29 Replies Latest reply on Apr 14, 2017 6:36 AM by shabaz

    PCB Stencil Printer - Alignment discussion

    shabaz

      Hi,

       

      For prototyping, I've noticed stencils are quite low-cost nowadays, which is fantastic.

      For those unfamiliar with the process, they are used to apply solder paste to the correct areas on a PCB prior to component placement and reflow soldering.

      The basic principle can be seen in this screenshot from a sparkfun stencilling video:

      stencil-printing-sf.jpg

      In that video, the PCB is secured in the XY plane using scrap PCBs that are temporarily taped to the desk, and then a laser-cut mylar template (it has laser-cut apertures which correspond to the pads on the PCB where solder paste is needed) is held in place (aligned by eye) and then paste is applied using a squeegee.

      So, that's the fundamentals of the process, but in practice it may need some refinement for certain use-cases.

       

      I'd like to use low-cost stencils (actually steel sheet stencils are low-cost nowadays, the Sparkfun video is six years old) but for perhaps half a dozen boards or more; in other words not production volumes, but "a few to a couple of dozen prototypes" volumes, and then discarding the stencil. Furthermore, nowadays tiny packages are very common for hobbyists to use too, so clamping of the stencil by hand isn't feasible. Perhaps alignment even needs to be inspected under a magnifier/microscope unless there are registration holes or pegs that can be used to obtain some accuracy without having to align by eye each time. How much accuracy is needed? I think an error around 0.1mm may be a suitable limit, because there are components that have 0.5mm or 0.4mm pitch and an offset of 0.1mm could mean that 25% less solder paste is on the pad.

       

      There are many other parts to the process that could be refined too, but for now I wanted to focus on the the alignment/clamping topic.

       

      The commercial solution is a stencil printing machine; for low volume prototyping there are manual versions of such machines. The way they work is that the stencil has a metal frame around it, and this is clamped/fitted into the machine on a hinge. The frame is lowered to do the pasting, then the frame is raised on the hinge to take out the pasted PCB and  insert the next PCB.

       

      However, framed stencils cost more. It is possible to self-frame, but this requires a stencil with lots of space around it. The squeegee may be 100mm wide or more, but hobby PCBs are often smaller than 100mm. A low-cost stencil may only be 20mm wider all around.

      Adding a custom frame to a low-cost stencil is therefore hard when the squeegee itself could be of a similar size to the stencil.

       

      I tried to think of a way to solve this issue. The diagram here shows the current thought process.

      screen-print-project2.png

       

      Basically the PCB would fit on a raised platform that is attached to a base. The raised platform could be swapped out for different PCB projects or drilled with more peg positions or have straight edges for aligning the PCB into a repeatable XY plane position. The height of the raised platform would be fixed for a certain PCB thickness but could be modified with some shim if needed. The raised platform could be screwed to the base from the underside. The base and raised platform would probably be  constructed from aluminium sheet.

       

      The stencil sheet would be clamped on one edge between two blocks shown in blue. This could be achieved using screws or winged nuts. The precise alignment would come from loosening the screws, moving the sheet manually and then tightening it.  If the stencil has 5mm holes then the clamping could be done with 3 or 4mm screws as an example, to provide for adjustment.

       

      The underside of the lower clamp block would have a couple of pegs that would fit into the base. These would allow for repeatable accuracy with insertion and removal of the clamp from the base. After a board has been pasted, the stencil and base would be carefully lifted and put to one side while the PCB is removed and a new PCB is inserted. Then the stencil/clamp would be placed back onto the base, and the alignment should not have changed.

      Although commercial screen printing machines use a hinged action for lifting the stencil, I'm wondering for small boards if this is needed or not; perhaps it is just as easy to lift up the stencil from the unclamped end slightly with a ruler for example, and then lift off the clamp from the base. I don't mind if I need to go carefully because I'm not doing production volumes; I can afford to take some extra minutes of care per board.

       

      I already have some ideas about how I will go about this which I'll jot down as I experiment (and eventually write it up in a blog post so that it can be replicated), and I'm curious if others were to do this, what tips/suggestions would you have?

