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Author Topic: FPGA -- DIY @ Low Cost  (Read 7905 times)
MrTeal
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May 03, 2012, 09:02:20 PM
 #21

Not to derail this, since 1) More competition is better and 2) I love DIY, but this would be a challenging board to DIY. Surface mount components like resistors and caps aren't tough, but even with a reflow oven doing the BGA FPGA would be difficult without a stencil. Even if you got the process down well enough that you weren't wasting any, after you add up the cost of the LX150s, the PCBs and all the other components, it would be really difficult to save any money vs something like the proposed 4xLX150 @ $640. If you're ordering singles that would be what you'd pay just for FPGAs at Digikey.

Great idea for a project, but just be aware this might not save much money.
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||bit
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May 03, 2012, 09:22:06 PM
 #22

Not to derail this, since 1) More competition is better and 2) I love DIY, but this would be a challenging board to DIY. Surface mount components like resistors and caps aren't tough, but even with a reflow oven doing the BGA FPGA would be difficult without a stencil. Even if you got the process down well enough that you weren't wasting any, after you add up the cost of the LX150s, the PCBs and all the other components, it would be really difficult to save any money vs something like the proposed 4xLX150 @ $640. If you're ordering singles that would be what you'd pay just for FPGAs at Digikey.

Great idea for a project, but just be aware this might not save much money.

Thanks none-the-less for your thoughts.

Where do you find a proposed 4xLX150 option? I found one posting, but it seems to be a time limited offering proposed. I think selling at manufacturing cost for a short term.

I'm not fully aware of the requirements to mount an FPGA chip. Just that it has a lot of pins to solder to the board. Could you explain in layman terms what you mean when you say "but even with a reflow oven doing the BGA FPGA would be difficult without a stencil"?

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May 03, 2012, 09:41:11 PM
 #23

You just can't solder a ball grid array by hand.

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MrTeal
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May 03, 2012, 09:42:33 PM
 #24

Not to derail this, since 1) More competition is better and 2) I love DIY, but this would be a challenging board to DIY. Surface mount components like resistors and caps aren't tough, but even with a reflow oven doing the BGA FPGA would be difficult without a stencil. Even if you got the process down well enough that you weren't wasting any, after you add up the cost of the LX150s, the PCBs and all the other components, it would be really difficult to save any money vs something like the proposed 4xLX150 @ $640. If you're ordering singles that would be what you'd pay just for FPGAs at Digikey.

Great idea for a project, but just be aware this might not save much money.

Thanks none-the-less for your thoughts. BTW: Where do you find a proposed 4xLX150 option?

https://bitcointalk.org/index.php?topic=78239.0

The biggest issue any of these projects is going to have is getting the volume needed for good discounts. Without volume pricing, everything doubles in cost.
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May 03, 2012, 09:54:41 PM
 #25

You just can't solder a ball grid array by hand.

Thank you for your response. I can see how that would be impossible by hand.

What does the chip mount to exactly? I like how in RPH's thread [see below] he has his chip mounted to a tiny board which then mounts to a larger board. This seems like a good approach in the event an FPGA needs to be replaced.

One of the reasons I see this whole thing as a possible for DIY, is just reading RPH's thread where he used a skillet or something. See here: https://bitcointalk.org/index.php?topic=44891.0
If he can do it, why not anyone? Despite the chances of losing a chip here or there, I think such a process could be fine tuned to be overall successful & repeatable.

What he has there in that thread is [seems] very elegant and simple. Though, I am uncertain about what happens under the FPGA... Is a multilayer PCB required for our purposes? (i.e. btc mining)

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May 03, 2012, 10:01:12 PM
 #26

You just can't solder a ball grid array by hand.

Thank you for your response. I can see how that would be impossible by hand.

What does the chip mount to exactly? I like how in RPH's thread [see below] he has his chip mounted to a tiny board which then mounts to a larger board. This seems like a good approach in the event an FPGA needs to be replaced.

One of the reasons I see this whole thing as a possible for DIY, is just reading RPH's thread where he used a skillet or something. See here: https://bitcointalk.org/index.php?topic=44891.0
If he can do it, why not anyone? Despite the chances of losing a chip here or there, I think such a process could be fine tuned to be overall successful & repeatable.

What he has there in that thread is [seems] very elegant and simple. Though, I am uncertain about what happens under the FPGA... Is a multilayer PCB required for our purposes? (i.e. btc mining)

||bit.out

It mounts to an array of holes that match the balls on the chip. It requires a bit of skill, and if you screw up you can potentially lose the chip. That is bad when each chip costs more than the rest of the parts combined.

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MrTeal
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May 03, 2012, 10:30:19 PM
 #27

It mounts to an array of holes that match the balls on the chip. It requires a bit of skill, and if you screw up you can potentially lose the chip. That is bad when each chip costs more than the rest of the parts combined.

It doesn't really mount on an array of holes, they're just exposed copper pads on the PCB. The only thing keeping the chip centered on the PCB is inertia and the surface tension of the liquid solder. It sticks to the balls and pads and tends to suck the chip into the right position if it's slightly off-center.

