Beginner Home Assembly Journey Episode 1: "Often Wrong but Rarely in Doubt" - Page 1
Forum section: electronics / general

author: meshtron
author_rank: Regular Contributor
timestamp: October 05, 2023, 03:53:16 pm
content: Against most of the advice Ireceived here(but, aligned with some of it) I've decided to assemble of my PCB product in my 3-car garage myself. The world is full of people like me; able to look back on experience with rose-colored glasses dialed up to 10 and venture undaunted into the future with high hopes, big dreams, and a very small chance of success. I thought it might be interesting - for entertainment value and for posterity - to document some of my journey here on this forum which provides so much good, sound advice often flavored with small doses of the salt of real experience.If this kind of post is undesirable or unwanted here, just let me know and I'll remove it.In today's episode: Humidity Wins and Re-Reflowing LosesHUMIDITYI've watched nearly every video on YouTUBE about the TVM802BX Pick and Place machine I have (at least in theory) on the way to me. Similarly, I've watched every video (all 5 of them!) about the Puhui T-937M reflow oven that I have in my garage right now. In addition to all these videos, I've done lots of research and something that kept coming up was humidity and parts storage.The storage part is pretty easy: I can keep unused parts indoors, in sealed boxes with desiccant packs in them. The challenging part is all the parts that will be mounted in reels on the Pick and Place. Especially with the TVM802 where a single shaft carries all reels on a side AND the load/unload process is "fiddly" (to be generous), there's no way it was going to be practical for me to have to set up/tear down the PnP every time I need to use it. So, those reel parts will be exposed to whatever ambient RH is and I need to get it under control!As I will be doing this work in my garage, and as I live in the Pacific Northwest (known for its constant rain), I decided to start working on measuring and controlling humidity right away. So, I bought some inexpensive wireless (Zigbee!) hygrometers off Amazon and placed one in the garage where the PnP will sit, one in an adjacent cabinet where I intend to store parts, and another in the filament housing of my (new for this project also) 3D printer.The datasheets I looked at seemed to indicate I wanted <50% RH for storage, so that's been my target. At this time of year in the PacNW - which is about the wettest - outside RH stands around or a bit over 60%. So, I spooled up a pretty large dehumidifier in the garage and placed a number of rechargeable desiccant packs in the cabinet. Between those two things, and with the dehumidifier running essentially 100% duty cycle, I was able to get RH to dip just below 50% (as low as 47%) and stay under about 52% for a week. But then it rained all night one night and even with the dehumidifier running flat out, RH climbed over 55% for most of the night.A few paths I had in front of me at this point:1) Add another (bigger!) dehumidifier and run it flat out also2) Find a spot in the house where the PnP could live instead3) Come up with another way to manage "hyper-local" RH at the PnPTurns out #2 just isnt' an option. Nearly the whole house is carpeted, cats have free reign over all, and there really isn't a good space (other than the one my even more moisture-sensitive 3D printer occupies) where I could fit a PnP with the right amount of access to 3 sides of it. I mused to my wife one evening that if we got rid of the dining room table, I could probably construct a small cat-proof booth there... That idea didn't even survive the entire trip out of my mouth before it died. RIPI came close on #1, but definitely feels like I'm in the mode of "if I throw enough money at it should be okay" and the fact that we still use the garage as a garage sometimes (meaning the doors open and close at least several times a week) combined with the fact I likely haven't seen the worst RH mother nature has to offer told me to save the roughly $700 I was going to spend on a bigger dehumidifier (not to mention the cost of running it 24/7) and explore another option.So, I've decided that I will rig up some simple clamps on top of (on one side) and underneath (on the other 3 sides) the workbench the PnP will live on, and I will simply "bag it" when not in use. I can put more, big desiccant packs in there and it has the added benefit of protecting it from airborne dust. I'm pretty confident this can work, but it remains to be seen how much cussing is involved in wrapping/unwrapping it when I want to use it.So, that is the current plan for humidity control:- Store unused parts indoors, sealed container with desiccant packs- Wrap and desiccant pack the PnP when not in use, hopefully in a way that's easy-ish to apply/removeRE-REFLOWLast night was a pretty big first:- I received my bare boards from JLCPCB on Monday of this week- I received the (unframed) stainless stencil for my boards on Tuesday- I received my GC-10 solder paste on Wednesday- I got my shipment from DigiKey with enough components to build 30 prototype boards yesterdaySo, it was with much excitement that I ventured into the garage after finishing my normal 10-hour day-job (I work from home) to hand-stencil and hand-assemble my first boards last night for reflow! I've probably watched 15 videos on how to hand-stencil things, so I had the gist of it down (I do not own a stencil printer). I placed some unused boards on the table and taped them in place, then I fiddled with the stencil to get it to lay exactly over the target board.First pass didn't go great. Board was a bit skewed so the some of the 0.5mm pitch pins had solder paste sort of next to the pad, but barely touching. No good - alcohol wipe to clean it all off and try again. 