      I'm still researching some materials and methods for assembly. For peg alignment I'd clamp the materials together and drill right through, so that the base and clamp are all drilled identically, unless I can find off-the-shelf pegs and sockets that I could incorporate into the design.

      I might use bits of optical breadboards etc to save having to drill too many holes precisely.

       

      Many thanks!

       

      Message was edited by: shabaz (Corrected the diagram to indicate the scrap PCBs to prevent damage to the stencil on edges)

        • Re: PCB Stencil Printer - Alignment discussion

          Never seen this approach before. For "alpha" prototypes, or just for when a basic PCB would be useful rather than veroboard or the likes, this'd be an excellent and affordable option. Particularly combined with one of those cheap, mini laser CNC machines built from an old DVD drive   Cut your own templates, and whip up simple circuit boards in no time!

           

          Nice one Shabaz! Great heads up, as always

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          • Re: PCB Stencil Printer - Alignment discussion
            mcb1

            Nice idea.

            If you used offset cams, you could align the stencil much more accurately.

             

            Mark

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            • Re: PCB Stencil Printer - Alignment discussion
              COMPACT

              I just use an XY knife plotter to cut Kapton film.

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              • Re: PCB Stencil Printer - Alignment discussion
                shabaz

                Just an update, here are the latest thoughts. I've ordered most of the material now based on this design:

                 

                stencil-printer-rev2.png

                A lot still needs to be worked out while constructing it, but I think it *might* work. Here is a description:

                Material and Dimensions

                Most of it is constructed from aluminium, and one steel shaft. The whole unit is approx. 300x250mm in size, which restricts it to boards up to around 160x160mm (it could be scaled up for larger boards of course).

                Floating Table

                This is the surface for the PCB. The "floating table" is not firmly attached to the base - I might leave it unattached, or maybe loosely attach it so it has freedom in the X and Y planes. It only needs 2-3mm of freedom in the X and Y planes. Gravity prevents movement in the Z plane. Not shown, but the table has threaded holes on its surface so that scrap PCBs can be clamped in place to secure the actual PCB in a fixed position. The floating table has a maximum of about 2 or 3mm of XY plane adjustment by using thumbscrews which will push it. On opposite sides there are springs (these look like grub screws but inside there is a spring and plunger).

                Thumbscrews

                The thumbscrews have 100 turns per inch, so this should allow very fine movement of the floating table.

                Stencil Clamp

                The clamp mechanism is the same as before. Different thickness PCBs are handled by inserting a shim between the upper and lower clamp halves, so that the stencil can be raised. Instead of the previously discussed peg method to lift the clamp, now there will be a bearing block and a shaft, to allow it to be swung up.

                Things not shown in the diagram

                A few things are not shown, but there will need to be blocks to fix the clamp so that it doesn't slide left or right, and some lever at the back to swing the clamp, and support it vertically so it doesn't fall back down (I might use a magnet for that).  For examination I'll just use a magnifier (I might fit a battery-powered LED somewhere). The battery will last for years with normal use. Also there will be rubber feet on the underside.

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                  • Re: PCB Stencil Printer - Alignment discussion
                    michaelkellett

                    Hello Shabaz,

                     

                    Just for fun I've attached  a link to the cheapest similar 'machine' on Aliexpress - pretty rough and ready looking but at £81 including shipping !

                     

                    There are loads more at higher prices and some look quite reasonable.

                     

                    Almost all the really cheap ones expect you to use a framed stencil which has got me wondering if there is a cheap way to frame a cheap stencil.

                     

                    https://www.aliexpress.com/item/cheap-price-manual-silk-screen-printing-machine/32609744951.html?spm=2114.10010108.10000…

                     

                    I've found it hard to get a clean lift from the board with a floppy (unframed or untensioned) stencil.

                     

                    MK

                      • Re: PCB Stencil Printer - Alignment discussion
                        shabaz

                        Hi Michael,

                         

                        Interesting design, looks like they've incorporated thumbscrews too. It could be a lot more economical to go with the Aliexpress machine and tailor it.

