Need to pick a few brains: FTDI USB IC's

scotta

I have two questions regarding Future Technologies USB to
serial conversion IC's.

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First:
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The FT232RL is packaged in a SSOP-28,
the FT232RQ is packaged in a QFN-32.

Has anyone used a convection toaster oven to solder these packages ?

Is one package easier to re-flow than another ?


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Second:
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I see two differences between Parallax and Future Technologies schematics.

Parallax schematic:
www.parallax.com/dl/docs/prod/prop/PropDemoDschem.pdf

Future Technologies schematic:
www.ftdichip.com/Documents/Schematics/FT232R/UC232R%20Schematic.pdf

The first difference is the grounding of the usb connector (on pins 4 and 5).

The second differences are the connections between VCC5I/VCC3I/VCCIO/3.3VOUT.


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Desired results:
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The USB connection will be used for programming and high speed communication with a PC.

When USB is disconnected, I would like the FT232 <--> Prop pins available for other tasks
(like a connection to the MAX232 IC).


Scott

TomS

Scott,

I haven't used the QFN-32 package but I've assembled around 40 boards using the SSOP-28 package using a toaster oven for reflow soldering and haven't had any problems.· One thing I did do is decrease the pad size on my stencil as I was getting too much solder on the pads and causing shorts.· If you are using a syringe to put solder down you will have to inspect the solder joints closely and correct any shorts. My experience with QFN packages leads me to the same conclusion that the size of the stencil opening for each pad must be smaller than the pad.· I use Eagle to layout the pcb but then use a commercial Gerber file viewer/editor to decrease the pad size for the stencil layer. As for ease of reflow soldering a leaded package will always be easier.

Use the Parallax Demo schematic.· The other one is for an old style usb connector, not the newer mini style.· The power connections are correct on the Parallax schematic.

I don't know if the RXD and TXD pins go to a high impedance state with 5V removed but regardless you can always put resistors in series to isolate them.

Tom

scotta

Tom,

Thanks for your input on this subject. I have pulled and replaced a few
SMT parts, but have not done any production. Everyone who has done
it will say 'you can do it...', I just need to get my feet wet.

Does a toster oven produce more board/component stress than a hot
air rework station ? (www.celeritous.com/estore/index.php?main_page=product_info&cPath=6_14&products_id=49

Thanks again,

Scott

TomS

Scott,

Does your hot air rework station preheat the board? When I rework boards I use a hot air bath and a hot air pencil does the actual solder melting. I've heard of quite a few people using a hotplate for preheat.· One problem with the hot air pencil is getting the airflow correct so you don't blow parts all over the board.· That isn't a problem with reflow soldering.· Otherwise I doubt if there is much difference.· I know the temperature profile of a toaster oven isn't as good as a commercial reflow setup but it can be fairly good.

See·this link for tips: http://www.stencilsunlimited.com/smt_stencils/smt_stencil_article_page1.htm
You don't absolutely have to use a stencil.

I've used a $20 Black & Decker with no problems for literally hundreds of boards.

Some people actually use a soldering iron to solder smt's, although I shudder when I think of soldering small caps that way. Also I can't see soldering leadless chips with a soldering iron.
See this post if you want to give the soldering iron method a try: http://forums.parallax.com/showthread.php?p=675231

Tom

Phil Pilgrim (PhiPi)

If you're connecting the FT232R to a Propeller, be sure to use 'LVC open-drain buffers (e.g. 74LVC2G07), powered from VCCIO, on the TxD and RxD lines. The pullup associated with each output should go to the supply (VCCIO or Vdd) associated with the input it's driving. This will prevent a powered Propeller from powering a non-connected FT232R through these lines and resetting itself through DTR. People using the Prop Clip or other devices that omit the buffers have had problems with this.

-Phil

scotta

Tom,

I have heated a few boards in a convection toster oven, pulled parts, cooled
the boards down, then replace the components. My biggest concern was the
amount of thermal expansion. The circuit board 'clicked for a few minutes
after pulling from the oven. This scared me a little.

Phil,
Can the FT232R be isolated with small signal diodes (1n914) ?

