Is partial SMT board assembly a practical option? - Page 1
Forum section: electronics / general

author: Pete_UK
author_rank: Contributor
timestamp: May 07, 2020, 09:44:18 pm
content: Don't know whether this is an unusual suggestion and whether it's too problematic, but I've been wondering if its possible to get PCBs partially-stuffed, say with all of the basic Rs and Cs etc, plus the other jelly bean parts, leaving the more expensive and obscure parts to be hand fitted later? The main problem as I see it is that the you'd probably want to avoid solder paste on pads destined for later hand-soldered parts (especially if using fine pitch QFPs etc.), which would require the pick'n'place solder paste stencil to have these parts removed. It all seems quite feasible, but in practice I guess this could be problematic to organise with the manufacturer/assembler? Has anybody done something similar, especially with a Chinese manufacturer such as JLC for instance?

author: KL27x
author_rank: Super Contributor
timestamp: May 07, 2020, 10:04:14 pm
content: Yeah, I do that. Two very important things to remember. 1. Remove the cream layer from the pads/footprints of the omitted parts. That one is obvious.2. DON'T FORGET the finish! Immersion GOLD FINISH! If you don't spec this gold finish, then when the boards get partially assembled and baked, the unused pads will form a hard oxide layer in the oven. This makes putting on the remaining parts almost completely impractical. In a pinch, I have sat down and removed this layer using a rotary tool and felt polishing tips with compound. Is no fun, at all. In less dire circumstances, I have thrown out and redone entire batches of boards for this very mistake. For some parts with large spacing you can sorta get by with just turning your iron to super-hot and using an aggressive flux, but the soldering still crawls and takes way longer than it should; the residue you leave will be chock full of salt, and your iron will require non-stop attention/cleaning in the short time before the tip erodes into a blob.

author: Pete_UK
author_rank: Contributor
timestamp: May 07, 2020, 10:23:23 pm
content: Thanks, that's super helpful! Can I ask how you edit your paste file - do you do this in the PCB software package or with a separate Gerber editor? I'll have a play around with my PCB software tomorrow to see if I can figure this out. Thanks again!

author: Someone
author_rank: Super Contributor
timestamp: May 07, 2020, 11:57:01 pm
content: As above, if you're needed planarity for the later added parts then gold is the surface finish of choice. For less critical things like SOP8 or large surface mount passives, leaving the paste as the finish is ok if its washed well.My assembly house does all that work in the gerbers when they order the stencils.

author: KL27x
author_rank: Super Contributor
timestamp: May 08, 2020, 12:12:38 am
content: I edit it in the CAD software. I can't remember for sure if there's an easy box to uncheck for the paste layer. Or if I actually edited the footprints in the device layer. But it's the same thing if you want to make ICSP pads, so it's something a lot of people might have to do for other than this reason. So I think there's a box to uncheck, somewheres.Yeah, I think it's a check box IN the device layer/footprint of the library part. In Eagle, anyway. Then update library and spin a new gerber.^Good to know that HASL of solder paste isn't so bad, Someone. My nightmare mistakes were with OSP, w/e that is.

author: Pete_UK
author_rank: Contributor
timestamp: May 08, 2020, 08:35:38 am
content: Very many thanks guys, really useful info!

author: Siwastaja
author_rank: Super Contributor
timestamp: May 08, 2020, 09:43:17 am
content: It can be done and you can do it if you have some super secret sauce chip which you don't want to send to your CM.Otherwise than that, I don't see the point because hand soldering a lot of SMD parts is slow and expensive.If you have only a few units, and want to hand solder some parts, why not just hand solder everything, and save the expensive assembly setup cost, which dominates in small projects?Or maybe you think about letting the Chinese CM locally source "simple" parts such as passives, but don't trust them to source the more complex parts, and don't want the hassle and shipping cost of sending those to them, either? If this is your idea, I can assure you that if anything, the "obscure" parts gets properly sourced, while they might replace the trivial ones (such as electrolytic capacitors) with low-quality ones.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 08, 2020, 02:16:46 pm
content: Quote from: KL27x on May 07, 2020, 10:04:14 pmYes!

author: OwO
author_rank: Super Contributor
timestamp: May 08, 2020, 02:42:32 pm
content: I've always used JLC SMT for prototyping and never had the "oxide layer" issue described. I always used HASL finish. I don't even use a lot of flux, just the minimum amount to wet the pad, and usual soldering temperature (300C). JLC will solder all the passives, and I reflow solder the rest by tinning pads, placing components, and using hot air. It would be nice if JLC left the unsoldered pads tinned already because then all I need to do is add flux and do the reflow.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 08, 2020, 03:24:41 pm
content: With HASL, there is less of a problem since the finish is already some kind of solder. But HASL is not very adapted for fine-pitch QFNs and BGAs in general, and due to thermal stress, not recommended for thin PCBs either (that will tend to warp). In other cases, sure.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 08, 2020, 03:50:39 pm
content: Quote from: OwO on May 08, 2020, 02:42:32 pmIn that case just leave the paste pads there ( or maybe use smaller ones, to put anly a small layer of solder from the first reflow. )One trick that can help is to use a hard flat tool to flatten the tops of the pads so parts don't fall to one side

