg and laminate are used in the construction of the board. Tg is related to the CTE, except that it measures the liquid glass transition temperature, which is the absolute maximum temperature achievable before the board becomes electrically and mechanically inoperable. Lastly, the thermal conductivity of the board is also important because it determines a material's ability to dissipate heat efficiently.
The choice of a PCB's material should not be determined solely by its thermal properties. The dissipation factor is a measure of the ratio of the power that is lost or dissipated in the laminate material to the energy that is transmitted through the wiring channels. All things being equal, low-dissipation factor materials are desirable since they minimize the energy that is lost in the form of heat.
The dielectric constant is a measure of the energy loss due to the polarization of the dielectric layer at high frequencies. The dielectric constant is important since it will determine the line widths that must be used to ensure that the PCB's power distribution capabilities are not compromised. Another electrical parameter that must be considered is conductive loss, which occurs in laminate materials at higher frequencies. At high frequencies, all wiring traces must be treated as transmission lines. The designer must therefore consider crosstalk between adjacent wiring traces due to the high electric fields that are generated inside the dielectric. These effects, if not taken into consideration beforehand, will result in a loss of signal integrity and performance in critical areas.
Board Type and Board Thickness
From a designer's point of view, the easiest board to design is a single layer PCB that doesn't place any restrictions on the board size or number of components that can be used in said design. Unfortunately, such a scenario is a pipe dream for the average PCB designer today. The new trend is to pack as many features and capabilities as possible into each PCB. The designer must now think in 3-D and consider how many wiring layers will be needed to complete a design. Multilayer boards are both a curse and a blessing. On the one hand, it eliminates the need to squeeze wiring channels into the available board area in one level. On the other, it can make the task of connecting all of the necessary components together a nightmare. Best practices in PCB design dictate that the designer should attempt to determine the minimum number of layers that are needed, since the price of each board will increase as more layers are added.
The number of wiring layers required will determine the overall thickness of the board. Knowing this, a designer can now pick and choose the types of device components that can be used in their envisioned PCB layout. If thru-hole vias and contacts will be utilized, the designer can opt for components with varying lead lengths. For thru-holes, the lengths of individual leads must be longer then the corresponding board thickness. For device components with short leads, thicker boards are a problem. If the board thickness can't be reduced due to the number of layers, then surface mount technology might have to be used.
Wiring Channel Widths and Proper Thickness
Before the designer can determine the proper trace widths and thicknesses that can be used, he must lay out a preliminary circuit schematic and net list. After this has been completed, he can then run circuit simulation software tests that evaluate how each circuit behaves under various operating conditions. These simulations establish the worst case voltage and current ranges that the design can tolerate without failing. Once the minimum and maximum current carrying capacities are known, the designer can then determine the minimum trace thicknesses and widths that will be needed to make each circuit function properly.
As current flows through a trace, it loses energy due to localized heating that's dissipated into the surrounding dielectric material. This power loss varies as a function of the square root of the current times the resistance. These loses must also be taken into account when the designer decides on which trace thickness to use. Increasing the trace width by a factor of two will not double the current. In reality, the current increases by its value raised to the factor 'c', where 'c' is equal to 0.725. In the above case, the current will be increased by 1.6 rather than by a factor of two. Increasing the width of a trace doesn't result in a proportional increase in current either, largely because current flows on the surface of the material are governed by a phenomenon called the skin effect. To make the designer's job easier, he can use the generic industry standard PCB tables to determine which dimensions to use. The IPC tables give the minimum trace thicknesses, widths and spacing that need to be met for various board materials and laminate compositions.
Using IPC Tables
The designer also needs to determine the minimum spacing or width requirements that can't be violated for each new board design. The IPC tables give different values for each parameter depending on whether the trace width is an internal or external trace. Internal traces are wiring channels that are surrounding by dielectric inside the multilayer board. External traces are those that reside on both external surfaces of the board laminate composition. Determining the proper width, thickness and spacing involves trade-offs. Many designers always plan ahead for the worst-case scenario unless space isn't at a premium. A good rule to follow is to assume worst-case routing conditions, in which case one would use the internal trace width and the external spacing as a standard for the distance between adjacent traces. If the designer wants to refine his wiring methodology even further, he can refer to the IPC-2152 standards, which provide a wealth of information and data that's not available in the standard tables.
