please review my first kicad project (and pcb design): an ADXL335 accelerometer breakout board..

Discussion:

Fabio Varesano

2010-08-27 19:18:29 UTC

Hi guys,

I just finished working on my first pcb design and I did it with Kicad.

I'm a computer science master student so I'm pretty confident with
computers and I know my stuff in the software side of things .. but we
don't have courses on pcb designs nor on electronics (a part from some
phisics which covers circuits and electro-magnetism.. but nothing
deep), so that's been quite a challenge to get were I'm right now.

I'm working on my final thesis project and we are working with
different kind of sensors (accelerometers, magnetometers, gyros and
more) to develop nice kind of user-computer interactions such as body
movement recognition, tangible in objects UIs, etc..

Until now we used pre-made boards (Arduino and some breakout boards)
with some soldering or bread boarding.. but I'm now feeling quite
limited by this so I'm trying to move on and pass to the next step:
developing my own circuits boards that fit my needs.

So here I am. To make things simple I started creating something
similar to something I already know: Sparkfun ADXL335 breakout board
(http://www.sparkfun.com/commerce/product_info.php?products_id=9269#)

I used this accelerometer in the past so I know how this works also on
the electrical side of things.

So, I designed using Kicad the schematics, the pcb layout and sorted
out the libraries of kicad..

So, I'd like you to have a look at my Kicad project and let me know
what you think. As I said I'm a complete beginner so that's likely I
missed lot of things there..

Please note that the pcb design is going to be create with home made
etching on a single side.

This is the accelerometer I used:
http://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf
The schematic I wanted to copy:
http://www.sparkfun.com/datasheets/Sensors/ADXL335_v10.pdf

Here you find the project files
http://www.varesano.net/temp/adxl335_breakout_board.tar.gz

Thanks for your time,

Fabio Varesano

------------------------------------

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Andy Eskelson

2010-08-27 21:38:21 UTC

Permalink

Personally I don't like lots of filled zones unless that really are
necessary, but that's a fairly minor point.

several other points:

Do try to put your connection pins on the edge of the board. It makes
wiring up much easier. You could also consider some form of connector.

How are you going to mount the board? Some hardware mounting holes
might be useful.

The tracks to the pins look rather thin. Thin tracks tend to lift when
soldering, also a small speck of dust on the film can cause a
break in the track. Using a thicker track will help in both cases.

On single sided boards, don't be afraid to use a few links it makes life
MUCH easier.

An Easy way to do this is to treat the board as double sided, and use the
top tracks as wire links. Beef up the vias to a suitable size to take your
wire links. Obviously you need to manually route this.

I'm not going to comment on the actual circuit as I've never used this
device.

Regards
Andy

On Fri, 27 Aug 2010 21:18:29 +0200

Post by Fabio Varesano
Hi guys,
I just finished working on my first pcb design and I did it with Kicad.
I'm a computer science master student so I'm pretty confident with
computers and I know my stuff in the software side of things .. but we
don't have courses on pcb designs nor on electronics (a part from some
phisics which covers circuits and electro-magnetism.. but nothing
deep), so that's been quite a challenge to get were I'm right now.
I'm working on my final thesis project and we are working with
different kind of sensors (accelerometers, magnetometers, gyros and
more) to develop nice kind of user-computer interactions such as body
movement recognition, tangible in objects UIs, etc..
Until now we used pre-made boards (Arduino and some breakout boards)
with some soldering or bread boarding.. but I'm now feeling quite
developing my own circuits boards that fit my needs.
So here I am. To make things simple I started creating something
similar to something I already know: Sparkfun ADXL335 breakout board
(http://www.sparkfun.com/commerce/product_info.php?products_id=9269#)
I used this accelerometer in the past so I know how this works also on
the electrical side of things.
So, I designed using Kicad the schematics, the pcb layout and sorted
out the libraries of kicad..
So, I'd like you to have a look at my Kicad project and let me know
what you think. As I said I'm a complete beginner so that's likely I
missed lot of things there..
Please note that the pcb design is going to be create with home made
etching on a single side.
http://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf
http://www.sparkfun.com/datasheets/Sensors/ADXL335_v10.pdf
Here you find the project files
http://www.varesano.net/temp/adxl335_breakout_board.tar.gz
Thanks for your time,
Fabio Varesano
------------------------------------
Please read the Kicad FAQ in the group files section before posting your question.
Please post your bug reports here. They will be picked up by the creator of Kicad.
Please visit http://www.kicadlib.org for details of how to contribute your symbols/modules to the kicad library.
For building Kicad from source and other development questions visit the kicad-devel group at http://groups.yahoo.com/group/kicad-develYahoo! Groups Links

------------------------------------

Please read the Kicad FAQ in the group files section before posting your question.
Please post your bug reports here. They will be picked up by the creator of Kicad.
Please visit http://www.kicadlib.org for details of how to contribute your symbols/modules to the kicad library.
For building Kicad from source and other development questions visit the kicad-devel group at http://groups.yahoo.com/group/kicad-develYahoo! Groups Links

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Fabio Varesano

2010-08-27 23:10:59 UTC

Permalink

Hi Andy, thanks for having a look at this. Please read below..

