Idea for a DIY stencil printer - Page 1
Forum section: electronics / general

author: SWR
author_rank: Regular Contributor
timestamp: March 10, 2019, 12:48:44 pm
content: Dear allThis is my idea for a DIY stencil printer ($75 including shipment from China).It has height/depth/width/rotation adjustment & guided squeegee with two 60° slots.The idea is to have 3 sliders on LM8UU bearings on 8mm stainless steel rails.The front rails is also the handle and the rear rail is the hinge.The stencil sliders have MGNR9C stainless steel rails on top to fix the stencil.The dark grey parts are 3D printed, so they have lower stiffness.There are individual height adjustments on the 4 corners.The with ajustment is combined with the spring loaded tension of the stencil.There are 2 combined depth and rotation ajustments on the rear height adjustments.The front height adjustments just slide on hex nut standoffs on the base.The base is a small whiteboard to hold the magnetic PCB standoffs.The idea is to place the PCB within 1mm using the magnetic standoffs.Then small adjustmants can be made with the adjustment screws.After a swipe you manually move the Squeegee to the other 60° slot.If I want to experiment with a different angle I'll have to 3D print another slider.Sliding speed is purely manual and will propably require some practice.I'm a newbee at this so please tell me what you think ... both the good and the ugly.Best regardsSoren

author: HalFET
author_rank: Frequent Contributor
timestamp: March 10, 2019, 01:28:04 pm
content: Looks nice! I'd widen it a bit more past the rails, then you can clamp it down on the table with glue clamps so it doesn't move as easily. What'd also be nice is a clamp to hold a DE razor blade as squeegee for small boards.

author: SWR
author_rank: Regular Contributor
timestamp: March 10, 2019, 02:19:17 pm
content: Thank you for the feedback.It might be a better idea to make the squeegee slider more generic.I could make slider with a row of vertical steel-lined holes and then make a separate part for the squeegee.This would allow customization of different squeegee sizes and different angles without having to replace the complete slider.

author: HalFET
author_rank: Frequent Contributor
timestamp: March 10, 2019, 02:30:25 pm
content: If it's 3D printed you can have multiple parts, that's the nice thing about it! I wouldn't limit myself to a single option.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 20, 2019, 03:08:56 pm
content: Squeegee slider like that won't work, or it would make thing worse than with no slider at all. (Just holding the squeegee in hand). To be useful the slider should be able to exert downward pressure on it's own. Then you just use hands to move the slider along the track.In this design applying downward pressure manually will be hard, since you actually need to push squeegee at an angle, and in an opposite direction of moving. It will be very exhausting.Whiteboard is not rigid enough, nor is the frame. The important feature is ability to make small adjustments for aligning stencil to the board.

author: SWR
author_rank: Regular Contributor
timestamp: March 20, 2019, 05:48:34 pm
content: Thank you for the feedback.I will revise the design so that the squeegee will slide vertically on the horizontal slider with two extra LM8UU bearings.Then the pressure must be applied vertically and the squeegee holder only helps to maintain the correct angle.The 2nd step could be to apply the pressure via weight instead of manual pressure.The stability of the whiteboard comes from a bottom plate that needs to be a bit thicker than the corner brackets.When it is placed on a level stable table it should be able to transport the pressure to the table without bending.I'll make a sketch of the revised squeegee holder design and post the update.

author: SWR
author_rank: Regular Contributor
timestamp: March 20, 2019, 09:09:47 pm
content: Here's an updated squeegee and slider design.The squeegee now moves vertically with two short 30mm rails in two extra LM8UU bearings on the slider.It would be quite easy to add weight to the squeegee using the mounting holes of the two short rails.

author: SWR
author_rank: Regular Contributor
timestamp: March 21, 2019, 09:40:51 pm
content: Do you have an idea how much minimum weight is appropriate on the squeegee blade?It's 180mm wide, 50mm long and 0,40mm thick stainless steel at 60° angle.

author: rhb
author_rank: Super Contributor
timestamp: March 21, 2019, 10:39:09 pm
content: First of all I suggest getting some hands on silk screen experience. For precision work you should use stainless steel screen. I can see no reason for mounting the squeegee on a slider unless you intend to automate it. With very modest practice manual is fine. It only takes a few tries to get the feel of it. The thickness of the screen controls the thickness of the coating.Second, I suggest using registration pins whose height is set from underneath with a screw and locknut so that the tops of the registration pins are below the board surface. Use a large screw and file the tips down by chucking them in a drill press. Use a lathe if you have access to one. Make sure the registration pins are a close fit to the board holes.If you are doing resist, solder mask, labeling and solder paste you need multiple screens. Solder mask and labels can be polyester. Resist and paste should be SS. Use a rubber squeegee on polyester screen. I'd use rubber for all of them.If you have access to a CNC XY table, give serious thought to using a laser diode to expose photosensitive emulsion screens.You should mount the registration screws on the screens so that they can be set once and not changed until you reuse them. A cheap 1100-1300 psi pressure washer is very useful for cleaning screens for resuse.

