Introduction
From product conception, theory and circuit design, parts selection to production, every step of electronics design interlocks with the others. It seems like your circuit diagram is perfectly designed and the materials are properly selected, but when you finally move to production of your printed circuit board (PCB), you start encountering problems. In which step did it go wrong? Should we reselect materials or redesign? There are many limitations and difficulties.
In DigiKey’s powerful tool library, as long as you make good use of some gadgets and design aids to reduce mistakes and worries, electronics and engineering can be made much simpler and much less uncertain. Let us introduce you to several design-level gadgets that can help you easily create products.
Printed circuit board (PCB) Design
After the initial circuit prototyping on a Solderless Breadboard, the next step is to design the layout of the PCB. A PCB is a sandwich-like board composed of an insulating layer often known as a substrate and a conductive copper layer, which includes signal traces, power supply and ground layers. The routing layout design is as strict as the circuit design. It is necessary to consider the integrity of the system and understand the circuit’s characteristics to maximize your circuit design and avoid issues later in the process. Understanding the trace characteristics can help engineers quickly determine the circuit and layer requirements for their PCB. Refer to the following checklists for PCB design for two different systems.
The following is a sensing system using a 12-bit microcontroller, whose circuit includes analog-to-digital (A/D) conversion, LCD display and 5V external power supply. Although the current carrying capacity is not high, the wiring for analog and digital circuits need to be separated independent from each other. The checklist is provided for reference.
- Check device placement versus connectors. Make sure that high-speed devices and digital devices are closest to the connector.
- Always have at least one ground plane in the circuit.
- Make the power traces wider than other traces on the board.
- Review current return paths and look for possible noise sources on ground connects. This is done by determining the amount of possible noise present.
- By-pass all devices properly. Place the capacitors as close to the power pins of the device as possible
- Keep all traces as short as possible
- Follow all high impedance traces looking for possible capacitive coupling problems from trace to trace
- Make sure your signals in a mixed-signal circuit are properly filtered.
(Source :A Compilation of Technical Articles and Design Notes, Analog and Interface Guide, Microchip)
As well, here are some basic rules for PCB circuit design in making a high current power supply controller using MOSFETs. Since different route areas have different current-carrying requirements, cost-effectiveness can also be achieved in designing routing for different areas.
- When delivering power to the slot, parts should be placed together.
- Provide sufficient current to the load.
- Ensure noise immunity of load and sense circuits.
- Current sensing considerations.
- Temperature Rising considerations.
- Consider trace impedance to reduce derating effects on high current rail groups.
(Source : PCB Layout Guidelines for Power Controllers, Texas Instruments)
From the above two totally different types of product designs, we can see that certain requirements remain the same for the width of the traces, including temperature changes, current carrying capacity and impedance values. Next, we will introduce the following gadgets for PCB design to help you quickly calculate the required trace width and trace impedance.
Fig.1 Online Conversion Calculators by DigiKey
This tool uses formulas from IPC-2221 to calculate the width of a copper printed circuit board conductor or “trace” required to carry a given current while keeping the resulting increase in trace temperature below a specified limit. If the length of the trace is also provided, the total resistance, voltage drop, and power loss due to trace resistance are also calculated.
When doing these calculations by hand, you first calculate the area (A) using formula (1):
For IPC-2221 internal layers: k = 0.024, b = 0.44, c = 0.725; external layers: k = 0.048, b = 0.44, c = 0.725, where k, b, and c are constants resulting from curve fitting to the IPC-2221 curves.
Then, calculate the width (W) using formula (2). (Note: Traces on the inner layers of the circuit board require much greater width than traces on the surface of the board.)
As long as you enter the required values in our calculator, you will quickly get the resulting printed trace width (W), resistance value, voltage drop and power loss in both internal PCB layers or external layers in air. The resulting values for the two routing designs are compared side by side. Taking the power controller mentioned above as an example, if the current carrying requirement (I) is set to 0.8A, the ambient temperature is 25°C, and the copper layer thickness (t) is 0.035mm (such as in MG Chemicals’ 587 model prototype board, 1 ounce double sided copper), a TRise of 10°C, and a trace length of 10” (the expected length on a 6”x 4” PCB without the trace being close to or over the heat sink area ).
Fig. 2: T he " PCB Trace Width Calculator " I nput Interface
The calculation results are shown in Figure 3:
Fig. 3: Calculation results display of “PCB trace width calculator”
The results are estimates only. The actual results may vary and depending on application conditions. Both the internal layers and the external layers in air are displayed at the same time, forming a strong contrast, which not only facilitates engineers to design circuits, but also to consider economic benefits. The voltage drop and power loss of using the internal layer will be lower, but you’ll need a wider trace, which is to say the cost will be higher. You can modify your parameter values at any time according to your requirements, and the results will be updated instantly and are easy to compare.