Post by Andy Eskelson
Personally I don't like lots of filled zones unless that really are
necessary, but that's a fairly minor point.

Well, I read that this is good practice as this make less use of
etchant during the etching process. Isn't that true?

Post by Andy Eskelson
Do try to put your connection pins on the edge of the board. It makes
wiring up much easier. You could also consider some form of connector.

Yep, that's the best I've been able to come out with.. I do know that
it will be painful to plug this. Do you have any suggestion for a 6
pin connector? Somehow I didn't find something suitable in the library.

Post by Andy Eskelson
How are you going to mount the board? Some hardware mounting holes
might be useful.

Yep, good point. How do you draw mounting holes with kicad?

Post by Andy Eskelson
The tracks to the pins look rather thin. Thin tracks tend to lift when
soldering, also a small speck of dust on the film can cause a
break in the track. Using a thicker track will help in both cases.

You mean the tracks to the accelerometer IC? Well, they quite match
the IC pins size. I don't think I could have made them bigger. Maybe
you are suggesting making them thin below the IC and bigger on the
rest of the board?

Post by Andy Eskelson
On single sided boards, don't be afraid to use a few links it makes life
MUCH easier.
An Easy way to do this is to treat the board as double sided, and use the
top tracks as wire links. Beef up the vias to a suitable size to take your
wire links. Obviously you need to manually route this.

I understand. So you are using wires as bridges to pass over one or
more track, right? Interesting..

Thanks for your suggestions. Really helpful. You rock!

FV

------------------------------------

Please read the Kicad FAQ in the group files section before posting your question.
Please post your bug reports here. They will be picked up by the creator of Kicad.
Please visit http://www.kicadlib.org for details of how to contribute your symbols/modules to the kicad library.
For building Kicad from source and other development questions visit the kicad-devel group at http://groups.yahoo.com/group/kicad-develYahoo! Groups Links

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Andy Eskelson

2010-08-27 23:45:50 UTC

Permalink

Zone filling does help with the etchant usage, but I don't tend to go
overboard with it. For example I would not have bothered to fill the
small area to the bottom left and bottom right hand corner of the IC
(breakout board-back.pdf Also that long tail extending down between the
top right hand connection pin and the one below. It looks as if it should
be going to an IC pin, but it's part of the zone fill.

No reason to remove them, just my personal preference. You will find that
there is a lot of than in PCB work...

There are lots of 6 pin conectors. A simple 0.1 inch header can be used,
SIL-6 is one, or Head_4x2 (8 pins) pin_aray_3x2 to name a few.

There are dozens of connectors, some will not be in the libs, you might
find them elsewhere, but a simple connector is good practice for drawing
up your own module give it a try.

If you have not already done so, print out the module documentation from
CVpcb. ("display footprints list documentation) 3rd icon from right in my
version. It useful to have that by your side when choosing modules. You
can also find that document in : /usr/local/kicad/doc/help/footprints_doc
or under kicad in program files if using windows.

With mounting holes, so people just use a big pad and set the drill size
to what they want, while others draw it graphically. If you are going to
have the board made by a production house then you need to define it as
the correct size. For home production I just shove a normal pad down. The
hole when etched makes a good drill centre.

The tracks, yes you need them the correct size for the IC pin spacing,
But you seem to have made that the thickest track you have used. When
hand soldering, thin tracks lift very easily. Likewise, give yourself
plenty of clearance to avoid solder bridges when building the units. You
will not have any solder mask on the board, so the extra clearance makes
it less likely for solder splashes to happen, and if they do they are
easier to clear.

e.g.
The tracks from the other side of some of the connection pads look
thinner than the tracks from the IC, that could be a product of the
pdf...I will check... No the postscript looks much the same.

For such a small board you have a lot of PCB area, so make use of it.

The links, yes spot on, treat them as component side wiring. If you had
any wire ended components in the design, they can also be used be used to
bridge over tracks.

