Are these balls because of soak or did I squirt too much paste? (reflow help)

[funbags]

Hey all,
I'm new to reflow ovens. Wondering if you could help diagnose some solder balls. I've been back and forth all day with tests of my oven, and I'm struggling to get rid of the solder balls. So here's where I'm at..
Hand placed solder and parts 0603 caps
Controleo3 on a B&D toaster oven
247Solder lead-free paste low temp melt @ 137C (Sn42-Bi57-Ag1 NC 273LT Amazon.com)
160 second soak to 120C
65 seconds reflow to 155C (overshoot to 170C)
hold for 55 seconds

Is this the best I can expect? Its hard controlling the amount of paste because the syringe is so small... Or, am I just cooking off  too much rosin before flow... or something? Is there a better lead free paste out there? I like the low temp nature  because I have two SOP8 chips on this board and dont want  to cook them.
Thanks for any help!

[ataradov]

I'm no expert, but you seem to have way too much solder. The solder fillet must be concave. For hand placing the solder paste, this may be the best you can do.

[forrestc]

I'm new to reflow ovens. Wondering if you could help diagnose some solder balls. I've been back and forth all day with tests of my oven, and I'm struggling to get rid of the solder balls. So here's where I'm at..
Hand placed solder and parts 0603 caps
Is this the best I can expect? Its hard controlling the amount of paste because the syringe is so small... Or, am I just cooking off  too much rosin before flow... or something? Is there a better lead free paste out there? I like the low temp nature  because I have two SOP8 chips on this board and dont want  to cook them.

The cause of solder balls are twofold:

1) Too much paste.
2) Paste not on the pads.

When you reflow, the solder in the paste melts.   If there is just the right amount only on the pads (or contiguous with the pads) you'll get nice ball-free joints.

If you have too much paste the excess has to go somewhere and often it will create a ball, typically right beside to the component.

IF you have paste not on the pads, then when it melts, it doesn't magically move to the pad, unless it is contiguous with the pad and the pad isn't already over-full of solder.   In this case, it will form a ball wherever it is.

But, from your picture you have way too much paste.  Another hint is the excessive amount of flux residue.

My recommendation:  Just order a stencil whenever you order a board.   It's like $5 more and it's easy to use... Found a decent video about the process on youtube: 

BTW, almost all modern components are fine with the higher temperatures related to SAC305 (lead free) reflow, so if you want to try a different paste as some point in the future, you shouldn't generally worry about cooking a part.  You're actually more likely to scorch the PCB itself.

[DerekG]

Is your solder paste fresh? It will have an expiry date on the can.

Normally it is tightly sealed & stored in the fridge to ensure it does not absorb moisture. Moisture causes the paste to "spit" leading to "solder balls" which is what you have.

The specs for the paste will have the recommended application temperature (so you need to ensure it rises to this before applying it to the pads).

I hope this helps.

[funbags]

Thanks everyone.
The solder is new  (2019 exp) and kept in the fridge between uses, because at room temp it's too runny, and the cold helps make it manageable. I also have stencils coming next tuesday, but wanted to see how well I could get hand placed parts in case stuff fails on me for what ever reason. After these experiments, I'll be 100% stencils though lol.

Do you guys have a favorite paste manufacturer? I don't mind experimenting with different alloys. The stuff I got on Amazon was just because of its rating, but I'm open to any brand. Also doesn't have to be low temp. I liked it at first because the temps were so low, i couldnt possibly break things or blow fuses. As I get more confident, I'm not so intimidated by higher temp stuff.

forrestc: thanks for the video.. any information helps! I'll watch it a few time before attempting it on my own...

I ran the same profile, but this time with much less solder (and actually attempting to level the PCB tray) and this were the results before and after. Its much better, but still a tiny bit too much paste, and it sneaked off the pad a little... but this is the best one by far.

Thanks again for the input and help!

EDIT: Question... When I place the part, do I have to squish it into the paste, or is resting on top fine? I'd like to avoid tombstones (or are those more of themal mass issue)?

[funbags]

Not sure I can get better than this without a stencil... in stereo the solder blob looks like nothing, but under the scope it still looks like too much.

Do I have to squish the part in to the paste or just rest it on top? My gut tells me that resting it on top helps the surface tension align the part better.
Thanks!

[funbags]

I'm happy with these fillets, but the parts didnt self align, and the joints themselves are somewhat weak. If I stick a tweezer leg under the LED, I can pop it off without much force. I'll try an other solder paste...
Any recommendations?

EDIT: Wait, the joints are strong, I just tried again and they seem to be pretty solid, but the parts are still misaligned... so I guess I want something with better flow if I'm going to make an LED bar.

