Printed Circuit Board Isolation Routing System

Project Number: P14311

Printed Circuit Board Isolation Routing System

Abstract

At the Rochester Institute of Technology (RIT), students with needs for printed circuit board technologies are often faced with a series of challenges in creating unique boards. The typical fabrication process includes chemical etching by an external company, which can be time consuming when multiple iterations of a prototype board are needed. Sending designs to be fabricated off-campus can also be very costly. To reduce lead-times and cost, project P14311 has developed an isolation routing system to be used by the Center for Electronics Manufacturing and Assembly on RIT’s campus.

An isolation routing system has the capability to reduce lead-times while allowing students to gain first-hand experience with circuit board fabrication at a more affordable cost. The assembled PCB Isolation Routing System developed by the P14311 team has capabilities to fabricate boards as large as 5” x 5” in area, with tolerances of 0.001” between traces. Many of the system’s processes have been automated through the use of Mach3 software. Finally, debris management and safety systems were developed to manage material debris and protect the user from the machining process, meeting RIT’s Environmental Health and Safety standards for lab equipment.

Nomenclature

Backlash-sometimes calledlashorplay, is clearance or lost motion in a mechanism caused by gaps between the parts. It also can be defined as the maximum distance through which one part of something can be moved without moving a connected part.

Collet- a holding device—specifically, a subtype of chuck—that forms a collar around the object to be held and exerts a strong clamping force on the object when it is tightened, usually by means of a tapered outer collar. It may be used to hold a work piece or a tool.

CNC- Computer Numerical Control

Eagle (Easily Applicable Graphical Layout Editor) - a flexible, expandable and scriptable EDA application with schematic capture editor, PCB layout editor, auto-router, and CAM & BOM tools developed by CadSoft Computer

G-code- G-code is the common name for the most widely used numerical control (NC) programming language, which has many implementations. Used mainly in automation, it is part of computer-aided engineering. G-code is sometimes called G programming language.

Gerber- The Gerber format is an open 2D bi-level vector image file format. It is the de facto standard used by printed circuit board (PCB) industry software to describe the printed circuit board images: copper layers, solder mask, legend, drill holes, etc.

I/O- Input/output

Isolation Routing- a technique that removes copper material to separate or isolate copper fields to serve as wires or traces. In contrast to chemical etching, PCB isolation routing generally removes a minimal amount of copper only through milling and drilling to separate the traces.

Mach3- open source software capable used as a CNC control package to control servo and stepper motors, compatible with numerous types of hardware and customizable

PCB- Printed Circuit Board

Introduction

Printed Circuit Board (PCB) technology is used in many RIT student and faculty projects and applications to mechanically support and electrically connectelectronic componentsusingconductivetraces, pads and other features, which areetchedon copper sheetslaminatedon a layer of non-conductivesubstrate. Typically, users spend time creating a PCB layout design and send these designs to a third party vendor who completes a chemical etching process that creates the desired trace artwork on the board. This process can be costly to the user and the lead-time to receive a completed board can as great as three weeks. This poses a problem especially when the board design is only a prototype, and a new design may need to be purchased to correct any issues with the original PCB.

Project P14311 is a new project, focused on dealing with these issues through the development of a “PCB Isolation Routing System”. Isolation routing systems have been used for many years by electronics hobbyists to produce prototype boards with mixed results in regards to the quality of traces and operator safety. P14311 has expanded on these hobbyist designs to create a PCB CNC mill at industrial standards with a focus on board quality and operator safety.

Process & Methodology

The system had multiple requirements imposed on it by stakeholders. First, it was expected that the system would reliably and accurately locate all board component mounting features, lands, and traces to near etched board quality with ideal tolerances ±0.001”. It was also expected that these features could be etched on boards ranging in dimension from 1" x 1" to 5" x 5”, with thicknesses of up to 0.125”. Operator safety was also specified through RIT’s Environmental Health and Safety guidelines, through requirements such as the removal of hazardous debris from the system, elimination of the potential for injuries through safety systems, and a visual feedback display for system status and errors. Finally, the system was expected to be less costly than commercial systems already on the market, with a project budget of $3,125.