author: SWR
author_rank: Regular Contributor
timestamp: March 22, 2019, 06:29:19 am
content: Thanks for the comments.Quote from: rhb on March 21, 2019, 10:39:09 pmI'll give it a shot without the guided squeegee first and get a feel for it. The only reason for the guide was an attempt to reduce the variables. I've read that paste printing was the most frequent root cause of manufacturing errors, and that care should be taken to make this process step very repeatable.Some articles describe that blade angle, pressure and speed are very critical to get the optimum paste filling and release, and I thought that the speed would be the easiest to reduce the variability in a manual operation, but maybe it is over complicating things? I have no personal experience yet, but it would be easy to leave out the squeegee guide and try it with fully manual squeegee operation to get some experience.I guess that a careful microscope inspection of the printed solder paste will tell me if there are problems with the filling and paste height etc.QuoteI do have a Weiss VM290V-F lathe like this onehttp://www.weiss-us.com/index.php?route=product/product&product_id=91in my garage so I can trim off screws, but my Idea for stand-offs were to put a M3 countersunk screw and nut on magnets like thishttps://www.aliexpress.com/item/HYSAMTA-10pcs-10x3mm-Countersunk-Neodymium-Magnet-Ring-Hole-3mm-Small-Round-N52-Permanent-Super-Strong-Powerful/32920580102.html?spm=a2g0s.9042311.0.0.27424c4diL6UAXI will make the tip of the support pins and the shoulder on the guide pins the same height. The idea is that the magnet support and regitration pins are stationary and all the adjustments are done on the stencil position.QuoteI'm only doing paste screening so I'll buy finished PCBs and steel stencils.QuoteI have a Weiss VM-30 mill for chip cutting work. I bought 3 axis CNC control for it but I havent mounted it yet, so for now it's purely manual operation. For laser engraving I've got these units:https://www.aliexpress.com/snapshot/0.html?spm=a2g0s.9042647.6.6.467c4c4dW4f1c8&orderId=82570385367280&productId=32793436271https://www.aliexpress.com/snapshot/0.html?spm=a2g0s.9042647.6.2.467c4c4dW4f1c8&orderId=82570385357280&productId=32788408244But I don't think I will need them for the paste printing.QuoteI'm hoping that the N52 magnets will be strong enough to hold their position when I switch the PCBs. Otherwise I will have to get bigger magnets.Thank you for all the good points for me to consider.

author: nisma
author_rank: Regular Contributor
timestamp: March 22, 2019, 04:50:18 pm
content: If the pcb is bigger, it's important to have micrometric screws for aligning the stencil to pcb.Second important item is that pcb is separated horizontally from pcb.If you don't have that it is important that the hinge is far from used stencil apertures as possible.This could be ensured from the construction, using vacuum pods that press the pcb down, ... .Third is cleaning, sometimes for high tech assembly you must clean the underside of stencilfrom every N swipe to every swipe. Depending on the construction it could be tricky or impossible to dobecause at the same time with the other hand you must block the solder paste flowing into holes.

