mlf package ground pad

[Jamie Morken]

Hi all,

I am using a 64 pin mlf package (atmega128) and it has a large square
pad on the bottom of the package that is hooked up to the ground pins
electrically. I didn't know that this package has a large ground pad
when I designed the board so there are some vias under the package that
are different signals. To avoid electrical contact with these vias and
the ground pad would it be best to use some lacquer to insulate the
ground pad before soldering? I think it would be ok to do this but am
not sure if this pad needs to be hooked up electrically to the board.

cheers,
Jamie

 

[Paul Burke]

Jamie Morken wrote:

Hi all,

I am using a 64 pin mlf package (atmega128) and it has a large square
pad on the bottom of the package that is hooked up to the ground pins
electrically. I didn't know that this package has a large ground pad
when I designed the board so there are some vias under the package that
are different signals. To avoid electrical contact with these vias and
the ground pad would it be best to use some lacquer to insulate the
ground pad before soldering? I think it would be ok to do this but am
not sure if this pad needs to be hooked up electrically to the board.

The big pad is part of the heatsinking for the device. For hand
assembly, I put a square pad underneath with a hole big enough to get a
fine iron tip into, and that seems to work. With vias there, perhaps you
could rescue the project with a fine heat conducting insulating layer,
perhaps peel an old fashioned mica power transistor insulator apart.

Paul Burke

 

[Robert Baer]

Jamie Morken wrote:

Hi all,

I am using a 64 pin mlf package (atmega128) and it has a large square
pad on the bottom of the package that is hooked up to the ground pins
electrically. I didn't know that this package has a large ground pad
when I designed the board so there are some vias under the package that
are different signals. To avoid electrical contact with these vias and
the ground pad would it be best to use some lacquer to insulate the
ground pad before soldering? I think it would be ok to do this but am
not sure if this pad needs to be hooked up electrically to the board.

cheers,
Jamie

Solder mask will do very well as an insulator, provided there are no
high voltage differences.
Be advised that ther will be a significant increase of capacitance
from the traces to ground = lower impedance.

 

[Jamie Morken]

Robert Baer wrote:

Jamie Morken wrote:

Hi all,

I am using a 64 pin mlf package (atmega128) and it has a large square
pad on the bottom of the package that is hooked up to the ground pins
electrically. I didn't know that this package has a large ground pad
when I designed the board so there are some vias under the package that
are different signals. To avoid electrical contact with these vias and
the ground pad would it be best to use some lacquer to insulate the
ground pad before soldering? I think it would be ok to do this but am
not sure if this pad needs to be hooked up electrically to the board.

cheers,
Jamie


Solder mask will do very well as an insulator, provided there are no
high voltage differences.
Be advised that ther will be a significant increase of capacitance
from the traces to ground = lower impedance.

I'd like to try that, is solder mask sold in electronics supply stores
as a liquid that can be applied?

cheers,
Jamie

 

[Robert Baer]

Jamie Morken wrote:

Robert Baer wrote:
Jamie Morken wrote:

Hi all,

I am using a 64 pin mlf package (atmega128) and it has a large square
pad on the bottom of the package that is hooked up to the ground pins
electrically. I didn't know that this package has a large ground pad
when I designed the board so there are some vias under the package that
are different signals. To avoid electrical contact with these vias and
the ground pad would it be best to use some lacquer to insulate the
ground pad before soldering? I think it would be ok to do this but am
not sure if this pad needs to be hooked up electrically to the board.

cheers,
Jamie


Solder mask will do very well as an insulator, provided there are no
high voltage differences.
Be advised that ther will be a significant increase of capacitance
from the traces to ground = lower impedance.

I'd like to try that, is solder mask sold in electronics supply stores
as a liquid that can be applied?

cheers,
Jamie

No; i ASS-u-MEd that you were relating to "mass" production.
You could use mylar tape if the temperature does not get too high, or
Kapton tape if it approaches 185C (would need a high melting point
solder, then).
Glass fiber tape is fairly decent up to 125C, and the el-cheapo
plastic "electricians tape" is good for room temp work.
Or, one could try spraying on some General Cement Red Dielectric
Sealer ("fog" on thin swipes at a few feet or so away, for better
uniformity).

 

[The Al Bundy]

"Jamie Morken" <jmorken@shaw.ca> wrote news:2WIfd.8682$df2.1134@edtnps89...

Robert Baer wrote:

I am using a toaster oven and solder paste, I think during soldering it
hits about 230Celcius for around 30 seconds. I think "magnet wire"
varnish will hold up as good as anything.

cheers,
Jamie

In this case I would use Kapton tape to isolate the via's. This tape is also
used for covering the holes/traces when the PCB goes through the wave
soldering, so they dont get full of tin (e.g. a gold plated PCB edge
connector). The tape can handle quite high temperatures.

Al