Working with Xilinx BGA packages

[enso]

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.

[ElEctric_EyE]

So oshpark's via's look like they do have soldermask over them?

[enso]

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.

[ElEctric_EyE]

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.

[enso]

Slighltly OT: Can Express PCB do unplated holes with pads on either side?

[ElEctric_EyE]

Unsure what you mean. Can you draw a pic or something?

[enso]

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.

[ElEctric_EyE]

Ah. No, ExpressPCB doesn't have provisions for this either in the board layout program.

[Arlet]

enso wrote:

I mean just a drill hit, without plating.

Also known as NPTH, Non-Plated Through Hole.

[ElEctric_EyE]

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?

[Arlet]

ElEctric_EyE wrote:

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?

Not simultaneously, but you can mount one side first, reflow it, and then do the other side. Unless components are really heavy, they'll keep hanging on the bottom while the solder melts. I've even done it with fairly big inductors and 1812 ceramic caps.

[enso]

ElEctric_EyE wrote:

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?

Quite the opposite, in an oven it is possible. On a hotplate, you really can't. However, I was considering mounting BGAs and top-layer devices on a hotplate (just because I am so happy with the results), then flipping the board over, applying paste and bottom layer and sticking into the oven for the bottom layer.

I would have to use a special jig to make sure that the top components don't touch the rack, but that's not too hard.

[ElEctric_EyE]

How do you keep the parts from falling off, when you reflow for the opposite side?

[Arlet]

ElEctric_EyE wrote:

How do you keep the parts from falling off, when you reflow for the opposite side?

Surface tension keeps them on. Sometimes they slide around a bit, but BGAs have so many balls that I don't expect a problem there.

[enso]

ElEctric_EyE wrote:

How do you keep the parts from falling off, when you reflow for the opposite side?

Like Arlet says, surface tension. And being very, very gentle.

My oven, a convection toaster oven, has a nasty vibrating fan. I really have to rewire and PID it soon.

Of course, the easiest thing to do is place components on top only.