How to prevent solder ball formation during reflow - Page 1
Forum section: electronics / general

author: pylo
author_rank: Regular Contributor
timestamp: June 04, 2019, 07:18:02 pm
content: Hello,I face a regular phenomenon every time I do my own reflow. It is not a critical problem, but nevertheless it is a hassle and annoying. I'd like to ask you guys if somebody knows the reason and maybe if it can be remedied by myself.I have DIY reflow process based on a controller and a miniature oven. The basic description of the problem is, during reflow, solder gets pushed out of the pads and it gathers in small balls on the sides of components. It doesn't happen on every component, but very often, and it happens irrespective of the type and footprint. It happens with capacitors, resistors, and even ICs, regardless of shape and size. These solder balls just sit in the flux residue after reflow. I fear these might get loose and create shorts or shorten leakage paths, so I always remove them.I have attached a picture for demonstration. (Yeah, I know the footprint of the crystal on this specific board is wrong, please disregard that - the phenomenon happens for other parts too.)Naively, I guess this might be a problem of either the solder amount (stencil thickness?) or a mismatch between the type of solder used and the reflow profile, but either way, I have no clue which one it is or how to proceed. Though I guess if it were a problem of the stencil then everybody else ordering from redacted-well-known-chinese-pool-service would have this problem too. Or maybe something completely different?As I said, I've been dealing with this problem by simply manually removing / cleaning each ball after reflow. Tbh not a big deal, but there is always a risk of missing a few, and it is kind of a hassle. I appreciate any help in finding a "cure".Thanks

author: amc184
author_rank: Regular Contributor
timestamp: June 04, 2019, 08:41:43 pm
content: I had this issue with the home SMT soldering method I use. The main issue seemed to be inadequate pre-heating; improving the profile to not ramp so quickly reduced this a lot (maybe 90% eliminated).

author: Bud
author_rank: Super Contributor
timestamp: June 04, 2019, 09:27:01 pm
content: Happens when using expired solder paste or too much paste.

author: IconicPCB
author_rank: Super Contributor
timestamp: June 04, 2019, 09:46:13 pm
content: What amc said... preheat for longer to allow solder paste to dry out so that no outgasing or explosions of gas eccur within solder paste.Dry farts are ok.

author: mrpackethead
author_rank: Super Contributor
timestamp: June 05, 2019, 02:17:34 am
content: stencil design can help.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2019, 02:24:16 am
content: Combination ofStencil thicknessSolder paste ball sizeReflow profile.Age of paste.When this happened to me i just reduced to a thinner stencil and got some new (different brand) solder paste and it went away.Always remember, Chipquik paste sucks. There's a reason why it's cheap.(It's more unpredictable than anything)

author: wraper
author_rank: Unknown
timestamp: June 05, 2019, 02:41:12 am
content: Quote from: Psi on June 05, 2019, 02:24:16 amMaybe it sucks but it's not cheap at all.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2019, 03:26:15 am
content: Maybe they changed pricing.When i stopped using it a few years ago it was always right at the top when ordering by price low-high.

author: pylo
author_rank: Regular Contributor
timestamp: June 05, 2019, 05:06:41 am
content: Thank you guys for all the feedback.Stencil:At first I also thought this might be due to too much solder, but if that were the case it would mean the stencil is wrong. While it is true this is not a high quality stencil normally used by mass assemblers, it is the standard one you get from pool services. It would surprise me if it was too thick for general boards, because then all their customers would have the same problem.Solder paste:It was suggested the paste might be too old, but it is not (assuming Digi-Key didn't deliver for me an out-of-date package). I've barely had it for three weeks, whereas the datasheet suggests shelf life is over 12 months at room temperature. Likewise I haven't exceeded its stencil life either during assembly. It does worry me though that Chip Quik was mentioned as a bad quality solder paste that might exhibit such issues, because as it turns out, the one I use is from Chip Quik. So I guess the paste quality could be a problem, but until I've done some experiments, I'll give them the benefit of doubt that the paste itself is OK, only the reflow profile used with it is inadequate.Reflow profile:All the above leaves me with adjusting the reflow profile as amc and Iconic suggested, at least as the first step in debugging this problem. I'll try increasing the pre-heat duration. Of course if that doesn't make things better, I'll be back to the conclusion that it is either the paste, or maybe (imho much less likely) the stencil.TL;DR: After all I've read in this thread, it must be the length of the pre-heat or the quality of the solder paste. I'll make some experiments with the reflow profile first, and let everybody know how things went.