[forrestc]

EDIT: Wait, the joints are strong, I just tried again and they seem to be pretty solid, but the parts are still misaligned... so I guess I want something with better flow if I'm going to make an LED bar.

Reflow may or may not re-center the components.  Yes, smaller ones usually/often move to the center of each pad, but it's by no means guaranteed or even expected.   

I also don't know about surface tension on the low-temperature solder you're using, it might not be as strong - which might lower the amount of mass one can move.

Also, see http://www.circuitrework.com/guides/7-1-3.html (and others) to get an idea of what is considered acceptable.   Parts don't even have to be fully on the pad to be considered acceptable assembly-wise.

[IconicPCB]

Try to prolong the soak period a bit it will drive out the volatiles out of the paste without melting it . This should result in reduced solder balling.

What is the recommender soak temperature of the paste?

At what temperature does the flux get activated? This should give you a hint where the maximum soak temperature is.

[funbags]

Reflow may or may not re-center the components.  Yes, smaller ones usually/often move to the center of each pad, but it's by no means guaranteed or even expected.   

I also don't know about surface tension on the low-temperature solder you're using, it might not be as strong - which might lower the amount of mass one can move.

Also, see http://www.circuitrework.com/guides/7-1-3.html (and others) to get an idea of what is considered acceptable.   Parts don't even have to be fully on the pad to be considered acceptable assembly-wise.

Thanks forrestc, That's discouraging since I really need the LED's to be perfect. I was also looking at my PCB design, and I put traces and a via under each LED(and the silk screen goes over the via making even higher). So I think those might be making high spot "islands" that catch the component and keep it from centering. I moved the vias and traces for future prints, but sadly I'm stuck with these for the short term. If you look in the image you can see how I created a perfect storm of a high spot on the PCB. I might switch to a leaded solder for the LED bar because maybe the heavier lead will allow the LED to "float" higher since it would be more buoyant... maybe? Thansk!

Try to prolong the soak period a bit it will drive out the volatiles out of the paste without melting it . This should result in reduced solder balling.

What is the recommender soak temperature of the paste?

At what temperature does the flux get activated? This should give you a hint where the maximum soak temperature is.

Thanks for the input Iconic. I can't find any info on the paste. At least nothing about the temp profile. Just that its liquid at 137C.
I found the MSDS but it doesn't have much other than health risk info. THe website for the solder literally has nothing else.
http://www.247solder.com/wp-content/uploads/2018/06/247-LOWTEMP-15_SDS.pdf

Ingredients              %                          CAS
bismuth                              ≥25 - ≤50             7440-69-9
Tin                                      ≥25 - ≤50             7440-31-5
Rosin, hydrogenated          ≤3                         65997-06-0
silver                                   ≤1                         7440-22-4
Amine Decanoic Acid Salt   ≤1                         -


I'll try and experiment tonight and see what a lower/ longer soak will get me. Cheers.

[jmelson]

137C liquidus?  That is not standard solder, it is rework material (or for special temperature-sensitive devices).

And, you can't get repeatable solder doses with hand application.  It is plenty hard enough to get reliable solder amounts with a stencil.

Jon

[DerekG]

I was also looking at my PCB design, and I put traces and a via under each LED(and the silk screen goes over the via making even higher).

Several things to watch out for:

1/ Avoid placing any silk screen under a component, especially on any copper (tracks/vias/pads) as it can lead to poor solder joints.

2/ Avoid placing vias within a pad as this allows the solder to flow down the via, thereby leaving insufficient solder on the pad itself. This can lead to dry joints & components that easily lift off the board. Where the design has no alternative (ie mosfet heatsinking to the other side of the board), consider using a thicker solder paste stencil (ie 0.2mm) so that you lay down more solder, or consider plating the vias shut, or having their holes filled by the board manufacturer.

I moved the vias and traces for future prints, but sadly I'm stuck with these for the short term.

There are a few design procedures that can help:

1/ Try & have the tracks leave the component pads from the back of the pads (ie leading away from the centre of the component).

2/ Try & place the exiting tracks "symmetrically" from all component pads.

3/ If really critical, close up the width of the pad to be very close to the width of the pad itself. This aids in "self alignment" when the solder is molten. Make up for the reduced pad width by adding to its length to ensure adequate paste is layed down.

IconicPCB is quite correct. It is critical to preheat the solder paste to ensure the flux is not trapped under the surface when the melting point is later reached. Any trapped flux can lead to nasty "balling" which is what you are experiencing.

A Google search should lead you to the paste manufacturer's specifications.

I hope this helps.

