Klondike - 16 chip ASIC Open Source Board - Preliminary

[TomKeddie]

Are there any tracks passing from the under side of asic chips placement? If not why don't we use my cooling idea?
http://mining-bitcoins.co/simple-cooling-idea-for-klondike-project/

I don't think you can do this.  The pressure from the heatsink would probably lift the pads from the PCB and pop the ASIC off.  There are lots of excellent links here on thermal vias - seems like an industry standard for stuff like LEDs.

[ektwr]

OK. I recognize that you are right. It was just a thought after all. The best part of this DIY project which i love most is that we are thinking as a team. Also i like thinking that i 'm a member of this team. Gentlemen we make a history here! 

[WynX]

an aside, I'd like to know if using silver epoxy solder available at some electronics shops is viable for on the fly upgrades. It may be easy to paint a little bit on the QFN tabs and then carefully place and hold until hardened. My question would be heat transfer - whether silver/epoxy will conduct enough heat? )

Have updated original post now.

http://www.mgchemicals.com/products/adhesives/electrically-conductive/silver-conductive-epoxy-8331/

Hmm yeah I've not found the most up-to-date reference (1992) on the thermal conductivity, but here's a paper: http://infohouse.p2ric.org/ref/27/26227.pdf

Combining this with the info of alfabitcoin, it seems that conductivity of the conductive epoxy is 'high', while it is still very much lower than typical solder (SnPb or including Ag).

Roughly said;
1 Watt/mºK for conductive (silver) epoxy
30-50 Watt/mºK for SnPb solder, Ag solder would presumably be higher.

This seems to be significant to not recommend epoxy for at least the thermal via connection (base of asic chips)?

EDIT:
Quick guestimate:
Thermal difference between heatsink and chip;
Assuming 1W/mºK
Assuming 7x7mm surface
Assuming 2.5W/chip
Assuming 0.2mm epoxy compound between chip and heatsink

Delta T= (2.5/(0.7*0.7) * 0.02 / 0.01 = ~10 degrees Kelvin or Celcius

Roughly losing 10 degrees from chip to heatsink (in PCB here) is not nice... (plz check my calculations, it's late and I'm sleepy )

Same formula for solder with 30 Watt/mºK would result in;
Delta T= (2.5/(0.7*0.7) * 0.02 / 0.30 = ~0.34 degrees Kelvin or Celcius

[BkkCoins]

an aside, I'd like to know if using silver epoxy solder available at some electronics shops is viable for on the fly upgrades. It may be easy to paint a little bit on the QFN tabs and then carefully place and hold until hardened. My question would be heat transfer - whether silver/epoxy will conduct enough heat? )

Have updated original post now.

http://www.mgchemicals.com/products/adhesives/electrically-conductive/silver-conductive-epoxy-8331/

Thanks for that info. Their data sheets give some figures for thermal conductivity. The 10 min curing epoxy is not nearly as conductive as the 4 hour long-cure version.

10 min = 0.893 W/(mK)

4 hour = 1.75 W/(mK)

This one being more thermally conductive then their thermal adhesives.

For miners who want to pre-assemble boards and then add ASIC chips later when they arrive it may be useful to run some tests. It's a lot lower than SnAg solder (being about 50 W/(mK) ) but also a very thin layer would be used so it may have little net effect. Anyway, I don't want to side track things - it was just something I thought about.

[villex]

an aside, I'd like to know if using silver epoxy solder available at some electronics shops is viable for on the fly upgrades. It may be easy to paint a little bit on the QFN tabs and then carefully place and hold until hardened. My question would be heat transfer - whether silver/epoxy will conduct enough heat? )

Have updated original post now.

http://www.mgchemicals.com/products/adhesives/electrically-conductive/silver-conductive-epoxy-8331/

Thanks for that info. Their data sheets give some figures for thermal conductivity. The 10 min curing epoxy is not nearly as conductive as the 4 hour long-cure version.

10 min = 0.893 W/(mK)

4 hour = 1.75 W/(mK)

This one being more thermally conductive then their thermal adhesives.

For miners who want to pre-assemble boards and then add ASIC chips later when they arrive it may be useful to run some tests. It's a lot lower than SnAg solder (being about 50 W/(mK) ) but also a very thin layer would be used so it may have little net effect. Anyway, I don't want to side track things - it was just something I thought about.

I agree with you, for test proposals seems right but for final assembly it will be something to test later, the price doesn't look attractive either.

