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Comment by civilitty | Hacker News Reader

What exactly do you mean by "getting started with FPGA on custom PCBs?" Have you made custom PCBs with high density BGAs before? Most non-trivial FPGAs (to me that means you can easily fit a decent softcore processor with space left over for your FPGA logic) will be ball grid arrays and almost impossible to DIY without xray inspection equipment and a reflow oven. You can get what you need for a few hundred $ on eBay if you're patient and lucky but even then, getting to the point where you can solder the chips reliably will take time.

If you have the budget for professional assembly, then I would start with a Digilent product that has an available reference design you can copy later. First get used to the FPGA and how it works (they are largely incomparable to a CPU or GPU except for the fact that they both have transistors). Then, design a simpler board with a smaller FPGA and work your way up to the big 500-1k pin chips.

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You don't need XRay and your reflow oven can be a toaster oven. I do recommend a cheap microscope and a good pair of tweezers though. With this you can do 0.5mm pitch BGA although that is pushing it. I've made hundreds of prototypes this way.

The fear of BGA parts is seriously overblown. The only real expensive part is the finer pitch parts will require tighter tolerances on the PCB which will take you out of the PCB batch services price tier.

edit: toaster oven, not toaster

With this you can do 0.5mm pitch BGA although that is pushing it.

If you have actually figured out how to consistently make boards with BGA-256/512 0.5mm pitch parts with just a toaster, I'd love to learn more about your technique. Even with professional inspection equipment and a pick and place, it's rarely worth the effort unless I have an imminent deadline.

For a beginner, I doubt the cost of trial and error would be cheaper than just having someone else do it. He could go with a larger pitch [edit: and smaller pin count] but I have never successfully introduced someone to FPGAs without a relatively huge chip capable of running a soft core closer to what they're used to with in general purpose computing.

I use a 10x jewelers microscope and tweezers to get the alignment perfect. The hardest part is actually applying the solder paste there is no room for registration errors. I triple check everything.

As for the reflow profile I raise the temperature gradually, with the thermocouple taped directly to the PCB with kapton tape. I do this manually, not with an automatic controller. My toaster oven is a convection model with a fan which helps, and I had some trial and error with the rack position.

The part in question is: TPS62180 which is a dual phase step down converter.

Edit: I think you are conflating I've done both high ball count and 0.5mm. Never both at the same time. I've soldered high ball count Xilinx FPGAs at 0.8mm which was relatively easy. Also I stress these are for prototypes. You can do production runs and that is how the company behind 3DRobotics got their start in the early days of quadrotors but its a whole other ball of wax.

My appologies, I work largely on client/personal projects in the high speed digital/RF domain so my assumptions can be all over the place. Client budgets/deadlines leave little room for mistakes and my dexterity isn't what it once was so I use a PNP or make funky assembly jigs (my CV implementation/hands makes it easier to align with pins).

If you can, I'd really recommend talking to dentists who are retiring in your area or buying an XRay machine on ebay. They're pretty cheap and relatively safe and will open up a whole world of PCB fab. The problem with BGAs is the lack of feedback: if you have a chip with hundreds of pins and precise requirements for power boot up timing or dozens of impedance matched traces, figuring out what went wrong with your boards is literally impossible without proper inspection.

Since you have an intuitive feel for soldering BGAs already, I think that with a visualization of the solder joints you would be able to solder high pin count chips at small pitches. Most issues that I've experienced with complex BGAs are caused by mass manufacturing where you dont have the room to reflow every 10th board and you find out too late that a variable (from personal experience: el nino) has changed and reduced your yields by double digits. If you can take the time to do it right by hand, the sky's the limit.

If your back is against the wall you can also hotfix design mistakes like swapping two LVDS MIPI2 pins by using a laser drill to strip the plating on microvias at an angle to preserve the trace above it and resoldering them with wire only slightly bigger than IC interconnects. An almost completely useless skill but nothing beats the feeling you get when you can command RoHS compliant surface tension to do your bidding. Fair warning though: here be dragons.

I've noticed it a lot going to trade shows that the North American market has turned so much to the high end that most people assume you need all this fancy gear to make anything. Its really stifling innovation in my opinion. People think they needs tens of thousands to get an MVP prototype out the door and most of the time it kills the idea. If you are a big company and already have the resources then by all means use them, but someone with a good idea shouldn't think its required to build something new.