[funbags]

Thanks jmelson I ordered sac305 paste yesterday, and will do more tests it. It's probably better I abandon it, since I found out I'm crazy allergic to the low temp stuff I had. Maybe the  stuff will be easier on my skin if I accidentally get it on my forearm (I got a reaction like tiny little ant bites). My guess it's from the bismuth.

Thanks DerekG for all the info. I'll remove the rest of the inside silk screening from the LED's on the PCB. I don't really need them anyway. As for the via locations near the LED's pads, I'll try and get them as symmetrical as possible, but I'm super restricted in size of the PCB. I can't make it much larger than it is (well, maybe if I bring the LEDs closer to each other, the installation spot the PCB fits into, is somewhat trapezoidal)

Also, I didnt find anything for the solder, but I'll keep looking. It seems like the manufacturer doesnt want people to know how to use it best.
Cheers,

[asmi]

You might want to get room-temperature stable solder paste as it makes things to much easier! It doesn't need to be stored in the fridge (hence it's always ready to use), and it also doesn't dry out once dispensed on the PCB. The latter is super-useful for manual assembly as you have all the time in the world to place component without fearing that the paste will dry out.

[funbags]

You might want to get room-temperature stable solder paste as it makes things to much easier! It doesn't need to be stored in the fridge (hence it's always ready to use), and it also doesn't dry out once dispensed on the PCB. The latter is super-useful for manual assembly as you have all the time in the world to place component without fearing that the paste will dry out.

Thanks asmi, do you have any recs? I searched for GC10 but it isnt in stock anywhere in the US in jars...
Cheers,

[forrestc]

Thanks asmi, do you have any recs? I searched for GC10 but it isnt in stock anywhere in the US in jars...

Neobits seems to have some type 4 in stock... http://www.neobits.com/loctite_henkel_1993881_gc_10_solder_paste_no_clean_p8381215.html?atc=gbp&gclid=EAIaIQobChMIkJX4_9-D3AIVAsJkCh2HKQOuEAkYAiABEgLXRfD_BwE

So does jensen both type 3 or 4:
https://www.jensentools.com/loctite-1993881-gc-10-solder-paste-no-clean-sac305-t4-temperature-stable-500-gram-jar/p/480-098
https://www.jensentools.com/loctite-2023628-gc-10-solder-paste-no-clean-sac305-t3-temperature-stable-500-gram-jar/p/483-418

[dkonigs]

Not sure I can get better than this without a stencil... in stereo the solder blob looks like nothing, but under the scope it still looks like too much.

Do I have to squish the part in to the paste or just rest it on top? My gut tells me that resting it on top helps the surface tension align the part better.
Thanks!

I recently did my first full reflow-soldered board myself, using a similar oven. I did use a stencil.
However, these photos remind me of something I noticed on my boards. Specifically, that little "ball" of solder forming on the side of the resistor/capacitor components. I thought it was because of "pushing down" during placement, so I'll try to not do that next time.

During furious post-reflow circuit debugging, I wound up cleaning up many of these using a handheld soldering iron under a microscope. (The cause of the issues I was debugging turned out to have absolutely nothing to do with soldering defects, FWIW.)

[funbags]

Cheers dkonigs,
I found that the solder balls were caused by a few things in my process.
1. Using the wrong solder paste (fixed by going to a quality SAC305)
2. Using too much paste (fixed with a stencil.)
3. not enough soak time (fixed by adding 20C and 20 seconds to soak profile)
4. Not fast enough ramp to reflow temp. (fixed by changing heating element bias to use more boost element)

If you have any solutions that worked for you I'd love to hear them. Thanks for the reply!

[dkonigs]

I'm also using a Controleo3 B+D oven, the "reference design" for it I guess.  I'm still sticking to leaded solder paste for now, and so far have only used the built-in profiles. However, I do wonder if anyone has actually found useful tweaks to the standard profiles that I might want to try. (I asked the creator of the Controleo3, and he said no one has ever shared/posted any.)

I know you mentioned some tweaks, but I think you're also using lead-free paste (so the parameters may not be useful to my setup).

[funbags]

Well one tweak that seemed to work really well, that should also work for any profile.. I was fighting cold spots near the front of the oven. So I started to physically raise the boost element 2 or 3mm at a time and then test (basically 2 or 3 full turns of the nut) until I got the parts nearest to door to flow with in 4 or 5 seconds of the back parts. I also reduced the bottom element bias by 10 and proportionally raised the boost bias by 10. So for example, my "hold" element bias numbers look like 15/20/10 (btm/top/bst). That helped a lot with getting an even heat. Thpugh I still feel like parts of the pcb tray get too hot, it much more even now.

I'm going to experiment with stand-off so the pcb panel isn't directly touching the tray.
Hope it helps.