author: KL27x
author_rank: Super Contributor
timestamp: May 08, 2020, 05:42:34 pm
content: Quote from: Siwastaja on May 08, 2020, 09:43:17 amMaybe you have a lot of units including only one or two expensive microcontrollers that need to be flashed with proprietary code or bootloader. For Americans, if we have that assembled in China, then we have to pay an additional customs fee for importing these microcontrollers from China. We also have to either handle the logistics of getting preflashed chips to china, or we have to flash the boards after assembly, which this is a significant amount of labor involved in itself. Esp if you have your own chip flashing machine and know the easiest way to solder such ICs (can be fairly trivial).If you send the flashed chips from the US and back, you can file some paperwork to avoid the customs. But you spend on the shipping and you extend the leadtime from 5 weeks to w/e your end is going to take... at the minimum it's the 2 or 3 days of emails from random China-time to bounce back and forth to set that up plus your time to get the stuff there. (And if you have a shipping department to pawn that off to, that's great. Some people don't.) This lead time sometimes matters for your project. And that is more time your money is tied up with nothing to show for it. And who likes paperwork? It can be nice to just order the boards with the rest of the jelly beans on there, to get that ball rolling. Then you have 5 weeks to buy and flash the micros, to get those ducks in a row. And the first batch can be ready hours after the boards arrive.

author: OwO
author_rank: Super Contributor
timestamp: May 08, 2020, 06:31:45 pm
content: I've never heard of hand soldering SMT in production, but I guess it's feasible if your volumes are < 30 a month. The main concern would be soldering defects, as hand soldering is generally worse quality than pnp and reflow.Programming should almost always be done after assembly imo, the logistics of dealing with flashed chips is just a mess. You have to build a test jig for functional testing of the final product anyway, so why not combine flashing into the test jig?

author: KL27x
author_rank: Super Contributor
timestamp: May 08, 2020, 06:49:04 pm
content: Quote from: OwO on May 08, 2020, 06:31:45 pmWhat is 30 a month? Are you soldering 1 IC per day? Or are you spending one day a month to solder these 30 ICs?(I soldered 300 IC's yesterday, and that was only a few hours of my day; I guess I'm done for the year.)Programming after assembly can be in some cases be just as bad as soldering an IC. Every time you have to handle an individual board, that is costly. When flashing through ICSP, no matter how easy, it still means you handle every board and require feedback... in this case to look at and make a connection, then look at a machine to verify good.* The soldering iron can be just as easy and provide instant feedback. Dependent on the package type, this can go faster or at the very least more easily. When something works 99.999% of the time and it is completely obvious when it works or not as you do it, the throughput can be higher than something what is half the time but only works 95% of the time on the first try. You have to also deal with the 5% fail, and you are constantly taking a wrench out of the machine that could otherwise be cranking on minimal oversight (instead of daydreaming, you will have to deal with a boring and repetitive reality what demands your constant attention like a crying baby).*I have used a CNC machine to flash panels, and that helps a little. But it's costly in time to setup between different panels, and it only helps a little. You still have to clear the machine on a bad flash, either fixing it on the spot or marking the bad pcb for later processing. And you have to stand up and walk over to reload the machine every 5 minutes. With some more motors and robots, maybe that could be improved, but the set up time just gets worse the deeper you go that route.I know the cool kids are installing their micros with preflashed bootloader and mousebites. That is not necessarily cost-effective on every project, either, due to board constraints and panel layout or available I/O's (or learning/executing, i.e. work). And that pre-flashed bootloader costs minimum 10 cents per, as far as I can tell, for small customers.QuoteThe only thing worse is having to deal with the logistics of pre-flashed chips weeks earlier where they can creep into the lead time. Now physically, dealing with chips does not have to be difficult. They are very tiny, relatively sturdy, very dimensionally consistent, oxide-free, and they're flat. They don't come in weird, arbitrary shapes with burred up edges, some of which are not easy to handle in an automated way, esp with the much greater comparative tolerances in manufacturing. And chips come in a variety of nice packaging to help you out. Dealers choice. Dealing with boards/panels is the hard part. Whether flashing through ICSP or slapping a DIL chip on there in about 20-30 seconds per (including chip and board handling and fluxing from the second you sit down and working at a rate you don't mind while listening to some tunes), you handle the boards.True though, you should have it combined with testing, where that is practical. It's not always. And if the flash time is significant, you're adding this time to what might be the worst, most monotonous, most baby-sittingest part of the process. The more of this process you can do while the chip is still a chip, in a standard, easy-to-deal-with package, where the flashing can be essentially 100% successful without creating additional steps, without requiring human hands or much attention/feedback/wrenches-in-the-gears, the better.