On the high speed digital front we do have 1Ghz DDR3 and its worked very well - we don't even use controlled impedance. Of course we are extremely careful to keep traces short and length matched. Our application can handle a very small error rate, however in practice we haven't seen any corruption at all in testing.

Its not the right approach if you are pushing the envelope of technology. But if you need a bit more grunt than an atmega by all means design in the ARM SoC and DDR3. You don't need fancy gear to do it.

Edit: Also thanks, we're looking into expanding more manufacturing in house and you've given me some ideas. I'll definitely look into an XRay machine.

I agree on the trade show front. I feel like the worst offender is LPKF: I've come close to losing a significant fraction of my client base twice because several clients insisted that $100-200k capex/support expenses were worth slightly faster (theoretically) prototyping speeds and the vastly higher NRE of learning how to design for the damn thing. We ended up sending 90% of the boards to a third party assembler anyway - delaying the completion of the project two months past my initial estimate with twice as many revisions as I had predicted. Literally the worst project of my career was caused by inability to say no to flashy new tech. Twice (Yeah, still bitter).

On the other hand, I designed a STM32F4 based design last year and it was glorious. SnapEda for all symbols/footprints, Altium design vault for a specialized FPGA design, and github for a STM32 reference design PCB file using the same layers/copper weights as my requirements with imported fab design rules for good measure. The PCBs cost like $1500 and assembly was only $1600. 10 years ago I used to pay that much per single board in quantities of 30, for roughly the same complexity. This time, the whole thing (minus firmware) took a weekend.

Good luck with your mfg! If you have a chance, please blog about your experiences. There arent enough people spreading the art of solder.

It's been a little while since I did proto boards, but I remember a vast chasm between industrial & hobbyist, that basically forced me to wrangle packages I had no desire to deal with. You could get 4GB DDR2 DIMMs, or you could get a DIP 8kB SRAM. For a microchip you could get TQFP-32, or BGA-1000. So on and so forth. I remember a useful chip that came in nothing but QFN-16.

It seemed like it wasn't I need fancy gear to make blinky lights, but rather for anything better than 8-bit, kilobyte, and low MHz, the only parts available was the fancy commercial stuff.

SMT soldering is seriously easy, I teach it to kids - people are just too scared by it - I've done small BGAs - hot air reflow, a fine tipped iron, and if you have old eyes like mine a stereo microscope are all you need.

I do occasionally do small (100+ pins) BGAs - for that I get a steel solder stencil made and use solder paste and a cheap Chinese reflow oven

I'm trying really hard to communicate that you can use these parts pretty easily. Just get a stencil made with your PCB it's another $40. Add in a toaster oven, some solder paste and tweezers and your all set.

Once you see how easy it is to reflow a board you won't want to bother with hand soldering DIP packages anyway.

If you have access to a laser cutter you can make a pretty decent solder stencil by folding some clear package tape (so all adhesive is on the inside) and having it cut out the hole pattern. Lay it down on the PCB, squeegee some solder paste over it like you would a real solder mask, and carefully peel the tape up.

Been a few years, but IIRC a labmate of mine got a few units of .5mm highish ball count to work using that process and a toaster oven, but it is a bitch to test.

Would maybe do for a hobby project assuming you have access the equipment (e.g. a hackerspace or something) and are willing to drop a buttload of time into trying to get it to work, but if you are working for someone it is probably more cost effective to save the manhours and get it done by someone with the right equipment.

Things have changed a lot. You can get proper stainless steel stencils from lots of providers now, most PCB houses have it available as well - done to the same specs as your PCB.

The only time it really fails is if I'm lazy applying the solder paste or if I stop paying attention during reflow. Doing it in house saves a ton of money which is really important when you are trying to get your first prototypes up and running. Plus it also keeps you deeply in tune with what is possible to manufacture and what isn't. DFM stops being just a checklist.

We'd have a lot more hardware startups if people realized how cheap and easy its become. You need the prototype to get funding.

Agreed, if you meant a toaster oven. It does take precise temperature control if you want any chance at reliability. And a fine touch. It's easy to line the whole thing up one row off and have to scrap the part, which is a bummer if you can't re ball it.

If i have never reflowed a pcb before, how should i learn?

Reflow trash PCB's. Focus on packages like TQFP, and discrete components of human-visible sizes.