2nd pass was better, even the small pads were at least 2/3 covered with paste. But, the paste seemed to "sag" as it sat there and eventually, especially on the QFN28 pad the MCU sits on, all the paste sort of merged into one big blob. But, I know solder mask and flux are damn near magic, so decided to roll with it.I opened up all the DigiKey bags and extracted enough parts to do a single board. This taught me that good tweezers are a must, that workstation ergonomics matter, and that I definitely want to run this stuff with a PnP. For reference, I have 2 QFN parts, most of the passives are 0603 and everything else is bigger than that. In the world of PCBs, these are "relatively big" parts, but in my garage, with my tweezers and my hands, they are friggin tiny!But, placement actually went decently quick. I learned that 0603 resistors are labeled with hieroglyphics that I had to decipher, but once I learned the pattern not so bad. At one point, a resistor I picked up 'plinked' out of the tweezers and landed (fortunately?) in another bin that also contained a single resistor. It took me probably 10 minutes to get that sorted because I had also - unknowingly - placed the wrong resistor in the bin that received the new one. Anyway, finally got it sorted and won't have to do that again. It also occurred to me while I was muddling through that problem that those resistors are worth less than a penny each and very likely the right answer was to toss them and just grab another, but I wanted to at least understand what was happening first.So, got everything on the board, ran the reflow oven through one cycle just to get it warmed up a bit and make sure it was behaving as expected, picked a profile that looked like a lovely shape and then it was into the oven!! About 7 minutes later I opened the drawer expecting a beautiful soldered board... But what I saw was solder paste still looking like solder paste albeit a bit darker, a bit more tired, and in some places having flowed a little bit. This was a pivotal moment, and I am a man of action. So I lickety-split threw it back into the oven, picked a pointier reflow profile that went well above 250C, and hit GO again. I walked out of the garage and back in to reset time itself essentially erasing the first failed attempt.To my surprise, when I removed it the 2nd time, it actually looked quite a bit better. Most of the passives had flowed decently and even the MCU looked pretty okay-ish. Sadly, the most expensive chip on the board (about $6/ea) with 1mm pitch pins looked - well it looked like hot garbage. Kind of a "fillet" of still paste-looking but hard and nasty solder paste all the way around it. A quick check with the meter confirmed what I suspected, all of its 15 or so pins were shorted to each other.I fiddled around with it with a hot-air gun trying to get it to flow a bit more. And, I did do that, but it didn't change the fact that it's shorted underneath the chip. Also, the MCU, on closer inspection, has 2 sides where all pins are bridged. I tried touching them with a soldering iron last night and some flux, but it seems like after reflow the solder joints have a hard waxy substance on them and it didn't budge.So, board #1 is a bust. Not really shocking, but most importantly, what did I learn?- I should've read the Engineering paper for GC-10 solder paste BEFORE I selected a reflow profile and either picked one that was within its recommendations or created one of my own- I should NOT have accepted the 2nd pass at stenciling my board that had the MCU (and probably the other QFN chip too) solder paste join into an amorphous blob- I definitely SHOULD have stirred the GC10 before use - some of the flux was separated out, so the consistency of the paste I used was inconsistent and likely over-flux'd- Reflowing twice is generally a bad thing. I think I baked most of the flux out on the first pass but didn't get hot enough to reflow, so the 2nd pass tried to reflow but didn't have enough flux to flow nicely- My $70 Amazon digital microscope is hot garbage (field of view, lighting, work area, etc.) Maybe good for something, but not very good for this- I'm going to add resistor/cap (where applicable) markings to my BOM until I get the hang of it- It's worth buying the $20 tweezer set vs the $8 tweezer set- I bought 500 of each passive, this is a terrible quantity as it's a long tape but not on a reel - should've just bought reels of them or loose tapes- I don't think LEDs, in particular, enjoy extra trips through the reflow ovenAnd, next steps which will have to wait until Sunday:- I'm going to sacrifice (at least) one of my bare boards to just be a reflow tester. I'll place passives in a couple places (hand pasting the pads) and then try out various reflow profiles until I get one that looks ideal- I'm going to attempt to remove the MCU and the other QFN chip from the board I did. Good rework practice even if I'm not successful- I need to figure out if I can clean off whatever is keeping solder iron heat off the solder on these post-reflow boards (will try solvent, alcohol, toothbrush, etc.)- I'll mix (I think knead is the proper term) my GC-10, probably 3D print myself a better poor-mans stencil printer setup and try to get the right amount of the right consistency of paste on every pad, then assemble and flow another oneIn hindsight, yes this is a lot of screwing around to get to the point where I can hand-solder the through-hole parts and actually start testing firmware on my board. But, I've always learned best by mucking things up then fixing it, so hopefully that trend continues. The hill ahead, it appears, might be taller and steeper than it previously appeared.EDIT1: Added note about LED, the lenses seem discolored after the 2nd trip around the sunEDIT2: Part is actually 1mm pitch, not 0.5mm