[BkkCoins]

Quick guestimate:
Thermal difference between heatsink and chip;
Assuming 1W/mºK
Assuming 7x7mm surface
Assuming 2.5W/chip
Assuming 0.2mm epoxy compound between chip and heatsink

Delta T= (2.5/(0.7*0.7) * 0.02 / 0.01 = ~10 degrees Kelvin or Celcius

Roughly losing 10 degrees from chip to heatsink (in PCB here) is not nice... (plz check my calculations, it's late and I'm sleepy )

Same formula for solder with 30 Watt/mºK would result in;
Delta T= (2.5/(0.7*0.7) * 0.02 / 0.30 = ~0.34 degrees Kelvin or Celcius

Isn't the m in W/mK in meters? 7mm would be 0.007m but also the thermal pad on the ASIC is only 5.4mm (and for closer accuracy the power dissipated by the ASIC should be 1.86W only). And a thickness of 0.2mm would be 0.0002m. I guess they cancel out anyway.

Checking with Fouriers Law,

dT = q*s / (K*A) where q is Watts, s  is thickness, K constant, A area.

Using 1.75 for 4 hour silver epoxy,
1.86 * 0.0002 / ( 1.75 * 0.0054*0.0054) = 7.3 C
vs. solder,
1.86 * 0.0002 / (50 * 0.0054*0.0054) = 0.25 C

So pretty close to what you said. To be viable you would have to be able to use a much thinner layer of epoxy compared to solder. If a 3mil plastic stencil is used that would be 0.07mm thick, which gives a temp difference of 2.5 C.

Using a taller heat sink or more air flow may be able to offset that. It's an interesting idea for those who don't want to get involved in reflow soldering ASICs in multiple passes but would require some testing to see how well it works. I don't plan to do that any time soon.

[d0qtrx]

I really *REALLY* dig this project. Looks awesome, and I hope I can scrape enough coin together to join up a group buy if we get a run of these manufactured.

[BkkCoins]

I just pushed up my tentative final Gerber files including ground stitching, 5x5 vias grids, and other details. I haven't really got what I want on the silkscreen layer but that doesn't really matter now.

Don't order boards using these Gerbers unless you are ok with throwing them out due to mistakes - at this time. I'm sure I'll have a few edits and unlike a 2 layer board the edits may not be on outer layers. But on the whole this is very close to what will be my first proto boards.

Any experts on PCB layout - please feel free to comment or point out places I can improve, preferably in a PM to me. I need some guidelines on pcb fiducials if I'm to place them as I haven't had to do that for my boards before.

Note: there is a few DRC errors due to my use of a AGND - GND link component. This is by design since I don't have an easy way to connect two nets in Kicad without it re-naming one, and forcing a zone merge. So this should work. Unlike some this program doesn't let me tag errors as ignore.

edit: just pushed a fix in AVDD tracks - if you downloaded already (speedy you) then pull again.

[torzsy]

´thank you. One thing, can you have a small border around the board 5mm? with such a border it is much easier for assembly lines or something like a pulling border?

There is almost that now but there are a few parts within 5mm of the edge. Not sure about that as I didn't really design it for automated assembly lines (something which I know nothing about). I suppose you could use a slightly bigger board size.

As I asked several companies for manual / automatic assembly (EU), most of them said they would do only manual assembly because of the very small (0402) components. However there are larger companies can do automatic assembly, but you have to order 1000+ boards and pay for the programming and assembly line setup about 1500 USD!

Imho below 100 boards manual assembly is the first option, however its more expensive.

[t13hydra]

Hi,
Just in case you need a bit of precious info, use a good Nokia or Samsung charger to power the RasPi. Otherwise it will restart and it's hard to troubleshoot where the problem is. You need at least 800mAh to power it and a 1200mAh charger will work like a charm.

By the way, what options do we have for ordering heat sinks? I guess we will need eighter 4 small ones or one 10x10cm big one to mount on the Klondike board. I'd like to order boards from you and i'm thinking on my options.

Thanks,
Steve

[tuxfool]

Hi,

Are you also going to get Solderpaste stencils manufactured? Some Manufacturers sell produce stencils as well as PCBs. It would be nice for those of us going the DIY route to not have to get a stencil made somewhere else...

[Chefnet]

´thank you. One thing, can you have a small border around the board 5mm? with such a border it is much easier for assembly lines or something like a pulling border?

