CpE 307: PCB Design Process, Issues, and Etching PDF

COURSE/SUBJECT: CpE 307 MODULE TITLE: Computer Engineering Drafting & Design, lab TOPIC VI: PCB Design Process, Issues and Etching INSTRUCTIONS: 1.) Open the websites. Explore. 2.) Read and go through this material. 3.) In light of this pandemic, we are driven to introduce this new pla...

COURSE/SUBJECT: CpE 307 MODULE TITLE: Computer Engineering Drafting & Design, lab TOPIC VI: PCB Design Process, Issues and Etching INSTRUCTIONS: 1.) Open the websites. Explore. 2.) Read and go through this material. 3.) In light of this pandemic, we are driven to introduce this new platform c. As an aspiring engineer/programmer, you must be adaptable to change. Engineers/programmer, after all, is catalysts of change. Adjusting to this new mode of learning will be hard, but this is better than not being able to learn anything at all. 4.) You may contact me via email, Facebook, or phone message if you have any queries. 5.) Have fun learning! TOPIC Objectives V. VI. PCB Design Process, Issues 1. Define Printed circuit board (PCB) and Etching 2. Explain the designing process of a PCB 3. Describe the parts of PCB 4. Explain how a PCB Manufactured 5. Identify the common issues in designing a PCB Laboratory Activity No. 6: Making a 6. Explain how to etch a PCB PCB Objective: 1. Create your own PCB using the design schematic diagram from layout simulation tool STUDY Wiring and Cabling Diagrams Electronic Packaging WEB SURFING: What are PCBs? || How do PCBs Work?- https://www.youtube.com/watch?v=Z2LgmIGE2nI PCB Basics - https://learn.sparkfun.com/tutorials/pcb-basics/all Inside a Small Chinese Electronics Factory - From the Archives - https://www.youtube.com/watch?v=HF0E8WeEUVM Open textbook or ebook Printed Circuits Handbook, Sixth Edition and Seventh Edition by Clyde F. Coombs Jr. and Happy T. Holden for reference and explore INTRODUCTION What is a PCB? Those small green boards are what helps makes the electronic device function. Without them, the device would not work. The PCB connects all of the other components inside, so you can use your electronic device for what it is intended to do. Although they are small, the manufacturing process of a PCB is quite extensive. Whether you are making one yourself or going through a PCB manufacturer, multiple steps are crucial to the development of the board. Because each step is so critical to the process, let’s take a close look at the manufacturing process of a PCB. Printed circuit boards are typically made with copper. Depending on the requirements, the copper is plated to a substrate and carved away to expose the design of the board. Since there are multiple layers, they must be lined up and bonded together for a secure fit. PCB We typically learn about, analyze, and design electrical or electronic circuits using a diagram, called a schematic that consists of component symbols connected by lines. The symbols represent everything from basic passive components such as resistors or capacitors to sophisticated integrated circuits such as microcontrollers, and the lines represent conductive pathways that allow electrical current to flow freely from one portion of the circuit to another. One thing that all schematics have in common is the utter inability to drive a motor, or blink an LED, or filter out noise, or do any of the other useful and interesting things that we expect electrical systems to do. A schematic is, after all, just a drawing. To actually accomplish something with a circuit, we need to translate its schematic into physical components and physical interconnections. Simple schematics can often be realized on a breadboard, but the vast majority of circuit designs enter the physical realm in the form of a printed circuit board, or PCB for short. The Structure of a PCB A very basic printed circuit board is a flat, rigid, insulating material that has thin conductive structures adhering to one side. These conductive structures create geometric patterns consisting of, for example, rectangles, circles, and squares. Long, thin rectangles function as interconnections (i.e., the equivalent of wires), and various shapes function as connection points for components. A printed circuit board such as the example in the image has only one conductive layer. A single-layer PCB is very restrictive; the circuit realization will not make efficient use of available area, and the designer may have difficulty creating the necessary interconnections. Incorporating additional conductive layers makes the PCB more compact and easier to design. A two-layer board is a major improvement over a single-layer board, and most applications benefit from having at least four layers. A four- layer board consists of the top layer, the bottom layer, and two internal layers. (“Top” and “bottom” may not seem like typical scientific terminology, but they are nonetheless the official designations in the world of PCB design and fabrication.) PCB Stackup The stackup is the arrangement of conductive and insulating layers in a multilayer PCB. The following side-view diagram shows the stackup of a four-layer board. The conductive material of choice is copper. Prepreg is an insulating material that is pre-impregnated (hence the name) with resin, and the core (also insulating) is similar in composition to the prepreg. I recommend that you use a four-layer structure whenever possible. A four-layer board allows you to devote one internal layer to the reference potential (i.e., ground) and another internal layer to power-supply voltages. The top, and if necessary the bottom, will be a component layer. This arrangement facilitates PCB design and also helps you to achieve improved circuit performance. Understanding PCB Features and Terminology There’s quite a bit of specialized vocabulary that arises in discussions of printed circuit boards. This section describes physical structures found on PCBs and gives you the words that we use to identify them.  