author: SWR
author_rank: Regular Contributor
timestamp: March 22, 2019, 08:29:37 pm
content: Quote from: nisma on March 22, 2019, 04:50:18 pmThe maximum PCB size with this version is 18x24cm.I'm not quite sure what you mean by micrometric screws?The M4 thread is 0,7mm/turn and the outside diameter of the adjustment knobs are 16mm diameter.A 5° adjustment is about 10µm.QuoteI don't quite follow you here?The only PCB here is the one that is printed with paste?QuoteThe hinge rotation point is placed level with the screen to minimize side motion when opening the screen.QuoteWon't the screen and squeegee press the PC down onto the support pins during printing?QuoteThis might be a problem if the paste is so thin that it starts to slide down when I open the screen?Do you have any idea how many degrees you can tilt the screen before paste will start sliding?I think I need to try it out to get some experience and get a feel for how the paste is reacting.A solution might be a jig that holds the screen level while the bottom plate is rotating down to give access for cleaning the bottom side? I don't want to touch the magnets on the bottom plate between prints to avoid shifting their position, so they need to be out of the way.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 22, 2019, 08:56:56 pm
content: Quote from: SWR on March 22, 2019, 08:29:37 pmHe means vertically, while remaining parallel to the PCB. As opposed to being hinged. Nice, but really not required and unreasonable to expect from a DIY printer. When the distance from hinge to the nearest aperture is at least 15 cm, it approximates vertical lift close enough.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 22, 2019, 08:59:40 pm
content: I applaud the effort, but cannot see this concept working much better than the 'tape it to the table' method.In my experience, designing a stencil machine is not trivial if your expectations are for it to be much better than just taping it all down. If the PCB's have big components and a loose layout - it is easy no matter what you do. If the PCB's have fine pitch and high density, a solid stencil printing solution is important.I currently use a setup that is considerably more robust than this and it is still far from good in the context of 0402 passives and .5mm silicon. For the chunky boards with 0805's and other big parts - just tape it to the table and go. Alignment is not critical.For panels and small parts - fine and robust adjustments for every degree of freedom become really important.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 22, 2019, 09:10:07 pm
content: Quote from: SWR on March 22, 2019, 08:29:37 pmDon't worry, the paste is thick enough, it won't run, at least not much in the time required to clean the underside, even if you open it to 90 deg. It really might be beneficial for you to buy some paste and try and print few boards manually.Taping knife works well as a squeegee, so it would be helpful to have the holder (if you at all consider it necessary) to handle various width knifes.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 22, 2019, 09:11:22 pm
content: Quote from: rx8pilot on March 22, 2019, 08:59:40 pmTaping to to the table really sucks. Maybe if I do one board, I would do that. For more than that, no.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 22, 2019, 09:21:37 pm
content: Quote from: ar__systems on March 22, 2019, 09:11:22 pmAgreed - which is why I only do that for 1-off boards with big parts.For all others, I purchased and modified a printer - and still struggle. I guess my point was that a simple solution to a complex problem may not be much better than the tape it down method.

author: SWR
author_rank: Regular Contributor
timestamp: March 22, 2019, 09:44:24 pm
content: Quote from: rx8pilot on March 22, 2019, 08:59:40 pmThe smallest components I use are 0402 and 0,5mm pitch ICs.My main limitation regarding experience is that I have never (as in not once) tried to screen print a PCB since I started mounting and soldering PCBs back in 1978. Back then I didn’t use 0402s though.My main worry with the tape method is the lack of adjustability and having to fiddle with X, Y and rotation while holding the screen and fixing the tape. It sounds to me that I’ll quickly be running out of hands, but maybe I should just give it a try?QuoteMy main focus is to reduce variation on all the important parameters, simply to be able to tune the process. If I don’t have repeatability I will be chasing a moving target.My main problem right now is propably my lack of experience with this process?What I found on this topic on the internet was that the most important parameters are alignment, squeegee angle, pressure and speed. Maybe due to my ignorance I thought that speed would be the easiest to control manually, so I was looking for a way to control the other parameters by printer adjustments.QuoteI need to find where this design lacks robustness. The goal with this design is to make squeegee speed the only variable.

author: SWR
author_rank: Regular Contributor
timestamp: March 22, 2019, 10:02:53 pm
content: Thanks. I’ll get some paste and get my fingers dirty. 😊QuoteIt should be able to handle any width up to 190mm. Each sqeegee blade will propably need its own holder to make changing squeegees quick.

