Trouble with the PCM1804.. shorting?

Heya,

I've soldered a texas instruments PC1804 A/D converter to a SSOP28
adapter board. At the moment, all adjacent pins are measuring about
400-700 thousand ohms of resistance between eachother.

However, when I hook up the digital and analog voltages to the chip, it
appears that the analog voltage pins (Vcc, 5 volts) are getting very
hot and possibly reflowing the solder and shorting out? The leftover
flux will start to smoke. I'll disconnect everything and measure the
resistance between the Vcc and Analog Ground pins, and there's anywhere
from almost no resistance to 10 thousand ohms.

I'm hooking it up like this:
http://www.pixelwrench.com/schem3.jpg

Could it be that the pins are just shorted? or am I hooking something
up wrong and it's shorting internally? All I've had hooked up to the
PCM1804 was just the Digital and Analog voltages and grounds, nothing
else. Any ideas?

Regards,
Matt

 

[martin griffith]

On 5 Mar 2005 10:15:47 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

Heya,

I've soldered a texas instruments PC1804 A/D converter to a SSOP28
adapter board. At the moment, all adjacent pins are measuring about
400-700 thousand ohms of resistance between eachother.

However, when I hook up the digital and analog voltages to the chip, it
appears that the analog voltage pins (Vcc, 5 volts) are getting very
hot and possibly reflowing the solder and shorting out? The leftover
flux will start to smoke. I'll disconnect everything and measure the
resistance between the Vcc and Analog Ground pins, and there's anywhere
from almost no resistance to 10 thousand ohms.

I'm hooking it up like this:
http://www.pixelwrench.com/schem3.jpg

Could it be that the pins are just shorted? or am I hooking something
up wrong and it's shorting internally? All I've had hooked up to the
PCM1804 was just the Digital and Analog voltages and grounds, nothing
else. Any ideas?

Regards,
Matt
I think you have swapped pins 13 and 14

and you have probably let the magic smoke out of the 1804


martin

"An eye for an eye makes the whole world blind"
Gandhi

 

The datasheet is pretty clear that pin 14 is Vdd and pin 13 is Digital
Ground, so I don't think that's the case... Anyways, the magic smoke is
comming either from pin 22 or 23. I'm pretty sure I havn't swapped
those either.

 

[martin griffith]

On 5 Mar 2005 10:48:30 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

The datasheet is pretty clear that pin 14 is Vdd and pin 13 is Digital
Ground, so I don't think that's the case... Anyways, the magic smoke is
comming either from pin 22 or 23. I'm pretty sure I havn't swapped
those either.
pin 13 digi gnd in your pic is connected to your regulator out

martin

"An eye for an eye makes the whole world blind"
Gandhi

 

Ah, sorry about that, the schematic I drew up is incorrect, however
it's hooked up correctly on my prototyping board. Updated Schematic.
(Or at least trying to.. my computer doesn't want to cooperate with me)

 

[Active8]

On 5 Mar 2005 11:00:18 -0800, mattcarpenter@gmail.com wrote:

Ah, sorry about that, the schematic I drew up is incorrect, however
it's hooked up correctly on my prototyping board. Updated Schematic.
(Or at least trying to.. my computer doesn't want to cooperate with me)

You have to be smarter than whatcher f*ckin' with
--
Best Regards,
Mike

 

[martin griffith]

On 5 Mar 2005 11:00:18 -0800, in sci.electronics.design
mattcarpenter@gmail.com wrote:

Ah, sorry about that, the schematic I drew up is incorrect, however
it's hooked up correctly on my prototyping board. Updated Schematic.
(Or at least trying to.. my computer doesn't want to cooperate with me)
well,

I assume you have checked with a magnifying glass for shorts, so all
thats left is to replace the chip/ remove and resolder it.

Never work on a project at the weekend, when the shops are shut, and
you have no spares at hand

martin

"An eye for an eye makes the whole world blind"
Gandhi

 

[Active8]

On 5 Mar 2005 14:41:00 -0800, mattcarpenter@gmail.com wrote:

I am finding it somewhat strange, however, that I'll re-work some
solder joints, then check the resistance between two adjacent pins, and
it'll be somewhere around 400k ohms to 700k ohms, but I'll hook the
power up, apparently it'll start shorting somewhere (the chip doesn't
get hot but the pins do, as do the voltage regulators, and I can hear
the leftover flux sizzling, letting out smoke, etc..) then I'll unplug
everything and check the resistance between the pins again and there
will be none.

So could a 500 ohm connection between +5v and ground be able to
generate enough heat to reflow the solder and create a bridge?

No. That's only 50 mW.

If there
are any bridges, they're probably under the chip. It's a SSOP package
which is pretty small and almost impossible to see between the
fluxed-up legs. I'll try to resolder the part again I suppose.

Pull the chip, fire up the regulators and check the voltages. It's
easy to get a regulator hooked up wrong when 2 regs have different
pinouts. You can thread a piece of wire behind the pins and tie one
end off to another part. Then pull the wire under the pins as you
heat them. It'll lift the pins off the PCB.

Maybe it would be easier for you to solder the thing by first
soldering the PCB pads. You don't need much solder and you can wick
the excess off the pads with Solder Wick. Then flux the chip pins
and hold it down on the footprint. Just heat the ends of the legs
and the solder will reflow. I'd remove the flux afterwards.
--
Best Regards,
Mike

 

[Rich Grise]

On Sun, 06 Mar 2005 11:47:49 +1300, Terry Given wrote:

mattcarpenter@gmail.com wrote:
I am finding it somewhat strange, however, that I'll re-work some
solder joints, then check the resistance between two adjacent pins, and
it'll be somewhere around 400k ohms to 700k ohms, but I'll hook the
power up, apparently it'll start shorting somewhere (the chip doesn't
get hot but the pins do, as do the voltage regulators, and I can hear
the leftover flux sizzling, letting out smoke, etc..) then I'll unplug
everything and check the resistance between the pins again and there
will be none.

So could a 500 ohm connection between +5v and ground be able to
generate enough heat to reflow the solder and create a bridge? If there
are any bridges, they're probably under the chip. It's a SSOP package
which is pretty small and almost impossible to see between the
fluxed-up legs. I'll try to resolder the part again I suppose.


Do the maths, dude. 5^2/500 = 50mW, so clearly not. Look at your pcb
with a decent microscope. Or crank the power supply current limit up to,
say, 20A and fry the short. Dont do what one of my techs did once though
- he couldnt fry the short because of its impedance, so cranked the
voltage up to about 35V. It fried the short, then fucked the micro. oops.

Maybe while it's melting its own solder, he could dab it with solder

wick.

Good Luck!
Rich