How to not make solder paste spread out during reflow? - Page 1
Forum section: electronics / projects

author: CookieEater
author_rank: Contributor
timestamp: July 25, 2023, 03:31:51 pm
content: Hello,I have been doing my first steps using stencils and paste, and I have a question I would like to ask, and some thoughts that came to mind as I tried to find reasons on my own.Looking at paste reflowing videos on YT, I see two categories:1) Where the paste spreads out (heavily) onto the solder mask during reflow and is then pulled back-2) Where the paste does not spread out during reflow. It immediately becomes this matte, dry paste, without spreading out and then reflows.I see 2) as more favorable as it reduces the chance of bridging, as well as less cleanup (because of less stray solder paste balls on the board). I, too, have not managed to do this and instead always ended up with situation 1). I'm currently just reflowing with hotair from below the board.Possible explanations for me:1) Different paste.2) Different reflow profile, as in it stays longer or less in certain temperature ranges.3) Different angle, speed and force when the paste is spread out over the stencil. (Temperature-wise mine is at room temperature when applying it)4) The amount of paste (I have checked with my microscope, and it is not more/thicker than in the videos I have seen where it does not spread out)5) "Seasoning" the paste by letting it rest for some time after application. Is that a thing?6) Different heating method? Oven (resistive heating with convention or no convection), IR, vaccum?It is a bit difficult googling this as google steers you towards the actual reflowing/melting temperature of the solder.Thank you!

author: Bud
author_rank: Super Contributor
timestamp: July 25, 2023, 04:43:07 pm
content: Can we start from what paste brand/manufacturer you are using.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 25, 2023, 04:52:04 pm
content: I do not necessarily see the second category as "desirable".My first guess is that it's just another brand of solder paste.When solder paste has more flux, it tends to flow more, and often this is a good thing.I've seen several video's where a "sausage" of solder paste is put over a row of pads for a QFP, and it solders perfectly in a reflow oven. The most important step here is to have just enough solder on the pads.Solder has a very high surface tension and it also "wets" the pads (when the flux does it's job). This combination help reduce the amount of shorts during soldering. Those shorts mosty happen when there is too much solder on the pads, or some other factor of the soldering process is not handled properly.

author: ajb
author_rank: Super Contributor
timestamp: July 25, 2023, 08:55:40 pm
content: Regardless of what the reflow process looks like, how do your results look? Lots of factors go into good pasting and reflow, but how the paste slumps during reflow isn't something you'd typically pay attention to (would be hard to watch in a conveyerized oven anyway!) as long as your results are good.QuoteNo. Solder paste has a limited open time, depending on the environment. Many pastes are hygroscopic and under excessive humidity will absorb moisture from the air--this makes them runnier, and more prone to voids and worse from the excess water in the paste. Under conditions that are too dry, the volatiles and solvents will evaporate out of the paste--that can cause printing problems, and may make the flux ineffective.Different application techniques may change the amount of solder printed, but will not cause the paste to spread more or less. Different reflow profiles may cause more or less spread: the paste's viscosity will decrease as it heats up, so a profile that spends more time in the soak period will give the paste more time to spread before the flux activates and the metal melts.