                         

                        In light of your comments, maybe effort is better expended on designing a framing method. I saw a cool video where an industrial machine used special frames that have approximately a bicycle inner-tube inside the frame, which when blown up with an air line will cause four levers angle to change. The lever has dozens of tongues, and there are full-length levers on all four sides of the frame, and the stencil has lots of holes around all sides, which fit around the tongues.

                        So, when the air is applied, it stretches and firmly holds the stencil from all sides. I'll try and find the link..

                         

                        Maybe dozens of screws pulling on a lever would achieve the same result.. I'll try to draw something up..

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                        • Re: PCB Stencil Printer - Alignment discussion
                          shabaz

                          Here is what I was thinking, I don't know how feasible this is to make, probably needs to be constructed of steel or thick aluminium.

                          This diagram shows the frame upside down. Basically there are four bars (shown in brown) that can be pulled against the inner sides of the frame using loads of screws.

                          The stencil (would have to be steel, I'm guessing mylar will just tear through) would have holes (e.g. 4mm) all around the edges, which would allow pegs to fit.

                          The pegs would be permanently pushed into the four brown bars, protruding about 0.8mm. This would allow the stencil to fit, and would not hit the surface where the PCB

                          was placed (unless the PCB was thinner than 0.8mm).

                           

                          It would be quite a non-trivial build though : ( Every hole would need to be super-perpendicular and accurately drilled. I'm going to search if off-the-shelf steel bar exists

                          with convenient holes..

                          frame-idea.png

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                            • Re: PCB Stencil Printer - Alignment discussion
                              COMPACT

                              Whatever system you devise you it is preferable to have the stencil lifted of your work orthogonally (usually vertically) without any lateral movement as not to smudge the solder paste.

                                • Re: PCB Stencil Printer - Alignment discussion
                                  shabaz

                                  I agree, and I think a frame will help there, but I'm hoping that can be retro-fitted if I really can't achieve a good result with the unframed stencil.

                                  In terms of the tilt mechanism, there is a good 60mm distance from the shaft to the edge of the 'floating platform' and from there perhaps another 10mm before the edge of the PCB, so a tilt of 0.2 degrees should lift off the entire stencil (assuming around 0.12mm for stencil thickness). However, an all-flat lift-off will be hard without a frame. I can't find a solution to solve this currently : (

                                   

                                  On the frame front, it is possible to get pegs with a slight taper to try and implement a frame design as drawn earlier, but I want to do some calculations to figure out how thick the frame would need to be and what material. But to do that, I need to know how much force I want to exert on the stencil, and that I have no idea currently. I don't have a lot of space so I want a thin frame. I have no brazing tools either, to build the frame in the first place : ( So building a frame is currently a last resort if I fail to get any reasonable result without it.

                                • Re: PCB Stencil Printer - Alignment discussion
                                  mcb1

                                  Looking at the cheap example and the issues it seems that there are several parts to this:-

                                   

                                  1. You need to hold the stencil to allow the paste to be spread.

                                  2. The stencil then needs to be lifted free of the PCB, and a fresh one inserted.

                                  3. The accuracy of the board v stencil needs to be held within 0.1mm (or whatever figure you decide)

                                  4. The size of the board and stencil are not consistent.

                                   

                                  The stencil holding arrangement needs to accomodate a stencil that may be only 20mm wider than the PCB itself.

                                  This suggests that it needs to be a simple clamping method, with no allowance for the postioning aspect.

                                   

                                  One simple method for increasing the size is to superglue it to a larger standard sized holder that fits the frame.

                                  If you're using mylar, then perhaps a larger sized mylar sheet with the centre cut out and the stencil superglued in place.

                                  Steel stencils could also be attached in the same manner.

                                  You may also get away with masking tape around all four sides.

                                   

                                  If the stencil is locked, then the PCB or the fixed portion of the hinge needs to be moved to accomodate the final position accuracy.

                                   

                                   

                                  The pcb could be held with 4 simple plates.