IPC 2141 Trace Impedance Calculator
Another useful gadget is our IPC-2141 trace impedance calculator. The IPC-2141 trace impedance calculator will help make initial design easier by allowing the user to input basic parameters and get a calculated impedance according to the IPC-2141 standard. Seven trace types – such as “Microstrip”, “Embedded Microstrip”, “Edge Coupled Microstrip”, “Stripline”, “Asymmetric Stripline”, “Broadside Coupled Stripline” and “Edge Coupled Stripline” are available.
| Trace Type | Cross-sectional View | Description |
|---|---|---|
| Microstrip |  |
For a simple two-sided PCB design where one side is a ground plane, a signal trace on the other side can be designed for controlled impedance. This geometry is known as a surface microstrip, or more simply, microstrip. |
| Embedded Microstrip |  |
The structure is similar to Microstrip, but the signal trace is placed between dielectric layers. Pros to this type are higher trace protection and lower impedance. Cons are that this trace type is difficult to decouple, and impedance may be too low for easy matching |
| Edge Coupled Microstrip |  |
Common technique for routing differential traces. This trace design has advantages in reducing electromagnetic interference. The electromagnetic interference fields produced by opposing low-voltage differential signal currents tend to cancel each other out. |
| Stripline |  |
This arrangement embeds the signal trace between a power and a ground plane. The low-impedance AC-ground planes and the embedded signal trace form a symmetric stripline transmission line. |
| Asymmetric Stripline |  |
This structure is similar to Stripline. The signal traces in the dielectric layer close to one of the conductive layers to form an asymmetric distance from the ground layer and power layer. Generally, the signal traces will be close to the ground layer. |
| Broadside Coupled Stripline |  |
Commonly used in BGA areas, it is composed of two parallel traces with equal width, distance between traces and distance from the conductive layer. For differential pairs routed on adjacent signal layers, broadside coupling will be stronger if there is any trace overlap. |
| Edge Coupled Stripline |  |
This style consists of two signal traces. Both signal traces are symmetrical strip lines. There is some coupling between the traces, which refers to two differential pairs routed on the same signal layer. |
Table 1 Cross-sectional View and Description of the Trace Types
Examples
After selecting the trace type on the calculator, you input the required parameters to find the target impedance. In this example, Microstrip is chosen. The known parameters are as follows:
- TRACE WIDTH (w) = 8.693 mil ; TRACE THICKNESS (t) = 0.035mm ; HEIGHT (h) = 0.79mm
- DIELECTRIC CONSTANT (εr) = 4.2 ( Reference Datasheet, Dielectric Constant @1 GHz )
TARGET IMPEDANCE (Zo) = 114.0170 Ω is obtained.
Another benefit of this gadget is that it can solve for “trace width” in reverse. For example, if you need to find a trace width that produces 50 Ω for the reason of impedance matching, you can easily find the most suitable trace type and the PCB requirements.
SMD Code Calculator
When the PCB routing design work is completed and the materials for your BOM are selected, purchasing parts is an important step. Many engineers may continue to use old parts for saving costs; however, part identification by marking is a cumbersome and time-consuming process. Surface marking or silk screen printing is generally used as a method to identify parts. The “Resistor Color Code Calculator” is for the longtime standard axial through hole resistor color code identification. In this article I will introduce two gadgets, namely SMD Capacitor Code Calculator and SMD Resistor Code Calculator for the SMD code identification.
Both calculators offer 3 code formats (3 Digit EIA, 4 Digit EIA and EIA-96/EIA-198). Simply select the code format and then the surface marking/screening numbers or letters on the resistor/ capacitor, or enter the resistor or capacitance value directly below and reverse search for the actual marking. Please ensure to choose the correct unit during the value input. Resistance unit options: Ω, kΩ or MΩ on SMD Resistor Code Calculator and capacitance unit options: mF, µF, nF or pF on SMD Capacitor Code Calculator.
Fig 5: Unit option interface of “Chip Resistor Code Calculator” and “SMD Capacitor Code Calculator”
If you want to a detailed description on the applications of 3-digit EIA, 4-digit EIA and EIA-96 standard, as well as examples to learn from, you can read these two techforum posts - “SMD Resistor Code; Surface Mount Resistor Part Markings” and "Reading a SMD resistor code”.
EIA-198
Due to the trend of component size is getting smaller and smaller, sometimes the surface space can only accommodate two codes. The EIA-198 system of part marking uses two characters (one number and one letter) in which the letter represents the value and number represents the multiplier. However, there are a few caveats:
- This system is case sensitive. You will notice that by using some of the lower-case versions of letters they could eliminate many of the troublesome numbers such as I and O. They too easily get confused for 1 and 0.
- The capacitance codes do not directly mimic the capacitance value like the others. The first digit is used for capacitance.
- Please be aware: while this system is still measured in picofarads, the multiplier code is one more than what we are used to in the other two methods.
For example: the mark is “G4”, the code “G” = 1.8 and “4” = 104, the capacitance value is “1.8” x 104 = 18000 pF or 18 nF.
For a detailed explanation of code numbers and multiplier letters, please refer to Knowles’ EIA-198 Standard Marking Code Table.
Summary
The gadgets introduced above are only a small portion of the DigiKey online tool library. We will introduce different types of converters or calculators to you in future posts. Although the calculators provide a very broad data reference, they cannot consider real application issues, such as the inductance or thermal effects of your heat sink, analog/digital ground management or signal attenuation, and damaged or old resistor and capacitor identification, etc. But in general, if you make good use of these gadgets, they will be a good helper.
For more technical information on PCB design and resistor and capacitor identification, please click on the links below. You are also welcome to leave a message at the end of the articles for discussion.
PCB Milling Tips
Resistance Values of PCBs
Keeping your circuit board safe from moisture and humidity
Circuit Board Trace Repair
The Golden Rule of Board Layout for Switch-Mode Power Supplies
SMT Electrolytic Capacitor with no Voltage Rating
