Electronics Production

Characterizing the specifications of our PCB production process and PCB production

To get started we started with characterization of our machine i.e SRM 20. We downloaded the PNG file from the Fabacademy Schedule page .... then Using Fabmodules I made .RML file and finally it was milled...... It was a group assignament...... After the final milling the results were not so good ..... we realized the tool was not sharp from the end and it needs to be replaced ......

Conclusion

This test was performed to check the capability of the machine..... In this mill job test tracks from 1 Mils to 20 Mils were made.... and from the image it may be concluded that the machine was able to print 1 Mils also, but the track quality is not upto the mark, It was observed that the tracks above 12 Mils were perfect.

Replacement of tool at such an urgent basis was not possible so we decided to grind the tip of the tool using dremel..... The results were totally uncertain but some how we managed to make that tool usable ........

WARNING

Be very much careful while using grinding tool...... If possible do it under supervision .... and keep some water near you .... The bit may get really hot ......

This week we will be using a pre-designed circuit to mill a circuit board & will not be designing one. Hence, wewill 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, wewill re-draw another pre-designed circuit in Autodesk EAGLE and that is where wewill 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). wewill 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 , you first have to attach it to the bed , and that is done in a surprisingly crude way (at least on these little precision milling machines). We use 2-sided tape , 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 . 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. westarted out with a single-sided blank FR-1 board.

Technically, these are bare single-sided copper-clad laminates in paper phenolic material. Under your board, there is an underlay , 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.

Then the set screw, this is a common beginner mistake, there is a set screw in the collet that holds the tool. wehave 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. wehave 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 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), weused 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 welike to do that … this is a stainless steel rule that has a very straight sharp edge, and so when weam done machining wejust use this, wepull 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.