                                  Image result for hold down screw

                                  They would have a slot to allow movement, and be locked in place using a holdown screw.

                                   

                                  This would ideally be outside the stencil frame area.

                                  Several different sized plates could be made to accomodate varying sized pc boards.

                                   

                                  We need to assume that all the PCB's outside dimensions are accurately.

                                  The alternative is to use a registration pin plate that could utilise the same lockdown slots.

                                   

                                  Once the PCB is located within 1mm of the correct place, then the adjustment system only needs to move +/- 1mm.

                                  It may be that the upper right corner is +1mm, while the upper left is -1mm if the pcb v stencil has a rotational component.

                                   

                                  The next decision is to lift the stencil, or use a hinge.

                                  If the frame is a set size it becomes easier to use a tapered pin for registration alignment which is both easy to produce and provides good accuracy and repeatability.

                                  Two simple plates with a lockdown could be used and minor adjustment made until the 0.1mm tolerance is achieved, and then they are fully tghtened.

                                  The tightening mechanism should only push down so any rotational force needs to be removed (bearings, multiple washers, etc)

                                   

                                  If the hinge is desired, the allignment needs to be incorporated into the fixed portion and an accurate pivot needs to allow hinging, but no movement.

                                  I would use two rod ends as they are easier to fit, and give perfect alignment

                                  Image result for rod ends

                                  They come in varying sizes and male or female threads to suit the frame attachment point.

                                  A simple bolt into the side of the fixed block attaches the rod end, but allows full removal when the task is complete.

                                   

                                  This just leaves the fixed portion positioning, which can be two oversized holes.

                                  The stencil is attached, the pcb is placed and adjusted, and then the fixed portion of the hinge is tightened and then tweaked by gently taping into postion before final tightening to lock it in place.

                                   

                                  This makes the base plate a very simple piece of aluminium plate with some threaded holes at varying points.

                                  You could use a wooden base, but you'd need to add somethign for the threads, and the long term stability (flatness) could be an issue.

                                   

                                  One big advantage I see is that the holes don't need to be accurate, and neither do any other aspects of this design.

                                   

                                   

                                  Mark

                                   

                                   

                                  If it's any help there is a blog here about mylar films as stencils

                                  Nerd Club: Creating a solder paste stencil with ExpressPCB and Inkscape

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                                    • Re: PCB Stencil Printer - Alignment discussion
                                      shabaz

                                      Hi Mark,

                                       

                                      This is useful information. I'm not very mechanically minded, so I'm glad for all this advice. It does seem like the frame will be fairly important : ( I'll have to give this some thought, how to add it into the current design.

                                      For the PCB securing mechanism it will use the screw and plate method as you describe (currently it will just have normal screws without the knobs, but I might upgrade that once I've got it working). For the shaft holder, I hadn't thought of those end rods but I'm doing something similar except with a block. The block is £10.15+VAT, so not as cheap as the end rods, but not super expensive (I'm only using one block, it is 30mm wide). The block has just a bore bearing (brass I guess), not a ball-bearing like those end rods, but hopefully this should be fine.

                                      For the base I've got a slab of aluminium, but if it turns out that I need to make the entire thing larger to accommodate a frame, then I will swap out to a slab of wood since big slabs of aluminium get pricey. As you say, long-term stability might not be great but I can always align before I start using it. It is great that nothing needs to be critical, since everything can be trimmed.

                                        • Re: PCB Stencil Printer - Alignment discussion
                                          mcb1

                                          shabaz

                                          Happy to help with ideas.

                                           

                                          Screws will work just fine, add two washers so that it doesn't try to rotate the plate.

                                          My thought was to make these outside the area so that it doesn't impact on the stencil.

                                           

                                          Your stencil holder could be made from two layers of pcb material.

                                          I have seen countersunk screws glued into aluminum to provide a flush surface but have the ability to use wingnut or similar on the top.

                                           

                                          The biggest obstacle will be your access to some engineering tools or methods.

                                          You may be able to convince someone with a CNC router or a Laser to make you a frame from wood.

