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FPGA to PCB
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secretstep



Joined: 17 Jul 2008
Posts: 2
Post16 Aug 2008 22:30   

FPGA to PCB
Hi!
I'm new to here!

I took a course in VHDL, but I'm having a trouble by doing a real project.
Basically I ordered the Cyclone II TQFP 144 pin, but then it says "BAKE" the chip. The moisture sensitivity strip became pink from blue when the seal is opened. So I guess I have to bake the chip before soldering.... but what does it mean by "BAKE" the chip? How can we BAKE the chip? I'm pretty sure we don't cook these.

Also, why do we need a socket for TQFP or any other FPGAs?
What are the "burn-in socket" and "test socket"?
What are the advantages of using the sockets?
Will the FPGA be damaged if I just solder them on a PCB?
and the sockets seem to be very expensive.. anyone know a good cheap socket?

It's my first time buying FPGA and putting it into a digital system. Please help me!
Thanks!
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TechSpec



Joined: 15 Aug 2008
Posts: 3
Post16 Aug 2008 23:12   

Re: FPGA to PCB
Forget the PCB's. Buy evaluation board (Xilinx etc.) cheapest one is 150$. It includes everything needed to make a real project without thinking of PCB's, soldering and other difficult tasks for a newbie. I think there is enough work without all this building your own board and debugging it.

Tech
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Sinisa



Joined: 03 Mar 2003
Posts: 337
Helped: 66
Post16 Aug 2008 23:17   

Re: FPGA to PCB
Baking is meant for production soldering to produce consistent results on automated soldering line. Any moisture on the pin will raise required time for proper soldering and because it's not uniform it might impede automated soldering. Baking is process where compromised parts are put in the oven and temperature is raised to completelly dry parts before soldering.
If you are attempting to solder this chip manually with soldering iron, you might want to practice soldering for a while before atempting something that dense. I am not sure what was the motive to go with this chip for the project, but might be too difficult for first attempt o soldering.
I would suggest to select some pre soldered board with FPGA and all needed chips arround it already soldered on the board. When you decide to practice soldering, take some simpler project with .1" pin density of IC's and through-hole components and as your skills improve make it harder.
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secretstep



Joined: 17 Jul 2008
Posts: 2
Post17 Aug 2008 3:37   

Re: FPGA to PCB
thanks for reply.
This FPGA must be implemented on a PCB board with a limitation on its size.
10cm width and length with other components such as analog muxes etc... so development kit wouldn't be an ideal choice.

For the soldering, I've done some of them (SOIC) as an electronics engineering student, but not the pins as tiny as FPGA.

So to bake the FPGA, I just put it in a oven?
What should be the temperature of the oven and also how long should I bake the chip?

Also is it better to use the socket? why is that?

Thanks
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FvM



Joined: 22 Jan 2008
Posts: 5151
Helped: 766
Location: Bochum, Germany
Post17 Aug 2008 9:28   

Re: FPGA to PCB
The moisture sensitivity classification of electronic devices is standarized by IPC/JEDEC J-STD-020. See: http://www.jedec.org/download/search/jstd020d.pdf

Baking as standarized procedure means 24h at 125°C. The TQFP packages usulally have been assigned a floor-lifetime of seven days, that means, after removing from the dry protective atmosphere, they have to be processed within one week or should be baked again. Another option is to store mositure sensitive devices in a gas-tight container (e.g.a screw-cap glass) with drying agent bags.

As discussed, the danger of cracking a device package due to absorbed moisture is mainly in reflow soldering (or manual hot-air soldering). During manual soldering, the package as a whole doesn't get that hot.

Quote:
Forget the PCB's

Designing your own FPGA board actually has some traps, as any complex design, e. g. with a microprocessor. A cheap Terasic DE2 evalboard would be an option to practise FPGA techniques without designing a board. But there be many reason to design a custom board. When I started working with FPGA, I typically needed some special signal processing hardware interfaced to the FPGA, that involves designing a test board anyway. So it was most simple, to include the FPGA with the design. If development time has highest priority, it can be meaningful to uitilize both options in parallel.
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