Protoype pcb population - Page 1
Forum section: electronics / general

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 13, 2020, 09:49:35 am
content: I needed a prototype pcb fitted with two largish SMD devices (the rest was TH)Best price I could get was £75 for one pcb.So on the next pcb revision I decided to have a go myself.I added rows of vias next to the pins so I could buzz for shorts.The smaller of the SMD was 0.5mm pitch but I managed to get it soldered ok after a couple of tries.Some pins shorted and others were open circuit when I powered up the pcb.So plenty of flux and run up and down the pads a couple of times and that seems to work ok.Of course if I got it done in quantity it would be much less than £75/pcb but I don't need big quantities.

author: rea5245
author_rank: Frequent Contributor
timestamp: January 13, 2020, 01:00:13 pm
content: I, for one, am happy to pay money to avoid soldering 0.5mm pitch parts. I just wish it were easier and faster to do so than by ordering assembled boards from China.- Bob

author: KL27x
author_rank: Super Contributor
timestamp: January 13, 2020, 02:39:13 pm
content: If you save 75 pounds half a dozen times, you can buy a fully kitted stereomicroscope.You might still want to outsource many of these types of jobs.But, a few weeks of soldering under a stereomicroscope and experimenting with the proper iron tips, you will learn things you simply don't know right now. And you can solder 0.5mm pitch easily, even without the scope, anymore, simply because you'll know what you're doing.Anyone designing with SMD parts and occasionally paying for this kind of assembly, I don't know how you can afford to not have a stereomicroscope. Even if you outsource the assembly 100% of the time, it is still going to be a good investment for design/debug/rework.Pre-microscope watching John Gamil video: I wonder if I could do that.Post-microscope: Why is John Gamil so slow?

author: OwO
author_rank: Super Contributor
timestamp: January 13, 2020, 02:50:53 pm
content: Above 0.4mm pitch QFP/MSOP is trivial to drag solder. No microscope needed and bridging is usually not a problem if you use decent flux. QFN can be done by tinning the center pad (on the board), placing the component using hot air reflow, then the 4 sides can be done with drag soldering. The hardest part of soldering a SMT prototype board is the passives (not hard, just time consuming).

author: rea5245
author_rank: Frequent Contributor
timestamp: January 13, 2020, 02:54:34 pm
content: My experience is that drag soldering is easy. Cleaning up bridges is not. With 0.5mm pitch, my failure rate is around 33% (i.e. after trying for a while to get rid of the bridges, I give up, throw out the board, and try again) :-(- Bob

author: thinkfat
author_rank: Unknown
timestamp: January 13, 2020, 03:05:10 pm
content: I just recently had to do an FPC connector with 0.5mm pin pitch. I've tried drag soldering but no matter the amount of flux, bridges would always form requiring a cleanup with a solder wick. 0.5mm is really where it starts to suck. At least for leaded packages. QFN or something - no problem with 0.4mm, especially if the package has a center pad.

author: OwO
author_rank: Super Contributor
timestamp: January 13, 2020, 03:08:09 pm
content: Removing bridges on QFP is easy as long as there isn't a big solder blob. Apply flux, clean off ALL solder from your iron tip, don't tin it, touch it to the solder bridge and lift after 1 second. Repeat if the bridge isn't removed. If there is a big solder blob then you will need to use solder wick, but otherwise avoid using solder wick because you are likely to bend the legs.

author: thinkfat
author_rank: Unknown
timestamp: January 13, 2020, 05:12:21 pm
content: I might have the wrong tip for that. It just doesn't seem to take.