Focusing on these requirements, the team researched, benchmarked, and designed multiple solutions to meet the challenges of the problem. Ultimately, an enclosed product using an off-the-shelf CNC mill and router was designed and proposed. Modifications to this mill implemented to meet requirements included a vacuum table base to hold the PCB in place as well as to collect debris, and the installation of sensors to sense when the stepper motor axes limits were reached. A specialty collet was utilized on the router to ensure the required milling and drilling tolerances. Off-the-shelf software and motor drivers were used to control the system, along with a designed input/output interface board, and electronics were mounted inside secured electronics boxes attached to the enclosure. Finally, the enclosure was outfitted with an interlock and emergency stop button to ensure operator safety during machine operation.

CNC Milling Application

To create PCBs, the system required the physical removal of copper to create circuit traces as well as component holes. A consumer CNC mill, manufactured by Zen Toolworks, was selected as the central component of the PCB Isolation Routing System. The Zen Toolworks CNC DIY Kit is a three axis CNC machine capable of seven inches of travel in the X and Y directions (transverse movement) and two inches of travel in the Z direction (vertical movement). Each axis of the machine uses M8x1.25 steel lead screws in conjunction with NEMA 17 stepper motors, providing 0.000246” of movement for every stepper motor increment. Backlash and friction from the lead screw assemblies is reduced through brass anti-backlash nuts. Steel guide rails and linear bearings support the weight of the cutting table assembly as well as X and Z axis gantry assemblies. The frame of the CNC machine consists of high density PVC that is rigid enough to support high accuracy cutting. Components of the milling machine could be easily reproduced in aluminum or other metal for enhanced rigidity if necessary in future iterations.

Supports for the selected cutter and work platform were designed to interface directly to the CNC machine. By attaching the milling machine frame to 0.30” plate steel, the machine was secured to the enclosure, enhancing the rigidity of the overall assembly. Limit switches mounted to frame components protected machine components by restricting movement of each axis based on actuation of the switch via a magnet. Optical interrupt switches were similarly mounted to frame components to provide homing ability of the X and Y axes.

The Zen Toolworks CNC milling machine provides a reasonable level of accuracy while retaining a relatively low cost. The unit is shipped unassembled with assembly instructions and comprehensive product documentation available on the manufacturer’s Wiki page. Spare parts and other additional components are low priced and readily available from the Zen Toolworks webpage as well as Amazon. A 3D model of the milling assembly was also provided to assist in the design of custom components.

Router and Specialty Collet

TheA Bosch Colt PR20EVSK palm router was selected to provides the rotational power required to cut circuit traces and drill through holes. Lightweight and compact in size makes This router was the Colt an ideal selection for the milling machine.the system as it is lightweight, compact in size, reasonably priced, and provides the accuracy (TIR) required for PCB routing. A Bosch claims a top rotational speed of 30,000 RPM from the Coltis claimed by Bosch, although our testing found this value to be lower at 26,000 RPM. This is, still fast enough to support moderate feed rates, and the speed may increase as the router bearings loosen with continued use. Reasonable priced at about $100 and available at home improvement stores the router provides the power and accuracy required for PCB routing. Mounting of the Colt router is provided byThe router is secured to the CNC machine by aluminum brackets and a mounting plate, which attaches directly to the Z axis. designed to attach directly to the Z axis of the milling machine.

The stock collet provided withpurchased with the router is compatible with cutting bits with a 0.25” ¼” shank, and without published run out values were not provided. In order to accommodate 0⅛” .125” shank cutting and drilling bits more commonly used in PCB cutting, an aftermarket collet was purchased from PreciseBits (Think and Tinker Ltd.).com replaces to replace the stock collet. Specifically designed to reduce the run out of the Bosch Colt to less than 0.0004”, (0.01mm) the specialtythis collet allows secures the smaller 0.125”smaller bits and allows for higher feed rates without increasing cutting bit breakage. Depth rings installed on the cutting and drillingl bits enable the plunging depth to be consistently maintained between bit changes.

