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Questions relating to designing PCBs
By rcooke
Hi Folks,

Up until today, I've been fairly successful in soldering SMD parts for
my prototypes but it looks like I've got a short in one of the QFN32
parts. How do you place them? I guess my hand shook a bit when I was
placing the thing and now one of the Vcc pads are shorted to a GND pin.

I've managed to get the 0603 parts down without much problem but this
QFN32 part is tough. I've placed the ATMEGA48 (TQFP package) and it
soldered fine.

I'm looking for some suggestions/words of wisdom on what other folks
do for their prototypes.

I went ahead and ordered the hot air station from Sparkfun today. Is there a way to re-work a QFN32 chip?


Richard Cooke
By silic0re
I am no expert in soldering tiny parts with lots of pins, but I have soldered a few components with 0.4 and 0.5mm pitches, some as high as 100 pins, onto Schmartboards.

I know the Schmartboard is specifically designed to help you solder these things easily, but I only ever get the "non-EZ" versions of their boards, and use solder paste with them. Typically I'll place the component on, get it nice and square, tape it on, square it up again, and then solder one or two pins on either side of the device to hold it in. From there, I just apply a tiny bit of solder paste to the pins (often dispensed from the head of a pin!), and solder a few pins at a time.

I did have one pretty bad bridge once, when I applied too much paste. The solder actually worked its way behind the metal part of the pin, between the pin and the flatpack. It took a couple tries, but by applying a small gob of paste, solder-sucking it up, and using an exacto-knife to clean out the rest, things went okay. It also was a lesson to avoid trying to solder 20+ pins in one go with a ton of solder paste... :)

I hope that helps a little bit. I know that if you have a rework station you can sometimes just heat the chip up and lift it right off, but I'm just using a good soldering iron with a fairly fine tip.

best of luck,
User avatar
By leon_heller
Tacking down two opposite corners and then drag-soldering with plenty of gel flux works for me.

By NleahciM
leon_heller wrote:Tacking down two opposite corners and then drag-soldering with plenty of gel flux works for me.

Ditto, except that I use liquid flux.
By winston
I've found that all SMD parts are easy to get off with hot air - easier than pin-through-hole - less solder to melt. I've only removed leaded-type packages, not a leadless one, but I can't imagine a leadless package is all that hard to get off with hot air.

My challenge tonight is to solder an LQFP80-10 (80 pins, 20 pins a side, 20 pins crammed into 8mm)... onto a home made PCB. I've found for hand soldering things like SSOP which are not much bigger pitch, drag soldering and solder wick to suck up the excess works very well for this kind of thing. For fluxing, I have a 'no clean' flux pen, which seems to dispense the proper amount of flux.
By blarggstar
Hey, I'm about to start soldering QFN32 so I'll keep you posted on my experiences. I'm going to be ordering the solder station / hot-air combo for $175 off of sparkfun this weekend.

My IC is the AT86RF230. Infact, I created my own breakout board on and they just arrived.

I'll keep you posted!
rcooke wrote: I'm looking for some suggestions/words of wisdom on what other folks
do for their prototypes.
User avatar
By bigglez
funnynypd wrote:I used hot air removing parts before, curious can it be used for soldering? Anyone tried?
Greetings (No Name Supplied),

Sure! Its my prefered construction method for PCB prototypes.
I use an ordinary heatshrink gun after placing the parts
on solder paste hand applied from a syringe.

There no automation so the results are based on skill and
practice, much like soldering with an iron. The only learning
was to use such a small amount of paste (compared to
TH soldering with an iron).

Solder wick is a good tool for removing excess solder.

Hot air gives better results than a soldering iron as there is
no 'tail' caused when the hot iron is pulled off the solder

If the hot air is too strong or the gun held too close to the
PCB it is possible to 'blow' the parts off the pads or have
them tip up (aka "tombstoning").

So far I've stuck to 1206 and SOIC footprints where possible
but also done well with TQFN32 packages.

Problem parts are those with large mass (in particular
inductors) and parts with little or no exposed pads (most
SMT electrolytics, trimmer caps). I have changed the
pad geometry for some of the parts I use often. Doing
these and any TH parts after hot air soldering and with
a hand iron works.

There is a risk of damaging the PCB if the hot air is
stationary for too long. The skillet method probably
gives more even heating.

Comments Welcome!
By FlipFlops
I solder leadless microcontrollers like the 28 pin QFN PIC18F2520 with a Weller heat gun and some no clean flux. Just put a teeny amount of solder on all the pads, position the IC, weight it down with an insulated pair of pliers, and use the heat gun.

I keep it about 3" off the PCB and slowly move it in circles until the solder melts and the IC drops down onto the PCB surface. Afterwards I allow it to cool, then test for shorts, although it's pretty rare. If I do find any, I simply reheat the IC and PCB again and it typically takes care of the problem.
By funnynypd
Anyone use solder paster or liquid water-soluable flux for hot-air gun?
By Azoore
I use solder paste and a hot air rework station for the 2nd side of my two-sided boards. I use a griddle for the first side with the most components. The smallest I go are 0603 components. I manually apply solder paste to a group of pads I'm soldering, lay the parts down and apply heat across the pads. You have to be careful that you don't scorch your PCB though. Just have some patience and you'll be fine. Also, keep the airflow low or your parts will fly off :wink: