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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Screen printing Solder Mask

#33042

Screen printing Solder Mask | 4 March, 2005

Anybody have any ideas on the proper way to screen solder mask onto a PCB? I am going to cover SMT parts that were soldered to the btm side of the PCB, now I have to mask them in order to run this animal through the wave solder process. I know it will work because of my experiments, but that was hand applied solder mask. I am going to have a stencil cut for machine applying of the mask. Anyone have any pointers? Blades to use? Stencil Thickness above 0603 components? etc.....Thanks for the help.......

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Slow Ride

#33045

Screen printing Solder Mask | 4 March, 2005

I would use a spot solder machine. If not, why not spray the mask onto the board? Use a Camalot machine or something similar. Using a stencil over placed components wil yield a thick mask - right? That's why I'd spray.

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pr

#33046

Screen printing Solder Mask | 4 March, 2005

I believe DEK manufactures stencils that will do this. I'm sure their Apps. engineers could help you with squeegee material etc.

good luck, pr

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RDR

#33047

Screen printing Solder Mask | 4 March, 2005

Glad to hear a little success from ya Duckmann! i have stenciled mask in the past and I can tell you that stencils are expensive. They are basically a step stencil, you will need full thickness for the area(s) you are going to apply mask and you will need the stencil etched down (up) to clear your tallest component. For example, if the 0402 parts are .3mm in height you will need a stencil at .5mm thickness, have the apertures cut for your components and then the stencil will need to be etched out from the bottom side to clear any other components that are present. If you don't have any other components besides the ones being masked then this will just be a standard stencil. I would then make the thickness just over the max heigh of your parts. I used worn out urethane squeege blades or spatulas or anything that would work, the mask only needs to touch the body and the board somewhere, you don't need a perfect fully covered application of this.

So why did you not use a stencil anyway?

Russ

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#33050

Screen printing Solder Mask | 4 March, 2005

Mrduckmann2000: Why do you want to do this? * Removing a non-washable / peelable temporary solder resist [TSR] is going become very tedious, because they do not release well from small / tight / isolated features. * Clay from washable resists will gum-up your deionizing beds and wear your washer pumps.

We run second side SMT components across the wave every day, using no mask or anything, except glue. Waving most passives and small actives is not a problem [although they will take-on a little more solder than we like if we use 'standard' pad designs]. If you plan to wave QFP, it requires more thought and observation of limitations.

Applying TSR is not rocket science. We have used a squeeze bottle. This goes back to a point made by an earlier poster. Why not dispense TSR?

Finally, becareful selecting TSR. Many are loaded with ionics from the ammonia used in curing.

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#33059

Screen printing Solder Mask | 6 March, 2005

Davef,

You have a good question. If we had perfect design engineers out there that would use the proper component spacing and not put SMT parts between and around thru-hole parts - glue and wave would work perfectly. I have done this with great sucess in the past. But on this job we have over 200 0402's that have been placed between the legs of thru-hole connectors and various other thru-hole parts. Our Gemini II will not put down glue dots small enough for 0402's. I worked with Electroverts tech's and app engineers but we could not get the machine to put down small enough dots. Thus the solder mask idea. Our in-line DI wash will remove the water soluble mask with out issues. This is an issue that I have dealing with for several months. I hope to slove this building nightmare by next week, if I get a stencil cut to meet the needs. Thanks for your input.

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#33060

Screen printing Solder Mask | 6 March, 2005

Russ,

I didn't use a stencil for my experiment because I was not sure that the solder mask idea would work. Now that I have treid a few experiments I feel dangerous! LOL. I will not have to use a step stencil, I will just mask all the parts. Like I've stated beofre more than 95% of the parts are right next too or between the legs of the thru-hole connectors. I figure that I might as well mask all of them. Thank you for your input. I will let you know how this animal works.

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#33061

Screen printing Solder Mask | 7 March, 2005

The Camalot Gemini II is capable of dots for 0402 chip. What would an Electrovert person know about that? With a 32gauge needle you can put down as small as 0.006" dia dot. For information about dots for 0402 chips, please contact Camalot applications engineer Jim Kaler 603-641-8666 x7208, jkaler@speedlinetech.com.

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RDR

#33068

Screen printing Solder Mask | 7 March, 2005

Duckmann, I was actually asking about using a stencil for the glue.

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#33072

Screen printing Solder Mask | 7 March, 2005

Russ,

I tried to get a screen for glue but the design engineer who laid this board out made up his own rules. The 0402 pads are to close for a proper dot size.....per the stencil house. That is why I am going the solder mask route. I would rather stencil the glue.

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#33073

Screen printing Solder Mask | 7 March, 2005

Pete,

I was speaking to the Electrovert/Speedline/Camalot/Gemini factory tech's and App Eng's. I now have new machine files which slowed the pumps down to half speed, I have several needles....all of which still will not place a small enough dot. I have also tried reducing the air pressure to the machine. I have been told that the machine should and will put down dots for 0402's, but have yet to see anything close on our machine. I have also had several people post comments that this machine will not do it, one mentioned his would but it wasn't worth the mess because of clogged needles. This is why I am seek info on screen printing solder mask....This has been a real fun project thus far. If I had my way the PCB would be in relayout right now but that cost money and our customer feels that the board is ok as is. So we are stuck at this point.

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