Stencil life and efficient small production? - Page 1
Forum section: electronics / general

author: vespaman
author_rank: Contributor
timestamp: August 11, 2024, 03:28:33 pm
content: Hi all,End of last year I started to manufacture some small products, doing everything myself basically, including printing housing.Now, the step that I dislike the most, is the stencil printing, and cleaning, and in order to try to clean as little as possible, I thought I should see if there's any thoughts on my process, and optimization of it.I consider myself a newbie on the solder process, and I am sure the lot of you has much more knowledge.So far, I need about two full days to assemble 20 panels with boards that lasts 2-3 months worth of product sales, and this fits me well. (each panel holds 10 boards)I have a manual stencil printer, a charmhigh chmt 48vb with OpenPnP, Neodon IN6 oven. Over the year, I normally have 20-24C 35-45%RF in the shop.I normally start in the morning by printing 1 panel, put it into the PnP machine, and then I print maybe 2 more panels while first panel is being populated, I then do optical inspection of the panels as they are populated, mixed with printing 2 more panels as needed. When I have 3 populated panels, I start the IN6, and once it is stable, I put all available boards into the oven as the are cleared optical inspection.At the end of the day, I put the remaining solder paste into a jar, that I reuse the following day. What's left day2 I normally put in a third Jar, that goes to the local recyling station as toxic stuff.This works, but the extra time I spend with the printing and cleaning does not feel optimal. And removing the stencil from the manual printer means new alignment procedure day two..The GC10 solder paste has a long stencil life, but what about printed life? This must be less, I presume?Would it be possible to print all panels day 1, and keep half of them for day two, if I put them in a closed box. Slightly cooled if needed?Second alternative would be to leave the GC10 on the stencil over night, but it is more open while it sits in the printer, so I am not sure if this is better.Third alternative is to print and populate all the boards on day one and also clean the stencil and tools. Then on second day, do optical inspection and put them into the reflow oven.Or is there any other/better way about this?Any thoughts and suggestions are welcome!

author: jmelson
author_rank: Super Contributor
timestamp: August 12, 2024, 02:06:20 am
content: I have a commercial P&P machine, so that may make a big difference. I have a Quad QSA30A made by Samsung, similar to their CP30 but with Quad alignment cameras and Quad electronic feeders. It is an amazing machine, but programming is quite complicated, as it needs to know the size of each component, and rejects any that are out of tolerance.Anyway, I built my own stencil printing frame. I used to make my own stencils from 0.003" thick brass shim stock, and etched them just like a double-sided PC board but with no laminate in the middle. But, now I buy stencils with my PC boards from PCBway, they are so cheap.I have never had even one of my crummy home-made stencils wear out. I clean them with paper towels with either acetone or denatured alcohol. I always take the stencil off the printing frame at the end of every day and do the cleaning from both sides.I can generally do several panels per hour with the P&P, but with large panels I can only fit one in the oven, and my batch oven takes about 10 minutes per cycle.Jon

author: vespaman
author_rank: Contributor
timestamp: August 12, 2024, 08:12:45 am
content: Hi Jon,Yes, I also use PCBWay framed stencils, cleaning them with IPA and paper. The last couple of times I used pressured air to blow them clean, to make sure there's no residue in the smallest openings in the stencil.So, even if you are doing the same boards the following day, you always clean your stencil/tools end of day?Have you considered leaving the paste on the stencil overnight, and what are your thoughts about printing boards for population the next day?- Micael

author: SMTech
author_rank: Frequent Contributor
timestamp: August 12, 2024, 08:21:22 am
content: With the right climate you can leave printed boards for over 24 hours, in fact you should find maximum times for on a stencil and printed in the datasheet in your paste. I would not leave paste on a stencil overnight. Paste you intend to use again should the kept sealed up in its tub for as much as possible.With framed stencils, it should be trivial to clean your stencil in situ and be able to leave the alignment setup ready for the next day. Personally I have not found that pure IPA is good at cleaning solder residue, I use the official cleaning fluid from my solder paste manufacturer. If you can build yourself a small handheld ultrasonic puck, you can use that to help get paste residue out of small holes.

author: vespaman
author_rank: Contributor
timestamp: August 12, 2024, 08:48:55 am
content: That sounds promising!There's lot of information in the GC10 manual, but I cannot read anything about "time after printed". (But this may be due to my newbie knowledge)What is easily available is "24-hour abandon times, up to 72-hour stencil life".Could the abandon time be used as "time after printed", perhaps?If I try to leave some printed panels over night, what conditions would be the best, do you think? Dry and 15C or something the like?

author: SMTech
author_rank: Frequent Contributor
timestamp: August 12, 2024, 10:30:22 am
content: I believe abandon time is how long paste can sit on a stencil unworked, paste that time it would need a fresh mix/kneeding cycle before a print. 24 hours is pretty long but that is one of Gc10's party tricks. Solder manufacturer pick and choose the advertised and tested features of their paste so it can be hard to compare. 2 I use both claim 72 hours tack time which is probably the number you are looking for. However they do not, in the TDSs I have to hand, give guidance on how to achieve it and in my experience, just leaving out a printed board in a PCB magazine means the paste is decidedly less soft and tacky than it was the day before. I see no advantage in leaving out printed board to save on 10mins of cleanup. I will make use of this potential time however to manually fit a parts shortage or awkward part the following day if that makes my workflow easier. But then my case is different, I am more likely to be building 100 panels than 5, I have a machine and I have a good idea before I start how many printed boards I can place in a day.If you can experiment with environmental control, do so. I would avoid sticking it in a fridge for the obvious humidity reasons.One of the risks with leaving printed boards is slump, but this doesn't seem to be a common issue with the pastes I have tried, this could be an old issue or one associated with pastes that can work at high speeds or finer pitch.

