TSSOP Exposed Pad - How do I solder?

[Nick.]

As a hobbyist, I've never needed to use SMD before. I now want to make a
circuit based on the Linear Lt3474:

<http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1094,P13452,D9377>

Only availiable in TSSOP. However, I notice that the exposed pad ("pin
17") must be connected to ground according to the datasheet. To me that
means soldered.

How might I do this?

 

[[email protected]]

Nick. skrev:

As a hobbyist, I've never needed to use SMD before. I now want to make a
circuit based on the Linear Lt3474:

<http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1094,P13452,D9377>

Only availiable in TSSOP. However, I notice that the exposed pad ("pin
17") must be connected to ground according to the datasheet. To me that
means soldered.

How might I do this?

put a few large(r) vias in the pad for the exposed flag, then you can
solder if from the underside
side using a nomal soldering iron through the vias.

maybe a hack, but better than nothing...

-Lasse

 

[Georg Acher]

|> Only availiable in TSSOP. However, I notice that the exposed pad ("pin
|> 17") must be connected to ground according to the datasheet. To me that
|> means soldered.
|>
|> How might I do this?

Make a wide (>1.5mm) plated via in the middle of the pad (ie. a double sided PCB)
and use a powerful soldering iron at the backside...

 

[martin griffith]

|> Only availiable in TSSOP. However, I notice that the exposed pad ("pin
|> 17") must be connected to ground according to the datasheet. To me that
|> means soldered.
|>
|> How might I do this?

Make a wide (>1.5mm) plated via in the middle of the pad (ie. a double sided PCB)
and use a powerful soldering iron at the backside...

I'm doing the same, is it better to use 2 or more vias, with smaller
holes?



martin

 

[Chris Jones]

I'm doing the same, is it better to use 2 or more vias, with smaller
holes?



martin

I like to use several vias large enough to be able to fit very fine solder
wire down them. If I feed solder down one hole and it comes up another
hole then I figure that at least part of the paddle must be properly
attached.

Some other people have different ideas from me on the subject of the via
size.

Chris

 

[martin griffith]

I like to use several vias large enough to be able to fit very fine solder
wire down them. If I feed solder down one hole and it comes up another
hole then I figure that at least part of the paddle must be properly
attached.

Some other people have different ideas from me on the subject of the via
size.

Chris

Thinking along the same limes here, wonder if there is a spice model
(joke)



martin

 

[[email protected]]

Chris Jones skrev:

I like to use several vias large enough to be able to fit very fine solder
wire down them. If I feed solder down one hole and it comes up another
hole then I figure that at least part of the paddle must be properly
attached.

agreed, solder moves toward the hottest place, so heat one via and
adding solder at
another should work.

and if the flag is there to help get the heat out of the chip its
probably a good idea to
have as many vias as possible (with no thermal relief) anyway

Some other people have different ideas from me on the subject of the via
size.

Chris

size is always up for discussion

-Lasse

 

[PeteS]

I have a local guy who puts things like this down for me. I put the
recommended pad on the board (both sides, because it's for heat
transfer anyway) and he simply puts it in position after dropping some
solder on pins and pads and blasts it with a hot air gun.

Works a treat.

Cheers

PeteS

 

[Noway2]

This may sound a bit off the wall, but you may find it easier to
pre-heat the board prior to / while soldering. While there is
professional equipment to do this, it is way too expensive for the
hobbyist. However, a electric hotplate works too, you just have to be
carefull not to burn yourself while working. I have seen articles
(searched for solder and hot plate) showing how to use this technique.

Another solution that works well, is the use of a toaster oven in place
of a reflow oven, but this requires the use of solder paste. If you
are interested in this method, again suggested temperature time
profiles can be found on line. I actually know someone who sucessfully
built a DSP based system board this way, so it isn't all that far
fetched.

 

[James T. White]

I'm doing the same, is it better to use 2 or more vias, with smaller
holes?

I've used two 70-mil vias with the LT1976 with good success. The 70-mil vias
allowed me to get a tip all the way down into one hole and feed solder through
the other. To do this you pretty much need to have a solder mask on the top of
the board to isolate the heat tab from the pins. When you have enough solder,
you see the level begin to rise in the vias. Surface tension and solder mask
keeps the solder from flowing beyond the tab. Two things that I have found make
this work better are injecting some liquid solder flux in the vias from the back
and using a hot air gun to preheat the board before starting to solder.

Good luck.