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Author Topic: Cheap and not simple repair of S7 hash board  (Read 2053 times)
Amortis
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March 28, 2017, 03:16:14 AM
 #21

They do make actual QFN test sockets.  Kinda pricey, especially for 0.4mm, but if you lurk on eba+aliexpress long enough one should come along for under $50.  Or else make your own using ENIGold PCBs from eg OSHPark.

But... why?

I mean are you going to desolder all the chips from your PCB, test them individually, then solder them back on?  The SMT pads are incredibly fragile, even a very skilled operator with the right equipment will damage the board ~5% of the time in a multi-cycle rework operation like this.  Multiply that by 45 chips/board and you're not likely to end up with any more working boards than you had to start with.


Yes, i agree with mjosephs,  I'd rather make pcb from a pcb manufacturer than buy from ebay or aliexpress.
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lightfoot
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March 28, 2017, 03:23:03 AM
 #22

The real problem is re-placing QFN chips just sucks. With BGA you have nice little balls that you can place on the pads, preheat, heat, and they drop with a nice little thud, but QFN is a crapshoot. Either there is too much solder on the pads, not enough, almost enough and you have to drag solder around the edge with an iron, and then you pull up a pad and life just loses meaning.

Bigger question: Is it worth it? Replacing the QFN48 power chips on a KNC controller board is worth it (although I prefer swapping the BGA FPGA by a long shot and have never screwed one up) but is it worth it for a S7?
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March 28, 2017, 10:16:00 AM
 #23

The real problem is re-placing QFN chips just sucks. With BGA you have nice little balls that you can place on the pads, preheat, heat, and they drop with a nice little thud, but QFN is a crapshoot. Either there is too much solder on the pads, not enough, almost enough and you have to drag solder around the edge with an iron, and then you pull up a pad and life just loses meaning.
Yep. And the leadframe on this damn chip is quite thin, so there's essentially no wicking or heating from the top. Hey, everyone needs a hobby Wink

Biggest problem I have at the moment is I have no way to prove that the chip I'm putting in worked _before_ I touched it.

Again, these PCBs are quite good - I haven't seen a delamination without extreme heat being applied. And the "interesting" pads have external access - technically you can rework, but it's a real pain at .4mm.

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Bigger question: Is it worth it? Replacing the QFN48 power chips on a KNC controller board is worth it (although I prefer swapping the BGA FPGA by a long shot and have never screwed one up) but is it worth it for a S7?

I agree completely. It's not a business, it's a hobby. The only bit that's easy and worthwhile is Frankensteining the 14v supply (boost modules on ebay are really cheap and it doesn't appear to be a high current or particularly sensitive rail). Also, why the heck does that area of the board always corrode?
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March 28, 2017, 12:30:39 PM
 #24

The 14V only sources power to local LDOs on the top couple nodes; the rest are fed by Vcore from nodes higher up the chain. Those LDOs provide 1.8V for control and IO and 0.9V for the PLL. Total current requirement per chip is almost nothing, on the order of 10mA. On the 2Pac I use a resistor divider for the 0.9V and it works fine. As long as your boost rail is at least about 1.5V higher than the Vcore of the top node (since ground reference there is about 0.7V lower and a good 1.8V LDO will work at 2.1-2.2V) and it can source ~100mA it should work. You're right, that is easy.

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March 28, 2017, 10:31:58 PM
 #25

So...

Because everyone deserves to laugh at someone else occasionally... I present to you, the worlds least efficient stickminer.

33w at the wall, one linear supply powering a SMPSU powering a single bm1384. Hash rate of 11Gh/s or so at an approximate efficiency of...

0.03GH/J

Err... There is a point to this, trust me.

http://i.imgur.com/saIXivj.jpg
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April 07, 2017, 03:08:31 PM
 #26

Right. Finally got another few minutes to look at this.

https://lh3.googleusercontent.com/LwFGnYJ_ZAJJGS9bmSvpmBLZ9qW1FbnyvMxRueCCxtyzT4lIeKMiOsSUi-KrQEfeQyqSgGxXJapAdLthqfcl8HTkMmYQrDynrQZmvvfHn0SM_6d2Vi7nS3Ec9KIAL7S2BdQoqltYEOnSexAFkJG_NPmxSqfAMUUWCId4rfFhbNduOsLUrAVYe5b8m8Favk8VyJ1uiuR4sA_D7K66_uHVl5jxdlPXsLGEttCKUlRitZF95hRnxnGi0-_NEVXEf_XjKLdgzoNMg-H5mHSBGgFnmDxsnG6i-Ma39NasX8AoBUjJRbV9_F2_LPYcAiE_9rXF2kP3dSnU4wPF4yG8KMKUEIPHpeCM4ua9T7oRqvS8szWguzA2fQvNdhhaWPto_F8TIy2iz6NGM2XU03i24TKGJIWWSEBlMEzv8pT7G7Xg-1PFCOnynjR-xaVNVZbQqqMZ-pNgKmwknlYCJHS9pbjzxwXxpRQn_KQeLOLS6H1-5eNdEfM933-MT0ylnDMe_vwloIBjI9A0zI-1UotaFc6xKQ2w_FIBp99qYqeM0r39CPLBmJ6GbOSXeJKSrjAD81NGf3413SnEeDzFdKdYzqtRwygBcrtZblTYN5-flav-v3wL9OZhDqk7H6CsA1zDzr3KzflXHcYtRDEJtXXGY5juZcSPuROw_1GtbutSu-4_HQ=w361-h641-no

CP2104 does 1.8v IO voltage natively. Of course, it's never that simple - the dongle I bought bridges 3.3v and the IO voltage under the chip. Aaaargh.

Reflow, slice, reflow, kynar, variable IO voltage yay.
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