author: KL27x
author_rank: Super Contributor
timestamp: January 13, 2020, 07:53:45 pm
content: Quotethinkfat:It's the part. FPC connectors don't have the luxury of pins that are shaped to work with drag or wave soldering. The FPC connector pins are an extension of spring contacts; thus, the cross section is dictated by the mechanical properties of the spring contact part. This makes the pins on the tight pitch FPC connector too tall for the spacing. This is one of the few exceptions to the rules.For this part, what I do is flux, first. Solder the side anchor-tabs (or the right tab, in the least, if right-handed). Then I wipe/fling excess solder from the bevel tip and wipe on the brass wool. Touch the tip to the two of the first two pins. A bridge will form between these first two pins. Then wipe the tip with a paper towel. Go back and drag that initial blob across the toe of the pins. This is enough solder to do about 8-10 pins. (So for a 16-20 pin connector, you would dab and bridge two pairs of pins to start with.) If you run out, then touch the tip to the side tab (or the stupid thin angel hair solder, if you have that) to pick up a wee bit more solder, then finish the job. If you get too much solder on there at any point, yeah, you need solderwick.FPC (and many other fine pitched) connectors stink. They're very finicky. Most all other SMD parts are designed so they don't bridge when wave soldering.The 2.8mm tinned face only bevel tip one of the best for drag soldering fine pitch IC's and resistor arrays and the like. Parts with gull wings and tight enough pitch, you can actually get too much solder on there, somewhat like with the FPC connector. But it has to be a crapton. With half a mind to what you're doing, there is no problem, at all. The amount of solder on (the correct types of) tip is very forgiving with a wide zone of success. I just dab solder on the tip and gravity/inertial fling the excess, and that's good to go. Even if you have to much solder, forming big blobs that go all the way up into the shoulder, you will usually notice and stop in time, wipe the tip completely dry, and be A OK.*Another thing with FPC connectors, you will notice most of them float the pins slightly above the surface of the pads. And you can rock the pins slightly higher or lower, as you want, to distribute the solder. If you feel like you started with too much, you can rock the pins slightly higher. And if you run out of solder, you can press the pins down while reflowing/redistributing, to get the last couple pins to connect. All this, you will notice if you have a stereomicroscope.I would never not have solderwick on hand, just in case. But I regularly solder hundreds of 0.5mm pitch FPC connectors without ever touching the wick. It's of course much easier when you can batch them out, a bunch at a time, to get in the flow of things. I have a project where I have the one half assembled, but the other half has only FPC connector and an IC. And the quantities/cost are such, I find it beneficial to just buy the bare board of this half and assemble them myself, is/as needed, say a batch once a month. So I might have more experience at doing these things than anyone else on the forum.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: January 24, 2020, 01:20:45 pm
content: I work under a stereo trinocular AmScope microscope with a long working distance and LED light ring. I have 2 soldering irons running, one small one and one with a larger flat end. I pre-treat the solder wick by running a no-clean flux pen along it so that it's wet. With 0.5 mm pitch, I find it impossible to avoid solder bridges so I go back afterwards and use the solder wick with the large iron. The technique that works best for me is to hold the solder wick lengthwise across many pins and then apply the iron to the outer edge of the braid, the solder wick gets to a point where it's filled with solder and that it becomes a sort of reservoir that floods a reasonable amount of solder to each pin. Hard to describe in words but it work pretty well for me. I clean up afterwards with acetone.

author: OwO
author_rank: Super Contributor
timestamp: January 24, 2020, 01:40:59 pm
content: QFP/TSSOP won't bridge if there isn't excess solder on the tip. It takes a little bit of experience to get right but the aim is to clean the tip, tin it, drag-solder one side of the QFP, and end up with little to no solder left on the tip. If the tip is completely dry afterwards you didn't apply enough solder, but if the tip ends up just wetted with a thin layer of solder then you did it right. If you do end up with a bridge, a usual conical tip or flat tip that is well cleaned should wick solder away from the bridge.FPC connectors are a bit harder to remove bridges on, so the aim there is to not get any in the first place, which should be easy to do once you have enough experience practicing with QFPs.

