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  1. #1
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    Which bits to use for PCB milling?

    Hi all,

    ... so I set up my CNC for my first PCB milling today and was successful drilling the holes, however, milling the actual traces was much less of a success ...

    Using a 10 degr. 0.1 mm V-bit the result overall looked like the PCB I was supposed to mill but unfortunately not much more than that. And the V-shape bit clearly had changed its end shape when the milling was completed.

    Which brings me to my reason for posting here .. I hope someone here have some good experience on milling PCBs and may advice on:

    - which mills to use for carving high-precision PCBs with traces down to 0.2mm widths and similar trace spacings? V-bit mills or regular low-diameter end mills? And which metal types/coatings are particularly useful for this?

    - which rpms and feeds to use for a successful milling?

    - how much may the Z-axis depth be increased per pass?


    I would much appreciate some advice on this - and if possible a link to a calculator of some kind that may help estimate these values for various bit sizes.

    Cheers & thanks,

    Jesper

    A P.S.: I guess part of successful CNC milling is using good end mills ... might one of you have a suggestion for a good end mill vendor in Europe?

  2. #2
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    Re: Which bits to use for PCB milling?

    10-degree bits have very fragile tips, they break if you look at them sideways. 30 to 60 degrees work much better for PCBs.
    Regular end mills are pretty much out of question. Your two options are:
    - V-bits and pyramid bits off eBay - cheap but functional.
    - Specialized PCB bits (such as PreciseBits) - supposedly better than V-tips but $10 to $15 a piece.

    RPMs and feeds: depending on the machine and spindle you have. My "sweet spot" is 8 000 RPM and 150mm/min; yours can be totally different.

    Depth of cut: I use a single pass at 0.05mm. Depending on the copper thickness you may have to cut deeper and/or make multiple passes.

  3. #3
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    Re: Which bits to use for PCB milling?

    Hi C-O-D ... & thanks for your feedback.

    The thing - which I should have written, my apology - is that I will make quite special PCBs. Likely up to 0.5 mm copper thickness and still with 0.2mm - 0.25 mm spacings between some of the tracks. And I also need these tracks to be very reasonably impedance matched. So a higher angle degree V-bit or the like is less than optimal if another option may be available ... Which is what I somehow hope to find ..

    Cheers - Jesper

  4. #4
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    Re: Which bits to use for PCB milling?

    Wow. What are you making, high power strip filters? RF amplifiers?

    A 10-degree V-cutter may work, but definitely not in one pass. Probably not even in five passes.

    I have not tried cutting such thick copper, but I would try a stub 0.2mm mill like this one:

    eBay

  5. #5
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    Re: Which bits to use for PCB milling?

    Quote Originally Posted by evalon View Post
    Likely up to 0.5 mm copper thickness and still with 0.2mm - 0.25 mm spacings between some of the tracks.
    Wow...

    Normal PCB has 35µm (0.03500000mm) copper layer, some heavy duty electronics use 70µm layer, but that's not very common, so whatever you plan to do it must be very special because I have never heard about PCB with 500µm copper layer. To me that sounds like you missed a zero after the decimal point or is not a PCB but copper plate. Sorry, I can't help you with this sort of PCB, I only use standard PCB with 35µm copper layers and on those I am using 22k RPM, 30 degree V-bit with 0.2mm tip. It works well, but I would not use it on PCB with 500µm copper layer.

    https://youtu.be/nrW6Hq6V3VY

  6. #6
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    Re: Which bits to use for PCB milling?

    Hi both ... well, I am doing something where I need the GND level to be very firm for a wide range of frequencies. And I am also interested in trying out what happens when using such a thick PCB copper layer. Regarding the PCB laminate I intend to start out with thinner copper layers (175um) and then move up in size if I can make it work. PCB laminates are not readily available with such thick copper - as you A-camera mention - but I have some PCB prepreg and will try to make the PCB myself in due time.

    I think I will try with another V-bit and also the 0.2 mm endmill that CitizenOfDreams linked to.

    Thanks again for your replies ;-)

    Jesper

  7. #7
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    Re: Which bits to use for PCB milling?

    Quote Originally Posted by evalon View Post
    And I am also interested in trying out what happens when using such a thick PCB copper layer.
    Well, I can tell you the first thing that would happen: you will buy a preheater and the most powerful soldering iron you can find.

  8. #8
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    Re: Which bits to use for PCB milling?