A conductive interconnection is called a trace, and connection points for components are called pads (for pins that rest on the surface of the board) and through-holes (for pins that are inserted into holes drilled in the board). Basic PCB design consists of arranging pads and through-holes so that components can be properly installed, and then connecting these pads and through-holes using traces.  Not all drilled holes are for through-hole components. We often need to transfer a signal or supply voltage from one PCB layer to another, and this is accomplished using small, conductive holes called vias.  Many PCBs also include mounting holes, which have a mechanical rather than an electrical function and therefore don’t need to be plated. The term “plating” in this context refers to conductive material that has been deposited onto the interior of a drilled hole.  A copper pour is a relatively large section of a PCB layer that is filled with conductive material. Copper pours can be used to provide a very low-resistance or low-inductance connection between components and to improve thermal performance.  A PCB layer that consists entirely of one large copper pour is called a plane layer. We frequently use an internal layer as a ground plane and create ground connections by placing vias next to component pins.  A through-hole or via begins as a circle of copper and then becomes a hole when a drill bit passes through the circle (ideally through the center of the circle). The term annular ring refers to the width of copper that remains after the hole has been drilled.   Printed circuit boards include a variety of “supplemental” information that has no role in the electrical functionality of the device. For example, reference designators uniquely identify components, dots indicate proper component orientation, and project titles or serial numbers help us to keep track of the many circuit boards that accumulate in a lab. We refer to this information as the silkscreen. PCB Design Process Step 1 – The Design Before you begin manufacturing the PCB, you need to have a design of the board. These blueprints will be what you base the process off of. The design process is generally completed through computer software. Using a trace width calculator will help with a majority of the details needed for inner and external layers. Step 2 – Printing the Design A special printer called a plotted printer is used to print the design of the PCB. It produces a film that shows the details and layers of the board. When printed, there will be two ink colors used on the inside layer of the board: Clear Ink to show the non-conductive areas; and Black Ink to show the conductive copper traces and circuits. The same colors are used for the outer layers, but the meaning of them is reversed. Step 3 – Creating the Substrate Now is when the PCB will start to form. The substrate, which is the insulating material (epoxy resin and glass fiber) that holds the components on the structure, begins forming by passing the materials through an oven to be semicured. Copper is pre-bonded to both sides of the layer and then etched away to show the design from the printed films. Step 4 – Printing the Inner Layers The design is printed to a laminate, the body of the structure. A photo-sensitive film made from photo-reactive chemicals that will harden when exposed to ultraviolet light (the resist) covers the structure.. This will help align the blueprints and the actual print of the board. Holes are drilled into the PCB to help with the alignment process. Step 5 – Ultraviolet Light Once aligned, the resist and laminate go under ultraviolet lights to harden the photoresist. The light reveals the pathways of copper. The black ink from before prevents hardening in areas that will be removed later on. The board is then washed in an alkaline solution to remove the excess photoresist. Step 6 – Removing Unwanted Copper Now, it is time to remove any unwanted copper that remained on the board. A chemical solution, similar to the alkaline solution, eats away at the unwanted copper. The hardened photoresist remains intact. Step 7 – Inspection The newly-cleaned layers will need to be inspected for alignment. The holes drilled earlier help align the inner and outer layers. An optical punch machine drills a pin through the holes to keep the layers lined up. After the optical punch, another machine will inspect the board to ensure there are no defects. From here on out, you will not be able to correct any missed errors. Step 8 – Laminating the Layers Now, you will see the board take shape as the layers are fused together. Metal