[SOLVED] SOLDERING an UNDERPAD of AN IC on a DUROID PCB Using HOT AIR GUN??

[usman_mahfooz]

I Am using a hot airgun to heat the IC pad (I apply heat under the IC Pad) which contains vias for underpad heatsink. I am applying heat at 160 Degrees Centi.
The IC is being soldered. But PCB is little deformed. I am using duroid 6002 (20 mil thickness).
Working at 3 GHz, what will be the problems that will arise using above method for soldering.
I HAVE CURRENTLY NO OPTION/FACILITY FOR SOLDER REFLOW OVEN OR WAVE SOLDERING :-|
Only Hot Airgun to solder underpad:shock:. Think of it as a QFN chip with an under pad. What will be the disadvantages to heat the base of PCB to solder top Lumped Components/IC.

Need some ideas and comments
Thankyou

 

[biff44]

Yeah, we hear you on that one! These QFN packages are pretty hard to solder.

IF it is a qfn without a big ground slug, you can actually put it down with a hand soldering iron and a very fine pointed tip.

If it has a big ground pad in the middle of the QFN, then that will not work. You can put some solder paste on the pads and the big pad, and then either put it on a hot plate, or carefully heat up the underside of the board with the hot air gun. You MUST make sure that the hot air gun is not blasting out too much heat! You do not want the board to catch fire!

One trick I sometimes do is to use wire solder on the big ground pad with a normal soldering iron, and smooth it out as flat as I can. I leave the individual pads for pins dry. I then put the board on a hot plate, carefully put the QFN on top and after the qfn heats up and the solder is melted, I carfully position the QFN and remove from heat to cool off. Then I use a small handheld soldering iron for the pins that need attaching (most QFN pins on a microwave chip are no-connects, and do not need soldering).

Unfortunately, these QFN packages are made of plastic, and heating the qfn chip from the top side will not heat it up enough to get a big ground slug underneath reflowed before the plastic package melts!

 

[rfmw]

I regularly manually solder QFN chips (i.e. QFN-16, like ONSEMI's 10Gb/s logic). They all have a ground paddle on the bottom side. I always have a PCB with multiple vias for the ground paddle connection. First I position the chip. This one is a very precise operation. I use a magnifying glass to see that the chip is centered with its side pads to the traces. Then I solder all the side pads using a 0.8mm soldering tip and a flux syringe. Without the flux this would be a no-go operation. At last I solder the ground paddle from the back side through the vias filled with the flux. Unfortunatelly, some QFN chips have no side pads visible (like some Hittite's parts). The latter ones are a bitch.

 

[cmontoya]

I have done this a bit here are my suggestions.

Here is a video to help you get an idea of what to do How to Solder QFN MLF chips Using Hot Air without Solder Paste and Stencils - YouTube

1 Get a stencil for the solder paste.
2 get fine solder paste.
3 you can skip this 1 and 2 if the part and pad already have solder on it / or try doing it the way the video shows that will req. a bit more skill
4 preheat the board you don't have to go crazy getting the right ramp, get a hair dryer and have it blow on the other sided
5 do your self a favor and keep the air flow on low.
6 hot air gun to get the solder paste melting place the qfn on the pad with tweezers.
7 remain calm
8 capton tape any near by components so they stay put


I have gotten this technique to work good up to about 4ghz. The boards I work on are 100mhz to 6ghz, I don't see much trouble with the ones I have repaired until about 4ghz.
by the way I do this on rogers as well. Rogers has fairly week glue compared to say fr4 so make sure you have heated the crap out of it be for you try to lift a qfn.

Hope this helps

 

[usman_mahfooz]

Thank you guys :smile: