FR4 copper thickness?

[Lomax]

If buying a stack of blank double sided 1.5mm PCBs, without knowing specifically what they will be used for, would you buy 35u (1oz) or 70u (2oz) boards? I understand that thicker layers are needed for high currents (duh), but other than cost is there any drawback to using 70u boards? I will be "isolation milling" them, if that makes any difference.

[Lomax]

Never mind, I realised uncoated copper laminate is cheap enough to get a selection of different plating and laminate thicknesses - I even ordered some 0.6mm 18u stuff, which may be useful when you need a bit of flex.

Edit: In hindsight, this should probably have been posted in the "beginners" forum instead. Mods?

[chickenHeadKnob]

It definitely makes a difference when chemical etching 1/2 oz (17 mu) results in the finest detail and least undercut. As I understand it pro- board houses use 1/2 oz on the outer layers to etch and then typically plate up to 1 oz thickness to get a standard board. I have no experience with CNC milling and imagine it is less of a concern although maybe thicker layers are more subject to tear out with dull tools.

Edit: my mistake, as Monkeh points out below.  Plating has to come before etching for electrical continuity reasons as etching would create isolated nets  , I mis-remembered  something I read on the Think and Tinker web site about pattern plating.

https://www.thinktink.com/stack/volumes/volv/cpltcart/plating1.htm

[Monkeh]

As I understand it pro- board houses use 1/2 oz on the outer layers to etch and then typically plate up to 1 oz thickness to get a standard board.

They drill then plate to thickness, then etch.

[nctnico]

If buying a stack of blank double sided 1.5mm PCBs, without knowing specifically what they will be used for, would you buy 35u (1oz) or 70u (2oz) boards? I understand that thicker layers are needed for high currents (duh), but other than cost is there any drawback to using 70u boards? I will be "isolation milling" them, if that makes any difference.

If you want to mill boards then they will need to be extremely flat. FR4 will wear down your milling bits quickly. Also the copper and epoxy dust from the milling isn't healthy. I strongly advise to etch boards. It will be quicker and cost less to setup. Ferric Chloride (FeCl) etchant can be (re)used indefinitely by adding HCl when it slows down.

[mikeselectricstuff]

You won't get as fine detail on 70u, and you may also get issues with less even etching. And it will take longer to etch and use more etchant.
It's only for special occasions, not worth stocking by default.

[T3sl4co1l]

On the other hand, if you'll be cutting it by hand, 2oz is better -- it peels more readily.  It also takes care of the situations where you need high current or heat dissipation.

Tim

[Lomax]

If you want to mill boards then they will need to be extremely flat. FR4 will wear down your milling bits quickly. Also the copper and epoxy dust from the milling isn't healthy. I strongly advise to etch boards. It will be quicker and cost less to setup. Ferric Chloride (FeCl) etchant can be (re)used indefinitely by adding HCl when it slows down.

Thanks. I have etched many boards in the past. To say it is quick is complete nonsense; it is a slow, laborious and messy process - especially when using FeCl, which is a low grade etchant that will totally f**k up your workspace and anything you keep in it (vapours). The idea that this would be a less toxic/dangerous method than milling is just ludicrous. You are also completely disregarding the need to transfer the layout onto the board (photo resist. laser transfer, etc), and the extra work and cost this involves. You're also contradicting what you said about drill bits lasting "decades" in my other thread about PCB drills. Finally, etching in a low-volume/hobbyist type environment rarely yields consistent results, due to variance in process and chemistry, and it is very difficult to get professional looking results without advanced equipment (heated/aerated etching baths, high quality etchants, UV photoresist boards, vacuum exposure device, photographic film negatives...). Isolation milling by contrast is a single step to finished board process, which - if you allow for tool changes - includes precision drilled holes. The mess it creates is minimal, and it requires no dedicated equipment of any kind. It produces consistent results of superb quality.

[sleemanj]

They drill then plate to thickness, then etch.

Yes, although they only plate to thickness on the bits that won't be later etched away - that is, they use a negative photo resist mask (bits you don't want are covered, bits you do want are exposed) when plating the copper.

Then they reverse that mask by thinly tin plating the plated copper (which is the bits you want) while the unwanted copper is still masked, then stripping the negative photo resist mask which is covering the unwanted copper, and etching it away with the tin being a positive mask, then stripping the tin so you are left with just the copper you wanted.

[mikeselectricstuff]

If you want to mill boards then they will need to be extremely flat. FR4 will wear down your milling bits quickly. Also the copper and epoxy dust from the milling isn't healthy. I strongly advise to etch boards. It will be quicker and cost less to setup. Ferric Chloride (FeCl) etchant can be (re)used indefinitely by adding HCl when it slows down.

Thanks. I have etched many boards in the past. To say it is quick is complete nonsense; it is a slow, laborious and messy process

Not if you're properly set up with a dedicated area to contain mess and avoid need to set up & tear down the setup. Fresh, Hot FeCl in a bubble tank will etch a board in well under 10 minutes, Exposure time about 3-5 mins per side, but that can be left unattended. Develop time 30-60 seconds.

[Lomax]

If you want to mill boards then they will need to be extremely flat.

This is also incorrect; modern PCB milling software starts with a probe cycle, measuring the board's thickness/angle across the board and adjusting the G-Code accordingly. I bet you could successfully mill a board even if it is visibly bent.

[Lomax]

Not if you're properly set up with a dedicated area to contain mess and avoid need to set up & tear down the setup. Fresh, Hot FeCl in a bubble tank will etch a board in well under 10 minutes, Exposure time about 3-5 mins per side, but that can be left unattended. Develop time 30-60 seconds.

You're forgetting about the time required to produce the negatives. In my experience it is difficult to produce a good film on inkjet (too low density black) and you need some other process. Laser might be better, but I don't have a laser printer - nor do I have a heated aerated etching tank. Perfect registration for double sided boards is also elusive. While a CNC router is a general purpose machine that can be used for many many things, all the equipment needed to chemically etch high quality PCBs is dedicated to that one use only.

[mikeselectricstuff]

Not if you're properly set up with a dedicated area to contain mess and avoid need to set up & tear down the setup. Fresh, Hot FeCl in a bubble tank will etch a board in well under 10 minutes, Exposure time about 3-5 mins per side, but that can be left unattended. Develop time 30-60 seconds.

You're forgetting about the time required to produce the negatives. In my experience it is difficult to produce a good film on inkjet (too low density black) and you need some other process. Laser might be better, but I don't have a laser printer - nor do I have a heated aerated etching tank. Perfect registration for double sided boards is also elusive. While a CNC router is a general purpose machine that can be used for many many things, all the equipment needed to chemically etch high quality PCBs is dedicated to that one use only.

Laser print onto tracing paper produces excellent films.
Registration is easy - just line up & staple the 2 sides together to form an envelope.