author: CookieEater
author_rank: Contributor
timestamp: July 25, 2023, 09:33:23 pm
content: Yes, of course! I'm currently using CHIPQUIK TS391SNL50 (SN96.5, Ag3.0, Cu0.5 with T4 balls).The datasheet for reference:https://www.chipquik.com/datasheets/TS391SNL50.pdfI currently do have some bridging but it is due to my no experience and the stencils. But my question wasn't really about my issues, but a general question based on the material I habe seen online.I suspected the excessive spreading out to be unfavorable because it creates more stray solder particles left to be cleaned out and I have seen this more "sintering" looking like reflow process in more professional videos, as opposed to the running away of the paste. I have seen people manually applying paste for larger packages by just putting it on there in one pad-crossing line, as someone mentioned. Before using stencils I tried that once and it didn't create good results for small footprints for me like qfns.The "seasoning" aspect I came up with because I imagined that while there is a timeframe after which the applied paste is not useable anymore, there might be chemicals in there that make the paste easier to apply that might also cause this if reflowed immediately.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 26, 2023, 12:55:17 am
content: With a bit of exaggeration I think this, for example, is an abomination:https://youtu.be/N_195d7bP9MYour 2) is the correct way. The phenomenon of spreading is called (hot) slumping, although DIY methods go far-far beyond that.That being said, as others pointed out if you get good looking joints, you have no shorts (or can clear them after) and you are able to sufficiently inspect your boards then none of this really matters. If it looks stupid, but it works, it ain't stupid. At least for DIY.The paste flowing over the solder resist and combining in puddles makes it impossible to control the amount of solder each joint ends up with. It also increases the risk of beading and bridges. In a mass production environment it is cheaper to curb it, so you can guarantee optimal joints.For one-offs or rework, where stencil printing isn't possible slumping doesn't matter since the paste will be smeared everywhere anyway. The key difference is that those jobs are being manually inspected for defects and can be corrected on the spot.If after this you still want to "fix" the issue, other than trying a different paste, I would look at the amount of paste deposited onto the pads. If the apertures are too big, the stencil too thick, paste is not wiped correctly, then too much paste will end up on the pads which will slump more. Modern, fine pitch components can use extremely thin stencils. Of course that brings other challenges as your board planarity start to be a concern or the part placement accuracy and the force with which they are placed etc.-Are you making an arduino LED blinker shield? -Smear some paste on it fam and throw it in the fire!-Are you making 10,000 of it, or only one but it has a $1200 FPGA on it? -Well, you better make that paste stay in place like it's bolted there.HTH, good luck!

