On Friday, 28 February 2020 21:18:52 UTC, Robert Baer wrote:> jrwalliker@gmail.com wrote: > > 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 > > > WHY do they mis-call it "no clean" when it needs to be cleaned up?Where does the data sheet say that it needs to be cleaned up? If it doesn't say that what makes you think that it does? John
Best way to solder prototype pcb with thermal-pad ICs?
Started by ●February 27, 2020
Reply by ●February 29, 20202020-02-29
Reply by ●February 29, 20202020-02-29
jrwalliker@gmail.com wrote:> On Friday, 28 February 2020 21:18:52 UTC, Robert Baer wrote: >> jrwalliker@gmail.com wrote: >>> 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 >>> >> WHY do they mis-call it "no clean" when it needs to be cleaned up? > > Where does the data sheet say that it needs to be cleaned up? > If it doesn't say that what makes you think that it does? > > John >Data sheet does not say; experience shows the crap let behind can screw up (increase) conductivity on surfaces, and may carbonize. May i suggest you get and read: Proceedings of the International Conference on Soldering and Reliability (ICSR) 2016, published by SMTA WHY CLEAN A NO-CLEAN FLUX Mike Bixenman, DBA, Kyzen Corporation Mark McMeen, STI Corporation Bruno Tolla, Ph.D. Kester Corporation See page 5, "When running a no-clean process, rosin/resin is an important ingredient for reliability. After reflow, the rosin/resin forms a protective layer, encapsulates active residues and provides a water impervious coating. Leakage currents and dendrites are more prone to form and propagate with active residues trapped under bottom terminated components. The problem is compounded when the standoff gaps are below 2 mils. When outgassing channels are blocked, flux residue accumulates and starts to underfill and bridge conductors. The residue is typically wet and active. As the distance between conductive paths is reduced, the problem is compounded." Damming with faint praise. "Just use No-Clean and you will be fine." Especially read the last 3 paragraphs on page 6 that essentially requires cleaning....
Reply by ●March 3, 20202020-03-03
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 > > 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?Would conductive paint or glue work there.
Reply by ●March 6, 20202020-03-06
Once I started using a preheater thermal pads were no longer a challenge. Heating the PCB to about 100 C by putting it on an IR preheater before hitting it with the hot air helps way more than you would think it should. I have one of the baby Aoyue units w/o termocouples--I have no idea what temperature the display on the front panel is claiming to measure. I just monitor the board temperature with an IR thermometer. When I used a wirebonder in a previous life I found that heating the pad to 80-100 C made a huge difference. Which seems surprising given that the melting point of Au is ~1000 C until you realize that getting rid of the water film that lives on everything is the point. But I don't think that is a factor here. I have switched over to a ($$$) Zephyrtronics air bath. The virtue is that an air bath can't overheat anything the way IR can, and you don't have the temperature differences due to emissivity differences. But my in-house prototype PCB assembler (wife) still prefers the Aoyue. Before I discovered preheating I would put a pad on the back side of the board to hit with an iron.
Reply by ●March 7, 20202020-03-07
On Friday, March 6, 2020 at 8:02:34 PM UTC-5, st...@qprinstruments.com wrote:> Once I started using a preheater thermal pads were no longer a challenge. > > Heating the PCB to about 100 C by putting it on an IR preheater before hitting it with the hot air helps way more than you would think it should. I have one of the baby Aoyue units w/o termocouples--I have no idea what temperature the display on the front panel is claiming to measure. I just monitor the board temperature with an IR thermometer. > > When I used a wirebonder in a previous life I found that heating the pad to 80-100 C made a huge difference. Which seems surprising given that the melting point of Au is ~1000 C until you realize that getting rid of the water film that lives on everything is the point. But I don't think that is a factor here. > > I have switched over to a ($$$) Zephyrtronics air bath. The virtue is that an air bath can't overheat anything the way IR can, and you don't have the temperature differences due to emissivity differences. But my in-house prototype PCB assembler (wife) still prefers the Aoyue. > > Before I discovered preheating I would put a pad on the back side of the board to hit with an iron.Interesting, thanks. So you preheat the pcb then take it out and solder to it by hand? I will observe that solid things have about the same heat capacity/ volume. So a 1/16" sheet of Al holds about the same heat as 1/16" of FR4. This sounds a bit like Phil's hot plate suggestion. I've used hot plates to assist in all types of soldering. (never pcb's) Is there any advantage over a hot plate? I once measured the hot plate temperature (with TC) over time. It was dang good.. I think it was just a bimetalic switch. George H.
Reply by ●March 7, 20202020-03-07
On 2020-02-27 01:29, 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? > >Like Chris I solder those from the other side. However, instead of a thermally coupled back pad I place a large via in the center. No thermal reliefs to the pad. Then I pre-tin the pad and heat with "Big Bertha" (150W iron) from underneath, adding some solder as needed. This also allows for the removal of such an IC in case it got fried during some experiment. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 9, 20202020-03-09
On Saturday, March 7, 2020 at 12:45:08 PM UTC-7, George Herold wrote:> On Friday, March 6, 2020 at 8:02:34 PM UTC-5, st...@qprinstruments.com wrote: > > Once I started using a preheater thermal pads were no longer a challenge. > > > > Heating the PCB to about 100 C by putting it on an IR preheater before hitting it with the hot air helps way more than you would think it should. I have one of the baby Aoyue units w/o termocouples--I have no idea what temperature the display on the front panel is claiming to measure. I just monitor the board temperature with an IR thermometer. > > > > When I used a wirebonder in a previous life I found that heating the pad to 80-100 C made a huge difference. Which seems surprising given that the melting point of Au is ~1000 C until you realize that getting rid of the water film that lives on everything is the point. But I don't think that is a factor here. > > > > I have switched over to a ($$$) Zephyrtronics air bath. The virtue is that an air bath can't overheat anything the way IR can, and you don't have the temperature differences due to emissivity differences. But my in-house prototype PCB assembler (wife) still prefers the Aoyue. > > > > Before I discovered preheating I would put a pad on the back side of the board to hit with an iron. > > Interesting, thanks. So you preheat the pcb then take it > out and solder to it by hand? I will observe that solid things > have about the same heat capacity/ volume. So a 1/16" sheet of > Al holds about the same heat as 1/16" of FR4. > > This sounds a bit like Phil's hot plate suggestion. > I've used hot plates to assist in all types of soldering. > (never pcb's) Is there any advantage over a hot plate? > I once measured the hot plate temperature (with TC) > over time. It was dang good.. I think it was just a bimetalic > switch. > > George H.No, I use the IR or air bath to preheat the board from the bottom, and then use hot air from the top to reflow the part without removing it from the preheater. (Often under a microscope--I have a bent nozzle for my hot air tool.) If you look at ($$$$) BGA rework stations, they typically use a similar setup: preheat the board from the bottom, and then add heat from the top. The advantage of the IR preheater or air bath over the hot plate is that you can rework boards with parts on the back.