    If you're indeed working with multi-hundred micron copper layers I'm afraid not much of anyone's PCB milling experience will apply. No idea what kind of tool would work for you, although a 0.1mm tip 10 degree v-bit is certainly quite fragile even for normal PCB work. Also, expecting your traces to come out exactly as wide as designed is not realistic with a V-bit; you might get the results you're looking for in time if you experiment enough, but typical modest machines just can't control milling depth that precisely. Regarding milling bits in the EU, these are the ones I know of (no affiliation with any of them), I think all of them carry PCB-specific bits: Sorotec.de, FrezyCNC.eu, VHF.de

  9. #9
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    Re: Which bits to use for PCB milling?

    Hi,
    I use a double sided PCB blank with 12oz copper, that is 420um or 0.42mm. I can confirm that its not easy to do. Tapered engraving bits are hopeless.
    I'm using 0.5mm two flute uncoated endmills made by Kyocera Tycom and supplied to me by drillman1 on Ebay. The last lot I got were on special for only
    $2.65 US each so I got 30, wish I'd bought more.

    They are fragile and its taken a while and quite a few tools to work out a good strategy for using them. First in order to get them to last you must avoid cutting
    the underlying fiberglass as much as possible. The Autoleveller software utility improved my results not just with this heavy copper board but ordinary board as
    well. The second thing I do is use flood coolant. Just water with water soluable oil but it increases the tool life from about 1/2 to 1 hour to over 10 hours and
    a marked increase in cut quality.

    Craig

  10. #10
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    Re: Which bits to use for PCB milling?

    Hi,
    should add what cutting parameters:

    I spin the 0.5mm tools at 24000 rpm, I'd spin them faster if my spindle would go faster.

    On a full width slotting pass, WITH COOLANT, I will go only half depth, ie 0.21mm cut depth at 100mm/min plunge and 200mm/min slotting.
    The second pass using the same toolpath, ie cutting too full depth is done at the same parameters, 100mm/min plunge and 200mm/min slotting
    to 0.45mm deep. That is to say through the copper (0.42mm) and a fraction more (0.03mm) into the fiberglass to be sure of isolation.

    Subsequent toolpaths are done at 50% stepover ie 0.25mm at 200mm/min plunge and 400mm/min cutting.

    Craig

  11. #11
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    Re: Which bits to use for PCB milling?

    Hi,
    I should emphasize the flood coolant. If you try, particularly the first full width slotting type tool paths without flood coolant to remove the copper chips from the cut zone
    you will break the tool for sure.

    I have found that the flood cooling to be the difference between success and failure. Its more about removing chips than cooling. I have just one modest jet of coolant
    directed at the cutzone. I think I would get even better results if I had three smaller but higher pressure jets directed at the zone. I would need a different pump
    so haven't tried it.

    Craig

  12. #12
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    Re: Which bits to use for PCB milling?

    Hi,
    re-reading your post and you say that you are trying to end up with 0.2mm to 0.25mm between traces. That would require endmills of
    0.2mm and they are too fragile especially if you are cutting 500um deep. That would be a 100% width cut, ie full width slotting which is always a challenging
    toolpath but at 250% deep!

    My heavy copper boards have a SOIC with 0.6mm between the IC pin pads, ergo I can use 0.5mm endmills. I am glad I do not have to go any finer than that.

    You may have seen that there are companies that make these heavy copper boards. It seems that military customers are prepared to pay the premium.
    They use a combination of etching, plating and isolation routing. The cost of borads of that description complete with plated through holes is about $1000US/sq foot.
    One company published design rules and they specify that the minimum distance between tracks be at least 0.4mm or at least the thickness of the copper. In my
    case with 0.42mm copper if I got them to make the boards they would have a minimum spacing of 0.42mm.

    I think you will have to review your design requirements with regard to either copper thickness OR minimum practical spacing.

    Craig

  13. #13
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    Re: Which bits to use for PCB milling?

    Hi,
    PCB Material World - About Us

    It looks like they have sold the last of the12oz/12oz boards that I have used but they have some 11oz/11oz and some 10oz/10oz boards.
    Beware the total thickness, some of these boards are meant to be glued together in a stack.

    Good outfit, they had a minimum order of $50US which he was happy to bend a little for me in New Zealand but I bought two boards 18 inch x 18 inch and
    he posted to Oregon where my shipping agent is for about $90US. As I say a good outfit.

    Craig

  14. #14
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    Re: Which bits to use for PCB milling?

    Hi Craig ... First thank you very much for replying with so much useful information in your posts! I do appreciate this - your replies really covers many of the questions I have on this subject (and some that I didn't yet have)

    Importantly though is your second-last post where you mention that it would indeed be challenging to use as thick copper as I intend to with a bit size of 0.2mms ... Reading through your posts I get the impression that you have investigated this quite thoroughly so I will just take your word for it.

