Author Topic: Understanding gerber files - simple question  (Read 1707 times)

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Offline FriedMuleTopic starter

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Understanding gerber files - simple question
« on: August 12, 2019, 04:51:16 am »
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
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Offline chrisl

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Re: Understanding gerber files - simple question
« Reply #1 on: August 12, 2019, 06:10:03 am »
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
 

Offline Ysjoelfir

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Re: Understanding gerber files - simple question
« Reply #2 on: August 12, 2019, 06:12:43 am »
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 Drill


I'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.
Greetings, Kai \ Ysjoelfir
 

Offline Siwastaja

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Re: Understanding gerber files - simple question
« Reply #3 on: August 12, 2019, 06:26:03 am »
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 layer
Only has SMD pads in it, pads are slightly larger than the same pads in the copper layer -> is a solder mask layer
Only has SMD pads in it, pads are slightly smaller than in the copper layer -> is a paste layer

Isn't a gerber file at all, but includes X...Y... coordinates: is an Excellon drill file.
 

Offline Ysjoelfir

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Re: Understanding gerber files - simple question
« Reply #4 on: August 12, 2019, 06:49:07 am »
Just look at the content with any gerber viewer:

Has traces and pads in it -> is a copper layer
Only has SMD pads in it, pads are slightly larger than the same pads in the copper layer -> is a solder mask layer
Only has SMD pads in it, pads are slightly smaller than in the copper layer -> is a paste layer
Isn't a gerber file at all, but includes X...Y... coordinates: is an Excellon drill file.
That is pretty much what I did, thanks for writing down the little "tutorial" :)
Greetings, Kai \ Ysjoelfir
 

Offline FriedMuleTopic starter

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Re: Understanding gerber files - simple question
« Reply #5 on: August 12, 2019, 07:03:37 am »
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 layer
Only has SMD pads in it, pads are slightly larger than the same pads in the copper layer -> is a solder mask layer
Only has SMD pads in it, pads are slightly smaller than in the copper layer -> is a paste layer

Isn't a gerber file at all, but includes X...Y... coordinates: is an Excellon drill file.
Thanks,  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 Work

The 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. :-)
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Offline Siwastaja

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Re: Understanding gerber files - simple question
« Reply #6 on: August 12, 2019, 07:08:21 am »
I would interpret ("further work") that they have recognized an issue using a 2-layer board, and are planning to 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.
 

Offline FriedMuleTopic starter

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Re: Understanding gerber files - simple question
« Reply #7 on: August 12, 2019, 08:16:58 am »
I would interpret ("further work") that they have recognized an issue using a 2-layer board, and are planning to 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.
Oh in that way, so if I make two board sized ground layers, it should work better?
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Offline Siwastaja

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Re: Understanding gerber files - simple question
« Reply #8 on: August 12, 2019, 08:29:03 am »
I would interpret ("further work") that they have recognized an issue using a 2-layer board, and are planning to 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.
Oh in that way, so if I make two board sized ground layers, it should work better?

You need to actually calculate the trace impedances for said important traces. As of now, the impedance is likely too high, but if you just add the planes, you may end up being too 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.
 

Offline FriedMuleTopic starter

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Re: Understanding gerber files - simple question
« Reply #9 on: August 12, 2019, 09:47:28 am »
I would interpret ("further work") that they have recognized an issue using a 2-layer board, and are planning to 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.
Oh in that way, so if I make two board sized ground layers, it should work better?

You need to actually calculate the trace impedances for said important traces. As of now, the impedance is likely too high, but if you just add the planes, you may end up being too 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.
Okay 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?
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Offline Siwastaja

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Re: Understanding gerber files - simple question
« Reply #10 on: August 12, 2019, 11:54:08 am »
Using an online impedance calculator gets you quite far:
https://www.eeweb.com/tools/microstrip-impedance

Put 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).
 

Offline FriedMuleTopic starter

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Re: Understanding gerber files - simple question
« Reply #11 on: August 12, 2019, 12:27:26 pm »
Thank you a lot! Do you know where I can order boards of the Rogers high frequency with prepreg material type?
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