Thanks,

Scott

Phil Pilgrim (PhiPi)

Scott,

No, the diodes won't work. The pullup on the Prop's receive side will backfeed current through the diode to power the FT232R. The 74LVC2G07 is very tiny (SOT-363) and cheap, and it only takes one, plus a couple pullups. There's no good reason not to use it.

-Phil

TomS

Scott,

I've never heard of that.··Excuse me for asking but are you sure its not the oven making the noise? I'm also not sure that reheating in an oven is the best method of removing parts.· A hot plate or hot air bath coupled with a hot air pencil would put less stress on components that aren't being removed. The following link will take you directly to some examples of component removal:

· http://www.howardelectronics.com/navigate/videoclips.html

I think that reflowing in an oven puts less stress on a board because of the even heat application.·What you're trying to avoid is large temperature gradients across components and board alike.· Reflow soldering is the most widely used method in use today so it must have something going for it.

Tom

scotta

Phil,

Just spotted the 74LVC2G07 at digikey, they are ~$0.40 each, qty 1.
I'll give these a try, they look so tiny!

Tom,

The oven was contracting, but the board (an old linksys wireless router PCB) made
a few noises too. There are lots of connectors on this board with grounded shells. I
suspect some the noise was from this.

I think I'm going to pull a simm chip out of an old computer, remove all the chips,
replace and test with a memory checker. If I can do this, the rest should be easy
enough.


BTW, The Max232 documentation shows the two charge pump filter caps.
The "V-" cap goes to ground, the "V+"cap goes to VCC.

http://datasheets.maxim-ic.com/en/ds/MAX202E-MAX241E.pdf

The PropStick documentation below shows both "V-" and "V+" going to ground.
http://www.parallax.com/dl/docs/prod/prop/PropStick-v1.2.pdf

Any ideas if this makes any difference ?


Thanks again Tom and Phil.

Sincerely,

Scott

Peter Jakacki

What's wrong with running the FT232 in self-powered mode and isolating the lines with resistors? This way you can still override the RX/TX lines and not worry about the chip resetting inadvertently. Also saves having to add extra chips.

As regards the different packages I can say that the QFN can be a little trickier but by lengthening the pads a bit outwards I have found that the toaster oven reflow works fine. If on some occasions the QFN didn't solder properly I was able to touch it up with a soldering iron when I had the longer pads, plus the longer pads just seemed to have less problems.

If you are operating in a cool room then the cooling profile may be too rapid, you could try turning the oven off and leaving the door open in this case.

*Peter*

Phil Pilgrim (PhiPi)

Even through series resistors, the Propeller can power an unpowered FT232R (which doesn't draw much current) and reset itself. Any "solution" that permits powering an unpowered chip through its I/O pins has to be looked at askance. Why cut corners when the right way is so easy, small, and cheap?

-Phil

Peter Jakacki

Self-powered mode means the chip is powered not from the bus but from the same supply as the cpu. No corners cut here. Smaller, cheaper, better.

*Peter*

Phil Pilgrim (PhiPi)

Ah, right you are. I completely glossed over the "self-powered" bit in your comment. My designs have always had to accommodate both self- and bus-powered modes, so the isolation thing is always formost in my mind. But, yes, in self-powered-only mode, the isolation measures I suggested aren't really needed.

-Phil

SailerMan

Phil,

Do you have a schematic of what you've been talking about? It would help me to understand.

Eric

Phil Pilgrim (PhiPi)

Eric,

You can take a look at the schematic in the MoBoStamp-pe manual. This board is designed to be either completely bus-powered or partially (i.e. just the FT232R). It also has to support selectbale Vdd (3.3V and 5V). It uses the 74LVC2G07 for signal isolation and also for voltage translation, since VCCIO is always set to 3.3V. The reason the 'LVC family works for this purpose is that there are no I/O protection diodes running to Vdd — only to Vss. So no I/O voltages will end up on the power bus of an unpowered chip. (The schematic also contains a transistor structure in the TxD/RxD signal paths, which can be ignored for Propeller apps. It's there for switching the serial echo in and out. There's also a soft-start/brownout circuit for Vdd, which I highly recommend for totally bus-powered designs.)

-Phil