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Working with Xilinx BGA packages
http://forum.6502.org/viewtopic.php?f=10&t=2070		 Page 3 of 10
Author:  enso [ Tue Jun 11, 2013 1:32 pm ]
Post subject:  Re: Working with Xilinx BGA packages
I am not sure why you'd want to fill the vias.

However, maybe it's worth a try. Everyone always insists that they will wick solder away through capillary action. However, on the internet, everyone will generally just repeat what one person stated somewhere, often wrong. Pretty much everyone says that you can't do BGA at home, you need an X-ray machine, you need solderpaste, etc.

First of all, make sure it's worth the trouble. You can only pass one wire between the pads anyway using 6mil wires - anything smaller will cost a lot. So if you drop too many connections to the bottom with filled vias, you won't be able to route them out anyway. You may find that it buys you nothing.

So let's say you want to fill the vias. First, you'd have to make sure there is no soldermask. You'd have to do add a 'filling' step prior to placing the BGA. To apply an even amount, you can either float the board on a layer of molten solder and let the wicking action pull solder into the vias. Alternatively, you can apply solderpaste and let it get sucked in.

I'd go with solderpaste - it seems more precise. Maybe from the bottom. You will need a lasercut mask with really small holes. Then you will have to make sure that the surface is even - if some balls don't touch the surface because they are raised, they may not make proper contact.

Once again, I certainly would not do this - I've lost enough 700ANs already :x Pretty soon I'll start a 'reballing' thread...
Author:  Arlet [ Tue Jun 11, 2013 1:41 pm ]
Post subject:  Re: Working with Xilinx BGA packages
I haven't tried vias in BGA pads, but I've tried vias in/near regular 0603 pads, and I can assure you that they do suck up the solder paste to the point where the joint becomes unreliable. I imagine it will be the same for BGA.
Author:  ElEctric_EyE [ Tue Jun 11, 2013 1:42 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Arlet wrote:
I haven't tried vias in BGA pads, but I've tried vias in/near regular 0603 pads, and I can assure you that they do suck up the solder paste to the point where the joint becomes unreliable. I imagine it will be the same for BGA.

This is without filling in the vias prior correct?
enso wrote:
...First of all, make sure it's worth the trouble. You can only pass one wire between the pads anyway using 6mil wires - anything smaller will cost a lot. So if you drop too many connections to the bottom with filled vias, you won't be able to route them out anyway. You may find that it buys you nothing...

I'm thinking first of all, via's under a ball would be great direct connection for the power and ground pins to the power and ground planes.
Author:  Arlet [ Tue Jun 11, 2013 1:44 pm ]
Post subject:  Re: Working with Xilinx BGA packages
ElEctric_EyE wrote:
This is without filling in the vias prior correct?

Correct. For regular components it's easier to inspect the joints afterwards, and add some solder to the bad ones.
Author:  enso [ Tue Jun 11, 2013 1:46 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Arlet wrote:
I would try this: use the soldering iron and a bit of solder wire, and rub it all over the pads + vias. Then take some desoldering wick and clean it all up. The capillary effect of the small vias should hold the tin. Clean the board with flux cleaner, and apply fresh flux.

The first couple of my attempts were along these lines (I thought I'd need some solder on the pads). None of these boards worked (for various reasons, some without an explanation).

I was really concerned with invisible shorts or other damage to the board made while dragging and wicking solder. Since the remaining solder residue is not perfectly distributed, some balls get more than others, so you'll never be sure if they all get soldered down.

I really like my final approach; I am absolutely sure that every ball has exactly the same environment and don't expect any trouble from the connections - and I would never make any bets on boards made after dragging and wicking solder. I am pretty sure that I will never touch the pads again with anything other than balls.
Author:  enso [ Tue Jun 11, 2013 2:00 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Here is a close-up of a failed board with a little resistor for scale. You can see the power distribution buses under the mask, and get a sense of the vias. Note that on my board I did not need that many IOs - I connected an SRAM and every pad of the 6502, a 44-pin connector (a few pins for power) and two 'butterfly' connectors with 4 extra IOs, and have a dozen spares connected to tiny pads. All using the top layer. Only a few special connections (JTAG, power, clocking, configuration) are routed to the bottom.
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Author:  ElEctric_EyE [ Tue Jun 11, 2013 2:09 pm ]
Post subject:  Re: Working with Xilinx BGA packages
So oshpark's via's look like they do have soldermask over them?
Author:  enso [ Tue Jun 11, 2013 2:13 pm ]
Post subject:  Re: Working with Xilinx BGA packages
ElEctric_EyE wrote:
So oshpark's via's look like they do have soldermask over them?

You could do mask or no mask, it's up to your CAD package. The post sockets in the upper left are just big vias.

EDIT: I think they call them 'tented' vias if they have a mask.
Author:  ElEctric_EyE [ Tue Jun 11, 2013 2:18 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Oh OK. BTW ExpressPCB is very competitive. Their 4 layer ProtoPro board service for a 21sqin board is $195. You get 4 boards. Their via's are not soldermasked. You do have to use their free software though. I like it and have grown used to using it.

EDIT: Permit me to show off my video board to show their quality. I should have added it to the thread a long time ago. It's at the very bottom of the first post.
Author:  enso [ Tue Jun 11, 2013 2:45 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Slighltly OT: Can Express PCB do unplated holes with pads on either side?
Author:  ElEctric_EyE [ Tue Jun 11, 2013 2:55 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Unsure what you mean. Can you draw a pic or something?
Author:  enso [ Tue Jun 11, 2013 3:22 pm ]
Post subject:  Re: Working with Xilinx BGA packages
ElEctric_EyE wrote:
Unsure what you mean. Can you draw a pic or something?


I mean just a drill hit, without plating. This would have to be a second drilling pass after the plating process is done, and Oshpark's fab does not do it.

I happen to need drill hits piercing pads on top and bottom, without electrically connecting them.
Author:  ElEctric_EyE [ Tue Jun 11, 2013 3:27 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Ah. No, ExpressPCB doesn't have provisions for this either in the board layout program.
Author:  Arlet [ Tue Jun 11, 2013 3:27 pm ]
Post subject:  Re: Working with Xilinx BGA packages
enso wrote:
I mean just a drill hit, without plating.

Also known as NPTH, Non-Plated Through Hole.
Author:  ElEctric_EyE [ Tue Jun 11, 2013 3:44 pm ]
Post subject:  Re: Working with Xilinx BGA packages
Back to BGA mounting, in an oven. It's not possible to mount devices simultaneously to the top and bottom of the board is it?
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