Andy

On Sat, 28 Aug 2010 01:10:59 +0200

Post by Fabio Varesano
Hi Andy, thanks for having a look at this. Please read below..

Post by Andy Eskelson
Personally I don't like lots of filled zones unless that really are
necessary, but that's a fairly minor point.

Well, I read that this is good practice as this make less use of
etchant during the etching process. Isn't that true?

Post by Andy Eskelson
Do try to put your connection pins on the edge of the board. It makes
wiring up much easier. You could also consider some form of connector.

Post by Andy Eskelson
How are you going to mount the board? Some hardware mounting holes
might be useful.

Yep, good point. How do you draw mounting holes with kicad?

Andy Eskelson

2010-08-28 09:17:50 UTC

Permalink

One other thing I've just realised:

The IC is a surface mount device Usually the module is designed for the
tracks to be on the COMPONENT side of the PCB. If you intend to produce
the PCB in the conventional way with the components on the top, and the
tracks on the other side, then you may have a problem. Check the
orientation of the IC very carefully, as you will need to fit it to the
underside (copper) side of the PCB, and that will usually mean that you
have to flip the module.

Andy

On Sat, 28 Aug 2010 01:10:59 +0200

Post by Fabio Varesano
Hi Andy, thanks for having a look at this. Please read below..

Post by Andy Eskelson
Personally I don't like lots of filled zones unless that really are
necessary, but that's a fairly minor point.

Well, I read that this is good practice as this make less use of
etchant during the etching process. Isn't that true?

Post by Andy Eskelson
Do try to put your connection pins on the edge of the board. It makes
wiring up much easier. You could also consider some form of connector.

Post by Andy Eskelson
How are you going to mount the board? Some hardware mounting holes
might be useful.

Yep, good point. How do you draw mounting holes with kicad?

Bernd Wiebus

2010-08-28 09:20:50 UTC

Permalink

Hello Fabio.

Post by Fabio Varesano

Post by Andy Eskelson
Personally I don't like lots of filled zones unless that really are
necessary, but that's a fairly minor point.

Well, I read that this is good practice as this make less use of
etchant during the etching process. Isn't that true?

Actually it IS a good practice to use filling zones. Not only for saving
etchant but also for grounding, shielding and low resistant power
supply. For electromagnetic compilance zone fill is essential.

But you have to be carefull. Some structures, as an example long
singular fingers or slots, are prone to get resonant and producing
additional RF Problems. You can also get isoliting problems, and low
ground conection losses for power supply means also high shortcut
currents.
Sometimes you get even additional heat by eddy currents.

So you have to look for this issues. In my experience, you can look for
this long structures and try to get them small or remove them complete.
To get them smaller you can add bridges to the design. As an example, if
you have a long slot by a long shaped socket, try to shortcut this slot
by connecting it at one or severall places from one side to the other.

Try to connect each zone with more then one (better more than two)
connections to its potential. This connections should be spaced. As an
example at opposite corners of the zone.

Often it helps, to put additional GND vias to the board for creating
additional zones of GND potential. So you get a unregulary grid of
Ground zones and interconnections of them at your board. If some ground
zones are covering the same area at opposite sides of the board, connect
them with vias (stitching). Try not to place this stitchings regulary,
because any regularity could get resonant, but try to place them
randomly. This will avoid to create some resonators.

As an example, you have a line of 100mm where the ground zone covers
both, the primary and secondary side of the board. So you want to stitch
them together by vias through the board, and the vias schould be spaced
equal acros this line, perhaps at 1/3 and 2/3 of the line. But DO NOT
place the vias at 33,3mm and 66,6mm, because you have a regular
structure of following 33,3mm parts, which can resonate together. The
better solution would to place one via at 30mm and the next at 80mm.
so you have three parts with different lengs 0f 30, 50 and 20mm, which
can only resonant individual. If you think, 50mm is too long, put an
additional via to divide it. But not ecactly at the middle. ;-)
also do not place the vias in an exact straight line. Move them randomly
out of the center of the line.

Vias are expensive, if you designe for a great quantity of boards. Also
it consumes much time to create this fill zones carefully.
So i can understand Andys suggestion to avoid them. Especially if you
have to save room for isolating issues.

I have not tested yet, wether stitching works at KiCAD. ;-) Up to now,
the remaining vias of the ground net created enough vias for my simple
cases.

With best regards: Bernd Wiebus alias dl1eic

------------------------------------

Please read the Kicad FAQ in the group files section before posting your question.
Please post your bug reports here. They will be picked up by the creator of Kicad.
Please visit http://www.kicadlib.org for details of how to contribute your symbols/modules to the kicad library.
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