Design and Construction of Custom Pcbs Within the Cadsoft Eagle Environment

Design and Construction of Custom PCBs within the CadSoft Eagle Environment

School of Computer and Information Science

Research Placement Report: Study Period 5, 2009

13 October, 2010

James Walsh

Supervisor: Dr Grant Wigley

Abstract

The outcome of the placement is the selection, design, construction, assembly and testing of a moderately simple, single channel, audio amplifier circuit board.The documented processes following the circuit’s construction will serve as a guide for the steps required to understand the development of custom printed circuit boards within theUniversity’s environment.

Contents

Abstract

Introduction

Background

Methodology

Learning to use eagle

Project selection

Board design

Schematic Design

Component Layout

Board milling

Component soldering

Circuit testing

Review

Bibliography

Introduction

With the ever advancing complexity of even the most basic electronic devices within the research realm, the custom design and assembly of printed circuit boards has become a crucial component in the proof of concept and rapid prototyping capabilities within a university lab environment. The development of rapid prototyping tools, such as the T-Tech Quick Circuit Prototyping System[1], has given researchers the ability to go from concept to construction within a few hours. This has lead to the adoption of PCB design software, such as CadSoft Eagle[2], that provide the facility for schematic design, component layout and then generation of machining files within the one environment. The use of both Eagle and the T-Tech machinewithin the University of South Australia’s Wearable Computing Lab[3], provide the facilities for researchers to develop all required boards, of moderate complexity, internally.

Following this, the outcome of the project will be the selection, design, construction, assembly and testing of a 50W, single channel audio amplifier, based on a ‘kit’available from Jaycar Electronics[4].This report covers the processes and proceduresfollowed to develop the moderately simple circuit from scratch, using Eagle for the PCB schematics and design followed by the use of the T-Tech machine to mill the physical board. The report also covers the final assembly and testing of the board, followed by a review of the project’s success.

Background

The ‘real world’ electronic nature of the Wearable Computer (WCL) and other laboratories at the universitycreate the requirement for the ability to rapidly design, test and developmoderately complex circuit boards within the University. The adoption of the T-Tech system, in conjunction with the use of Eagle, allows researchers to develop the required boards.With numerous WCL projects requiring the use of custom circuitry, either as a primary or secondary goal to the project, the ability for lab members, both staff and students alike, toposes the skills required for theirdevelopment is of great importance to continued student involvement and research project development.

Even without direct involvement in any one project involving custom circuitry, the joint nature of the various labsmeans all members need to beat least be familiar with the schematic and layout design processesthat are involved in that project. The introduction of a new electronics development course, scheduled for introduction in 2010 as an elective, will introduce students to the processes required for circuit manufacturing and assembly. The existence of this course demonstrates the requirement for students(the University’sfuture researchers) to obtain the necessary skills required for involvement in many of the various labs’ projects.As such, the aim of this placement was to obtain the same skills being taught in the course, expanding the horizons for involvement in future lab projects.

Methodology

The processes required for board development follow from the schematic design, component layout and track routing, milling and soldering of the board,followed by testing of functionality. The logical steps in learning any new skill revolve around the concept of working towards a basic goal, and then increasing the complexity once that goal has been obtained. Therefore, many of steps involved in this process revolved around learning how to perform simple steps correctly, so that in future, these skills would serve as a basis for more complex work.

Prior to any actual work or involvement in the workshop, one must read, understand and acknowledge the University’s Occupation Health and Safety requirements for those involved with the relevant machinery. This serves to ensure all staff and students can work confidently in a safe and productive environment.

Learning to use eagle

CadSoft’s Eagle Light Edition is available as a free download and is free to use for personal and student projects (supporting a maximum board size of 100x80mm with only two signal layers). The University of South Australia’s wiki(VonItzstein 2009)hosts a very basic outline of the steps required to develop PCBs using Eagle in conjunction with the university’s T-Tech prototyping machine. This served as first step in understanding the required processes whilst also learning some key definitions for users new to both Eagle and electronics in general.

The Eagle tutorial, providedwith the software’s default installation, served as a basic guide in becoming familiar with the Eagle environment and itsbasic functionality. Moving through each set of features one by one, the tutorial covers all the functions required to design and test a board from scratch.The approach by the tutorial writers, in covering the features one by one, despite offering thorough coverage of each feature, lacks the ‘glue’ required to link the featurestogether in real world use. The tutorial would have benefited deeply from taking a project approach, where the user is taken through the steps required to build a test board, with each step following its predecessor, with more advanced test projects used to explain the more advanced features. However, the isolated step approach chosen does also serve a purpose as a reference manual for each feature. After the completion of the tutorial, a relevant first project was selected.