Your comment reads as if you equate 'BGA' with 'SMD', a BGA is a ball grid array with up to 1,000 tiny pads that have been pre-dipped in solder. Your toaster isn't going to work.

Yes, it will. Sure, it won't do wonders with 0.5mm pitch bga's, but 217 pin 0.8mm ones are doable even with a cheap Chinese hot air station. That's how I did these boards.

https://brainyv2.hak8or.com/

You can use a toaster, but your yield will be poor. Slapping some insulation on the oven and a PID controller will improve things significantly.

If you screw up the solder job, you can use a hot air gun to remove the BGA part, reball it (using a stencil, fairly cheap if its a common package), then try reflowing again.

If you practice for a few hours on scrap electronics, you can get good enough at it.

BGA is actually easier than leaded SMD parts. The tiny leads tend to bridge easily. With BGA you can be up to half the pitch off and it will center itself. I actually only go for leadless and BGA now because anything else is more of a hassle.

It is a bit rude to assume I don't know what a BGA part is.

The tweezers bit is what triggered that. Handling a large BGA with tweezers is going to scratch the PCB if you're not ultra careful.

Anyway, if you do this stuff often enough then I see no reason why you wouldn't get the proper tools, a rework station and an actual reflow oven or something with a PID controlled heating element would make your life so much easier. Working with bad tools would drive me nuts.

The reason the larger BGA center themselves is as soon as the solder goes fluid there is a lot of accumulated surface tension trying to reduce the size of the bridge and that will center the part all by itself. For that to work properly though everything has to become fluid more or less at once and stay fluid until the part has shifted to the right position.

The tweezers are for the 0802 passives and nudging the FPGA itself. It takes forever but then so does programming the pick and place for a one-off.

As for the reflow you can actually get more consistent results with the toaster oven - It just won't be able to handle the volume of actual production. Whatever you do just don't try going "semi-pro" and getting one of those IR ovens from China. Stick with the $40 walmart special. The toaster oven when heated slowly is much less likely to have hot and cold spots. Stenciling and placing parts take up a lot more time and are much more error prone.

Someone told me a spartan 6 might be enough for what I need, will qfp make my life easier?

I personally find them more annoying than just BGA. But conventional wisdom is they are easier since technically you can solder them with a soldering iron and eventually you'll get lucky and remove the last solder bridge without creating a new one.

The trick with BGA parts is to use solder paste and a stencil. Once you try it you will never go back. By the way make sure you read the xilinx documents extremely carefully. Its easy to forget to wire PUDC_B and prevent the thing from ever being programmed. There are lots of little gotchas.

IMO start with QFP, yes. It's not trivial and has a lot of hand work, but critically you can both see & fix your mistakes.

Aside:

Have you ever considered a DIY youtube channel?

If you, @slededit, others on this thread did something like Louis Rossmann, that'd be fantastic. I've never even seen a dentist's x-ray machine, much less seen it used for DIY design. That'd be amazing to watch.

Louis Rossmann https://www.youtube.com/channel/UCl2mFZoRqjw_ELax4Yisf6w

(But maybe y'all would dial down the rhetoric from an 11 to a 8 or 9.)

The project I'm working on right now is with a FLIR tau 2 thermal camera. Our university got it donated but the only decent expansion board is 1400 euro so i figured I might use that budget to build my own. I basically just need to convert LVDS to mipi csi-2 or ethernet. You can find the LVDS spec on page 16 of this spec sheet https://www.flir.com/globalassets/imported-assets/document/f... I need to make a board with the 50 pin hirose. Do you have any suggestions on getting started? I figured a qfp spartan 6 would be enough.

Man I hate those connectors. I don't have a problem soldering most things down to 0.4mm pitch & 0402 components without magnification, but these connectors love to bridge. Either lay down a (thin!) bead of solder paste, or try drag soldering (I prefer this).

With LVDS, make sure to keep tracks short,have some impedance control (doesn't need to be super strict), and match trace lengths. One issue I had before was a clock (90Mhz I think) that wasn't quite in sync. Ended up ordering a new board with a 8 channel inverter, and snaking the clock through a few of the the gates to get a slight delay.

I'm curious, my uni satellite club is using the same camera for a cubesat, is that the same domain you're using it for?

We are building ours for a drone. Please feel free to contact me if you want to work together to figure this camera out.

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