Best way to solder prototype pcb with thermal-pad ICs?

[Winfield Hill]

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?


--
Thanks,
- Win

 

[Gerhard Hoffmann]

Am 27.02.20 um 10:29 schrieb Winfield Hill:

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

I usually solder the power pad first using hot air and
a surprisingly small amount of tin. Using too much will make
the tin protrude from underneath the package. Then I measure if
there are unwanted connections. I everything is OK, I solder
the other 8 connections.

My experience is with ADA4898-2, and it's good. My Aoyoue (?)
hot air station also removes the chip easily if needed.
And I love that Metcal 5xxx. I think it would also do the job.

When there are a lot of GND vias under the chip that may
behave differently. Then a hot plate could be required from
below. Only 100°C or so, we don't want to bring the entire
board above the glass temperature.

I have a lab-scale vapor phase machine but it was never
ever needed up to now.

regards, Gerhard

 

[Michael Kellett]

On 27/02/2020 09:29, Winfield Hill wrote:

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

I put the solder paste on with a stencil now, I've tried using an air
powered sysringe type thing and you just can't control the amount of
paste properly. Stencils are cheap compared with boards and time. Use
leaded solder if you can. Use decent solder paste and don't keep it out
of date.

The best way to reflow is to use a proper temperature controlled IR
thing. Ideally you want a belt type job with several zones - there are
cheap Chinese ones but I didn't dare buy one. I bought a CIF FT03 from
Farnel or Rs (forget which) - it cost loads compared with Chinese (about
ÂŁ2k) but if you are reflowing a prot pcb with a component cost of maybe
ÂŁ1k and 2 days work in putting the bits on, how much risk do you want to
take ? The downside of the CIF is that it doesn't have forced cooling.

If you want to spend less then use a cheap hotplate, I used to get this
to temperature and place the board on it and watch the paste reflow.
Dont' attempt this with lead free - the margins aren't big enough to get
away with it.
On prototpyes I much believe in putting vias through thermal pads, it
lets excess solder out, it lets you check if the pad is soldered
(visually and with a needle probe (does work - I've done it)) and it
makes rework easier.

If you need to rework then use temperature controlled air heating from
below (mine cost about ÂŁ500 from Metcal) and then you need much less top
heat.

MK

 

[Mikko OH2HVJ]

Winfield Hill <winfieldhill@yahoo.com> writes:

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

I'm usually doing SnPb paste + preheating to around 130°C + hot air to
reflow. On our board with extended pad you might be able to reflow with
an iron.

Dont't know if that's the best way, but failures are quite rare and
usually due to rushing and burning components with air.

--
mikko

 

[George Herold]

On Thursday, February 27, 2020 at 4:30:10 AM UTC-5, Winfield Hill wrote:

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

With a bigger 8 SOIC pac. I had two plated holes where the pad was
and hand soldered it from the back side.

George H.

--
Thanks,
- Win

 

[Phil Hobbs]

On 2020-02-27 05:02, Gerhard Hoffmann wrote:

Am 27.02.20 um 10:29 schrieb Winfield Hill:

  After soldering the chip's 8 pins, I used a large flat
  tip to wick solder under the IC onto its thermal pad.
  At test, the circuit malfunctioned.  Did I damage the
  IC while soldering?  Removal was a pain.  Using a talon
  tip failed to heat the pad, ditto for my hot-air system,
  only a giant flat tip applied to an exposed ground plane
  above the chip (see image) got everything hot enough to
  release the IC.   I think I'll forgo soldering the pad,
  as the converter only dissipates 70mW with a 300mA load.

  But the experience makes me wonder, what's the best way
  to do prototype assembly of pcbs having thermal-pad ICs?


I usually solder the power pad first using hot air and
a surprisingly small amount of tin. Using too much will make
the tin protrude from underneath the package. Then I measure if
there are unwanted connections. I everything is OK, I solder
the other 8 connections.

My experience is with ADA4898-2, and it's good. My Aoyoue (?)
hot air station also removes the chip easily if needed.
And I love that Metcal 5xxx. I think it would also do the job.

When there are a lot of GND vias under the chip that may
behave differently. Then a hot plate could be required from
below. Only 100°C or so, we don't want to bring the entire
board above the glass temperature.

