Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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Clamshell printers


Clamshell printers | 3 August, 2000

Does anyone out there use a semi-automatic clamshell printer in production with pitches down to 20 mil? I would appreciate any advice you have on process control and setup parameters for this type of printer.


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Re: Clamshell printers | 3 August, 2000

I presume you have no vision alignment. Buy yourself a little monocular measuring microscope ( about $100 ). You are not going to get the registration you require between stencil and pcb for each print if you are running off tooling pins , edge clamps or whatever. Pcb manufacturers aren't that accurate with punchining/drilling or edge cutting. Once the pcb and the stencil are in print position, stop the machine before the squeegee starts to move. You may have to lift a cover, hit an emergency stop or trick a solenoid/relay to do this ( most printers don't have a pause mode ). Get the little microscope in there and align your job, especially on the finer pitch stuff. You can use print/print mode so the squeegee assembly is always out of your way. You will get good results if your set-up is correct. Metal squeegee blades will help. Use a 24x magnification scope with a 5mm to 8mm field of view and divisions of 0.05mm. Great for measuring all sorts of things. I've got two, one with 6.5mm field of view, the other with 1mm field of view - 0.01mm div. Couldn't live without 'em.


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Re: Clamshell printers | 4 August, 2000

So Darby do you use your clamshell printers in production, if so what volume? Do you use a snap off distance? With our current printers the board is held with tooling pins, when the stencil is lifted the board has a tendency to stick and lift to some degree with the stencil, this causes release problems, any ideas on how to stop this? I've tried making the tooling pins tight but this makes it very difficult to fixture the boards. What is the smallest pitch you are doing? The monocule sounds like a good idea but I bet it really slows you down. Oh, we don't have any vision alignment.


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Re: Clamshell printers | 4 August, 2000

James, we used to use 3 clamshell type printers (one of which did have a two camera vision system, and did all our fine pitch), but had limited success with 20 mil pitch. 25 mil pitch was no problem. I suspect that you'll find people that have had success, albeit with some close attention to details. We also use metal blades.

This is the stuff we had to make sure of:

lift timing: on our machines each front corner lifts on it's own cylinder, and if they come up at different times it skews the bricks....bridges up the kazoo.

squeegee pressure: too little and you obviously don't get a clean pass. Too much, and not only does it tend to stretch the mesh and misregister, but it can also lift the stencil mounting frame depending on how stout it is. In our case it lifted it off the locating pins, so all alignment was lost.

snap off: our machine had 4 independant adjustments, one for each corner, and around here that spells trouble. Too much detail for our operators (we are trying to keep 4 shifts running and rarely have experienced operators on the odd shifts). Try to set yours up with no snap-off, and lock it down so they can't screw with it. Beat up stencils, of which we still have a few, are a little tougher to deal with. Can't replace them all at once, though.

We tape the corners down on the panelized boards, mostly to compensate for warpage, but that would reduce the sticking problems you have.



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Re: Clamshell printers | 4 August, 2000

Ditto to everything said. If everything in the process is correct you will have good success at 25 mil but it will take much care. I have done 20 mil on this type of printer but, as Steve says, I wouldn't want to on a daily basis. Those wipe-offs are a pain. Our clamshell had no provision for snap off but rather just progressivly peeled off as it went up. I believe this is why we never had any trouble with a board lifting off on release. Unfortunately it was also the cause of difficult paste release and irregular brick shape. Trapezoidal aperture profiles were a great help here. Another problem was operator fatigue and back pain from leaning over the printer to verify and adjust registration. An absolute necessity with fine pitch. Our operators love the new (used)printer I convinced management to spring for. There is something to be said for true vertical snap off, closed loop motion control and vision. Best of all it didn't even cost twice what the clamshell did new! Wipe-off, what's that? John Thorup

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Re: Clamshell printers | 7 August, 2000

James, I only use the clamshells for fine pitch if my other printers are non operational. Not reccommended but achievable in an emergency. In answer to your questions. We use two clam shell printers and two vertical lift printers. All are used for 8 hrs per day. The MPM 20 is squeegee only operation so the operator has to clamp amd unclamp the clamshell. 250 prints per shift would be the max. Panellise the pcbs up if you can to cut down the number of prints. On contact printing with no snap off. 0.005' thickness stencil max. I nest all our pcbs on 6mm thick perspex using tooling pins. This allows you to undercut the pcb at each end to get your fingers underneath. 0.020" pitch is the smallest I've gone with any repeatable success. Bloody oath it slows you down. I agree with the other comments, you need a conscientious operator with the back of a bullock - and you probably could pick up a 2nd hand vision/vertical lift for the price originally paid for the clam. Good luck. Darby

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