Project selection

Given the aim of activity was to learn about and become familiaroperating within the Eagle environment;a pre-designed ‘ready-made kit’ project was selected. The use of a kit project ensured the focus of the activity remained on the development process itself, not the design and development of an electrical device.Jaycar’s focus on home level enthusiastelectronics results in it carrying a number of kits, ranging from those completely new to electronics, to those experienced and looking for a challenge.The project select was a 50 Watt Amplifier Module kit[5] (featured inSilicon Chip magazine March 1994). This kit was selected for numerous reasons. Aside from being moderately simple, it provided all the components required along with an already made, two layer PCB that could be used as a basis for the new board’s design. The kit also ensured we had a working electronic circuit to work from.

Board design

Schematic Design

Prior to the actual layout and design of components on the PCB, a schematic of the components must be entered into Eagle. This provides two functions. Aside from being able to prompt the user with which connections are still remaining to be routed\laid on the board, Eagle can more importantly ensurethe circuit design is logically correct, meaning no crossed signals. Primarily this just involves selecting the components (microprocessors, capacitors, resistors, etc.), playing them in a logical arrangement in the schematic and then connecting wires between the required pins and busses.An issue arouse where the National Semiconductor LM3876T chip was not included in the Eagle libraries (either installed or online). Searches online resulted in the use of a third party library[6]containing the chip.The chosen kit, despite providing the board design, did not provide a schematic for the circuit. As such, the board had to be reverse engineered to create the original schematic from its final product. This was done by using the manufacturer’sdesign specification for the LM3876T[7]. Once the circuit schematic has been entered, Eagle now has enough information design the physical board layout.

Component Layout

The component layout in Eagle exists of positioning the components that were entered in the schematic.Placing the components in a similar fashion to the original Jaycar board, the auto router was used to recommend routing placements for the connections.To simplify track routing, the router had placed tracks on both sides of the board. Given the nature of the project, to keep it simple, manual tracks were instead laid in similar positioning to those on the reference board. An issue occurred for the input, output and power pins of the amplifier, where single pins pads (not attached to any device) were required. This lead to the creation of a custom library containing only one part, a single via pad, with which tracks could be connected.

Board milling

The CAM process to actually mill out the circuit onthe copper-covered, fibreglassboard was done using the T-Tech Quick Circuit Prototyping Systemwith the assistance and direction of Dr Robert Kong. The move from the EAGLE board files to the T-Tech platform required the generation of three purpose specific files. These files contained the machining information for the circuit border and dimensions, how to mill out the tracks and pads used for connecting components andfinally the drill layer used for creating mounting holes.

Due to the variable nature of materials used, the milling of the tracks was primarily a trial and error process.A small amount of oil was rubbed across the board to ensure the foot of the T-Tech drill could easily glide over the surface. To obtain an even depth around all the tracks, numerous passes,at various depths by the drill bit, were required to successfully drill the tracks.The drilling of the mounting holes was also a trial and error process, with two drill bits breaking due the drill being set to a lower than required RPM setting. Following the final cut out of the PCB from the rest of the sheet, the board was cleaned with a small amount of dishwashing detergent. This helped to remove any lubricant remaining on the board, which would prevent the solder from fluxing.

Component soldering

The soldering of components on the board was a fairly direct process.The soldering of certain pads was problematic due to the creation of pads that were too small for use by a beginner. Also, when placing the same components in Eagle,the default hole size created forthe chips and fuses were not large enough to support the actual pins of the physical components.To remedy this, small pins and files were used to manually increase the pin diameter.

Circuit testing

The unusual power supply required for the circuit (35V), meant the use of the transformer in the WCLworkshop. Following the original kit instructions, the assembled board was tested by first removing one of the two 2A fuses, and using a multimeter in the amps range to measure the current across where the fuse was positioned. Expecting areading between 30mA and 70mA, the device fell well within the accepted range.After removing the ear-buds from a set of ear phones and attaching the bare wires to the circuit inputs, the standard 3.5mm end of the cable was plugged into a generic MP3 player. Again using the multimeter, we observed that the output from the board varied as expected withthe audio.

Review

All goals that were set for this project initially have been achieved. The development of the PCB within Eagle followed by the physical milling of the board using the T-Tech machine and finally the soldering and testing has been successful.After utilizing various resources to become familiar with the Eagle environment, the move to developing a moderately simple board was minor.With the assistance of Dr. Robert Kong, the milling process was successful, creating a board with identical circuitry to the provided kit board.Following the soldering of the board, difficulties caused by pads that were too small for an amateur to solder followed by the incorrect sizing of component holeswould be rectified in the next project, to ensure the defaults are updated to adequate values.

The skills involved in this project have wide ranging applications, especially to a multitude of projects within the WCL and other university laboratories, including those participated in by students as part of an honours thesis.

Bibliography

VonItzstein, DS 2009, Circuit Board Prototyping, updated 24 April 2009, University of South Australia, viewed 16 October 2009, <

[1] T-Tech Incorporated,

[2] EAGLE Layout Editor, CadSoft Computer GmbH,

[3] University of South Australia’s Wearable Computing Libratory,

[4]Jaycar Electronics,

[5]Jaycar 50 Watt Amplifier Module Kit,

[6]LM3876 Eagle Library,

[7] National SemiconductorsLM3876 Specifications,