author: Psi
author_rank: Super Contributor
timestamp: June 05, 2019, 05:12:26 am
content: Quote from: pylo on June 05, 2019, 05:06:41 amWhenever i buy a stencil i am asked for the thickness. 0.2, 0.15, 0.12 or 0.1mmQuote from: pylo on June 05, 2019, 05:06:41 amOut of interest, what T number is your chipquik paste. It's the T4 that i had the most issues with.T3 worked most of the time.Quote from: pylo on June 05, 2019, 05:06:41 amIf you ramp the temp up too quickly any moisture boils instead of evaporates. This throws the paste around a bit and instead of a single pool of molten solder you can get multiple blobs, some of which are not in contact with the main blob so don't recombine. They set as their own ball of solder around the chip.

author: Berni
author_rank: Super Contributor
timestamp: June 05, 2019, 05:26:23 am
content: So far this looks like it might be too much paste having gotten off the pad.This could be due to your stencil(too large openings) or paste application method (you push the paste in too much and it squeezes out and spreads underneath the paste stencil) but could also be a temperature profile issue as mentioned before. Its also possible you have gotten too much paste under the component when placing it, the excess paste ending up between the pins and it gets squeezed out once paste on the pins melts and sucks the component down to the boardsGenerally the sign of old crappy paste is that it forms lots of smaller balls around pads that don't combine together into the large blob on the pad. But we found out that you can "revive" old under-performing paste by simply adding in extra flux and mixing it in thoroughly

author: Spragnut
author_rank: Contributor
timestamp: June 05, 2019, 06:18:24 am
content: We have gone from using 0.15mm stencils to 0.1mm and that change alone has made an improvement on consistency and quality. Not only of the finished results but also paste release from stencil and paste quality on board. It's also eliminated the majority of balls we had on our boards. Although if you look at consumer grade equipment you'll often still see these lurking around. So it isn't just small scale production that have the issue.IPC standard or manufacturer's recommended for all our apertures.We use that material thickness across all our boards. Its amazing how much extra paste you get from that 0.05mm material thickness.Paste we use is from BLT Circuit Services, we run vapour phase so use a paste suitable for that reflow process.Sent from my SM-G975F using Tapatalk

author: IconicPCB
author_rank: Super Contributor
timestamp: June 05, 2019, 07:06:55 am
content: We too had a problem with solderballs earlier in the piece.We identified stencil cleanliness as the issue causing the problem.We now use a stencil cleaner to wipe the stencil clean ( bottom side of course) and that has eliminated our solderballing problem.We use Chemtools stencil cleaner and importantly LINT FREE applicator.

author: SMTech
author_rank: Frequent Contributor
timestamp: June 05, 2019, 07:38:34 am
content: Mid chip solder ball is the term generally used to describe this and would be the term you could google to learn more. Footprint and stencil design are going to be the primary areas you need to focus to work out why its happening (and printing technique/skill) as excess paste will be the primary driver. Excess paste can be there because your squeegee angle is wrong and your pushing too much paste through, your gasket between stencil and PCB is poor, the stencil is too thick or your apertures present paste too far underneath the device body. If you are boiling your flux causing it to blow off solder, that is quite impressive, but so is getting one on a SOT23 where the paste has plenty if opportunity to escape without forming a random blob.Be careful with getting your stencils too thin however, while it can help with manually printing fine pitch devices it can also mean you end up with too little paste under things with a center pad that require the paste height like some QFPs.

author: mrpackethead
author_rank: Super Contributor
timestamp: June 05, 2019, 07:55:36 am
content: All of the above ^^^I would say that 90% of the problems i have on my line come back to problems with paste. Getting a decent semi-automatic printer has helped no end. I use 0.15mm stencils, Henkel GC10 paste. I have done a lot of work on my stencil apature designs. I do not leave it to the stencil fabricator to do. Each and every pad is thought about. Yes, its a lot of work, but if you want reliability...

author: Harjit
author_rank: Regular Contributor
timestamp: June 06, 2019, 12:37:32 am
content: Thank you for the insights! I've had the same issue and will try the suggestions on the next board.I make small hobby boards and was wondering if there are alternatives to Loctite GC 10. Purchasing a $100 tub is a bit much for what I do.Thanks.

author: Berni
author_rank: Super Contributor
timestamp: June 06, 2019, 05:12:36 am
content: We used to use some ChipQuick brand leaded paste in a syringe before and it worked well enughBut since we used quite a bit of it we bought a 500g tub of Loctite 2006905-M and it worked even better than the stuff we used before. But this amount of paste does last a long time and makes no sense for hobby use as most of it will just end up getting old and going bad before you get around to using it.The place where we see the difference between of a really good paste is when it comes around to stubborn components. For a lot of prototype stuff components end up sitting around in open bags for a year or more so the pins can get a bit of corrosion. The nice Loctite paste doesn't care and solders all of them just fine, but the ChipQuick stuff did seam to struggle here and there with one of those crusty components.