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2023, 04:02:38 pm
content: I think this is one of the most valuable kinds of posts to have on the forum. Real world experience and very good teaching material.

author: Kean
author_rank: Unknown
timestamp: October 06, 2023, 04:02:59 am
content: No photos?When placed in the oven, was the PCB directly sitting on the metal drawer?I always sit my PCBs on some brass M3 standoffs laying on their side and placed away from any parts, usually placed along edge rails.Not sure about the T-937M, but on my T-962A there are definitely "zones" of the tray which heat up better than others. Easy to identify if you try reflow a panel of smaller PCBs.

author: meshtron
author_rank: Regular Contributor
timestamp: October 06, 2023, 04:13:27 am
content: Quote from: Kean on October 06, 2023, 04:02:59 amSaved visuals for a later episode - this is the rising action. 🤣I did put the board right in the middle of the tray sitting on the metal. I will need to experiment with standing it off, makes sense it could be helpful.

author: asmi
author_rank: Super Contributor
timestamp: October 06, 2023, 05:37:10 am
content: Quote from: meshtron on October 06, 2023, 04:13:27 amNever place PCBs on a metal tray/drawer for reflow, especially high-Oz boards and/or multilayer ones with several power/ground layers. Steel has crazy high thermal conductivity and will wick away a lot of thermal energy. I use standoffs made from M3x8 bolt (or longer if there are some tall parts on the bottom side), or just a bunch of scrap PCBs if there are no mounting holes on PCB I'm reflowing - though I try to avoid that and in case of small/oddball shape PCBs I panelize them with mounting holes on a tooling strips so that I can reflow entire panel at a time.Also, just a heads up - some folks here complained that T-937 oven tend to destroy some plastic connectors, so whenever you intend to use a new type of connector, try subjecting it to a reflow profile on a scrap board to see if it survives, and if not, solder them manually after reflow.

author: MR
author_rank: Regular Contributor
timestamp: October 06, 2023, 07:47:38 am
content: Single sided PCBs I just put on some meshed stainless wire, double sided on standoffs, usually some brass parts yes.I prefer to use my 8L PID controlled BBQ grill instead of my 4K USD reflow oven (especially since the smaller oven fits better for smaller prototypes).

author: TIOUK
author_rank: Contributor
timestamp: October 06, 2023, 02:30:51 pm
content: Paste is thixotropic (think non-drip paint), so always needs mixing. Get yourself a normal kitchen type glass cutting board and a stainless filling knife (flat end fishtail type not the angled putty type) about 35mm wide. Use the underside of the glass cutting board, it will be flat and shiney, the top surface has ripples in it for some reason (who would even use a hard glass sheet as a cutting board, other than someone that wants blunt knives?).Work the paste with the knife like you were filling holes and then scraping it back in to a pile, do that for a few minutes. It also helps getting the paste to room temperature after being in the fridge, but leave it in the pot until the chance of moisture condensing on to the cold paste has passed.If working cheaply, a wider stainless filling knife the size of your pcb plus 20mm is a good squeegee. Make sure the edge is good by rubbing it down flat with water and wet & dry silicon carbide paper (600 grit). Keep the blade 90 degrees to the wet SC paper on the glass cutting board, you want that flat, not angled like a knife or chisel.Apply the paste in ONE go, don't ever go back over it. Start the drawing of the paste with enough room from the start of the aperatures that it forms a roll of paste, you don't need lots of it, but it must roll as you pull the blade TOWARDS you, not sideways and keep it at an angle of 45 degrees to the PCB surface, the paste will flow in to the stencil apertures. You are not trying to force the paste like filling cracks in a wall, forcing will tend to push the paste under the stencil aperature edges leading to bridging, too much paste and bad results. If you load the paste onto the knife by drawing it through the paste that you spread out thinly on the glass cutting board, it will have already formed the roll. Practise on your PCB by putting a piece of copier paper on top of your PCB under the stencil, then lift the paper out and in bright light look at the results with a small magnifiyng glass to see how the paste is flowing, you are looking for straight sided blocks of paste right into the corners of the aperatures. You will then see if it needs more mixing without messing up a PCB and having to clean it again. I always do at least one paper test before I paste a batch of boards.Even if you think you eyes are 20:20 use the magnifying glass to check your stencil alignment and if you're using the taping down method, the tape will stretch enough to mis-print by a fraction of a millimeter, so you migh need to compensate in your alignment. When I used this method in the past, I found using aluminium adhesive tape to be very accurate as it has no stretch, however it is also weak where the stencil hinges uo and down, so put a length of plastic electrical tape over the top of the hinge area to get the benefits of both non-stretch and flexibility.Make sure you have plenty of support around the edges of the stencil using spare PCB, beware of taping it down under the stencil as this will cause uneveness in the stencil laying over your PCB, there sould be no tape between the pcb surface and the stencil, it will lead to paste leaking between the aperatures.I hope that helps you obtain better results.

