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

Started by Winfield Hill February 27, 2020
 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
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
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 £2k) but if you are reflowing a prot pcb with a component cost of maybe £1k 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 £500 from Metcal) and then you need much less top heat. MK
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&deg;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
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
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 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 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 2020-02-27 05:02, Gerhard Hoffmann wrote:
> Am 27.02.20 um 10:29 schrieb Winfield Hill: > >> &nbsp; After soldering the chip's 8 pins, I used a large flat >> &nbsp; tip to wick solder under the IC onto its thermal pad. >> &nbsp; At test, the circuit malfunctioned.&nbsp; Did I damage the >> &nbsp; IC while soldering?&nbsp; Removal was a pain.&nbsp; Using a talon >> &nbsp; tip failed to heat the pad, ditto for my hot-air system, >> &nbsp; only a giant flat tip applied to an exposed ground plane >> &nbsp; above the chip (see image) got everything hot enough to >> &nbsp; release the IC.&nbsp;&nbsp; I think I'll forgo soldering the pad, >> &nbsp; as the converter only dissipates 70mW with a 300mA load. >> >> &nbsp; But the experience makes me wonder, what's the best way >> &nbsp; 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&deg;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"