clamps hold the layers together as the laminating process begins. A prepreg (epoxy resin) layer goes on the alignment basin. Then, a layer of substrate goes over the prepreg followed by a copper foil layer and more prepreg resin. Lastly, there is on more copper layer applied, which is the press plate. Step 9 – Pressing the Layers A mechanical press is then used to press the layers together. Pins are punch through the layers to keep them properly aligned and secured, these pins can be removed depending on the technology. If correct, the PCB will go to the laminating press, which applies heat and pressure to the layers. The epoxy melts inside of the prepreg that, along with the pressure, fuses the layers together. Step 10 – Drilling Holes are drilled into the layers by a computer-guided drill to expose the substrate and inner panels. Any remaining copper after this step is removed. Step 11 – Plating The board is now ready to be plated. A chemical solution fuses all of the layers together. The board is then thoroughly cleaned by another series of chemicals. These chemicals also coat the panel with a thin copper layer, which will seep into the drilled holes. Step 12 – Outer Layer Imaging Next, a layer of photoresist, similar to Step 3, is applied to the outside layer before being sent for imaging. Ultraviolet light hardens the photoresist. Any undesired photoresist is removed. Step 13 – Plating Just like in Step 11, the panel is plated with a thin copper layer. After this, a thin tin guard is layered to the board. The tin is there to protect the copper of the outside layer from being etched off. Step 14 – Etching The same chemical solution from before removes any unwanted copper under the resist layer. The tin guard layer protects the needed copper. This step established the PCB’s connections. Step 15 – Solder Mask Application All of the panels should be cleaned before the solder mask is applied. An epoxy is applied with the solder mask film. The solder mask applies the green color you typically see on a PCB. Any unwanted solder mask is removed with ultraviolet light, while the wanted solder mask is baked on to the board. Step 16 – Silkscreening Silkscreening is a vital step since this process is what prints critical information onto the board. Once applied, the PCB passes through one last coating and curing process. Step 17 – Surface Finish The PCB is plated with either a solderable finish, depending on the requirements, which will increase the quality/bond of the solder. Step 18 – Testing Before the PCB is considered complete, a technician will perform an electrical test on the board. This will confirm the PCB functions and follows the original blueprint designs. OBJECTIVE OF THE PCB DESIGN PROCESS The objective of the PCB design process is to engineer a PCB, including all of its active circuits, that functions properly over all the normal variation in component values, component speeds, materials tolerances, temperature ranges, power supply voltage ranges, and manufacturing tolerances and to produce all of the documentation and data needed to fabricate, assemble, test, and troubleshoot the bare PCB and the PCB assembly. Doing less than this in any area exposes the manufacturer and user of the PCB assembly to excessive yield losses, excessively high manufacturing costs, and unstable performance. Achieving the objective involves carefully designing a process that matches the end product, selecting design tools with controls and analytical utilities, and selecting a materials system and components that match. DESIGN PROCESSES Figure 14.1 is a flowchart of the major steps in a complete PCB design process, beginning with specification of the desired end product and continuing through to archiving or storing away the design database in a form that permits subsequent design modifications or regeneration of documentation as necessary to support ongoing production. This process takes advantage of all the computer-based tools that have been developed to assure a “right the first time” design. The basic process is the same for either analog or digital PCBs. The differences in the design process for the two classes of PCBs center around the differences in complexity of these two types of circuits Commonly Seen PCB Design Issues The most basic form of design for manufacture as it applies to PCBs is the use design rules and design rule checking in PCB design software. Design rule checking (DRC) is the process of looking at a design to see if it conforms to the manufacturing capabilities of a PCB fabricator. Typically the designer will get the highest tolerances that a PCB fabricator supports from the fabricator, load these tolerances into their design program and then run a design rule test on their prospective design. Design rule checks are commonly integrated into PCB design software and are not typically considered as add on service. More advanced design for manufacture analysis software is also available to look for more complex and less obvious design flaws. Typically, DFM software checking is offered by PCB fabricators to customers as an extra service. The reason for this distinction is because of the plus cost of high end DFM software and the additional training required for using it. 1. Starved thermals Starved thermals occur when the thermal relief traces connected to a pad are not properly connected to the associated copper plane. Quite often, the spacing between vias will pass a basic design rule check, but the attached thermal relief traces will be interrupted and the effected vias will be inappropriately isolated from their assigned copper pours. This issue is most commonly seen when multiple vias are placed in proximity to each other. 