    As it is I am capable of - most often - etching 120 um copper with the precision required for TQFP parts:

    ( https://www.google.dk/search?q=tqfp+...uIN-0x9BdJPtM: ).

    I do these etchings using a combination of Na2S2O8, a non- NaOH containing developer and Kontakt-Chemie's Positiv 20. All in all a very feasible and quick process when things run well - as they actually often do. Also it is environmentally very clean as (at least here in Denmark) all chemicals can be submitted for handling by the public waste collection system. In practice I guess that for a large PCB board (e.g. 20 * 20 cms) just a fraction of a drop of etchant is released into the environment.

    Unfortunately I cannot etch thicker copper boards with this method. During the etching process the etchant "digs" in under the track sizes defined by the photo resist and thus thicker copper tracks cannot be made with 0.2 - 0.25 mm spacings. One possible solution to this could be to etch the PCB with HCL - apparently this etchant should not dig in under the copper to the same extent. But it is much less pleasant to work with - with fumes being released during the etching process.

    Anyway - this is my reason for seeking to find alternative methods that also don't cost a fortune. As you may have experienced a design is not always completed "first-off" and so using commercial PCB manufacturing facilities for such special PCBs likely would cost thousands of dollars - and likely also add some undesired stress to my design process.

    ... I have also taken a look at the PCB blank company you linked to and it does indeed look interesting for specialty PCBs. So also a "thank you very much" for this link. I have actually been searching for such thick-copper PCBs for quite some time and the only vendor I have found has been abcfab on ebay who - just very, very occasionally - have had such PCBs.

    In any case - I will let go of trying to mill 0.5mm copper thickness PCBs with 0.2mm traces. Thanks for clarifying this!

    Have a good day wherever you are

    Jesper

  15. #15
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    Re: Which bits to use for PCB milling?

    Hi,
    my understanding is that commercial PCB companies can etch up to about 4oz copper with the same limitations you have described. They can
    electroplate up to about another 4oz, which incidentally is about the maximum wall thickness that can be achieved with plated through holes.

    At even thicker copper layers the traces are cut out of foil and glued in place. With such methods 1mm thick traces can be had and also variations in one layer.
    Thus the signal traces may be etched 4oz copper but 1mm thick power traces could be added.

    Such a hybrid board was used as a manufacturing example, it was for a military radar RF board.

    I see the PCB company has some 11oz/11oz board of 0.059 thickness. I think the thickness refers to the fiberglass layer which makes 0.059 (1.498mm)
    about perfect for a double layer board and the copper layers are about 385um thick. When I bought my 12oz/12oz (420um/420um) boards about two years ago they still had
    about 30 pieces left. They must have sold them.

    I have used 0.4mm endmills and they are even more fragile than 0.5mm. I haven't tried 0.2mm, they are likely to fragile for me. You may have noted also that the smaller they
    are the more expensive they get. Most small diameter tools are 150% long, that is to say if they are 0.2mm in diameter then standard (150%) flute length is only 0.3mm.
    Thus you could not cut greater than 300um copper. Stub length endmills are commonly 100% long.

    My 0.5mm endmills are standard length so I could potentially cut 750um copper. I personally think that's pretty daunting, it took a lot of time and experimentation
    and lots and lots of failures before I managed to get good boards with 420um copper.

    Craig

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    Re: Which bits to use for PCB milling?

    Hi,
    https://www.ebay.com/itm/5-0-50mm-01...cAAOxy1VlRFPam

    Sorry they are longer than I thought, about 400%

    Craig

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    Re: Which bits to use for PCB milling?

    Hi Craig ... once more thanks for your elaborate and very helpful replies ... I was thinking if you by any chance know of a commercial PCB company that makes PCBs in the 4 oz + 4 oz electroplating like you mentioned? I have searched for such companies myself only to not find any. And since the PCBs I need are pretty small maybe - just maybe - it could be made at a reasonable cost when the design otherwise is ready.

    Cheers,

    Jesper

  18. #18
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    Re: Which bits to use for PCB milling?

    Sounds difficult to me!
    Remember that a spindle TIR of 50 microns will destroy a 200 micron cutter kinda quickly. For a TIR of 10 microns you are starting to talk about some expensive gear.

    Cheers
    Roger

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    Re: Which bits to use for PCB milling?

    Hi Jesper,
    there was a Chinese company that offered that service, they still email me all the time, I'll try to find a refenence to them.
    There was also a company in California specializing in this sort of thing, military stuff and very expensive.

    Hi Roger,
    my spindle with the Rego-Fix collet has less than 5um TIR, it wasn't cheap.

    Craig

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    Re: Which bits to use for PCB milling?

    it wasn't cheap.
    Just so!
    But very nice to have.

    Cheers
    Roger

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