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2019, 10:29:38 pm
content: If you're using a steel stencil and a steel taping knife there should not be any paste on the stencil. If the stencil is in close contact with the PCB, there should not be any paste on the underside except for what might adhere to the sides of the openings. Some neodymium magnets under the PCB would help keep the stencil against the board if the SS is magnetic. Some are, some are not.I can't see how the magnetic PCB mounts can keep position from board to board with the precision needed for fine pitch parts such as 0.5 mm spacing packages. For those you're going to need 30-50 um repeatability. That means the PCB holes have to be a light press fit on the pins. That in turn requires reaming the holes precisely to size. I expect the friction will shift magnetic posts. I think a pins on rails on rails mount will be required.Ream the board holes to size, place on pins and lock the pins in position. Then align the stencil to the board using fiduciary marks at the corners and lock the stencil in position.I just tried some 10 um (one graduation) adjustments of a Swedish ACOX 0.5 mm pitch micrometer. That's about as close as I could place the spindle without good visual feedback.You will need to regularly true the edge of the taping knife so that is is straight and square. A skilled metal worker can do that with a fine pitch file, but it takes some practice to acquire that skill. Clamping the knife to a carriage and sliding it past a grinding wheel may be a better option if you haven't done much bench work with a file.Rather than spend money on solder paste, I suggest practicing with a stencil and drywall joint compound mixed to the same consistency. Place the stencil on a photocopier and then try to apply mud to the image of the holes. Be aware of paper distortion going through the fuser. Hand align it, tape it to the paper and apply the mud. The let it dry and look at it with a microscope and scrape segments of lines off to see how well placed the mud was.This is not hard in the sense that it is routine industrial practice. But it's not easy either.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 22, 2019, 11:30:27 pm
content: Quote from: SWR on March 22, 2019, 09:44:24 pmTo be perfectly clear, I am NOT advocating the 'tape method' or claiming that it is a good idea for anything other than very basic projects.It does, however, provide a way to practice with the process and get a feel for the variables. I went from manual soldering to manual paste syringe, to pneumatic paste syringe, to cheap Chinese printer, to cheap Chinese printer that I modified. Each step was a forward move in precision and speed. My next step is either buying a 'real' printer or building my own.The ideal printer would be rigid overall, able to repeatably hold/position PCB's, have 6 degrees of alignment motion between the PCB platter and the stencil. I have learned through blood, sweat, and tears why well built commercially available manual printers are many thousands of dollars. My volume is not all that high, so I can live without the brutally robust production designs - but I would certainly like to have solid precision.The quality of the printing is directly related to the quality of the end result.I say that because early on, I did not understand that and I paid in constant re-work of the more delicate designs. That eats a lot of time and is frustrating in general.The quality of the hardware and the skill of the operator both come together to determine how well it all works. There is definitely a learning curve in how you pull a squeegee across a stencil. The squeegee is also an important part of the tool. Mine is basic, but all dialed in - sharp and square edge on 316 Stainless which is about .025" and backed by a strip of urethane.I made magnetic holders that really work well. Very solid and they do not require holes - they hold the edges and corners of the PCB. Any shape or size.

author: gasmeter
author_rank: Contributor
timestamp: March 23, 2019, 09:52:49 am
content: HiI have been through this process over the years.I started with a PCB pool system .This is a bed with a hinge, they supply a free stencil and you align the board against two edge's then the stencil is manually aligned and magnets used to hold it in place on the hinge.Its OK for one or two offs but the repeat-ability relies on the pcb being accurately milled which of course it isn't.Lately I have been using a system from eurocircuits (15 euros) , this relies on pins and cones to align the two.It works amazingly well.http://Https://youtu.be/HBWtqZro_fgPeter

author: SWR
author_rank: Regular Contributor
timestamp: March 23, 2019, 12:14:56 pm
content: Thank you for the relevant points and good examples you’re mentioning here.This design has focused on the vertical forces from the squeegee.I think this design has two separate callenges: stability of the PCB and stencil position.Regarding the PCB this is propably the easiest of the two. I think the magnets I chose are strong enough for the support pins, because they will receive very little forces in the XY plane. I might need stronger magnets for the positioning pins to handle the forces during positioning and removal of the PCB without moving.Regarding the stencil stability in the XY plane the only support are the two hinges on the rear side. They will need to take up all forces in the XY plane. There are two sources of force that will have components in the XY plane: the hand dragging the squeegee and the squeegee blade dragging on the stencil.The hand can mainly transfer forces in the Y direction, and the big unknown to me is how much I can avoid pushing and pulling in the Y direction. The ideal would be to only pull the squeegee in the X direction, but it is propably unavoidable to push and pull a bit in the Y direction as well. I think this will be easier if I can apply down pressure with weights instead of pressure from the hand. The down pressure will also have less variation using weights.The transfer of forces by the handle in the X direction should be minimal. The LM8UU roller bearings slide quite easily if the rails are kept clean.The big unknown is the dragging of the squeegee blade on the stencil. It depends on squeegee angle, pressure and speed plus maybe the solder paste viscosity? I imagine that the viscosity depends on paste type and temperature? Maybe there’s also a paste preparation process that will influence the viscosity?This will generate a rotation force that will have to be taken by the rear hinges. To minimize this force I need to minimize the squeegee pressure and speed. I don’t know if the squeegee angle will also contribute to the generation of drag force on the stencil?I have more questions than answers right now.But I guess thats to be expected when I have no experience. 😊

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 23, 2019, 06:32:09 pm
content: Quote from: SWR on March 23, 2019, 12:14:56 pmYes you do, and you wasting time worrying about non-issues, like "the dragging of the squeegee blade on the stencil".It is kind of like trying to design a bra for someone who never saw a live woman... It is all useless exercise until you start actually experimenting yourself.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 23, 2019, 06:35:33 pm
content: Quote from: ar__systems on March 23, 2019, 06:32:09 pmAgreed - Go and experiment. The details of the problems and challenges will jump out quickly. Iterate until you run out of problems, time, or money.

