Layer Alignment Target questions


#1

Hi,

Could someone please explain firstly what the purpose is of Layer Alignment Targets?

And secondly, what should I expect from my PCB manufacturer?
The reason for the second question is that the boards made by OSH Park had no markings on them related to the target, however the boards made by PCBWay had large drill holes through the board at the location of the target - which is not what I had expected.

Thanks
Steve


#2

Could someone please explain firstly what the purpose is of Layer Alignment Targets?

Good question. My thought was that it is something that appears on all layers, so you can print them out and stack them up and align them on the target. But they don’t appear on all layers (at least in the latest BZR version), only on the edge cuts layer. This is based on exporting Gerbers with a design which has the target added.

I will ask on the developer’s list.

And secondly, what should I expect from my PCB manufacturer?
The reason for the second question is that the boards made by OSH Park had no markings on them related to the target, however the boards made by PCBWay had large drill holes through the board at the location of the target - which is not what I had expected.

If the target is on a drill layer (check the drill report), then the board vendor should drill a hole at that location, and I agree, it’s not what you want!


#3

Thanks Andy.

no it is not listed in the drill file. But because it is on the edge cut layer, PCBway determined that it is a cut out, and as a result cut it out of the board. You learn these things the hard way, but thankfully it isn’t an issue in this case.
In fact I am not sure why OSH Park didn’t cut out the hole.

At this stage it looks like these Targets are not of much use, and should be avoided, unless there is something more to them.


#4

But why is is on the Edge.Cuts layer in the first place? This layer is supposed to be routed out by the manufacturer (me, in this case). In my opinion it should be on any layer but this one.


#5

I have a question regarding Layer Alignment Targets and since this thread didn’t seem to reach any conclusion I thought I would revive it.

Is there any reason why layer alignment targets are not enabled on all layers? There doesn’t seem to be any point having them otherwise. I realize that when plotting, the edge cuts layer is combined with the other layers by default so they do end up on every layer in the Gerbers. That is until my fab requests that I not have my board’s profile on my silkscreen, soldermask and copper layers. In this case I check the option to exclude the edge cuts layer from the other layers when plotting but then I lose the layer alignment targets as well.

Does anyone actually use layer alignment targets or do you leave it up to the fab to add their own?


#6

See this ancient thread
https://bugs.launchpad.net/kicad/+bug/1323899


#7

So it was raised as a bug nearly 3 years ago and left at that.

So what does everyone else do for layer alignment?


#8

Strangely I was thinking about this today. I found out that the China fab that makes my boards has to send four layer stacks to be pressed together before the second round of etching. This means lots of opportunity for the inner layers and outer to be misaligned. I was thinking about four quadrants with one filled on each layer and the other three clear. Make each slightly undersized and shine light through to ideally show a cross


#9

I would assume that a Layer Alignment mark would be to align all the different layers in a PCB. In that case, using this tool should mark all different layers at the same time and it should appear in the Gerber files or any other file used to produce the graphic artwork for production. Ideally, the user should put at least 2 of them in opposite corners of the design, but outside the body of the actual PCB. The farther the better to allow for an easier and better alignment of all the layers.
But if the Edge Cuts layer can be included in the output of every Gerber file, it would produce the same results.


#10

I always wondered what Layer Alignment Target was for, I have never used it. It seems to be a hangover from the past, no longer needed.

As always, you need to design for your fab requirements, and if they don’t need it, don’t use it.


#11

I remember back in the day we had a board manufacturer request layer alignment targets on all the layers. They wanted 3 of them, not on a perfect square/rectangle. They described to me the reason for them was when manually aligning artwork during the fabrication process they would help ensure that the artwork wasn’t flipped or rotated.

I’m not sure why they needed that on my boards since they were the ones who panelized my designs for the size sheet of FR4 (or pre-preg) that they used in their fabrication process, and they could have added their own panel targets outside my design. We were, after all, paying $200/layer as an NRE artwork fee… (Maybe it was a known design issue with their pannelizing software that would allow board layers to get moved independently of each other?)

I did get an opportunity at that job to visit one of the board houses to see how they manufactured our 4 layer designs. To ensure the copper layers and machining processes all lined up they did use indexing holes (and a jig with pegs) in the border of their panel outside the customer’s artwork. I forget how they aligned the plastic mask sheets for the photolithography process in preparation to etching.

Granted, this was also back in the mid '90s…


#12

I need these alignment target holes, when I mechanically etch the copper on front and back. This is what we call CNC PCB Milling using GCODE.

So first I have to mechanically grind the copper out from the front PCB. The next is I flip the board. but when I flip the board I make sure that the PCB is aligned perfectly from the original position. The way to do that is use two pegs. So that when I place the PCB for the back part, the front will still be in the same position as before. The pegs go to the alignment target holes.

This is how the alignment target holes works for me, at least that I know of.