Need some advice for Sn63/Pb37 solder and technique for first reflow project - Page 1
Forum section: electronics / projects

author: kreyszig
author_rank: Regular Contributor
timestamp: February 17, 2023, 05:27:55 pm
content: Hi,So in the past I worked on projects where I could manage to solder all the components using a soldering iron and regular Sn63/Pn37 "44" resin core Kester solder and then clean the residual flux with a toothbrush and alcohol. Now I have this project where I will have to resort to 1005 SMD components and tiny chips such as MP4576GQBE with underneath pads and no legs. I have a decent home-made reflow oven, so this part is ok I guess, but I am not sure what to do about the choice of solder and application technique.I want to stick to SN63/Pb37 solder, but I am not sure about the flux. I assume it is better to go with a no-clean flux for SMD, as I will not be able to clean underneath the chips with alcohol. Are there no-clean fluxes that have a performance as great as rosin? Note that I would probably want to clean the board afterwards, with alcohol if possible, but I don't want to worry about corrosion underneath the SMD components.Also, if time is not a factor, is it better/easier/more reliable/less error-prone to go with solder paste + stencil, hand-applied solder paste with a syringe, or pre-applied solder with an iron, then some new flux and then reflow, if this is even a thing?Thanks!Edit: I have watched some videos such ashttps://youtu.be/qyDRHI4YeMI, and he seems to get good results even with a bad spatula, a soft mat and cheap hot air station, but I have also read in the past reports of people having difficulty with stencils, so this is why I am asking. I am also not a big fan of 6 month shelf life for solder paste, but if it is the best way (regardless of available time), that will do.

author: crgarcia
author_rank: Regular Contributor
timestamp: February 17, 2023, 09:22:29 pm
content: I should not answer in here. From this entire forum, I am probably the worst one at solderingI built boards that include some small things (at least for my skillset) like SHT41 or 0603 components (for example:this one).Some things that work very well for me:1. I always do it with stencil and a regular credit card. It is much easier in my opinion than doing it manually. I use the standard JLCPCB and PCBWay stencils with and without frame and I have never had a problem (better without frames though!)2. I fix my PCBs from all sides, because I really push the credit card to the stencil. In the video you mention they fix it only laterally. I add 4 PCBs to hold my PCB (will take a photo later to show you)3. Use a solder paste that is firm, not a too-liquid one. I watched in a video that for reflow with stencil, the ones sold in a pot trend to be better than the ones coming in syringes4. I always struggle with getting the USB-C connector done right at first shot, without any pins solder together. In that case I follow "less is better than too-much". For me it is much easier to add solder afterwards than to remove solder5. I try to buy the flux/paste that is easier to work with. I don't care too much about noclean or nolead (I use mask, gloves, fume extractor, etc)6. I see a lot of people pushing the components after they place them. I don't that with the small ones, they always stick to my tweezers. I just softly drop them in place and let gravity do it's job once the solder is melted. Maybe if one day I have a microscope I will be able to do it betterIf I can do it, I am completely sure everybody can

author: kreyszig
author_rank: Regular Contributor
timestamp: February 17, 2023, 09:41:24 pm
content: Quote from: crgarcia on February 17, 2023, 09:22:29 pmThanks for your feedback! Nice project by the way. What flux/paste have you had success with (Sn63/Pn37 in particular)? Do you use extra flux in some situations when working with solder paste?

author: kreyszig
author_rank: Regular Contributor
timestamp: February 17, 2023, 10:01:00 pm
content: I am considering this one:https://www.digikey.ca/en/products/detail/chip-quik-inc/TS391AX50/7802229

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2023, 03:40:16 pm
content: A stencil is the only way to go if you ask me. Applying paste with a syringe by hand is inconsistent unless you use some kind of dispensing machine. The smaller the part, the less solder you need, and the less excess solder the joint will tolerate.By 1005, you mean 1005 metric, aka 0402? Those are already small enough to be hard to paste by hand. You pretty much always end up with too much.Now that stencils are cheap to get, I don’t know why anyone wouldn’t use one.For hand stenciling, where it’s easy for the stencil to not be 100% flat against the board with no gap, where a bit of shifting is possible, etc, the tendency is to end up with more paste than you need. I have had much better results once I set my layout program to reduce stencil apertures by about 20% (rather than the typical 10% or so that the PCB maker will do by default).Also, excess solder means much higher risk of solder balls, and it also means more flux to clean up. With the correct amount of paste, you shouldn’t even see flux residue with the naked eye. (And in that case, I wouldn’t clean it.) When you apply by syringe by hand, you’re going to have a much bigger mess to clean.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 19, 2023, 02:10:15 pm
content: Thank you. So I will go with a stencil. You mentioned reducing the aperture size. What about stencil thickness?

author: tooki
author_rank: Super Contributor
timestamp: February 19, 2023, 02:52:32 pm
content: Quote from: kreyszig on February 19, 2023, 02:10:15 pmI meant to mention that and forgot! Yes, that’s also an approach, though I haven’t gotten around to testing it yet. I would assume that thinner stencils are even easier to damage, so there’s that. Even with them, reducing aperture size a bit is still desirable, so that you don’t have to align with the pad absolutely perfectly like with no reduction.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 20, 2023, 01:15:03 am
content: Quote from: tooki on February 19, 2023, 02:52:32 pmThanks! But what are you using for stencil thickness when you reduce the aperture size by 20%?

author: tooki
author_rank: Super Contributor
timestamp: February 20, 2023, 06:09:36 am
content: Whatever the PCB manufacturer default is. (JLCPCB and Beta-Layout’s PCB-Pool.)

author: jmw
author_rank: Frequent Contributor
timestamp: February 20, 2023, 07:41:41 am
content: The Chipquik thermally stable paste works very well. I've had no problems with it and 0402 pads. I do recommend refrigerating it between uses to extend its life.