Please Sanity-Check My BGA Fanout

Making PCB Layouts, Manual routing, Auto-routing, Copper pouring, Updating from Schematic, Manufacturing Output
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joen
Posts: 9
Joined: 04 Mar 2013, 17:06

Please Sanity-Check My BGA Fanout

#1 Post by joen » 11 Mar 2013, 16:32

I am a very new PCB layout hobbyist. So far, I have successfully made about half a dozen boards and sent them to both OSH Park and Bay Area Circuits and they are coming back just great. My workflow is probably naive, but it works - I get the schematic right, send it to PCB, use auto-place, then place the parts myself to get a nice looking logical board consistent with how users would want inputs and outputs to work and keeping despike caps next to their ICs, etc. Then I do auto-route, which works fine. Now I want to try a BGA part (having soldered some QFNs and TQFPs with no problem) to test my hot-air soldering skills there. Atmel's ATMega2560 CBGA-100 totally defies using this workflow - auto-route will not work. I changed my trace width and via size to be OSH Park's minimums. No go. So after reading some of the threads on this forum, I see a suggestion to use the fanout feature in PCB editor. Now, this confused me a bit because it doesn't provide a total fanout for BGA parts, as far as I can see, it just fans to vias. And I take that to mean you are expected to do the fanout yourself at that point using the vias to the bottom level. So this is what I did. To make this work on Atmel's part I had to use 0.006 trace width and clearance, OSH's minimum. I had to use 0.013/0.007 vias, OSH Park's minimum. And on top of that, to keep to 0.006 clearances, I had to change the BGA pads to 0.012. The library pattern had them at 0.016. But it was essential to keep the clearances from violating. Anyway, here is the design:

Top:
BGAFanoutTop.png
Bottom:
BGAFanoutBottom.png
Given this, I have a few questions:

1. Given that I changed the pads, do you think this will solder OK? Is it more or less likely to create solder bridges or might be more prone to other types of soldering problems or failures?

2. Now that I manually routed this one part, do I have to manually route the rest of the design? To do this test, my schematic was the 2560 itself and nothing else. Once I add additional parts and wires, will the software know how to route to these trace ends or is it going to try to route to the pin (an impossibility)?

3. What else can I do better? Is this a sane design?

Would love any criticism. I am totally new at this and I don't understand all the ramifications. Just designing what seems to make sense.

-- 11 Mar 2013, 19:09 --

Now I found this video you posted some time ago:

http://www.youtube.com/watch?v=ahUhpnCI8Xk

For some reason, when I tried this the last time it didn't seem to want to autoroute as easily as this video implies. Maybe it's because this is a small pitch device and OSH Park has moderate pitch requirements so it couldn't make it happen. In any case, I will be trying to autoroute a BGA automatically again and see if I can get it to work.

-- 11 Mar 2013, 19:18 --

I think I didn't reduce the pads the last time I tried to use autoroute. This time I did and autoroute works pretty good but leaves three ratlines that it can't seem to route (and takes up enough space routing it that there is no way to route those pins at all now other than to use jumpers). Bit I am making progress at least.
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Alex
Technical Support
Posts: 3102
Joined: 14 Jun 2010, 06:43

Re: Please Sanity-Check My BGA Fanout

#2 Post by Alex » 13 Mar 2013, 08:54

The fanouts look good but pads are quite small. I'm not sure if it is good for soldering.
You can create fanouts manually, then check routing and net class settings. If the settings are OK autorouter can do the rest of job. The BGA package is simple and autorouter can cope with routing. But if you need perfect routing you can do it manually.

joen
Posts: 9
Joined: 04 Mar 2013, 17:06

Re: Please Sanity-Check My BGA Fanout

#3 Post by joen » 13 Mar 2013, 14:20

This BGA package is 100 closely spaced pads. Is it simple? It's not coping with it, at least on a 2 layer board, until I make the pads smaller. Here is my test scheme, please tell me what I am doing wrong:

I place two headers and one BGA 100 on a schematic. The parts are:

Type: IDC2X25M
Pattern: IDC50M

Type: ATMEGA2560_CBGA
Pattern: BGA-100/10x10x0.8_9x9
BGATest0.png
I convert to PCB. I make the traces as small as OSH Park allows. 6mil traces 6mil clearance 13/7 mil holes.

Using 16 mil pads, the default, I fan the BGA out and I autoroute. It leaves some ratlines:
BGATest4.png
I change to 12 mil pads, fan back out, autoroute, much better:
BGATest7.png
When I go to 10 mils it actually autoroutes to completion.

I have more images, the board software only allows 3. I hope my explanation gives you enough information. Where am I dropping the ball? Is this just an impossibility on a two layer board without obscenely small pad sizes?
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Alex
Technical Support
Posts: 3102
Joined: 14 Jun 2010, 06:43

Re: Please Sanity-Check My BGA Fanout

#4 Post by Alex » 14 Mar 2013, 10:26

100 pin BGA is smaller than 1000 pin BGAs, isn't it? I think your package is simple.
Anyway, if you selected BGA package you should be ready to use thin wires and clearances, small vias, multi-layer boards and manual routing. Your board can be routed in two layers, I'm sure.

joen
Posts: 9
Joined: 04 Mar 2013, 17:06

Re: Please Sanity-Check My BGA Fanout

#5 Post by joen » 14 Mar 2013, 12:05

Alex wrote:100 pin BGA is smaller than 1000 pin BGAs, isn't it? I think your package is simple.
Anyway, if you selected BGA package you should be ready to use thin wires and clearances, small vias, multi-layer boards and manual routing. Your board can be routed in two layers, I'm sure.
Oh, I am sure it can be. You can see the manual fanout I did above. That meets DRC. So I bet I can route from there no problem once it is broken out to that level. I am just concerned that I had to make the pad size smaller than the default and you indicated that you were too. Is it possible that 6 mil traces / 6 mil clearance is just too big for this part? Any idea how standard that is? Do other manufacturers go smaller and it is just a higher charge? Sorry for the giant number of questions. Once I get a few of these questions ironed out in my mind I think I will be OK for at least this complexity of part which is about as far as I want to go.

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