author: KL27x
author_rank: Super Contributor
timestamp: January 27, 2020, 09:52:19 am
content: Gadalf_SrQuoteI think you have never tried a TFO bevel. Anything from between 2-3mm, preferred. Not that the smaller ones aren't useful for specific things and sometimes necessary for space reasons, but the 2.5-3.0mm are especially good at sucking bridges out and distributing even amounts of solder for miles.QuoteThe large bevel tip is also excellent for heating braid. If there's room on the PCB, I will lay the braid down o blank FR-4 next to the pins but without actually touching; maybe a half mm gap. Then I place the CF tip over the braid with the edge of the bevel touching the pins/bridge. This will remove any bridge, even from an FPC connector, with very little potential of bending a pin or overheating a pad/component.Owo:QuoteYeah, this is helpful. But technically, anything with gull wing pins can blob if you get too much solder on it. It's just the threshhold limit of what is too much is super tiny with the FPC connectors.

author: SMTech
author_rank: Frequent Contributor
timestamp: January 27, 2020, 10:13:41 am
content: It should of course always be noted that not all QFNs have either a center pad or pads that extend to the outside of the device, and that even when they do have such pads, they are not technically intended for soldering to(so are not tinned).The same is true for some FPC connectors & things like memory/SIM card holders, RJ45 sockets etc. Sometimes you just need paste and an oven and even then, without automated placement or a fancy rework station you are trusting a little to luck as you can't see what you're lining up.A 3rd party incurs some cost to do that and will have a minimum standard they want to work to that might be higher than the one a one man band might be happy with for a small project. £75 is a lot for two parts but it really isn't much when you factor in the logistics, it could have been interesting to see how the price changed if you wanted 200 parts on the board or 200 boards...

author: KL27x
author_rank: Super Contributor
timestamp: January 27, 2020, 11:37:04 am
content: QuoteI haven't used my oven in almost ten years. I threw out all my paste when I bought my last fridge. I'm sure I'll eventually need to solder or desolder something my hot air station can't manage, but that'll probably be more due to the board size/dissipation than the parts. So, the oven I might need, again.But the paste out of a syringe? I suspect there is possibly no place where solder paste in a syringe is the easiest way to do anything. I guess that just depends on the board and the person to a large extent. But I bought the paste and got the stencils and the oven because I thought spending that money would make things faster/eaiser in general. Maybe I'm too good at hand soldering for that to help me without a pnp machine.

author: thinkfat
author_rank: Unknown
timestamp: January 27, 2020, 11:55:46 am
content: For soldering passives I'm almost always faster with applying paste through a syringe to a number of pads, then placing the components and finally reflowing them using hot air. But not for anything smaller than 0603. SOT-23 is doable for me with paste/syringe, but MSOP/TSOP - difficult.What I find helpful is heating the board up a bit and then dipping paste from the syringe on the the mildly hot pads. That usually gets me just the right amount of paste. The biggest problem using a syringe is to get the paste to "break off" and stick to the pad. Heating the board helps here.

author: SMTech
author_rank: Frequent Contributor
timestamp: January 27, 2020, 01:13:05 pm
content: Quote from: KL27x on January 27, 2020, 11:37:04 amNo never a syringe, but then again we all have our priorities, mine is running the SMT line, those with soldering irons on their desks, their job is stuffing and soldering through hole. For us the best way is the right way so always a stencil and always the oven, maybe if the engineer designs a prototype with like 20 parts he might make it himself (and frankly be wasting his valuable time) but otherwise at the very least get the shitty stencil from china with the board.

author: KL27x
author_rank: Super Contributor
timestamp: January 27, 2020, 09:07:55 pm
content: Quote from: SMTech on January 27, 2020, 01:13:05 pmI hand solder tiny SMD parts, regularly. I agree, it has to be a very few components and modest volumes to consider hand soldering beyond one-off/protos. 20 unique parts sounds about right. Some of the PCB I populate with an iron have only a few unique parts for which I have plenty of experience, including micro QFN, QFP, TSSOP, 0.5mm FPC connectors, and SOT parts, as well as the passives. FPC connectors are the only pain. If you batch them out, there are a lot of ways to "cheat," though. It's just a matter of finding the way to make them flow without stoppages. Almost anything other than connectors, soldering by hand is trivial, if you use the proper tools and techniques. Align, swipe, verify. Like breathing. It just takes a bit to sort out the most efficient order of operations for the particular PCB.