TI TUSB8041 - Hot air or oven required?

[timgiles]

Hi all,

I am shortly testing a 4 port USB hub I have designed using the TI TUSB8041 IC. It is a 64 pin QFN (0.5mm pitch) with a large exposed thermal pad underneath. Having not soldered one of these before, I was wondering, if it is possible to solder well with a hot air station?

I have a Xtreme ZD939 hot air station which comes with a 10mm QFN nozzle. On a 'normal' QFN I understand how the hot air is placed around the outside of the package and causes the soldering to take place. However - with a QFN that has a ground plane / thermal plate underneath, I dont see how the hot air is going to reach / melt the solder on this pad.

Any advice? I am thinking maybe a cheap reflow oven will get it done and help me if I want to run a number of these off, with consistency.

Regards Tim

[stmdude]

You can definitely solder QFNs with EPs using a hot-air station. I do it on pretty much every board I build.

You just treat it as a small localized oven. Working in circles around the chip, you'll gradually heat everything (including the PCB) to the point where the solder will flow. Piece of cake.

If you live close to Malmo or Landskrona, I can even show you.

[timgiles]

Cheers, Umeå for me :-)

So not easy to take you up on the offer. I am ordering my boards and components 25th. I currently get my boards from Oshpark - do you have any tips for anywhere in Sweden that can produce reasonable 1-5x PCBs?

Regards

[rx8pilot]

You can definitely solder QFNs with EPs using a hot-air station. I do it on pretty much every board I build.

You just treat it as a small localized oven. Working in circles around the chip, you'll gradually heat everything (including the PCB) to the point where the solder will flow. Piece of cake.

If you live close to Malmo or Landskrona, I can even show you.

Yes. True. Not hard at all. The only thing else that I do is use Kapton tape around the area to reduce the heat to other components - not critical though.

[stmdude]

There are a few PCB places that can produce boards with well over "reasonable" quality, but I assume you're referring to price.

The only one I've found is electrokit in Malmo, but, basically, what you're doing is paying them to send the order to China. (Well, there's actually a middle-hand as well, but that's not important).
URL (as it's not very easy to find it): http://www.electrokit.com/monsterkort-kundspecifikt.48155

Never used them for PCBs though, so I can't say anything about quality or delivery-times.

Personally, I order from ITead. Never had a problem with customs/taxes/etc.  I use DHL shipping though, as I'm quite impatient.

[stmdude]

Yes. True. Not hard at all. The only thing else that I do is use Kapton tape around the area to reduce the heat to other components - not critical though.

Ah, yes. That might be (somewhat) important.  I usually start with the QFN-EPs (and other components that must have hot-air), and after that, I hand-solder the passives and other ICs with leads, as to avoid heating other components unnecessarily.

[coppice]

I don't entirely disagree with the others, but as has been said if your ONLY tool is a hot air tool then you'll have to try to wave the thing around, adjust distance and airflow to try to emulate a proper reflow profile at the device without thermally damaging the PCB.  That certainly isn't best practice, and at worst it could be nearly impossible to do correctly depending on what thermal properties your footprint and PCB has (thermal reliefs used?  heavy ground plane heatsinking some or all of the pads? gap clearance used?).

An oven equivalent reflow (hot plate, oven, ...) with suitable ramp control will certainly be able to more uniformly preheat, reflow, and cool down the PCB particularly if there are planes or large thermal asymmetries (central pad vs. periphery et. al.).

If you have a hot air tool and wish to use it, also using a proper (or improvised) preheater is recommended since then at least you can get the preheat stage of the reflow taken care of at a controlled temperature and timing despite the thermal mass of the central pad and planes and then you can use much less temperature and time with the hot air tool just to achive the ramp from preheat to reflow which will be easier in that fashion.

I agree with this. Simply soldering these things is a piece of cake, and for a prototype its generally fine to be quite sloppy. The problem is trying to ensure a low level of thermally induced stress as a production board cools down. You can have some seriously reliability issues if you don't. With a thin PCB you might even make it curl.

[dmg]

I have soldered quite a lot of these chips manually with a Hanstar 858 hot air station and it's way way easier than soldering TQFP's and many other packages. Surface tension makes things smooth.

Important things:
-You need solderpaste and need to know how to apply it. A thin strip over each row of pads and a dot pattern in the thermal pad. The amount is somewhat critical, too much and the chip will lift or the EP's solder will overflow, too little and the thermal vias will wick it away creating voids.

-You need a hot air gun that can blow as much volume of air as possible and use it without a nozzle, or alternatively with the highest diameter nozzle you have or the one that outputs the most turbulent flow. You want lots of turbulent hot air, not a fine jet stream like some stations produce. Jets and fine nozzles are for rework, not for soldering. Here you want a constantly relplacing cushion of hot air surrounding your chip.
-You need to watch the temperature. It's important to at least hit two critical temp spots: first ramp it to 100 C for some minutes to drive off moisture, then ramp to the solvent evaporation temperature of your solderpaste (typically around 150-180C) and when the paste starts looking dull,dry and ash-like ramp it to peak temp. Never set the hot air gun above 260 C for low thermal mass PCB's or 300 C for tougher jobs. 350 C is only for rework, not for soldering the first time
-And do the job over an insulating surface, for example holding the board with a board holder with wooden clamps or over a piece of wood.