SMD soldering with solderpaste

[Guni]

I use hotair and hot plate for smd soldering. I'm going to build reflow oven very soon. But at this moment I have some problems with soldering of the both used method.
Almost everything is ok but after soldering I have a lot of small balls around joints. I don't know why - quality of solder paste, wrong temperature, too fast heating.
I use lead solder paste ASN9337Sn63Pb37 Rosin Flux Rma
I'll be very grateful for any advice or suggestion
   

[kingofkya]

I have limited experience, but i would suspect you need to keep it under heat a bit longer,and maybe add some more flux.

And/or heat it more slowly. Heating too fast will evaporate the flux too fast.

[wraper]

Most likely expired/crap solder paste. If you put a some amount of solder paste on hot resistant surface and heat it with hot air, it must become 1 or maximum few big balls. If there appears mess with a lot of small bolls, discard the solder paste, it is junk. Do you store it in refrigerator?

[wraper]

ASN9337Sn63Pb37 Rosin Flux Rma

Do not mean anything, actual brand/model?

[Guni]

Yes, it's what I suspected. I store paste in the cool place but I don't know the "history"
Paste is branded by Marbell Litd. Technologies, England, Victoria Court 123 but I can't find this company on the net.
So probably it's a fake, chines low quality old over-timed product.

I'll buy something "known" but as you know soldering paste is quite expensive

[tautech]

have a lot of small balls around joints. I don't know why -

Could it be your paste application method?

Any little smears of paste will produce this.

[wraper]

Actually cheap chinese solder paste from ebay is not that bad. Usually solder paste have a shelf life of 6 months. If stored properly, might get year or maybe a bit more from it without serious degradation. Depends on actual solder paste a lot.

[Guni]

have a lot of small balls around joints. I don't know why -

Could it be your paste application method?

Any little smears of paste will produce this.

I've tied manual application (I don't have special dispenser) and with stencil.
It's limited control during "cooking" on the hot plate but I made few experiments with the hotair. Every time the same effect - balls   

[tautech]

have a lot of small balls around joints. I don't know why -

Could it be your paste application method?

Any little smears of paste will produce this.

I've tied manual application (I don't have special dispenser) and with stencil.
It's limited control during "cooking" on the hot plate but I made few experiments with the hotair. Every time the same effect - balls   

There was a thread a while back ~6 mths, where the flux in old paste was a problem IIRC.
Mix the paste well and experiment with addition of flux.
Sorry I can't remember the thread, do some searching, the answer is in the forum I'm sure

[wraper]

Adding additional flux is basically useless. If paste is crap additional flux won't make it any good. And imo, it is not flux what becomes bad but solder particles become oxidized or some another chemical reaction destroys the solder paste.

[nanofrog]

Adding additional flux is basically useless. If paste is crap additional flux won't make it any good. And imo, it is not flux what becomes bad but solder particles become oxidized or some another chemical reaction destroys the solder paste.

What brand/s are you using, and in what format?

Perhaps the metal powder is settling out of the flux, screwing up the ratio (would be a bigger issue with a syringe)?  Heard of some being able to fix it to some extent by stirring their jars, but rather more difficult to try and fix in a syringe without introducing other issues, namely air pockets.

[cncjerry]

I am using low temperature paste and seeing balls because I believe the surface pads and chip legs don't get hot quickly enough for the solder to whet them prior to it balling up.  Leaving the heat on longer gets them to flow.  If it is an indication of poor quality paste, I am using chip quick brand I think, then that is nice to know as I often worry about one of those little balls getting somewhere it shouldn't.

[zapta]

Adding additional flux is basically useless. If paste is crap additional flux won't make it any good. And imo, it is not flux what becomes bad but solder particles become oxidized or some another chemical reaction destroys the solder paste.

I just did it last weekend. My little chipquik paste got crusty (I don't bother with keeping in a fridge) so I mixed it with tacky chipquik flux (both have 219 in their names so I figure they must match) and was able to print and reflow. It was not like new but much better. Two weeks earlier I had similar problem with the paste, it came out of the reflow oven non shiny and some solders were cold. I added tacky flux to the board and did another reflow cycle and it got nice and shiny. This suggests to me that the problem is a lack of flux.

Anyway, not recommended for professional settings. I should get a new jar soon. 

[Kjelt]

The 6 months is just a disclaimer from the manufacturer the same as other products like cookies.
Now I don't know about you guys but I did eat some cookies that were months over date (still closed ofcourse) without problem.
For pro application they just toss the stuff after the shelfdate since if there is a problem with one jar of 500g paste you are talking about $100k or more products that can be scrapped, not an option for a paste jar worth $40.-

For us hobbieists, I use paste that is one and a half year old kept in the fridge which has the same result as my new jar I bought a few weeks back just to be sure.
If you want to add something to get the paste a bit jucier don't add the flux but the solvent, AFAIK plain IPA is the best to use, very little bit is necessary otherwise the paste becomes too juicy and runs.

@TS: do you try at least to imitate the reflow profile, so start with ramping up to 160^oC and stay there for almost a minute then rampup to 230^oC for 20 seconds and let it cool? Or are you blasting 230^oC right away for 40 seconds and stop, I would suspect that this might be an issue. The paste has a wetting phase were the flux becomes active and than starts to evaporate in that process the balls are coming together again.
Another cause could be the pcb it self, do you have made your own pcb or professional pcb with solder mask? The soldermask helps keeping the melting solderpaste at its place.

If nothing helps and you want to continue just use a solderiron and do it pin for pin, works great but takes a lot of time.

[DanielS]

i keep reading about solder pastes having a lifespan of 6months ... but what is it that makes it 6months? is it some kind of evaporating solvent? would it be longer if we vacuum the air out before freezing?

Solder paste is a colloidal suspension of microscopic solder balls in solder flux. The biggest issue would be non-uniform distribution after sitting still for too long and possibly clumping. Depending on the type of flux, drying might be an issue too.

For hobby uses where most jobs are very low volume and you can afford going over each board, this is not much of an issue. But in a production environment where you go through hundreds if not thousands of boards, you do not want potentially "stale" solder paste to cause abnormal defect rates.

[mazurov]

I solder a lot using old paste and old PCBs sometimes.  In my process, small balls are caused by insufficient soaking time, like when I accidentally reflow large board using a profile for a regular one.

I also have a tub of very strange  paste, it's Amtech NC-560-LF, fresh, bought locally. Per Amtech info, it is formulated specifically for ENIG and it always gives tons of small balls, on any  profile and any finish, with or without nitrogen. I only use it to thin old paste.

[Guni]

Tons of small balls... sounds known    It's exactly my case. Of course I don't have any temp controller but more or less I have some control with hot air operation. It looks like some problems with my paste. This most of the response suggests (thank you very much). So, I've ordered different one and I'll see.