Understanding gerber files - simple question - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: August 12, 2019, 04:51:16 am
content: Hi I need someone to look at these gerber files, to tell me what the 4 layers is containing.I know the top layer and bottom layer but what about the two middle layers?https://github.com/dkramnik/Eagle-Public/raw/master/instrumentation-active-probe-makemit-2014/MakeMIT%202014%20Wk1/Active%20Probe%20Proto2.zip

author: chrisl
author_rank: Regular Contributor
timestamp: August 12, 2019, 06:10:03 am
content: Looks like it is a 2 layer board as I only see 2 layers; top and bottom.Other layers in the zip file are:- assy dwg- solder mask top- solder mask bottom- NC drill

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: August 12, 2019, 06:12:43 am
content: The file endings of gerber files aren't fixed in any way, but I struggle a bit to make a sense of those you linked to. From looking at it it seems like the following:.plc: Silkscreen layer, plc = "placement"?.sol: bottom layer, sol = "solder"?.cmp: top layer, cmp = "component"?.stc: Solder Mask Top.sts: Solder Mask Bottom.drd: NC DrillI'm not sure about .stc and .sts. It definitely looks like the soldermask layer, but I wouldnt name it ".stx", since I would read it like "stencil component/solder", which could be the paste mask.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 12, 2019, 06:26:03 am
content: Yes, the naming can be anything. If the file names are not clear enough as they are, the contents must be specified separately, like using a human-readable text file.Lacking that, you need to reverse-engineer. Just look at the content with any gerber viewer:Has traces and pads in it -> is a copper layerOnly has SMD pads in it, pads are slightly larger than the same pads in the copper layer -> is a solder mask layerOnly has SMD pads in it, pads are slightly smaller than in the copper layer -> is a paste layerIsn't a gerber file at all, but includes X...Y... coordinates: is an Excellon drill file.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: August 12, 2019, 06:49:07 am
content: Quote from: Siwastaja on August 12, 2019, 06:26:03 amThat is pretty much what I did, thanks for writing down the little "tutorial"

author: FriedMule
author_rank: Frequent Contributor
timestamp: August 12, 2019, 07:03:37 am
content: Quote from: Siwastaja on August 12, 2019, 06:26:03 amThanks, have tryed that but with not much result:http://www.gerber-viewer.com/I was expecting 4 layers because of this text:"Conclusion and Further WorkThe likely reason for this is high dielectric losses in the FR4 PCB material (solution: send out the PCB using Rogers high frequency material) and an impedance mismatch in the 50 ohm trace that connects the final amplifier’s output to the SMA coax connector (solution: use a 4-layer PCB with thin prepreg material so that the trace width required to reach the 50 ohm target impedance is low enough to place on the board; with a 2-layer board the required trace width is too large)."I do not understand the part about 4 layers, please help. :-)

author: Siwastaja
author_rank: Super Contributor
timestamp: August 12, 2019, 07:08:21 am
content: I would interpret ("further work") that they haverecognized an issueusing a 2-layer board, and areplanningto use a 4-layer board in the future. So the downloadable design likely still is in 2 layers.The issue with 2 layers is that the separation between the layers is too large to practically build impedance-controlled 50 ohm traces. With 4 layers, they could get a ground plane just some 100 - 200 µm away from the top and bottom layers.

author: FriedMule
author_rank: Frequent Contributor
timestamp: August 12, 2019, 08:16:58 am
content: Quote from: Siwastaja on August 12, 2019, 07:08:21 amOh in that way, so if I make two board sized ground layers, it should work better?

author: Siwastaja
author_rank: Super Contributor
timestamp: August 12, 2019, 08:29:03 am
content: Quote from: FriedMule on August 12, 2019, 08:16:58 amYou need to actually calculate the trace impedances for said important traces. As of now, the impedance is likelytoo high, but if you just add the planes, you may end up beingtoo low. It may work better or worse.You should really do this on the PCB layout level, modifying the original design with proper calculations, instead of adding layers to the gerber set. You can't do it without learning a bit more on the subject.

author: FriedMule
author_rank: Frequent Contributor
timestamp: August 12, 2019, 09:47:28 am
content: Quote from: Siwastaja on August 12, 2019, 08:29:03 amOkay thanks a lot, the problem is that it is a circuit that I would really like to build, to help me in other projects. I have changed all the schematics from Eagle to KiCad but as you politely state, I do not know enough about it at all:-)May I please ask on how it is best to measure it all?

author: Siwastaja
author_rank: Super Contributor
timestamp: August 12, 2019, 11:54:08 am
content: Using an online impedance calculator gets you quite far:https://www.eeweb.com/tools/microstrip-impedancePut the numbers in and you'll see the problem with the 2 layer design.Note that if you order 4-layer prototypes, you need to carefully check the manufacturer's website for the actual stack-up, to get the actual number for the layer separation. It can be as low as 0.1mm, or as much as 0.25mm. This affects the required trace width.It's still not perfect; for really accurate impedance, you need to pay extra for impedance controlled PCB manufacturing (meaning more precise process control, so you get accurate track widths).

author: FriedMule
author_rank: Frequent Contributor
timestamp: August 12, 2019, 12:27:26 pm
content: Thank you a lot! Do you know where I can order boards of the Rogers high frequency with prepreg material type?