author: SWR
author_rank: Regular Contributor
timestamp: March 23, 2019, 08:25:13 pm
content: I’ll build it and get some experience that way.Thanks for your input.

author: nisma
author_rank: Regular Contributor
timestamp: March 24, 2019, 07:11:07 pm
content: I suppose you have seen these two printer systems.

author: SWR
author_rank: Regular Contributor
timestamp: March 24, 2019, 11:38:31 pm
content: No I haven't. Very interesting and a lot of similarities. Especially the last one looks very similar.Thanks for the info. Do you have any links?

author: rhb
author_rank: Super Contributor
timestamp: March 25, 2019, 01:55:35 am
content: Those look very well thought out and executed.

author: nisma
author_rank: Regular Contributor
timestamp: March 25, 2019, 09:04:34 am
content: You want clone the sd-240 printer.Both displayed product images are single side only, but could be upgraded to double side similar to the original sd-240 if required.The first image is the sp2421 .It is interesting because it use the stencil as hinge and it is simple and cheap to build including the optional tensioning improvement. Further without hinge it is a great frame for frameless stencil to be used on the so named high precision China stencil printer.The second is a diy version and this is the link for the instruction and part sources:https://www.cnclips.net/video/_GMGjjmfx3U/stencil-printer.html

author: SWR
author_rank: Regular Contributor
timestamp: March 25, 2019, 02:58:33 pm
content: Excellent video. Thank you for that.Here are my immediate thoughts after watching it.Advantages:-The rigid bottom frame removes the need for a flat sturdy table surface.-Moving X & rotation adjustment to the base simplifies the frame.-It confirms that the drag forces from the squeegee is not a big issue.Drawbacks:-A lot of DIY work instead of buying finished parts...but the advantage is that you don't have to wait 4 weeks for china parts.-Extra hinge on the frame instead of using one rail as hinge.-Unguided squeegee operation: might be ok with good manual skills?-Clamps on the PCB edges:-Not always accurate: I prefer drill holes for alignment.-Can't accept irregular shaped PCBs.-Breaks adjustment each time you remove a PCB.Strange that the stencil holes are larger than the pads during alignment?I'm used to reducing them a bit in my layouts.Great video for inspiration. I'll have to check out the bending stencil hinge idea.

author: SMTech
author_rank: Frequent Contributor
timestamp: March 26, 2019, 01:33:44 pm
content: I quite like the DIY one although it maybe doesn't look as robust as one might like.As to your drawbacks -The number of custom parts could probably be reduced for greater outlay, the trick is knowing the names for the kinds of parts you might be looking for to achieve a similar result, you could lose many of the 3D printed parts by using solid Aluminium parts & a tap instead of profiles+plastic inserts for instance.Unguided squeegee really shouldn't be and issue on the boards you would be printing on this scale - it wouldn't be a very big squeegee either.Clamps on edge - has its downsides, but there is nothing about this design that stops you swapping magnetic edge holders for a selection magnetic "nipples" that locate in screw holes, the downside in doing it that way is the nipple & hole dimensions need to be consistent enough that the board sits on them flat and they don't poke through the board. If your designs use a lot of different mounting hole sizes or different PCB thicknesses you might need different nipples to suit. The is another downside to nipples and that is lining them all up to begin with however you have better support for double sided printing, swings 'n roundabouts.Breaks adjustment each time you remove a PCB - you don't need to move (all) the edge clamping to extract the PCB so the adjustment should be pretty accurate between prints, if not it shouldn't need more than a tiny tweak, not exactly a hardship.Can't accept irregular shaped PCBs - I don't see why not, you can print the magnetic clamps in any shape you please, or just use naked magnets (or fix it any other way you choose). More to the point you shoudn't be ordering PCBs with awkward shapes outside of a panel that makes them square - a total pain in the rear end for every process that follows.For reference I print 1000's of boards with a manual printer using an unguided squeegee and edge clamping on a flat deck. The difference is the printer base and tensioning frame add up to ~20KG so everything is very rigid and unlikely to move during a print.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 27, 2019, 12:04:30 am
content: Quote from: SMTech on March 26, 2019, 01:33:44 pmMe too - the manual squeegee works well. It has a learning curve, but I can now paste PCB's very close to 100% perfect every time - even the fine pitched versions. If I was going to DIY a printer, the semi-automated squeegee would be really far down on my list of hopes and dreams. Rigidity, ease of use, precision, repeatability on the base would dominate my efforts.