author: vespaman
author_rank: Contributor
timestamp: August 12, 2024, 12:53:30 pm
content: OK, I guess I will have to try then. If Tack time is this, then according to the GC10 manual, it gets slightly worse between 8 and 16h, then it is steady. (GC10 Tack-life is specified as >48h)Maybe I can optimize the cleaning process instead, maybe keeping the stencil in the printer as you suggest. This is probably a useful task anyway, given that it has to be cleaned regardless.And of course optimize the PnP a little bit more, but I doubt I will be able to do more than 15 panels a day, keeping my (manual) optical inspection before reflow.I'll try this workflow the next batch;Paste print all 10 panels in the morning, load the machine with the first panel, and then clean the stencil+tools directly.And for each panel populated, do optical inspection.Maybe if I don't do the printing spread out over the day, I get more focused on the optical inspection, and don't fall behind the PnP machine like I have so far. Once I am behind, I kind of stress out..Thank you for reasoning with me!

author: Hawaka
author_rank: Regular Contributor
timestamp: August 12, 2024, 01:19:43 pm
content: From what I know you can keep printed PCB for a few days in a fridge without issue. At least we have done it at work without any issue.Stencil are always cleaned every day and paste is put back into the pot for next time, stored in fridge. I don’t think we discard paste.

author: Accu-Sembly
author_rank: Contributor
timestamp: August 12, 2024, 02:38:09 pm
content: Given your situation, I think the best thing to do is print all the boards you need in a 48 hour period right away. Do not leave paste on your stencil overnight and avoid leaving paste idle as much as possible (works best when it's constantly kneaded).I haven't done this in many years (it was not uncommon practice back in the day), but given the volume and build time of your production, any PCBs you don't populate that day should be put in the fridge. I know it seems counter intuitive given the humidity factors, but it helps (probably for the same reasons keeping paste refrigerated until use increases shelf life). I have sustained PCB tack time and reflowed without issues up to 48 hours. Again, it's not ideal, but I think it's totally viable given your situation. The printed paste will be drier and if needed, you can add liquid flux on top of any high risk areas before reflow. Be more cautious during PnP because if you lose tack time the components may bounce.

author: vespaman
author_rank: Contributor
timestamp: August 12, 2024, 04:24:43 pm
content: OK, so all of you think this is possible. Good; it would be much more efficient, only having to do the printing one time. The actual printing of a panel does not take long time (as you all know), it is the stuff around that takes time.Another big benefit with the "print all the panels at once", is that it is more flexible overall - maybe a good first day, I can do more, and then have less to do the next day. (And the other way around; if something messes with me, I already have the setup for continuing the next day, and don't have to do super long days if I feel tired). Or wipe panels (which I have done a couple of times - something that I don't want to do again).I think I'll 3D print a panel storage container, so it is easy to manage all printed boards, and keep then in their own micro climate.Maybe make a space for a couple of silica bags in the bottom, to see if this help. And while writing this, a future expansion might be to add a peltier to this container directly!? But maybe that is over the top, next thing I'll be adding space for a can of beer and what not.

author: jmelson
author_rank: Super Contributor
timestamp: August 13, 2024, 12:00:49 am
content: Quote from: vespaman on August 12, 2024, 08:12:45 amWhen stuff went wrong, I have held boards overnight before reflow, but they didn't come out as nicely as when done promptly. I know that the paste will absorb moisture, and that may cause the paste to "explode" on occasion, tossing parts around. So, I try not to do that. Also, the solvents in the paste evaporate over time, and the paste turns crusty. So, I do NOT want to deal with paste that has turned into some kind of goo. Just using enough solvent and clean paper towels will get the apertures clean. Hold it up to the light to check if the apertures are clean.Jon

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2024, 12:08:16 pm
content: I did rework and repair once in an production environment. and once PCB's were pasted, they had to be inserted into the P&P machine and soldered pretty quickly. Preferably within 15 minutes, but there was definitely never a lunch break in between. when this wet time was over 15 minutes the PCB's were still solderable, but yield started getting down, resulting in more rework. It was just not worth it.But that was 20 years ago, modern solder pastes are much more tolerant.But even so, I would not let a used stencil dry before cleaning it. I would wipe it off within half an hour of the last use, and then give it an extra wipe with solvent / alcohol. There is no need to start by diluting the alcohol with flux that can be easily wiped off.Have you tried cleaning the stencil without taking it out of the printer? I don't know if it works for you, but you should at least try it 2 or 3 times. If it works, it saves you time, if it does not work, it won't cost you much extra time. It can help to put an old towel over most of the printer, to prevent it from getting dirty from stuff dripping from the stencil. Then let the towel dry and reuse for the next time.

author: Aspartame
author_rank: Contributor
timestamp: August 25, 2024, 03:36:53 pm
content: This is an interesting case.You can upgrade to semiautomatic printer and 10 heads pnp machine instead of your toy machine and reduce your workload to less than a day from 2 full days.It will cost you $20K+shipping and duties.Then the question is, is it worth it?You need to answer that.