Just done soldering the 8 boards my friend ordered. didn't take all that long after all...
Tried different techniques... first used (tacky) flux to hold the components into place while soldering. Kinda worked but hard to dose the stuff using my big 30cc syringe, and time consuming. Plus the stuff expired 3 years ago so it turns into kinda glue. Once heated/soldered it's a huge dark brown sticky mess that even my powerful dedicated chemical can't really clean perfectly. Board still remains a bit sticky afterwards... so I gave that up. With fresh flux, in a smaller syringe, or a flux pen maybe (have one but can't find it and even if I did it's even older than my syringe...), maybe it could work, dunno.
So next I tried putting a dab of solder on one single pad, soldering it while holding it flat onto the PCB with the tweezers, then soldering the other pad. Kinda works for the MELF diode and 1206 resistors, but the SOT23 trannies have wayyy smaller terminals and the pads are ridiculously tiny, minuscule, so doesn't work. Plus, even on the resistors and diode I would still feel the need to reflow with the hot air soldering station (crappy cheap 858D+ , but does the job...) to make the joints look better and be sure the components were well in place and that there was no tension/stress on the joints.
So then I tried third method : since I reflowed everything, might as well solder everything directly that way ! The less thermal stress on the components, the better. So I put a bit of solder on every pad using my regular soldering iron and my smallest solder (0.38mm diameter). Pads of the SOT23 are so small you can barely put solder to them, it's ridiculous.
Then I placed all the components at once, very roughly, next to the solder pads. Can't put them ON the pads anyway, because of the solder blobs. Components just can't stay in place on blobs, they just slide around.
Then I just go over each component one by one with the hot air gun, holding the components with the tweezers. Airflow set low, 3 out of 8, to try and not blow the components...
Temperature set to 350°C because I don't know any better, so had to start somewhere.
Soon ran into a problem : board is so tiny that once you have soldered the first component, it keeps the solder liquid until you get to the 4th component at least... I was soldering the 4th component, and accidentally hit the first component with the tweezers... component MOVED ! Its solder still had not solidified !
So I laid the PCB onto a soaking wet sponge to keep it cool as I solder, and let a second or two pass between each component. Helped a lot.
First board, looked like the resistors looked a bit "cooked"/overheated...oops...
So I lowered temperature a bit, to 325°C, and also lowered the air flow a bit as well as I would still blow components every now and then. Set it to 2.5 / 8.
Seemed to yield better results, no cooking anymore.
How hot do you adjust your hot air station for small components and low thermal mass boards like this ??
Anyway. Next time might try ordering a stencil and some solder paste to see how that goes. Maybe build a little DIY reflow oven, unless the chinese by now have come up with a ready made affordable one ??
Of course getting JLC to solder for me is just as good...
Once done I did some testing. Checked all resistors. All fine at 10K.
Checked all diodes and trannies.. all fine except for one diode that read fine in forward, but read crap in reverse : 2.2V instead of 0.6V... because it's in reverse parallel with the base-emitter junction of one of the tranny (to protect it from large negative voltages from the RS232 serial port). Reflowed tranny, problem solved, was a bad joint.
Question : anybody has a link on the interweb for decent yet affordable tweezers ? Mine are crap. Tips too coarse for SMD stuff, was a pain to use here. Plus although they are made of stainless steel (I think... looks like it), they might as well be made out of butter : they bend so easily that they are now completely out of shape and any and all attempts to straighten them with pliers, failed miserably. Now even if I press them hard to grab say a thin mod wire.. wire will slip ! Can't even get a hold on that !
So need tweezers with finer/sharper tips, and that don't bend too easily...
Any suggestion / link welcome ! What do you have that works ?!
Also, bench top is made of OSB but it's not very suitable for SMD work... the irregularities of the OSB surface are enough for the SMD parts to hide into ! Once lost, never to be found again !
Plus it gets dirty and then impossible to clean.
So I would like one of these fancy mats that everybody seems to have now.. "cutting mat" it's called ? The large mat that's blue or grey or green, with a grid on it and other stuff... where can I get a good one ? I mean that can stand the heat of a solder iron, , and with edges that lay flat on the bench, don't like mats (mouse or else) that don't lay perfectly flat on the bench, looks cheap and stuff can get underneath it and hide as well !
Would like a large grey one with white grid, if possible, yes I am it choosy I know !