The new RF (and USB/RS232) modules look awesome–great capability in a tiny package. But they seem so tiny, is it possible to solder them up without any sophisticated hardware? For example, do you need a reflow oven; they appear too small to hand solder.
-Jeff
Jeff & Forum,
Small pitch IC packages look really scary to the beginner, and I suppose we all live in fear of messing them up with solder shorts between pins.
However the solution lies in the use of a good quality sufrace mount flux, and a little help from physics by way of surface tension.
Buy a spray can of SMT rework flux, a pair of fine tweezers and a x7 magnifying glass.
First step is to apply flux liberally to the IC footprint pads on the board - it should end up sticky with flux.
Then using tweezers and perhaps a X7 magnifying lens, line up the chip so that it lies exactly over the pads. Do not try to rush this step - it must be 100% aligned.
Apply vertically downward pressure to the middle of the chip package - using a pencil or similar prod instrument. The flux will help to hold it down.
Lightly tin the bit of the soldering iron - not too much solder however.
With a smooth wiping action - slowly draw the bit of the soldering iron along the leads of the IC. If you move at about 1/16 or 1/8 inch per second - you will actually feel the solder melt under the pin and the pin almost spring into position on the pad.
Tack down a few pins in one corner of the IC and then recheck the alignment of the IC on the pads. Work your way methodically around each side of the chip applying more solder sparingly and more flux liberally when needed.
If 2 pins get bridged by solder, clean the bit of the iron and gently wipe the offending bridge away from the centre of the chip. If there is enough flux - it should clear OK.
It takes practice to master these skills - but the main thing is not to panic. Any chip can be removed or reworked using flux and simple hand soldering iron - or hot air gun with small nozzle.
I regularly place 64 pin quad pack onto boards - it just takes practice - and good alignment, and good rework flux.
I have watched Chinese production workers remove 64 pin chips with a 1/4" soldering iron bit, by flooding all the pins together in a blob of liquid solder and lifting the chip clear with a scalpel blade. Do not be afraid to stick all the pins together with excess solder, you will always be able to clean the chip up afterwords.
For 0603 SMT parts, apply a little solder to one pad on the pcb, slide the resistor or capacitor into place using fine tweezers keeping the one pad hot with the tip of the iron and then solder the other side down. For 3 pin transistors and diodes, solder the one lead down using the same technique and then solder down the other 2 pins.
You could try practicing first on an old PC motherboards or 5 yr old mobile phone.
regards ,
Ken
Ken,
EXCELLENT! Thank you very much for describing in detail how you’d go about soldering those tiny little buggers onto a board. I’ve been soldering SOIC packages onto a board with a clunky $10 iron not having any exposure to the things you mentioned.
I’ll give those things a try in my current stuff and am going to consider ordering those little guys once I get more confidence.
Thanks again,
Jeff
It’s funny - we are getting braver and braver with our soldering skills. Hence, we are getting smaller and smaller packages. QFNs are now standard issue around the office. Our eyes hate us for it :shock:
I was shaking like crazy the first time I soldered a ‘surface mount’ part. The ADXL 202 was so small. But with a little practice, you’re confidence comes up and soldering smaller and smaller devices becomes cake.
These are my recommendations in order of importance:
Get a good soldering iron. $10 is too much to spend! What I mean is this - you can’t do squat with a $10 iron if you need to do SMD. Throw the thing away and spend $90 and get a Weller WES51. I’ve got a WES50 that we’ve been using for years without a hickup. You can always drop the big one and get a $500 Metcal but I did not experience that much of a difference.
Get good solder wick. I use ‘TechSpray 1815-5F’? No clean wick, .055" width. But the part number is not that important. Get a good grade wick. $0.90 is useless. $20 is ridiculous. This was $2.51 for 5’ or something. Works like a charm!
Hemostats are good. Tweezer work well.
Lighting is important - you’ve got to see!
Magnifying glass is nice, but I never use it. Nothing against ken (old man 
) but if you’re eyes are good, or if your prescription is up to date, you can see even the smallest jumper problems without the Ghost Busters’ eye magnifiers.
