PCB Layout with Amarula Electronics

Why choose Amarula Electronics for your PCB layout?

Amarula Electronics engineers have a combined experience of 25+ years in the Electronics and PCB design Industry. Over this time we have gathered data from previous designs and generated our own internal Design Checklist which our Engineers follow each and every time. You not only receive a professional PCB design for your product, but your company can also benefit and take advantage of our vast amount of experience and reduce outgoing costs by getting the design right in short timescales.

What is PCB Layout?

Printed Circuit Boards (PCB’s) are manufactured from single or multiple layers of etched copper. Each layer of the PCB Layout is isolated from the next using a variety of materials, each of which has certain properties which is selected depending upon the requirements of the product.

Some of these properties we consider would include:

  • Electrical properties
  • Thermal properties
  • Mechanical properties

PCB Layout usually begins with careful consideration into the mechanical housing that the Printed Circuit Board needs to be assembled within. Based on the Schematic Diagram, a board shape and PCB land area can be estimated. The size and shape constraints will then provide a good indication as to the following key areas which can all affect final PCB cost:

  • Number Of Copper Layers required to route the traces efficiently?
  • Materials used to fabricate the PCB and their dielectric properties?
  • Via drill size requirements?
  • Do we need to place components on both sides of the PCB?
  • Do we need to use Controlled Impedance and/or Differential Pair techniques?
  • Are there any Slots/Tooling holes required?
  • Are there any BGA components?
  • Do we need to use any special techniques? (Via-In-Pad etc…)
  • Do any components have high speed busses, requiring trace length matching?

Timescales:

Using the above as a formula, we can then better estimate timescales based on complexity. We now generate a PCB Stackup, which holds key information such as number of layers, thickness of copper and isolation materials. Answers to the above also generate our Design Rules which are fed into the EDA software package. (Also known as DRC)
A typical 6 – layer Stackup with Blind and Buried Vias would look something like the following:

Each Layer (Either Copper, or Dielectric/Pre-Preg Substrates will have properties that need to be set according to the Design Rules):

layers-PCB-layout

Planning and Layout:

When we have generated a suitable PCB board outline, we can begin to place any critical components, while paying close attention to any requirements such as interconnect components to the outside of the mechanical housing. Having placed critical components, the remaining passive components are placed, again following any rules suitable to the circuit they are relevant to, such as decoupling capacitors that should be placed as close as possible to certain supply pins on Integrated Circuits.
There are many other important rules to consider when planning the PCB Layout such as:

Copper Layers:

The number of copper layers is a very important first step to allow enough space for routing the traces between all of the components. Also, some designs require Ground Planes under certain traces in order to effectively create and calculate controlled impedance for the traces on an adjacent layer. The copper thickness of the layers also has an effect on the impedance properties of the trace in question.

PCB Layout and High Speed requirements:

High speed signals require careful attention for controlled impedance (Mostly used on Antenna Feed lines, USB Data Buses, and other Address/Data buses), Differential Pairs (Pairs of traces that are comprised of signals where the difference in voltage levels create the digital value sent and received), Length matching (Ensuring that each signal on a particular data or address bus reaches the receiving node at the same time – typically used on devices such as DDR Memory).

High Current requirements:

Some paths on the PCB will carry High current depending on the application of the circuit. These paths or traces need careful consideration to ensure that the amount of copper used to create the trace is enough to carry the current that will be passed through it. There are many PCB Trace width calculators available online that can assist with this calculation.

Routing the Board:

Having placed all components on the board, the Netlist generated by the schematic design now allows us to route all of the individual traces.
An image showing the rats nest (Blue Lines) which is generated by the schematic design and shows us which pins need to be connected in the PCB Design:

pins-PCB-design

This includes adding items such as copper shapes for areas that require Ground Planes or increased current capability.
Some PCB designs will require additional attention to other key areas such as:

  • Via stitching for High Frequency – This can be on the sides of the board (For EMC reasons) or in specific areas on the board to reduce cross-talk and emissions. Also, vias stitching is an effect method to sink currents and/or heat from a small area or component pin situated on an outer copper layer, onto a larger internal layer of copper.
  • Thermal Balancing of the copper layers
  • Separate Analog and Digital Ground planes

Depending on the complexity of the PCB, for some designs we will utilise and EDA tool known as an Auto Router. The Auto Router requires user input such that rules can be set on how to go about routing the PCB traces such as which layers to use and which directions are allowed, where to place vias and how traces are allowed to leave a component pin.

Design Rule Check (DRC):

The Design Rule Check is possibly one of the most important steps in PCB design. During this phase, all of the rules previously generated from the requirements can be checked to ensure that the design is optimal. These days, most EDA software tools contain Online DRC functions that will flag an error whilst your are placing components and routing the board.
Example of an online DRC Error, where a via is too close to a trace (According to the spacing constraints already setup:

Similarly to the Auto Router tool, the Rules are required to be set up initially according to the design requirements and also the capabilities of the PCB Fabricator who will be engaged to manufacture the Printed Circuit Boards.

Some of the Design Rules necessary can be seen in the following image, although not all are used for every design:

pcb-layout-example

Preparing the Files for Manufacture:

When the PCB design is complete, we need to generate the PCB fabrication data (Known as Gerber Files). The Gerber format is an open ASCII vector format and is the standard format used for PCB data. These files contain 2D artwork for each of the copper layers, board outline, Silkscreen, Solder-mask etc…