author: OwO
author_rank: Super Contributor
timestamp: May 13, 2020, 06:53:19 am
content: Realistically at these volumes you won't be having dedicated personnel for it, which really means your engineers are going to be doing the manual labor. 30 a month means under 100 per 3 months, and 100 boards can probably be done in a day or afternoon. One day per 3 months of an engineer's time isn't too bad.How do you do functional tests if you don't handle individual panels/boards? I assume you already have a bed-of-nails or plug-in test setup, so it shouldn't be hard to add two more pins for SWD? You do not need a preflashed bootloader if you simply bring the SWD pins to the testing header or test points.Flashing time shouldn't be a problem if you have enough test jigs and the whole process is automated; the operator only needs to load/unload panels onto the test jigs, and can load more panels without waiting for completion. If you ever do burn-in testing you have to deal with long times on the machine anyway.

author: KL27x
author_rank: Super Contributor
timestamp: May 13, 2020, 07:40:03 am
content: QuoteRealistically, I don't do real engineering 29 days a month. The EE stuff is the part of what I do, but the PCB sourcing/assembly is probably more of my time. I wouldn't enjoy doing hardcore EE, day in and out. I solve problems. When that requires coding or PCB design, I might spend several weeks doing only that. I also solve physical/mechanical/production problems. I also solder.The DIL chips, as I wrote, are not difficult to put on there, if you learn how. I did 300 in maybe 3 hours, earlier this week. That's the easiest 3 hours of physical labor I can imagine. Play some music. Or YouTube talk show, more likely, these days. Get in the zone. Next thing you know, the hour hand has moved and the stack of boards are done. No problems. No frustration. Clock in, clock out. C'ya. This is a nice change of pace from coding, where basically I lose all track of time for potentially days or weeks and sometimes work for 14 hours a day, cuz that's what works for me.Yeah pogo pin testers. I've made and used them (thousands of times). To me, this is a special kind of hell. Sometimes the board has to be wiggled to get a connection. Sometimes the connection starts out good but gets lost partway through. Sometimes there are certain errors that have to be categorized because of the cost of the board and the fact that some errors can be fixed. Boards of various sizes and shapes have to be set in bins, moved to other bins, and there is just a sea of bins requiring massive bench space. And labels. Fing labels. Clear the deck to do this crap. Doing this, the clock hand doesn't move. It's stuck in time. And here you are stuck in the hell that doesn't end. Having experienced this, I spare no expense to make the best jigs and testing protocols that I know how for my clients. But other than the design and testing of the test, I do not ever want to see or use any of these testing jigs ever again. I choose to not do that part (but emergencies do happen). I feel a real and deep bond and sympathy for anyone who has to do this kind of testing.Production soldering is like being a trained monkey. BUT a monkey that dictates his own pace and who has control over everything in this domain. And who can set this up so that in these hours his mind is free to his own thoughts. Board flashing/testing is like being a trained pigeon enslaved by and hopping to the beat of machines. In some cases, the flash time is not significant. But when the flash time is 20 seconds, and the test is 2 seconds, then you're extending this hell. To someone. Yes, you can have more testing stations. This is more work. More organization. More clutter to setup and to clean up everytime this production phase has to be done. And more time, even when things are running right, because the longer the chain, the more kinks you have to sort out. And ideally, this will require a short enough time and few enough testing setups that one can do this from a chair rather than having to walk around, constantly.The soldering is extra work. But the other benefits of partial assembly are the logistics and commitment. When the board is partially assembled, you aren't committing as much in the basket in case of a major unforeseen problem or obscolescence. You can still change the firmware or whatnot without adding or redoing work (for me, anyways, with my equipment and in my current work). It's like production on demand, rather than storing up lots of inventory at once.This is how I see it, and I could be missing some major things. But this is the little world I know.In China, it is probably a little different. In America, you can't find someone happy to be a good robot pidgeon for more than a short while, unless they are personally invested in profit and loss. And unless you have your own, you can't use children. When you do find people who are halfway productive at this, they are so for exactly 3 months. Then they qualify for unemployment. I see company owners and managers who bitch about this; they expect human beings to do this day in/out. If someone is good, you better find them other positions after they put some good time in at this job.*** Good employees are hard to find, and this will burn out any American. Americans have alternatives other than jumping out of a window. This is why we outsource this work to your country... when we're ok with the massive long-term commitment, upfront cost of paying in advance, long lead times, the paperwork, and the extra 21 emails setup and customs forms, and the whole trusting a Chinese manufacturer with the gerbers and the firmware. Of course it makes sense to you. You can find someone to flash/test boards cheaper than an IC vendor charges to load a machine and press a button, per unit cost.***I work with a company who has had their own lessons to this problem. Imagine you have a worker doing assembly line, repetitive work to a device. They make, I dunno, sub $20 an hour. They do a procedure on a $100 part, once every 2 minutes or so. This worker willfully destroyed over a hundred of these parts by the time it was noticed. Been there a year, and has done this properly thousands of times, and all of a sudden he decides he would rather be on unemployment, and this is how he achieved his goal. Same guy, they talked highly of and were hoping to make him a manager, in the first several months of his employment. Repetitive work has a way of crushing the soul.