TSSOP 38 routing problems

Jon Russell wrote:

>Hi,

>I've created a dozen or so through hole boards with Eagle (mostly Arduino shields), but I need to create a surface mount board. The DAC I need to use is a TSSOP 38 package (AD5348).

>I created a custom library following an online tutorial for the DAC and copied the TSSOP 38 package for a different existing library.

>I laid out the board (with the DAC and some op-amps) but the auto route couldnt get past about 75%.

>I played for a while, but couldn’t get any further. So, I thought I'd test the part with a simple breakout board. I created a board with just the TSSOP 38 device, fanned out to 0.1" headers.

>However, to my surprise, the auto route failed to route this simple board as well. See below. Why would the auto route not be able to fan out this simple board ?

>What am I doing wrong ? Or am I expecting too much ?

>I'm using the free version of Eagle v6.3.0 with a two layer board.

routing grid settings?

--

Lorenz

There may be a way of getting Eagle to do this better (I don't use Eagle so I can't say) but I think you are expecting rather a lot from a free tool ! In my experience cheap auto routers (< £10k) are all very limited and rarely worth using at all. Why don't you route the board by hand - that way you can get the tracks to go where they should from an electronic point of view and it will work much better.

Michael Kellett

It's a good idea to read section in the manual on autorouting. The manual

is found in the .docs folder. There it describes the impact of the routing

grid. That is a different grid to the display grid.

When a board design was simple and had no fine pitched devices you could

start the autorouter with no consideration of its settings or those of the

design rules (DRC) and get a result many would accept. As pin pitch, trace

width and clearances get smaller then you have to prepare for these

requirements. If you don't then the autorouter cannot fully route because

all of the numerous rules cannot be met. The goal is to make it easy for

the autorouter otherwise it will appear to do odd things and not even get a

result when manual routing would.

One approach is to identify the smallest pitch device, your TSSOP, and

determine the maximum trace width and spacing it will support. Looks like

5mil for the TSSOP. Next you need to choose the routing grid pitch. Smaller

will ensure routing completes but it increases the routing time

exponentially so you are after the largest you can get away with.

For your simple example I would set the routing grid to match the pitch of

the TSSOP pins. I would temporarily set the display grid to this size and

move the TSSOP so that the pin origins match the grid, then restore the

desired display grid.

Keep in mind that the autorouter routes to the intersections of the routing

grid.

HTH

Warren   

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On Tue, 27 Nov 2012 08:37:59 +0000, Michael Kellett wrote:

There may be a way of getting Eagle to do this better (I don't use Eagle

so I can't say) but I think you are expecting rather a lot from a free

tool ! In my experience cheap auto routers (< £10k) are all very limited

and rarely worth using at all. Why don't you route the board by hand -

that way you can get the tracks to go where they should from an

electronic point of view and it will work much better.

I've found the Eagle autorouter to be quite useful.  Expecting it to do

the same thing as a super-zoot tool is unrealistic, but it does a

competent job when done right.

--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

On Wed, 28 Nov 2012 10:24:54 +1300, Warren Brayshaw wrote:

It's a good idea to read section in the manual on autorouting. The

manual is found in the .docs folder. There it describes the impact of

the routing grid. That is a different grid to the display grid.

 

When a board design was simple and had no fine pitched devices you could

start the autorouter with no consideration of its settings or those of

the design rules (DRC) and get a result many would accept. As pin pitch,

trace width and clearances get smaller then you have to prepare for

these requirements. If you don't then the autorouter cannot fully route

because all of the numerous rules cannot be met. The goal is to make it

easy for the autorouter otherwise it will appear to do odd things and

not even get a result when manual routing would.

 

One approach is to identify the smallest pitch device, your TSSOP, and

determine the maximum trace width and spacing it will support. Looks

like 5mil for the TSSOP. Next you need to choose the routing grid pitch.

Smaller will ensure routing completes but it increases the routing time

exponentially so you are after the largest you can get away with.

 

For your simple example I would set the routing grid to match the pitch

of the TSSOP pins. I would temporarily set the display grid to this size

and move the TSSOP so that the pin origins match the grid, then restore

the desired display grid.

 

Keep in mind that the autorouter routes to the intersections of the

routing grid.

What Warren said.

What happens is that the autorouter is only going to put traces on the

grid, and it'll only put a trace to a pin if it can do so without

violating clearances.  Where I've run afoul of this is with an inch-pitch

board and a mm-pitch part: even with a fine autoroute pitch, there are

inevitable mismatches between the board grid and the centers of the pins;

the autorouter will try to keep the traces on its grid, which means that

it comes off of the part with a little jog.  If that jog puts the trace

into another pin -- it doesn't get routed.