Here’s how I do a tiny, tiny, QFN package. We don’t do any board prep. Get your board and your iron, clean your tip, and throw a blob of solder on pads 1 and 2. Jumper them with a good blob. Next clean your tip. Next, take your QFN (or other) part with the tweezers. Next, clean your tip. Then heat the blob with the iron and slide the QFN part horizontally into the blob. You can do vertically, but it will tend to push the solder the wrong way. Once you have close alignment, hold the part still and take the iron away - pause, release the part. Does it look lined up? No? Hold the part, heat the blob, tweak the alignment ever so slightly. This is where practice comes into play. We can usually line up a part on the first or second try.
Once you’ve got good alignment, take a big blob of solder on your tip and run it down the opposite side of the chip. A little flux helps, and little practice is worth even more. Leadless chips are great because they tend to hold the surface tension of the solder better than SOIC or SSOP leaded chips. Repeat for all sides.
A quick inspection will show leads that you need to touch up.
I taught a history major how to solder in a few days. Through hole, to SSOP, to QFN. He’s now lightning fast and better than I. :roll:
With all that said, I am currently drooling over a $20k pick a place machine with reflow oven… Hmmm…
http://www.madelltech.com/m2-2.html
-Nathan
Nathan,
Your approach to the QFN packages is a good one.
Tacking one corner down first so that you can get the rest of the alignment right - good thinking.
I still would recommend the use of a good “SMT Rework” Flux - it really does encourage the solder to flow and prevents blobs.
I started soldering SMT about 6 years ago - and must admit that my eyes have deteriorated in the last few years - mind you I am 27 - but only when you count in hex
In February I had an accident pruning an apple tree and for a few weeks lost a lot of my visual acuity in my right eye. I recommend safety goggles for anyone working with trees!
This was a bit of a crisis, because I was right in the middle of building some very busy SMT prototype boards (200+ components - mostly 0603).
Fortunately my vision is almost back to normal in the right eye, because the left is a bit low res and contributes little.
It did however make me think, that it is time to seek some new skills - as the hardware is getting harder to see.
Hence my decision to learn C, so that I have another string to my bow.
The PIC 16F88 based wireless central heating controller is coming along nicely…
regards,
Ken
Ok, no more jokes about bad eyes - I am glad to hear you are ok!
0x17 here
We are awaiting a reflow machine and manual pick and place machine today. I’ll let you know how it goes!
-Nathan
Hi,
I noticed the QFN packages have a central ground pad. Does this need to be soldered? I think the easiest way to solder it would be to use a big
ground via and solder it from the backside of the board. Any other ways?! 
cheers,
Jamie
sparky:
http://www.madelltech.com/m2-2.html-Nathan
We’ve got a couple of pick and place machines at work, I could sit there for hours watching them…
Yeah, we like to solder that. Not just for grounding, but also for heat-sinking.
Pete
jmorken:
Hi,I noticed the QFN packages have a central ground pad. Does this need to be soldered? I think the easiest way to solder it would be to use a big
ground via and solder it from the backside of the board. Any other ways?!
cheers,
Jamie
Hot air rework does wonders. The ground pad underneath and all the surrounding pads liquify and the chip literally “reflows”.
Note : Most ICs have ground pins as well as a ground pad underneath. Therefore, you can get the ground pins and leave the ground pad unconnected (cause it’s already connected via the pin). Works fine for prototyping. In mass runs, of course stencil and reflow the ground pad.
I talked to a tech tody who had a couple vias underneath the chip. Using the hot-air gun from underneath the PCB, the vias transmitted the heat, and the chip reflowed and centered itself. Really cool to watch. I do the same heating the chip from above, no problem, but it heats up the chip quite a bit more.
Btw - we’ve got a $20k pick and place prototype robot in the works. We’ll post pictures as soon as we can get our grubby hands on it. It’s still 8 weeks away.
-Nathan
Some good info here guys. I do quite a bit of SMD prototype work here in my consulting business and I just sent out some boards that have a power QFN Li-Ion battery charger (bq24113) on them.
The boards will be in next week and I am going to give the “hot air from the backside of the board” method a try. They have untented vias on the ground pad and use lots of ground plane for heatsinking (they have to as the charger is capable of 1A+).
It will be interesting to see how it works out.