Commit 015076f4 by Ashish Sawhney

week5

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# 5. Electronics production
# 5. Electronics Production
## **assignment for the week**
1. Characterize the design rules <s>specifications</s> of your PCB production process
2. Make an in-circuit programmer by milling the PCB (/make a PCB by milling) (program it, so that you can use it to program your board in Electronics Design week, and in other weeks)
3. *Optionally, trying other processes (of making PCBs)*
## **learning outcomes**
1. Describe the process of milling, stuffing, programming, and de-bugging
2. Demonstrate correct workflows and identify areas for improvement, if required
## **have i**
1. Shown how I made and programmed the board
2. Explained any problems and how I fixed those
3. Included a 'hero shot' of my board
## **characterizing the design rules <s>specifications</s> of our PCB production process**
This week I will be using a pre-designed circuit to mill a circuit board, I will not be designing one. Hence, I will leave out that portion of the PCB production workflow. This week, the workflow will be from the point when you have 2 image files, one for traces and pads and another for outline, and you input those, one at a time, in to fabmodules.org and generate toolpaths for milling. In the Electronics Design week, I will re-draw another pre-designed circuit in Autodesk EAGLE and that is where I will talk about CAD for drawing and/or designing circuits, exporting those circuits as image (.png) files, and further processing those images in an image editing program (optional). I will then integrate steps from this week to present the complete PCB production workflow.
Machining or milling PCBs, as explained by Neil, is a multi-step process as follows: To machine a board (Figure 5.0 A), you first have to attach it to the bed (Figure 5.0 B), and that is done in a surprisingly crude way (at least on these little precision milling machines). We use 2-sided tape (Figure 5.0 C), and the reason we do that is if you clamp the board, it bows a little bit, and the flexing and the tension can break the tool. By using a tape, we hold it from below, and that keeps the board nice and flat, and keeps it relaxed. Mounting a bare board on to the sacrificial layer of the milling machine is called **fixturing** (Figure 5.0 D). Here, let me grab an example ... so here is an example of the boards you will be making, and you can see the fine features and the traces that you will be cutting out. So the board has to be nice and flat to make those fine features that we need. An important precaution while fixturing is to avoid any air bubbles between the tape and the board, and between the tape and the underlay. I started out with a single-sided blank FR-1 board, 4 in * 6 in (Figure 5.0 A). Technically, these are bare single-sided copper-clad laminates in paper phenolic material. Under your board, there is an **underlay** (Figure 5.0 E), there is a sacrificial material, and that is so that when you cut through your board, you don't cut in to the machine, you cut in to that sacrificial material. Those materials have a lifetime. Once the sacrificial gets old, it doesn't hold down your board very well, and you need to replace it. So depending on how busy your lab is, maybe once a month you put in a new sacrificial layer. When you do that everything needs to be clean and smooth so that the layer lays flat, and so if you have an old sacrificial layer on a dirty machine, your board will move around, and again that will break the tool, and your board will not have a good finish. If you have a fresh sacrificial layer on a clean machine, everything will be smooth (**orientation**), and it will cut nicely. Then, you need to set the tool right on top of your board. We are only going to cut a few thousandths of an inch deep, and so **zeroing** (Figure 5.0 F) is where you set the machine (actually the end mill) right on top of your stock/board. All the steps I am describing are shown in this nice [series of videos from Charles Fracchia](https://www.youtube.com/playlist?list=PL-xEsC0ZUCUM42QNHaOOdoOwYg0j251dU) (Neil's student). The way we zero it is you lower the tool to the surface, it helps to tap it a little bit just to make sure its in good contact, and then you tighten the **set screw**. And we have to get that very close to within a few thousandths of an inch. Now, one thing that can go wrong is if you don't hold the tool while you tighten it, it can move a little bit. So you hold it with one hand and tighten it. Another thing that can go wrong is when you lower the tool, if you drop it, it can actually shatter. These are hard but brittle so you lower it gently. Then the set screw, this is a common beginner mistake, there is a set screw in the collet that holds the tool. I have got a spare one here. This is the part of the machine that holds the tool (referring to the collet) and there is a little tiny screw that goes in to hold it. You do not tighten it as hard as you can. If you do that you actually strip the threads (of the set screw). It is just snug. You tighten it up snug, you don't tighten it so hard you are stripping the threads. If you have an old collet that has been over tightened the set screw does not hold very well, the tool wobbles, and again that will break the tool. Then when the tool is brand new, it is so sharp you get very fine shavings. After a little bit, it gets slightly dull and curves, and it actually machines more smoothly, and then when it gets very old, it doesn't machine nicely anymore, it just sort of smooshes the copper around and you get rough edges. That is a sign you need a new tool. I have come on people in fab labs who are actually trying to machine with a broken tool, and they say its not working well, and there tool is broken, it actually doesn't have the end (tip) of the tool there. And so you should look at your tool and make sure its in good condition, and be aware that it has a **lifetime**. To cut traces on the board (removing copper from the board), and perimeter of the board (basically, cutting through the copper and substrate layers on these bare boards), I used 2 different kinds of end mills (Figure 5.0 G): a v-bit (tip size, angle, equivalent to a 1/64 in end mill) for cutting traces, and a 1/32 in end mill for cutting perimeter of the board. Once you machine your board, there will be fine shavings left behind, and so you need to **deburr** it, and the way I like to do that ... this is a stainless steel rule that has a very straight sharp edge, and so when I am done machining I just use this, I pull it across, and it removes all the fine burrs, you can use a fine abrasive too. And then the last step is you **clean the board**. There are oils on your fingers that over time will begin to actually tarnish and then etch the board, and then there is the fine shavings. Oils from fingers also create problems during soldering components to the board. And so, you wash the board in soap and water before you assemble the circuit. So, those are all the steps you are going to go through to make your board by machining. And I want everybody to machine a board. I want you to learn this milling process because you can do it on a $ 1000 machine, the waste is just a little bit of dust, its easy to clean up, and its a very quick turn process.
## **PCB production process**
PCB fabrication/production by milling is one of several ways to produce PCBs. My laboratory has a monoFab SRM-20 compact desktop milling machine (basically a miniature CNC mill, Figure 5.1), and that is what I used to produce PCBs for myself. The mill is computer-controlled through a program called VPanel for SRM-20 (Figure 5.2). It is used to output toolpaths (cutting data) to the mill. The toolpaths were created at fabmodules.org. Here are the steps:
This week I worked on defining my final project idea and started to getting used to the documentation process.
## Research
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