Re: which one among the available FPGAs is best for a fresher?
- From: fpga_toys@xxxxxxxxx
- Date: 11 Feb 2006 14:22:53 -0800
John Adair wrote:
If you are looking to add a chip to one of your own circuits then you will
want to look for a package that is easy to mount on a board. None of the
modern packages are an easy solder onto a circuit unless you have profession
kit especially BGA packages. If you just want to buy a board to use a number
of vendors including ourselves have low cost products for students. We even
have student based pricing under our UAP scheme.
Actually, if looking to add an FPGA chip to one of your own designs,
BGA is by far the easiest for a hobbiest. Hand soldering a 100-200 pin
QFP varient package is a nightmare for a hobbiest without a stereo
microscope and a very fine low temp iron, and lots of aquired skill.
Even stenciling it is a nightmare, as it takes accurate placement to
avoid smearing the paste.
There are several tricks for prototype BGA assembly which work well for
a hobbiest, and a low volume commercial proto lab as well. Solder paste
for attachment is a super pain, as it requires accurate placement to
avoid smearing the paste - nearly impossible by hand without a
placement fixture. What works much better is to manually wet all the
BGA pads on the PCB by coating the PCB with ample water soluable flux
and dragging a large solder ball across all the pads with a fat
"screwdriver" soldering temp on a temp controlled iron -- do not
linger, to avoid heat damage to the pcb or device. Do this until all
the pads have a uniform solder bump and are shinny. This works equally
well for cleaning up a BGA device prior to reballing using the
SolderQuik preforms. Then wash off the expended flux, dry and bake to
remove any moisture trapped by the cleaning process to avoid popcorning
pads off. Next, apply a liberal amount of water soluable flux to the
pcb and BGA balls, and carefully mate the balls to the pads by eye. If
you have a device outline on the pcb that is accurately registered to
the pads, this helps. (either copper, soldermask or silkscreen). It
sometimes help to bring two ball pads on the outside edge of the board
to a land large enough to touch with the soldering iron, which allows
fusing a ball on opposite sides/corners of the package to hold the
device in place during handling for assembly. Ditto for QFP packages
-- prewetting the pads and leads with an iron, and then just fluxing
for assembly is sometimes easier than accurate placement with paste -
as you can always drag the iron with a little dangling solder across
the leads that didn't quite wet/attach with a fillet well enough.
If you have a toaster oven, take some time to calibrate it, and get a
temp stick.
http://www.pcbexpress.com/stencils/stencil_article_page1.htm
Stencils for all the small parts which can be easily hand placed is a
great idea too.
While I use a couple Chipmaster SMD-1000's in my lab, I've also use my
wifes table top convection oven and a toaster oven for demonstrations
for local hobbiests. I suggest preheating the oven/toaster to about
325F and setting the project board into the preheated oven, then cycle
the oven/toaster up above soldering temp by 15-25 degrees and leave it
there about 2-3 times longer than the traditional temp profile of a
commercial reflow oven as there isn't quite as much energy available to
heat the board and parts to temp as quickly. This is where a temp stick
is useful, but not required.
It will generally take a few boards before you find exactly the perfect
process that leaves all the parts properly wetted and attached with
minimum heat and process time. For BGA's this is easy to confirm, by
making sure the balls all have a uniform "squat" which confirms the
inside balls came to temp, and wetted, allowing the package to fall
slightly and left the balls slightly compressed in shape. All other
parts take a good magnifying glass or lab scope, and verify the wetting
left strong fillets on all the connections. Where the fillet is
obvious, but lacking in volume, take note as that pad needs a larger
paste mask opening next time.
Reballing BGA packages can be done the same way. But another process is
sometimes easier. That is to use a digital hot plate, or heavy electric
skillet and an inexpensive non-contact IR thermometer. Set the BGA in
the reballing frame on the preheated surface which is just over
soldering temp, and cover with a shinny preheated lid, and turn the
heat source off. The mass in the hot plate or skillet will bring the
BGA balls to temp, then cool back below plastic state. After a few
minutes remove the lid partially (lift one edge) to allow the device to
cool, but not hard shock it with cold air. Then remove and place on a
warm surface to further cool.
.
- References:
- which one among the available FPGAs is best for a fresher?
- From: chaitu11311@xxxxxxxxx
- Re: which one among the available FPGAs is best for a fresher?
- From: John Adair
- which one among the available FPGAs is best for a fresher?
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