Hand soldering 41-BQFN (SMT) part

[epiflow]

Hello,

Does anyone have experience hand soldering 41-BQFN parts?  What is your method?  Do you use stencils?  Most of the pads are UNDER the chip.  I have attached a picture and a link for viewing.  Also, does anyone have the EEVBLOG discount code?  Thanks in advance. 

http://www.digikey.com/product-search/en?keywords=296-34944-1-nd

[pmbrunelle]

At my work, for prototyping, we installed XX-DFN (don't remember the number of pins, 16 maybe), it was a square package with bottom terminations.

It was the first component to be installed on the PCB.

We applied solder paste (and flux), then placed the component on the top of the PCB.

Then, we placed the PCB on a hot plate. The heat went from the bottom of the hot plate to the top side. You could tell the reflow was done when there were flux fumes and the part repositioned itself.

We only did a handful, but there were no apparent problems with bridging.

[epiflow]

pmbrunelle,

Thanks for the reply.  How did you apply the solder paste and flux? 

[xyrtek]

Hello,

Does anyone have experience hand soldering 41-BQFN parts?  What is your method?  Do you use stencils?  Most of the pads are UNDER the chip.  I have attached a picture and a link for viewing.  Also, does anyone have the EEVBLOG discount code?  Thanks in advance. 

http://www.digikey.com/product-search/en?keywords=296-34944-1-nd

for RnD prototyping I solder and rework Thin QFN very often and use the same method as ever.

under microscope
mix some low temp solder paste with flux
cover chip with said mix and using a temperature controlled soldering iron tin the pads leaving an even amount of solder on them.

I sometimes use two solder tips if the thermal pins are big (during this process the tip is always submerged in flux)

Then clean the pcb traces (sometimes i also do the above procedure to the pcb), add flux and using hot air proceed to solder the chip.

Pre heating the pcb does help, but if you don't have a hot plate use the air gun to warm up the area then align the chip as best as you can and and continue pre heating. (short time)

Simulate a reflow profile as best as you can, watch the air speed nothing ruins the moment more than the chip flying away.

The temperature is important, and always use low temp solder paste. if your flux is burning then the iron is too hot.

Check the  recommendations for the part you are soldering and stay within those temps and times, if you use low temp solder paste you will not need to reach those limits.

Keep the components sealed in low humidity environment until you are ready to solder them. Most come with humidity indicators.

GL.

[michaeliv]

[...] always use low temp solder paste. [...]

What melting point / composition was the solder paste you used ?

[xyrtek]

[...] always use low temp solder paste. [...]

What melting point / composition was the solder paste you used ?

Nothing fancy, from good stuff at work to no name at home.

I have used this:
http://www.amazon.com/gp/product/B00AGOSL1U/ref=s9_hps_bw_g469_i1

Leaded solder paste maybe better (by better I mean easier to use) but it will be higher temp. (That seller use to have a leaded type that was around 180C, ask him).

GL.

EDIT: EDIT:



*183C lead solder paste:
http://www.cyberdocllc.com/shop/quickalloy-smt10l183-solder-paste-no-clean-syringe-6337-10cc-ml-35g-melting-point-183c-361f/

[pmbrunelle]

Thanks for the reply.  How did you apply the solder paste and flux?

Paste with a syringe, flux with a squirt bottle (kind of ends up going everywhere).

The paste over the tight pitch pads didn't short, it figured where to go (thank you solder mask), but I didn't do it personally, so I don't know how much was applied.

[KL27x]

I tin the pads with solder from a piece of regular 63/37 solderwire and flux. Try to use less on the bigger pads so the bumps end up level. Then wipe off with alcohol and add fresh flux. Set chip down. Hit with hot air.

If the pads bridge while tinning them, so what? Put some flux on and run a clean/dry soldering iron over it to remove the excess solder and it will be close to perfect. Go back over the drier pads until everything is pretty level.

I'm not sure what the difference should be doing this with solder paste vs solid solder. 

I also have solder paste. Solder paste can smush and bridge. Its advantage is for mass production with a stencil and PnP machine. But I don't see why you would necessarily go out of your way to use it for putting a chip on the board by hand, using a hot air station. 

Check the datasheet to be sure, of course. But I have never burned a chip with a hot air station just getting plain lead solder to reflow.

[nanofrog]

Either the solder wire or solder paste techniques with hot air described above will get the job done, so user's choice. 

As per supplies, if you decide you want to use solder paste, I'd suggest using Kester EP256 (63/37) that's been repackaged into hobbyist friendly syringes by CML Supply (they also sell via eBay, here). They also sell smaller syringes of RF741 paste (it's a no-clean, but it uses a modified rosin as the base resin rather than a synthetic = easier to clean).

[epiflow]

Thanks for the info. and contributions guys!  The video helped immensely.   

[KL27x]

Problem with these parts is you will not be able to see a short. It is a near certainty that out of 100 tries, you will have X number of installations that look perfectly fine but which have a hidden short. Using paste will most likely increase your chances of making bridges. Getting it on there in an even manner is not easy to begin with. And even with soldermask, solder paste does not automatically suck to the pads. Little bits of smeared paste will bead up into little isolated bits of solid solder and potentially cause future problems. Between any of those pins you might have a lot of tiny bits of solid solder that can move if/when the board is cleaned of flux residue. You will never have this problem when pretinning the pads.

Even if you get the paste perfect, you can easily screw everything up when setting the chip down or when applying the hot air if the chip moves from the air or the flux bubbling. If you accidentally smudge the paste too thin during any step, you can easily get the isolated beading problem or complete bridges. Unless you have a lot of IC's to do and you can "get in a rhythm" with the paste, I think you will spend more time trying to use paste while getting a higher error rate.

The downside of pretinning the pads is the solder beads are rounded. And when the flux heats up and mobilizes, the chip can tend to slide off to one side or the other.  So on either the pcb side or the chip, you should aim for just a really thin layer of solder just to help wetting. Flux the pads and rub a clean soldering iron tip over it just to silver-flash the pads on, say, the chip half of the equation. But you can always remove the heat and nudge the chip back in place as many times as you need to, before the solder starts to reflow, since there's no paste to smudge.

So there are downsides, but I prefer to pretin the pads. Personally, I have a lower bridge rate when doing it that way. Which ends up saving time, overall. I feel like people go out of their way to find a use for solder paste. Outside of its intended purpose. And it's not really that great.