2. Acid traps When two traces are joined at a highly acute angle it is possible that the etching solution used to remove copper from the blank board will get "trapped" at these junctions. This trap is commonly referred to as an acid trap. Acid traps can cause traces to become disconnected from their assigned nets and leave these traces open circuited. The issue of Acid traps has been reduced in recent years by fabricators switching to the use of photo activated etching solutions. So, while it is still a good idea to make sure that your traces do not meet acute angles, the issue is less of a worry than it had been in the past. 3. Silvers If very small portions of a copper pour are only connected to larger portions of the same copper pour through a narrow trace, it is possible for them to break off during fabrication, "float" to other parts of the board and cause unintended shorts. The problems presented by silvers have been reduced in recent years by fabricators switching to the use of photo activated etching solutions. So while silvers are still to be avoided in designs, they are not as predominate of an issue as in the past. 4. Insufficient annular ring Vias are made by drilling through pads on either side of a board and plating the walls of these holes to connect the two sides of the board. If the pad size called out in the design is too small, the via may fail due to the drill hole taking up too large of a portion of the pads. Minimum annular ring size is commonly part of the DRC process. This issue is mentioned here because of the not uncommon occurrence of missed drill hits in prototyping boards. 5. Via in Pads Occasionally it may be convenient to design via to be positioned within a PCB pad. However, via in pads can cause issues when the time comes for the board to be assembled. Via will draw solder away from the pad and cause the component associated with the pad to be improperly mounted. The image below shows difference between via in pad PCB and normal PCB. 6. Copper too close to board edge Normally caught during design rule checks, placing copper layers too close to the edge of a board can cause those layers to short together when the board is cut to size during the fabrication process. While this sort of error should be caught using DRC features typically available in PCB design software, a PCB fabricator that does a DFM check will also catch this issue. 7. Missing solder mask between pads In very tightly spaced, small pin pitch devices, it is quite common for there to be no solder mask between pins due to standard design settings. The omission of said solder mask can lead to solder bridges forming more easily when the fine pin pitched component is attached to the PCB during assembly. The image below shows high precise solder mask between 0.4 pitch QFN pads. 8. Tombstoning When small passive surface mount components are soldered to a PCB assembly using a reflow process, it is common for them to lift up on one end and "tomb stone". Tombstoning can greatly affect PCB yields and quickly drive up production costs. The source of tombstoning can be incorrect landing patters and imbalanced thermal relief to the pads of the device. Tombstoning can be effectively mitigated by the use of DFM checks. Below image is a tombstoning sample and its schematic. ETCHING Open textbook or ebook Printed Circuits Handbook, Sixth Edition and Seventh Edition by Clyde F. Coombs Jr. and Happy T. Holden for reference and explore the other chemical used in etching Activity 6 The following questions have been designed to test the objectives identified for this module: 1. What is the acronym of PCB and define. 2. What is the objective of PCB design process? 3. Give 3 chemicals used in etching 4. How will you determine PCB size of your circuit? 5. What is PCB typically made for and why? Laboratory Activity No. 6: Making a PCB Objective: Create your own PCB using the design schematic diagram from layout simulation tool. Description: Create your own PCB using the steps on PCB design process and etch your PCB. After you etch and drill, put the components used in schematic diagram and connect all components by soldering and prove that your circuit is working A. RESULT: Input the images on how you create your own PCB I. SCHEMATIC DIAGRAM: II. PCB DESIGN: III. ETCHING PROCESS AND DRILLING IV. SOLDERING: V. FINISH PCB PRODUCT WITH COMPONENTS: B. OBSERVATION AND ANALYSIS: C. CONCLUSION: REFERENCES: Coombs, Clyde F. Jr., Holden, Happy T. 2016. Printed Circuits Handbook. Seventh Edition. McGraw-Hill Companies. Coombs, Clyde F. Jr. 2008. Printed Circuits Handbook. Sixth Edition. McGraw-Hill Companies. Keim, Robert. 2020. What is Printed Circuit Board (PCB)?. https://www.allaboutcircuits.com/technical-articles/what-is- a-printed-circuit-board-pcb/ PCB Manufacturing Process. 2018. https://www.candorind.com/pcb-manufacturing-process/