author: MarkS
author_rank: Unknown
timestamp: July 26, 2023, 01:15:35 am
content: Quote from: Marsupilami on July 26, 2023, 12:55:17 amHis faith in the surface tension of solder paste is greater than the faith of any theologian who has ever lived!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 26, 2023, 07:03:45 pm
content: Quote from: Marsupilami on July 26, 2023, 12:55:17 amWhat an annoying video.First nothing happens for three and a half minute (except paste getting a bit thinner), and then when it starts soldering, most gets obscured by silly text so you can't see what is happening. Just yuch!The video quality is also quite marginal.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: July 27, 2023, 02:59:00 am
content: I did some reflow soldering using "ZB" brand small reflow oven (probably around 100 times), heated plate and hot air. I think that OP different heating method hypothesis may be right. Unlike DIY reflow oven, semi-professional reflow oven doesn't have a glass door window for inspection, so there is no way to say what's going on there. My observations: a new "Mechanic" brand Pb-paste gives factory quality solder joints, with a very few solder bridges caused by my imperfect paste application. No easy way to see what's going on during melting process without opening the oven.Heated plate allows visual inspection during heat-up. In this case paste melts badly and covers solder mask, adjacent pins melted paste overlaps, etc. Again, I don't know if this happens in my reflow oven and then somehow fixes itself. But for obvious reasons we can conclude that components and their pins temperature is lower than PCB surface temperature. And PCB surface temperature is lower than hot plate temperature. Obvious reasons are: imperfect contact between these three objects, and cooling of everything of top with surrounding room-temperature air. So my speculation would be that in infrared reflow we have opposite temperature distribution: chip surface and pins heat up faster than PCB surface, because components shield underneath board areas from infrared radiation (shadow from components). Sooner or later every will heat up to the same temperature, but there is still some time difference between what heats up first This may affect behavior of melted paste. In reflow oven (heating by IR element from above), paste melts and have opportunity to "climb" a little on the heated pins first, and only then soldering pads heated up, maybe with very small 0.5 second delay, but this delay may be enough for a totally different outcome. Imagine a water droplet on the window during the rain: if you "help" it with a toothpick to move in a certain direction downwards, e.g. lower-left, then it trajectory would be altered and there is no way for a droplet to "climb up" and go through unaltered trajectory. Or a small piece of ice on a frying pan with a screwdriver touching it from above. Outcome would be different for two cases: if you heat-up frying pan first, or heat-up screwdriver (e.g., with a lighter's flame). I am not sure if this logic may be applied to solder paste melting process, but thought this idea may be worth sharing.Another observation, this ugly overlapped melting is not always caused by excess amounts of applied solder paste. In my case, I observe that solder joints of 0805 components are very small, and kind of solder-starved and asking for more solder. This makes me think that the second OP hypothesis about heating profile may be right. But I don't have explanation for this. I can't understand how this melted paste volume may be decreased with a preheating. Evaporation? Probably not.It seems that professional factory reflow soldering process achieves good results thanks to hood preheating and rapid speed final heatup. First, solder pad and pins are heated to some temperature right below melting point, so everything is almost prepared to be wetted with solder. Then, everything is heated up rapidly above melting point, so pad and pin are heated up above melting point and wetted with solder simultaneously. Melted paste to goes in both directions, and there simply no more liquid state solder left to escape and cover solder mask area.To sum up, it seems that we need to preheat everything right below melting point and keep paste in non-liqud state. We are doing it slow and wait until everything settles around some common temperature value. And only then, quickly ramp-up temperature. In some sense we are trying to make solder pad and component pin heat up similarly to soldering iron tip. If final heat-up is too slow, or something heated slower, there is a chance that liquid solder escape joint area. With a heated plate it is very difficult to ramp-up this temperature due to poor thermal contact: pin is thermally "isolated" from heat plate by PCB itself, and by thin layer of paste. Adding additional flux often improves DIY reflow, probably by reducing thermal resistance and improving wettability. With infrared reflow, we can illuminate pin and uncovered pad area much faster and almost simultaneously.problem 1 to be solved:if pin or pad temperature raise have significant time lag, then we have excess of liquid solder during this lag. E.g,. pad heated up properly, but pin is too cold. Liquid solder only bonded to pad, but not to pin, like component is not there. Moreover, "cold" pin prevents solder to form a half-sphere droplet, by tipping into middle with it's non-wettable surfaceproblem 2 to be solved:final heat-up is too slow. If final temperature raise rate is slow, then paste goes to liquid state while pin and pad are not wettable. In some sense, during this period pad is not different from solder mask, because there is no bonding. If we cool down things at this stage, solder is easily peeled off from pad. Very similar to faulty solder joint made by poorly heated soldering iron tip.In short, heat-up everything to below-meltimg point, then do some "shock" rapid heat-up to make pads and pins work as mini soldering iron tips, their surface must become wettable fast, preferably before paste melted.

author: coppercone2
author_rank: Super Contributor
timestamp: July 27, 2023, 03:21:12 am
content: lmfao that video should be a gif saying 'real life' with that capacitor bobbing up and down.so easy to make a board am I right? i swear people think its knitting

author: CookieEater
author_rank: Contributor
timestamp: July 27, 2023, 05:11:03 pm
content: Thank you all for the very interesting inputs. It is nice to know there is a term for this phenomenon, as well as the detailed observations and deductions in regards to different reflow techniques.I was also wondering if surface finish of the boards can make an important difference here? I would assume gold nickel plated pads to be very flat and due to the etching of the copper and plating process maybe have a roughness to them, whereas HASL treated pads are very smooth and convex in nature.

author: ajb
author_rank: Super Contributor
timestamp: July 27, 2023, 05:35:37 pm
content: ENIG (and I guess OSP) are indeed generally more flat than HASL, and this can be an issue for BGAs and leadless packages, but would not really cause any difference in spreading of paste. HASL isn't THAT convex to the point that the paste will slide off of it.