I have a lab-scale vapor phase machine but it was never
ever needed up to now.

regards, Gerhard

I've been known to tin the pad on the board, then use a flux pen and hot
air. It's an easy way to control the solder volume.

If I'm stuffing more than a couple of parts, I use a ~120C hot plate to
help out.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

On 27 Feb 2020 01:29:55 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

I'm using these to go from 24 or 48 volts down to 12.

https://www.digikey.com/products/en/power-supplies-board-mount/dc-dc-converters/922?k=srh05&k=&pkeyword=srh05&sv=0&pv1525=87718&sf=0&FV=-8%7C922&quantity=&ColumnSort=0&page=1&pageSize=25

It can do up to 72 volts in.

My tiny pulse generator can be powered from 24 or 48 volt warts, so
this gets me down to the internal 12 volt rail, for secondary
regulators and fans.

I think it will also do +48 to -12. Gotta try that.

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package.

That sure has a lot of personality.


On the

pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

Maybe use a baby-board, so if it fails, it doesn't have to be
unsoldered. Unsoldering power pads by hand is about impossible.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[Winfield Hill]

George Herold wrote...

On February 27, 2020, Winfield Hill wrote:

... what's the best way to do prototype assembly
of pcbs having thermal-pad ICs?

With a bigger 8 SOIC pac. I had two plated holes where
the pad was and hand soldered it from the back side.

Yes, I've done that before, seemed to work OK. Just
forgot about it this time - getting used to doing it
right for automated assembly. I appreciated and liked
the advice from the three previous answers. I need to
step up my game, look into more and better soldering
equipment, but the hole-under trick is much easier.


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. [ snip ]

Looking at my layout, it was poorly done, perhaps even
causing the malfunction. Here's a redo from scratch.
https://www.dropbox.com/s/tv19qk065uu47k7/RIS-796_4_Fan-supply_pcb.JPG?dl=0


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. [ snip ]

Looking at my layout, it was poorly done, perhaps even
causing the malfunction. Here's a redo from scratch.
https://www.dropbox.com/s/tv19qk065uu47k7/RIS-796_4_Fan-supply_pcb.JPG?dl=0

The design is crammed into an 11mm slot. The inductor's
current paths are better above the IC rather than below.
The two FB resistors are supposed to be close to the IC,
and with the up-to-1MHz speed and up-to-100V input of the
converter, it's a good idea to have a ground plane under.


--
Thanks,
- Win

 

On 27 Feb 2020 07:07:11 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. [ snip ]

Looking at my layout, it was poorly done, perhaps even
causing the malfunction. Here's a redo from scratch.
https://www.dropbox.com/s/tv19qk065uu47k7/RIS-796_4_Fan-supply_pcb.JPG?dl=0

Do you need to solder the power pad? Maybe use a bit of silicone
grease or gap-pad if power is low.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[Phil Hobbs]

On 2020-02-27 10:44, jlarkin@highlandsniptechnology.com wrote:

On 27 Feb 2020 01:29:55 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0


I'm using these to go from 24 or 48 volts down to 12.

https://www.digikey.com/products/en/power-supplies-board-mount/dc-dc-converters/922?k=srh05&k=&pkeyword=srh05&sv=0&pv1525=87718&sf=0&FV=-8%7C922&quantity=&ColumnSort=0&page=1&pageSize=25

It can do up to 72 volts in.

My tiny pulse generator can be powered from 24 or 48 volt warts, so
this gets me down to the internal 12 volt rail, for secondary
regulators and fans.

I think it will also do +48 to -12. Gotta try that.


The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package.


That sure has a lot of personality.


On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

Maybe use a baby-board, so if it fails, it doesn't have to be
unsoldered. Unsoldering power pads by hand is about impossible.

A hot plate plus hot air usually works OK. Fancier hot air tools have
rectangular nozzles for different package sizes.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

On Thursday, February 27, 2020 at 4:30:10 AM UTC-5, Winfield Hill wrote:

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?


--
Thanks,
- Win

There are assembly houses that will work these prototypes for way less than what it costs for you to do it, if you can wait a few days. Lots of really cheap vendors in China too.
https://www.pad2pad.com/assembly/

 

On Thu, 27 Feb 2020 07:47:21 -0800, jlarkin@highlandsniptechnology.com
wrote:

On 27 Feb 2020 07:07:11 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. [ snip ]

Looking at my layout, it was poorly done, perhaps even
causing the malfunction. Here's a redo from scratch.
https://www.dropbox.com/s/tv19qk065uu47k7/RIS-796_4_Fan-supply_pcb.JPG?dl=0

Do you need to solder the power pad? Maybe use a bit of silicone
grease or gap-pad if power is low.

