Circuit Engraving (Precisely(For TQFP(Really!)))

Engraving circuits is not a new approach in anyways. There are handful of near perfect machines in the market(Wegstr,Bantam,LPKF). And as you may have already guessed, they are quite expensive. Looking from an novice CNC user perspective, engraving 0.1mm thick copper should not be that hard since machines nowadays can reach 0.01mm repeatability and 12k RPMs without any problems.

For now I'm only going to focus on the most delicate part, engraving routes. I may add drilling, double sided tips and tricks in near future but for now you can follow other guides on these topics since they are not that experience needy operations in my humble opinion.

There are couple of things to consider/fix/buy before rolling your sleeves up. Of course you need CNC machine, blank (FR2,FR4 depending on your needs) and routing bits. I'm going to summarize what you should consider as much as I can in coming stages.

Get ready to break some bits and hope to stay away from blood sweat and tears :) All jokes aside, please consider wearing -at least- an eye protection since we are going to play with sharp bits with an insane amounts of speed. Keep your hands in emergency stop (power plug :)) in first stages of trials since you can easily destroy your workspace and motivation.

Lets start with what to check before sitting on your beloved computer.

Have fun :)

Protection and patience

Sturdy CNC with ~12k Spindle

Blank copper board -FR2,FR4 etc.

Engraving bits -10,30 degree 0.1mm 0.2mm bits (Lower is better on both!)

Enemy to defeat! (PCB Design ..)

Flexing is ok, gap is not! What do I mean?

When you are forcing (not pounding, not hammering. just a kind push of a finger) your axis, you should not feel any sudden movements. It should not be clicking sound. NO GAPS ARE ALLOVED BETWEEN ANY AXIS! We are talking about TQFP44 packages my friends and 0.2mm gap between pins are not going easy with a wiggly machine. But worry not! We are descendants of an apes and we can overcome -at least some of- these problems like our ancients.

If you are feeling slight movements in axis, tell them not to do that. In my case it didn't work and I have to follow my roots methods. Brute force! Find the gap that causing the movement and fill it. In my cheap 3018 CNC I've stuffed two or three contact points with paper or toothpick. Just make sure after those alterations, all your axis are moving smoothly.

Most of the wiggles can be solved with home appliances but -unfortunately- there is another problem that needs more luck/skill/money then that. Spindle runout. There are a lot of content about detecting and solving this issue but for really cheap CNCs, its like a gamble. Spindle runout is basically how smoothly your bit turns. If it is not turning perfectly parallel to your spindle axis, your 0.1mm routing bit may behave like a 0.5mm and it will greatly reduce your CNC performance. Buying a new spindle might be the only choice if all else fails.

Attached video shows how NOT to check runout. You cannot check it from outer shell of a shaft but it will give you an idea :).

Copper Plates

Ok, we got enough confidence in our machine and ready to engrave some current carrying hieroglyphs. While choosing copper plates, pick one for your application needs. Thick copper for high current applications may need deeper dive for your engraving bits. Trial and error is your best friend on that matter. Pictures in this page are examples of 35µm FR4 type boards. You may expect your board to be fairly flat but it does not really matters. We will see why in coming stages.

Engraving Bits

Chisels of the modern world. They will cut, drill, engrave. Huge topic to master from collets to bits.

Summary: ER collets are preferable for runout performances, V bits with 1 or 2 flute works best, without flute will also work. Pointy(10° 20° 30°) and thin(0.1mm max 0.2mm) will work. I've tried micro flute end mills but couldn't make them work on 12k rpm. I would love to try them on 30k but ..

You can find my preferences in the pictures but they are not scientifically selected by any means. Hardness, precision and accuracy are matters of luck while you are buying from "certain" websites.

Ok, we got our material and ready for some destruction. But for making that robot do crazy sharp movements, we need to tell it what its supposed to do in machine language (G-Code). It's a profession all by itself so I'm just going to mention what you need from ComputerAidedDesign to ComputerAidedManufacturing for PCB.

CAD:

You can use anything Altium, KiCad, Eagle .. And luckily we have standard called "Gerber" as well accepted output. Definition of Gerber by Wikipedia is "The Gerber format is an open ASCII vector format for printed circuit board designs.". Each of those CAD applications that I've previously mentioned have their own tutorials for how to take Gerber Exports.

CAM:

Now you have output folder filled with unknown extensions. You can find documents like bill of materials, drill guides(you will need in future) and layer gerber files in this folder. For Altium .gbl is for bottom layer and .gtl is top layer gerber files (By the way, you can also create toolpath from png file). I'm not sure whether they are same in all CAD programs. We are ready to create path for our V bits. We can use hybrid applications (i.e. Fusion) for it but I'm strongly recommending Flatcam for this. It's combined CAD/CAM software for PCB that allows making changes in your design and creating G-Code's from that design. You can isolate, drill and cut your board with only Flatcam but I'm going to mention only routing problems since other parts are relatively easier.

FlatCAM is an open-source project so feel free to contribute. What a world we live in ..

It has steep learning curve but let me share some of my noobities with you;

• If some of the default values seems odd to you, check measuring systems. I know it is ridiculous but there are still millions of people around the world uses ancient methods (Kings foot!) for a standardized measuring unit.
• Don't tell real tool diameter. Lets say you have 0.2mm isolation routes in your design. Use 0.19. Check for your generated geometry, if there are some routes needs to be isolated, try 0.18 so on .. Make sure there is only 1 pass for the thinnest isolation.
• Try to dive with 0.02mm steps. I'm usually satisfied with 0.06 total depth. If you dive with 0.06 in one step you will flex the material and fail. 0.02 will skim the top of copper plate without too much pressure.

I've tried to summarize FlatCAM usage in 6' this video.

We are working with fractions of millimeters from surface with machines that are not build for it. I can give you couple of tips for reducing discomfort.

• If you have disposable surface, try surface milling. If you have metallic one, clamp some wood or MDF and mill that one.
• Don't clamp with inhuman forces. If you clamp with too much force, surface will compress and your copper plate will become concave in middle.
• If you can use CAM software like Candle, use heightmap. It's a perfect feature that measures your workpiece in array and create heightmap differentially.
• Don't go crazy with expectations. As surface goes bigger, height difference also goes beyond range. Start with small designs at least for the beginning.
• When route isolation is done, use 220-400 grit sandpaper for better results. Trust me .. Do it.

Photos of my failures and other attempts are attached to this step. So don't feel bad if you fail :)

I hope this long lasting poorly written guide will get you started with this adventure. Please feel free to address any question, commend or advise.

Happy tinkering,

Ozgur