Setting the autoroute grid to match the dimensions of the finest-pitch

parts (which are usually mm these days), and making sure to put all of

those parts on with the correct grid (be sure to pick them up at the

center with a control-left click), should make it all work.

The autorouter works by first doing a "boneheaded" route, with traces

going strictly up and down on one layer and side to side on the other. 

Then it optimizes a bunch of vias away.

If you're not getting 100% routing in that first step it'll go ahead and

optimize anyway (at least 5.x does).  So while you're making your board

fit for autorouting, it's a good idea to turn off the optimization

steps.  That way you can inspect what the autorouter has done and get an

idea of what you need to do to make autorouting possible.

Densely packed parts with lots of pins will spawn a lot of vias in close

proximity, and there needs to be enough space for all of those vias in

that first step.  So once you get the autorouter so it can connect the

pins at all, then you may need to fine tune the board so that it can get

past that first step.

Make sure to define the smallest vias you or your board house are

comfortable with. 

If that's not enough, and if the autorouter seems to be dying because

of not enough space between vias under or around the chip then decrease

the autoroute grid further, to let the autorouter squeeze in more vias.

If that doesn't do it, then make sure that your dense parts have lots

of space around them -- pack things too tightly around that 100 pin LQFP,

and you're autoroute will never happen.

And, if all else fails, hand route.  I find it's generally better to

spend time jiggering the board around so that it'll route itself, because

I inevitably get the board all routed then find out some circuit change

that Must Be Made; it generally doesn't take much to make a minor change

and hit autoroute again if you've already made it work once.

--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

Hi,

Thanks for the all the advice. My board still won’t route and my hand routing sucks. If I upload the files, does anyone fancy telling me if it’s possible to get the auto router to work ? And what setting I need to have to do that ... ?

I've got other changes / iterations I want to make to the board, and don’t want to have to keep hand routing it (badly :-)

Thanks.

Jon.

Jon Russell wrote:

Hi,

 

Thanks for the all the advise. My board still wont route and my hand

routing sucks. If I upload the files, does anyone fancy telling me if

its possible to get the quto router to work ? And what setting I need

to have to do that ... ?

 

I've got other changes / iterations I want to make to the board, and

dont want to have to keep hand routing it (badly

 

Thanks.

 

Jon.

Image:DACShield-sch.png

 

Image:DACShield-brd.png

Sure,  upload the files.

I'm faily sure you will run into the size limitation, which for the

newsgroup and Eagle Central is around 40k   You will likely have to deposit

them out there on the web somewhere for people to grab.

Warren

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Hello - I'd like to announce that posts exported to the CadSoft NNTP server from element14 Community will now have a footer containing a link to the post on element14.  This solution addresses the problem of missing inline images and file attachments for posts created on element14.  The URL in the footer doesn't require any registration or login to view.  Please contact me with any issues regarding the new footer.

Thank You,

Drew Fustini

element14 Community Dev Team

Drew Fustini (admin) schrieb:

Hello - I'd like to announce that posts exported to the CadSoft NNTP

server from element14 Community will now have a footer containing a

link to the post on element14.  This solution addresses the problem

of missing inline images and file attachments for posts created on

element14.  The URL in the footer doesn't require any registration or

login to view.  Please contact me with any issues regarding the new

footer.

Thanks for finally addressing this.

Two comments:

1. Please modify that link to display the particular post, not the

   start of the thread.

2. Once this problem is fixed, please correct the line wrap problem

   which is about the same age.

   (Your post, for example, consists of one single line!)

Tilmann

Drew Fustini (admin) wrote:

Hello - I'd like to announce that posts exported to the CadSoft NNTP

server from element14 Community will now have a footer containing a

link to the post on element14. This solution addresses the problem of

missing inline images and file attachments for posts created on

element14. The URL in the footer doesn't require any registration or

login to view. Please contact me with any issues regarding the new

footer.

 

Thank You,

Drew Fustini

element14 Community Dev Team

 

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To view any images and attachments in this post, visit:

 

http://www.element14.com/community/message/69093#69093/l/re-tssop-38-routing-problems

I can't agree that it solves the issue. Sure it's a short cut to the E14

postings, and I  that will help, should people get  really curious about an

image someone has posted.

For me the Element14 site is a bad experience to this side of the world so I

endeavour to keep away from it. You will appreciate this helps very little.

A similar but better solution would be to provide a link to the image files

somewhere at Element14 and not burdon us with the bloated web pages.

I have been watching your tests and they  promised a solution better than

this.

Thanks for trying

Sorry no prize.

Warren

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