Still, if you're serious about doing more than a couple of these, I'd seriously recommend spending the $100 to get a stencil made. It will make applying the paste so much easier. I have a LPKF Protoflow S, and even with that I hate doing too much SMT work without a stencil just because it's so much of a PITA to apply the solder paste. With one, a simple board like rph's would take 10 minutes to apply paste, populate and stick in the oven. It does a lot nicer job with getting a consistent amount of paste as well, so you're much less likely to run into issues of shorts between pins.
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May 03, 2012, 11:42:48 PM
 #28

If you wanted to build like rph's wouldn't it make sense just to pool some funds and have a batch of daughter boards made with the fpgas correctly soldered to them?  Leave the DIY to the board that the daughter boards mount onto.

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May 04, 2012, 07:20:21 AM
 #29

If you wanted to build like rph's wouldn't it make sense just to pool some funds and have a batch of daughter boards made with the fpgas correctly soldered to them?  Leave the DIY to the board that the daughter boards mount onto.

Yes. However, the main purposes here is to minimize all costs where possible. Also, if I really want to optimize, I need to be exposed to as much of the FPGA circuitry as possible.

About the miniature board that has the pads that he soldered the FPGA chip to. It has pins on the outer edge to plug the mini-FPGA board to the slightly larger board. Do you think the solder balls are all accounted for by a pin? (i.e. one for one.)
Is that mating pair a product of Xylinx? Can those boards be made using a regular DIY etching process? If it requires multiple layers, then I don't see how it could be practically done....but to be thorough, I prefer to ask anyway.
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May 04, 2012, 07:33:22 AM
 #30

Interested.
How about we create a fund to pay for research or pledge to a developer?
I'm in 10BTC for the start.

Sounds good in concept if we were to set a ~1MH/$ goal, and require proof that it would work.
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May 04, 2012, 07:44:31 AM
 #31

If you wanted to build like rph's wouldn't it make sense just to pool some funds and have a batch of daughter boards made with the fpgas correctly soldered to them?  Leave the DIY to the board that the daughter boards mount onto.

Yes. However, the main purposes here is to minimize all costs where possible. Also, if I really want to optimize, I need to be exposed to as much of the FPGA circuitry as possible.

About the miniature board that has the pads that he soldered the FPGA chip to. It has pins on the outer edge to plug the mini-FPGA board to the slightly larger board. Do you think the solder balls are all accounted for by a pin? (i.e. one for one.)
Is that mating pair a product of Xylinx? Can those boards be made using a regular DIY etching process? If it requires multiple layers, then I don't see how it could be practically done....but to be thorough, I prefer to ask anyway.

rph's boards are all custom and professionally produced. In fact, I'm pretty sure he ended up having the FPGAs soldered by a shop as well, but did the first ones himself by hand for prototyping (although I could be wrong about that). There is definitely an advantage to having them done by someone with the right tools. It costs money, but how much does your time cost? Not to mention the FPGAs that don't survive.

In my experience, DIY is not about saving money. In fact, most of my weekend projects have been a huge money sink. Smiley That's not to say it isn't fun and educational, though!

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May 04, 2012, 09:47:10 AM
 #32

rph's boards are all custom and professionally produced. In fact, I'm pretty sure he ended up having the FPGAs soldered by a shop as well, but did the first ones himself by hand for prototyping (although I could be wrong about that). There is definitely an advantage to having them done by someone with the right tools. It costs money, but how much does your time cost? Not to mention the FPGAs that don't survive.

In my experience, DIY is not about saving money. In fact, most of my weekend projects have been a huge money sink. Smiley That's not to say it isn't fun and educational, though!

Admittedly, there is an element of fun involved, even in the prospect of considering DIY...and a dash of frustration...or maybe better stated, a cup of frustration.  Wink

I was looking at the images of the X6500 board here: http://fpgamining.com/products/x6500-rev3
How many pins or balls or whatever are actually connected & used outside of the FPGA chip? It looks like only a few capacitors on the underside are used to connect some balls or pins, and a few traces on the top surface are used. Am I missing a lot of connectivity that is happening in middle layers?

I am curious about how many pins are utilized for the input and output to the chip in normal operation. I don't see why all the dozens of pins or balls need to used on the chip during mining. Seems just a single data input, a clock, VCC, common and a single output are enough. Unless of course you have many inputs/outputs that go to or come from different/parallel 'threads' for lack of a better term.

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May 04, 2012, 10:02:54 AM
 #33

There aren't many pins used for communication. Usually 2 for actual job/share transfer and 4 for JTAG or some other means of bitstream upload.
The vast majority of pins is for the three required power supply rails, one of which is carrying like 10 amps of current any may not have more than very few milliohms of series resistance. I'd think there are like 100 power supply balls total on that package.

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May 04, 2012, 10:46:14 AM
 #34

There aren't many pins used for communication. Usually 2 for actual job/share transfer and 4 for JTAG or some other means of bitstream upload.
The vast majority of pins is for the three required power supply rails, one of which is carrying like 10 amps of current any may not have more than very few milliohms of series resistance. I'd think there are like 100 power supply balls total on that package.

From that, I would think there is no need for a multi-layer PCB to support a FPGA chip like the Spartan 6x150. Is that an accurate notion?
lame.duck
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May 04, 2012, 11:18:36 AM
 #35

While you could try your luck with a 2 layer board, it should be clever to use a 4 layer design with massive power and ground planes and a lot of vias. If you look at the the thread by eldentyrell where he started with a board  using conventional capacitors he got problems when the design used higher  gate count and or frequencies.  Its not only a matter of ohmic resistance but also the inductance of the power and ground connections.
It could even worth a consideration to make a design that uses additional blocking caps 'under' the BGA and leave them unpopulated if testing shows they can left out.
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