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

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....

Would conductive paint or glue work there.

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 hitt ing it with the hot air helps way more than you would think it should. I h ave one of the baby Aoyue units w/o termocouples--I have no idea what tempe rature the display on the front panel is claiming to measure. I just monit or 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 me lting point of Au is ~1000 C until you realize that getting rid of the wate r 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 pr ototype PCB assembler (wife) still prefers the Aoyue.

Before I discovered preheating I would put a pad on the back side of the bo ard to hit with an iron.

On Friday, March 6, 2020 at 8:02:34 PM UTC-5, snipped-for-privacy@qprinstruments.com wrot e:

tting 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 tem perature the display on the front panel is claiming to measure. I just mon itor the board temperature with an IR thermometer.

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 wa ter film that lives on everything is the point. But I don't think that is a factor here.

at an air bath can't overheat anything the way IR can, and you don't have t he temperature differences due to emissivity differences. But my in-house prototype PCB assembler (wife) still prefers the Aoyue.

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.

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.

ote:

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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 t emperature the display on the front panel is claiming to measure. I just m onitor the board temperature with an IR thermometer.

d to 80-100 C made a huge difference. Which seems surprising given that th e 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 i s a factor here.

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-hous e prototype PCB assembler (wife) still prefers the Aoyue.

e board to hit with an iron.

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 p reheater. (Often under a microscope--I have a bent nozzle for my hot air t ool.)

If you look at ($$$$) BGA rework stations, they typically use a similar set up: 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 yo u can rework boards with parts on the back.