I’m about to get my very first batch of SMT pcboards for a new project, and I’m new to surface mount work. I started practicing today on some leftover ethernet cards from years gone by, first removing the chips and then soldering them back on. I have a Aoyue rework station. I have read through the SFE tutorials but still have a couple of questions.
My first question is about desoldering wick. I found I used this in a couple places. First when cleaning up the pads after desoldering an SMD device. Second, when trying to clean up some bridged pins after reworking that same device back onto the board. I didn’t have the greatest luck – the solder just didn’t seem to wick onto the braid. I’m using some generic “radio shack” desoldering braid. Is name brand braid (for example the techspray wick sold by SFE) better and will it help with my problems? I also had a few issues with using my through-hole iron on the braid and on a densely-populated board it’s not always easy to get room to apply the side of the tip to the braid. I generally use a very fine tip, and it seems like it might be better to find a cheap wide-tip soldering iron for use with desolding braid?
Next, I have some questions about solder paste. I haven’t seen a place that tells me what kind of applicator tip to use… I’m currently using an 0.024"-0.027" needle. Is this an appropriate size? should I go bigger or smaller? When soldering a 64-pin device I ended up with about a half-dozen bridges. Does that usually indicate too much paste? or are occasional bridges just a fact of life when using a hot air station?
Finally, about clean-up… I’m assuming it’d be wise to take the needle back off the syringe of solder paste and seal the paste back up after I’m done using it. Is there a particular way to clean the old needle out? or should I not bother?
I hoping to get all my practice in on old junk boards before my shiny new custom boards show up in the mail!
Thanks for any advice.
Scott
You need to use flux with solder wick.
Bridges are going to happen. Use the wick and flux to pull them up.
I’ve been using these syringe ends.
http://www.dealextreme.com/details.dx/sku.21947
TheDirty:
You need to use flux with solder wick.
Bridges are going to happen. Use the wick and flux to pull them up.
I’ve been using these syringe ends.
http://www.dealextreme.com/details.dx/sku.21947
I’m still using the syringe end that came with my little 5cc of solder paste. 
For solder wick, I use stuff from MG Chemicals that’s pre-fluxed. The TechSpray wick sold here seems to be prefluxed, too, at least according to John Laur’s comment on the cheap chinese flux product page. Wick looks like magic every time I use it.
One thing when I’m playing with tiny pitch is that I’ll swap to a larger iron tip to wick away bridges, simply because of the extra mass of copper with respect to the pins. It’s essential that the section of wick you’re trying to use to soak up the solder needs to heat up to the appropriate temperature. Sometimes I’ll even add a little blob of solder to the tip before wicking to more quickly transfer heat to it.
What are you using for an iron? A cheaper iron can’t source enough heat to reliably work with anything but the thinnest of solder wick.
Also, if the tip of your iron is dirty you may not be able to transfer enough heat into the wick to make it work properly.
-Bill
phalanx:
What are you using for an iron? A cheaper iron can’t source enough heat to reliably work with anything but the thinnest of solder wick.
My iron is a temperature-controlled Xytronic that I’ve had for about 20 years. It has a relatively new tip on it, but it’s also a very fine tip. That’s what makes me think I ought to either get my hands on a second iron with a larger tip, or dig out a bigger tip and swap it onto the Xytronic.
I’m pretty sure I tried wick both with and without fluxing. Without was when I had the most issues; my second attempt with flux seemed to go much better (it’s been a few weeks now, so I can’t remember for sure), but still wasn’t perfect. I have some of the techspray braid coming, and I’ll see how that performs.
It’s time to ‘practice’ on another old LAN card out of the junk box…
Thanks to everyone for advice.
smbaker:
…but it’s also a very fine tip.
That is more than likely your problem. Try switching to a medium screwdriver or chisel tip and you should have much better results.
Also, I always use the prefluxed solder wick so I don’t have to deal with a separate flux container.
-Bill
I find I always need less solder paste than I actually have put on, bridges are a sign of too much paste (especially on fine pitch devices). The other thing that indicates too much paste is small components like 0603 capacitors “tombstoning”, where the surface tension of the melting solder paste will make the capacitor suddenly stand on its end, well, like a tombstone.
It’s incredible how little paste you actually need. I’ve never had the problem that there was too little paste on a pad!
FYI, the techspray wick works much better than the original radio shack wick did. Gone are the problems I had with the solder not wanting to flow onto the wick. It soaks it up like a sponge. No need to switch out the tip on my soldering iron either; the fine tip is working great.
As far as the amount of paste, I agree with winston. My experiments have shown that a little bit of paste can go a long way.
My next project is going to be soldering down a QFN package. Any tips for how to do that? My plan was to pre-apply the paste to each pad and then set the QFN down on top. However, my practice with other devices (QFPs) has not shown me to have the greatest aptitude for setting a package down aligned properly the first time, and I usually have to use a pick to prod devices into correct alignment. Is that going to get me into trouble with solder bridging on the QFN? Or are leadless devices more resistant to solder bridging?
Moving it around afterward to align everything is pretty much a given. It usually takes me a little while with the loop lens before I’m satisfied all the pads are aligned with the alignment marks on the edge of the QFN. Just make sure you only put a tiny amount of paste on the centre pad, since there is nowhere for that solder to go, it will lift the whole chip up if there’s too much. Otherwise I have gotten bridges with QFN’s, but the solder braid works just as well with them. Bridges turn out as solder blobs on the edge of the chip that can be sucked up with the braid. I’ve never had a bridge with them that could not be fixed this way.