Vacuum Table

Holding down work pieces while machining is a simple task that can be done in many different ways. In this case,For this isolation routing system, a the chosen solution was a vacuum table was chosen to hold down the work pieces while machining. This solution was selected because it was the most operator friendly, as it is controlled directly within the Mach3 software.. It also The vacuum table accomplishes basic tasks that could be done other ways but also provides added features in the most operator-friendly way. The added features include eliminates anying vice operations that could potentially bow the copper bare board, ensuring that work pieces stay flat throughout the machining process.

The vacuum table has a seven inch by seven inch work space with a total of 78 holes that provide suction force to the work piece. The vacuum table system is composed of multiple parts including a pegboard sacrificial layer, an aluminum table top and bottom, pipe fittings, flexible tubing, and a shop vacuum. Air is transferred from a four gallon, 4.5 horsepower shop vacuum to the vacuum table through flexible tubing to 3D printed pipe fitting that connects the shop vacuum to the vacuum table. The shop vacuum is also shared with the debris management system, which will be further discussed later.

The vacuum tables ability of the vacuum table to hold the work piece in place is the most critical when drilling holes for the registration system pins as well as anyand through hole componentss. When drilling these holes, the only force holding the board in place is the supplied by the vacuum table. Once these holes areAfter these holes are drilled, the registration pins will beare inserted which willto help hold the board down while also keeping the board aligned during the milling process.

The vacuum table has a 7” x 7” working space with a total of 49 holes that provide suction to the work pieces. The vacuum table system is comprised of many parts, including: a sacrificial layer, table top, table bottom, pipe fittings, flexible tubing, and a vacuum. The sacrificial layer is made of nylon while the table top and bottom and made from aluminum. The air is transferred from the vacuum to the vacuum table through transparent flexible tubing. The vacuum being used is a 4 gallon, 4.5 horsepower Shop-Vac. The vacuum is also being shared with the debris management system which will be discussed later.

Registration System

The registration system was integrated into the design of the vacuum table through a 0.125” through-hole grid, with each hole of ⅛” through holes 1”.5” apart. As mentioned above, these holes are primarily used to allow a vacuum to hold down the piece of copper when drilling. Secondly, dDowel pins with a 0.125” diameter of the same size can be inserted into these holes to be used as registration pins. Dowel pin clearance holes based on the grid pattern must initially be incorporated into the design file for the PCB, preferably exterior to the actual design. While using the vacuum table to secure the copper bare board to the PCB Isolation Routing Systemmilling assembly, all through-holes in the PCB design, including the dowel pin clearance holes will be drilled into the copper board prior to milling. Once the drilling process is complete, registration pins can be inserted into the PCB to provide resistance to shear force while milling and assist in, and allow for zeroing when flipping over athe bottom side of a two sided board. Because the registration pins are not flush with the surface of the board, it is necessary to either place the registration pins outside of theexterior to the PCB design,, or the operator must stipulate that the Mach3 code for the design file does not move the Z axis independently. If Mach3 drives the X, Y, and Z axis simultaneously, the movement could cause a collision with a registration pin, which could potentially break cutting bits.

Debris Management

When this project was created, one of the mostAnother important customer requirement included the protection of the operator from machining debris, through the collection and removal of waste.s was for the machine to be able to discard of the debris being created when milling the copper bare board. In many other similar hobbyist systemsdesigns, there is no systet a system m to manage the debris be created and it causes a huge mess in the area surrounding the machineas it is not a priority. The PCB Isolation Routing System will be housed on RIT’s campus in the CEMA lab. This lab is required to maintain a clean work environment. In order for this machine to be used in the RIT CEMA lab, it was required to have a system that can remove a majority of the debris being createdcreated as the lab is required to maintain a clean work environment. This is done through the debris management system.