Additional Wiring Considerations
Most PCB design shops have a preferred board manufacturing vendor that they work with to produce their boards. During the concept phase, the designer usually determines which board technology he or she will use to complete his design vision. Once the board technology has been defined, the PCB designer now knows the number of wiring layers that can be used to connect all the various components that will be placed on the board.
As we discussed in the previous section, the trace widths, thicknesses and spacing are estimated either from the IPC tables or from additional proprietary board vendor algorithms. Based on these tracing ground rules, the board designer can start to lay out and route the wiring channels because he knows that he has clear parameters to which he can adhere. Unfortunately, the designer will sometimes determine that he doesn't have enough room to route all of the necessary wiring channels. At this point, the designer has basically two choices. He can either increase the number of layers in the board, or he can go to a higher technology board that has better dielectric properties.
If the number of layers cannot be increased, then a better technology must be used. This option can change all of the various wiring parameters that have previously been determined. Additionally, it can change the aspect ratio, angular size and pad dimensions that must be used to define the thru-holes that allow the various device components to be attached to the board. We'll discuss this in more detail in the following paragraphs. Regardless, both of the processes mentioned above can increase the total cost of each board and add additional time to the design and manufacturing schedule. This illustrates the point that the correct technology should be considered carefully at the beginning of each design cycle.
Thru-hole and Pad Technology
A golden rule for designers is to use the largest diameter thru-hole possible when laying out a board. Many vendors usually charge a premium for holes that must be drilled smaller than 0.4 millimeters. If you're in doubt, consult your board manufacturer to ascertain the minimum size hole that they can support with their tooling set. There are two basic types of thru-holes that can be used for the most part, although some manufacturers offer variations on either technique. Plated thru-holes are typically plated with copper to provide an electrical path to connect the copper traces between various levels. Non-plated thru-holes are unsupported since they don't have copper cladding inside the hole.
Plated thru-holes are either soldered or non-soldered. For non-soldered thru-holes, the board manufacturer will provide information on the aspect ratio drill hole tolerances he can guarantee. The aspect ratio is simply the ratio of the board thickness to the smallest hole that can be drilled. Plated thru-holes most often have a capture pad that's been defined in previous processing steps. The landing pad will be used to connect wiring traces on different wiring layers in a multi-layer board. The minimum size and diameter of the pad can again be determined by referring to the IPC specifications. An important parameter is the minimum angular ring, which is the distance that the pad must extend beyond the drilled thru-hole. If possible, the designer should design his board using dimensions that exceed the minimum specifications in the IPC tables. If questions remain, the board manufacturer should be consulted on what is feasible. All of the considerations mentioned above apply to soldered thru-holes, except that the external pads should be made larger to dissipate heat during the soldering process.
Non-plated thru-holes may or may not have surrounding capture pads. Since the thru-hole is not plated, the wiring pad must be larger than that used for plated thru-holes. This is because there's no support for the thru-hole other than the copper adhesive. The larger pad will provide more surface area to ensure that the pad adheres to the board during high-temperature processing or soldering. For non-plated holes, the IPC specs require a minimum angular ring of 0.152 millimeters. This number should be increased if the pad is to be soldered.
Non-Plated Holes Without Pads
A non-plated hole is nothing more than a hole in the board laminate that's used as a mounting hole or as a routing hole for external wires. As such, it doesn't require plating and doesn't need to satisfy any angular ring specifications. Because there's no pad image to use as a reference point, the drill hole tolerances are much larger than for plated thru-holes. In addition, the clearance or spacing around the mounting hole needs to be expanded to accommodate the registration errors that will occur during the drilling process.
The topics that have been discussed above are by no means a complete list of all of the design constraints that a PCB engineer will encounter during the design process. Along the way, additional complications can crop up that may force the designer to pursue a different path than the one that was initially planned. Like any complex engineering project, PCB design usually involves a series of trade-offs and compromises. Nevertheless, the designer must tackle each problem head on and weigh all of the available options to determine the best way to proceed at each individual step of the journey. Only by doing this will a PCB designer be successful in completing any given task.