There is almost that now but there are a few parts within 5mm of the edge. Not sure about that as I didn't really design it for automated assembly lines (something which I know nothing about). I suppose you could use a slightly bigger board size.

As I asked several companies for manual / automatic assembly (EU), most of them said they would do only manual assembly because of the very small (0402) components. However there are larger companies can do automatic assembly, but you have to order 1000+ boards and pay for the programming and assembly line setup about 1500 USD!

Imho below 100 boards manual assembly is the first option, however its more expensive.

Hi,

I have a company I have worked with for automated doing with small series from 1 to 200 normally.
normal price for a prototype board is 400€ for the pattern and then the price vary on how much and what components are used.
For example a Ztex Licence Production without the fpga chips (quantity orderd 25) 70€ per Board excl. VAT and when you want 1 > 250€ (I have done it last year)

Example calculate for 1 Prototype:
1. 400€
2. 250€
3. Avalon AsicChips (1-16)*7€
4. PCB Board: ~10€ (plus initial costs 50€)
5. Heatsink 10€
~ 850€ excl. VAT

mass production (from 25++) price for 1 Board inclusive asic chips:
1. 70€
2. Asics: (16) ~112€
3. cooler/heatsink 10€
3. PCB: 10€
~200€ excl. VAT

so 4.5gh/s should be possible with 2.5 BTC :-) or in € 45€ / gh/s

Please Note this is only a calculation the endprice will vary.

[KS]

´thank you. One thing, can you have a small border around the board 5mm? with such a border it is much easier for assembly lines or something like a pulling border?

There is almost that now but there are a few parts within 5mm of the edge. Not sure about that as I didn't really design it for automated assembly lines (something which I know nothing about). I suppose you could use a slightly bigger board size.

As I asked several companies for manual / automatic assembly (EU), most of them said they would do only manual assembly because of the very small (0402) components. However there are larger companies can do automatic assembly, but you have to order 1000+ boards and pay for the programming and assembly line setup about 1500 USD!

Imho below 100 boards manual assembly is the first option, however its more expensive.

Hi,

I have a company I have worked with for automated doing with small series from 1 to 200 normally.

You think you could get us a quote?

[Chefnet]

post updated: https://bitcointalk.org/index.php?topic=190731.msg2154669#msg2154669
similar price like burnin

[Bicknellski]

That seems good.

If there are many local options like this then the real savings in time and shipping / import taxes will be where these DIY avalons will beat the competition. No more waiting in a pre-order limbo either. Also if you can add reflow for the asics to that with heat sink let people get their own PSU I think you will see more people opt for this method.

Price comparable and time is lessened which makes all the difference. What I am keen on is January 2014 when I bet something gets unveiled... heck maybe even a small hint at the bitcoin conference from Avalon might even happen.

So will you be do this Chefnet or are you just giving everyone idea on prices?

[sensei]

We would need solder stencils with and without the ASICs in place as I bet a fair number of boards will be assembled without the ASICs and it would be better when placing the ASICs on unpopulated boards to have clean pads instead of solder puddled up on the pads.

[curiousone]

do the BOM/gerber files exist?  could I take a look please?

[loshia]

do the BOM/gerber files exist?  could I take a look please?

Dude all that exists is here:

https://github.com/bkkcoins/klondike

pls check to see if the file you are looking for is inside

[Bicknellski]

We would need solder stencils with and without the ASICs in place as I bet a fair number of boards will be assembled without the ASICs and it would be better when placing the ASICs on unpopulated boards to have clean pads instead of solder puddled up on the pads.

Many ways to skin the cat.

[BkkCoins]

We would need solder stencils with and without the ASICs in place as I bet a fair number of boards will be assembled without the ASICs and it would be better when placing the ASICs on unpopulated boards to have clean pads instead of solder puddled up on the pads.

When I get further along I'll be testing my computer cutter with synthetic papers to make stencils. I've had this thing for a couple years and never used it hardly. I have read online this works quite well for short runs, and at least one vendor sells this type for $10 each. Real stencils (steel) are quite expensive and would be worthwhile for someone doing a larger assembly run (these can be had for $70 in China).

With this cutter method you use postscript output to create the stencil so I can provide such a file with/without ASICs later on github. If this cutting method works well enough then I'll either include a stencil with each kit order or make it an optional item for a low added cost. A stencil without ASICs could have a separate footprint of a single ASIC to be used individually.