                                          Since it rests down on the frame any bending is eliminated.

                                           

                                          Your block will be equally as accurate ...

                                           

                                          Cheers

                                          Mark

                                • Re: PCB Stencil Printer - Alignment discussion
                                  shabaz

                                  Just to keep the links for future reference, some interesting stencil printer designs:

                                   

                                  SAB by ELPRO:

                                  The current model seems to be SAB 69, and costs 1513 euro.

                                  It is usable with mylar stencils:

                                  https://youtu.be/v6Ma8JJgsW8

                                  mylar.jpg

                                   

                                  Printer by CIF:

                                  I can't work this one out, looks like they don't stretch the stencil at all:

                                  https://www.youtube.com/watch?v=SRSeoSaJCHA

                                  cif.jpg

                                   

                                  'CYBRES' from Cybertronica Research:

                                  The design of this one looks nice, and shows that perhaps just steel threaded rod is good enough for two sides of the frame, to keep it taut in one axis.

                                  In the other axis, I guess they keep it taut by sliding the screws in the slot in the aluminium extrusion before tightening with the wing nuts.

                                  https://www.youtube.com/watch?v=HELIypW0P54

                                  cybres.jpg

                                   

                                  Both the CIF and CYBRES printers need manual alignment per PCB, which I think is acceptable for prototype quantities, but the alignment methods are not as easy to trim as the thumbscrew method.

                                   

                                  If the frameless method (which I'm committed to since I've already ordered the appropriate bits of metalwork) doesn't work well, I think I can make my design extendable to a frame, adapting the peg method described earlier with the technique using the threaded rods as shown in the CYBRES design.

                                  The peg method has the advantage that the stencil is at exactly the same height with or without the frame, meaning it can be retrofitted. The pegs just need to protrude less than the PCB thickness, which is not a problem.

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                                  • Re: PCB Stencil Printer - Alignment discussion
                                    shabaz

                                    Just a mini progress report.. Currently I'm exploring options..!

                                    I will chop the long aluminium rails tomorrow, so that the size of the entire thing is about 280x320mm.

                                    If I don't get decent stability from the rails as a base, then I'll throw them out and replace with a frame made of beams.

                                    The rails are easier to try quickly first, because all the punched holes mean I don't need to drill any holes in them at all!

                                    The aluminium bars on the lower-right of the photo are going to be tapped for the positioning screws that will push the floating platform into place.

                                     

                                    metalwork.jpg

                                    • Re: PCB Stencil Printer - Alignment discussion
                                      shabaz

                                      I thought it would be worth sharing photos of what is possible using the 'tape hinge' method (i.e. where sticky tape is used to secure a stencil along one edge, much like this photo from the Internet):

                                      hinge.jpg

                                       

                                      The actual results turned out to be extremely good. The procedure was to visually align the metal stencil under a microscope, and then while holding it in position, tape it down along one edge, and then paste.

                                      Afterwards the tape needs to be removed and then the visual alignment process needs to be repeated for any second prototype. So the procedure works well enough for prototypes. I still am working on the stencil printer though, because it could speed up the process slightly. But for one-offs, it appears that the tape method is not bad at all.

                                      This was actually a first attempt (not by me but by a colleague - I only did the visual alignment). For this pasting, a squeegee from Transition Automation was used, available from Somerset Solders in the UK.

                                      Here is a pasted result, where U2 is a TSSOP sized part, so there is 0.65mm pitch, and 0.3mm spacing between adjacent edges of pads.

                                      paste-example-mid.jpg

                                      Going smaller (apologies for the fuzzy photo), here the pads are on a 0.5mm pitch, and there is now just 0.22mm spacing between adjacent edges of pads:

                                      paste-example-05mm.jpg

                                      And another example, here are 0.4mm pitch spaced pads, with again 0.22mm spacing between adjacent edges of pads.

                                      paste-example-04mm.jpg

                                      In summary, going down to 0.4mm pitch parts is feasible with the taped-down stencil method for single prototypes, and even smaller may be possible.

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