Or thermally conductive double-stick tape. That pad is electrically
isolated.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[David Nadlinger]

On 27.02.20 9:29 am, Winfield Hill wrote:

thermal pad.
[…]
But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?

I've made good experiences doing it the other way round: Apply a little
bit of solder directly onto the exposed pad with the iron. Then, add
copious amounts of flux and use hot air to solder the pad while lightly
pressing the chip down with tweezers to make sure the contact is good
(and any excess solder is ejected to the side). Then, solder the pins as
usual with the iron (either drag-solder, or just one by one for
something fairly big like an SOIC-8).

If there are any doubts about connectivity (or shorts), just reflowing
the entire chip with hot air in the end tends to magically correct any
misalignment, even with a thermal pad. Of course, this is predicated on
having gotten the footprint right and using a sensible amount of solder.

Like this, I never really had any issues soldering things like the 2 mm
x 2 mm 6-pin LFCSPs that some of the ADPxxxx regulators come in, a
number of TI's SOIC/TSSOP op amps, etc. I haven't needed to manually
solder anything excessively sensitive to thermal stress in a while, though.

If the thermal conductivity of the PCB is on the high side (heavy
copper, aluminium substrate, etc.), doing the above with the board on a
hot plate to pre-heat it evenly makes all the difference, and also helps
to avoid thermal stress.

— David

 

[Winfield Hill]

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

[ snip ] I think I'll forgo soldering the pad, as
the converter only dissipates 70mW with a 300mA load.

That calculation was based on a 15V input, where the
upper high-Ron MOSFET, 725mR, is on most of the time.
W/o the thermal pad connected, I measured 46C package
temp for the chip and 49C for the inductor, 1.3 ohms,
at 300mA. But when I raised Vin to 60V the chip temp
rose to 100C and the inductor to 65C. So apparently
the LM5163's switching losses mean it does indeed need
its thermal pad connected.


--
Thanks,
- Win

 

[Chris Jones]

On 28/02/2020 01:50, Winfield Hill wrote:

George Herold wrote...

On February 27, 2020, Winfield Hill wrote:

... what's the best way to do prototype assembly
of pcbs having thermal-pad ICs?

With a bigger 8 SOIC pac. I had two plated holes where
the pad was and hand soldered it from the back side.

Yes, I've done that before, seemed to work OK. Just
forgot about it this time - getting used to doing it
right for automated assembly. I appreciated and liked
the advice from the three previous answers. I need to
step up my game, look into more and better soldering
equipment, but the hole-under trick is much easier.

I've also done MLF/LFCSP by putting vias in the thermal pad under the
chip and a large square pad on the back side of the board with a solder
mask opening all over it. There were 5 plated through holes in a domino
pattern in the thermal pad, ideally large enough to fit fine solder wire
(0.25mm?) down the holes (0.4mm?). I would flux everything, then tack
the part in place by soldering the pins, then flip it over and heat the
big pad on the back with a Metcal iron and feed solder down one of the
plated holes until it comes up through the other ones. I figure that if
solder goes down one via and comes up the others, then there is enough
solder on the thermal pad.

If you have to, and provided the board is designed as described above,
you can remove small LFCSP/MLF packages by reheating the thermal pad
from the back with an iron, and eventually the solder on the pins should
melt and you can pull it off, but hot air would be better as it would be
quicker.

The only parts I can recall damaging by heating were some LTM DC-DC
converter modules that I failed to bake before reflowing the board. They
were already soldered to the board when I wanted to add another BGA by
vapour phase reflow. I baked the new parts that I was adding but it had
not occurred to me to bake the populated board as well. Nearly all of
the LTM parts died. Next time we baked them.

 

[George Herold]

On Thursday, February 27, 2020 at 5:53:32 PM UTC-5, Chris Jones wrote:

On 28/02/2020 01:50, Winfield Hill wrote:
George Herold wrote...

On February 27, 2020, Winfield Hill wrote:

... what's the best way to do prototype assembly
of pcbs having thermal-pad ICs?