author: meshtron
author_rank: Regular Contributor
timestamp: October 06, 2023, 02:41:23 pm
content: All right - a bit more detail (and a picture!) on the chip that didn't flow right. In the pic, IC1 is an SOT223 LM7805, flowed nicely. All the other passives in view are 0603 and flowed nicely (on the 2nd pass mind you). The unlabeled other part between Battery In and Load Out is aDRV8144and is connected to 3 top-and-bottom matched, via-stitched pours for heat dissipation. 2-layer board with 2oz copper both sides.So, my first thought when it didn't flow nicely was that it had to do with all the thermal dissipation wicking heat away due to the pours. Sounds like that jives with setting it straight on the metal tray which would then have a LOT of good area to pull heat from. The small communication pins at the end flowed okay-ish (still some bridging, but they were close) but it seems like the big pins didn't flow and I've got solder contact between VIN/VOUT/GND.But I talked myself out of it being a thermal problem because I could not find any resources online talking about reflow considerations for higher "thermal mass" parts and because I thought "well, the pours should be seeing the same temperature as the rest of the board - no thermal delta should mean no wicking." Maybe I just picked the wrong terminology to search for, and I definitely wasn't considering the metal tray itself.So, based on the input you all have shared, I've ordered some stainless mesh that should be stiff enough I can easily shape it into some standoffs (my board has no holes).I suspect I'm going to want to build my own reflow profile that closely matches what's suggested in the GC-10 literature. They have 4 similar profiles, the main difference being the amount of time spent in the "soak" region. Am I right to think that, because I probably AM fighting a thermal mass/dissipation issue, staying in soak longer should help my cause? Experimentation required regardless, but feels like I'm getting close to getting my head around at least the basic factors at play here.And as always, thanks for the input!!EDIT: Added copper thickness

author: meshtron
author_rank: Regular Contributor
timestamp: October 06, 2023, 02:50:41 pm
content: Quote from: TIOUKHoly crap THANK YOU! That helps a ton!!- I did not knead it at all (nor did I realize that was more than "stir until it doesn't look separated")- The putty knife I used was slightly narrower than my actual area- I made at least 2 passes, maybe 3I did order a manual stencil printer (a ZB304H) but it won't arrive for about a month. I have aluminum duct-tape here, I will try that in the meantime. Part of my problem is the board is small, stencil is huge, and I didn't put enough blank-board spacers around it to take out the crown as the stencil sags over the stuff in the middle. I might even 3D-print something the size of the stencil with a pocket in the middle for the board to sit and enough support to keep everything flat - will play with that Sunday. And yeah, I looked as close as I could, but between the stencil lifting slightly from the board, mediocre lighting and inability of my microscope (with a built-in base) to look at it, I have no doubt it was "close-ish" but not better than that.

author: asmi
author_rank: Super Contributor
timestamp: October 06, 2023, 03:38:50 pm
content: Quote from: meshtron on October 06, 2023, 02:41:23 pmYou will need to increase soak time for larger parts with higher thermal capacity (like big BGAs, especially those with heat spreaders), or if you reflow boards with high thermal capacity (thick copper, many ground/power layers). The point of this stage is to ensure an even transition of water's boiling temperature for all components (and PCB itself), as uneven heating can cause thermal cracks with moisture evaporating and creating pressure pockets. Also since you have high humidity workshop, you might want to bake larger (and more expensive) parts before reflow (2-3 hrs just above water's boiling temp - about 120°C - is typically enough) to drive moisture out of them. Most reflow ovens have a constant-temperate mode, so it's usually just a planning problem, as you need to reflow parts within 2-3 hrs after baking, otherwise they'll reabsorb moisture.