With a bigger 8 SOIC pac. I had two plated holes where
the pad was and hand soldered it from the back side.

Yes, I've done that before, seemed to work OK. Just
forgot about it this time - getting used to doing it
right for automated assembly. I appreciated and liked
the advice from the three previous answers. I need to
step up my game, look into more and better soldering
equipment, but the hole-under trick is much easier.




I've also done MLF/LFCSP by putting vias in the thermal pad under the
chip and a large square pad on the back side of the board with a solder
mask opening all over it. There were 5 plated through holes in a domino
pattern in the thermal pad, ideally large enough to fit fine solder wire
(0.25mm?) down the holes (0.4mm?). I would flux everything, then tack
the part in place by soldering the pins, then flip it over and heat the
big pad on the back with a Metcal iron and feed solder down one of the
plated holes until it comes up through the other ones. I figure that if
solder goes down one via and comes up the others, then there is enough
solder on the thermal pad.

Exactly right, at least two holes so you can see the solder flow
via the other hole. And hey, then the big vias are filled with solder.

George H.

If you have to, and provided the board is designed as described above,
you can remove small LFCSP/MLF packages by reheating the thermal pad
from the back with an iron, and eventually the solder on the pins should
melt and you can pull it off, but hot air would be better as it would be
quicker.

The only parts I can recall damaging by heating were some LTM DC-DC
converter modules that I failed to bake before reflowing the board. They
were already soldered to the board when I wanted to add another BGA by
vapour phase reflow. I baked the new parts that I was adding but it had
not occurred to me to bake the populated board as well. Nearly all of
the LTM parts died. Next time we baked them.

 

[Jasen Betts]

On 2020-02-27, bloggs.fredbloggs.fred@gmail.com <bloggs.fredbloggs.fred@gmail.com> wrote:

On Thursday, February 27, 2020 at 4:30:10 AM UTC-5, Winfield Hill wrote:
I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

After soldering the chip's 8 pins, I used a large flat
tip to wick solder under the IC onto its thermal pad.
At test, the circuit malfunctioned. Did I damage the
IC while soldering? Removal was a pain. Using a talon
tip failed to heat the pad, ditto for my hot-air system,
only a giant flat tip applied to an exposed ground plane
above the chip (see image) got everything hot enough to
release the IC. I think I'll forgo soldering the pad,
as the converter only dissipates 70mW with a 300mA load.

But the experience makes me wonder, what's the best way
to do prototype assembly of pcbs having thermal-pad ICs?


--
Thanks,
- Win

There are assembly houses that will work these prototypes for way less than what it costs for you to do it, if you can wait a few days. Lots of really cheap vendors in China too.
https://www.pad2pad.com/assembly/

you might be waiting more than a few days for assembly in China this month.



--
Jasen.

 

On Thursday, 27 February 2020 20:51:30 UTC, Winfield Hill wrote:

Winfield Hill wrote...

I'm struggling with issues prototyping PCBs having
ICs with a thermal pad. I assembled a new 100-volt
buck converter powering a 12V fan, see schematic:
https://www.dropbox.com/s/ypvtp2z74nudps7/RIS-796_3_Fan-supply_sch.JPG?dl=0

The circuit uses the elegant NSC LM5163, a 100V 0.5A
1MHz converter in an SO-8 PowerPad package. On the
pcb layout, I had extended the thermal pad beyond
the IC to allow access for a soldering-iron tip.
https://www.dropbox.com/s/z13q7l2uiigi40r/RIS-796_3_Fan-supply_pcb.JPG?dl=0

[ snip ] I think I'll forgo soldering the pad, as
the converter only dissipates 70mW with a 300mA load.

That calculation was based on a 15V input, where the
upper high-Ron MOSFET, 725mR, is on most of the time.
W/o the thermal pad connected, I measured 46C package
temp for the chip and 49C for the inductor, 1.3 ohms,
at 300mA. But when I raised Vin to 60V the chip temp
rose to 100C and the inductor to 65C. So apparently
the LM5163's switching losses mean it does indeed need
its thermal pad connected.

Thanks,
- Win

Win,

You could try something like this:
Chipquik SMDLTLFP

This is a no-clean solder paste made from a tin, bismuth, silver alloy
with a melting point of 138degC, available from Mouser, Digikey, Farnell etc.

https://www.chipquik.com/datasheets/SMDLTLFP.pdf

John