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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Step Stencil

Felix

#3379

Step Stencil | 5 August, 2000

Dear Netters,

Could anybody kindly advise : 1. what step stencil means 2. what are its characteristic ?? 3. What special process makes it? 4. What is the application ?? 5. Advantages and limitation ??

just short breifing is enough.

Thanks

Best Regards, Felix

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JAX

#3380

Re: Step Stencil | 5 August, 2000

Felix, 1-5: A step stencil is simply a stencil with area's of different thickness( step down's ). They are used for multi-package boards( BGA, fine-pitch, ..... ). They keep you from deciding whether or not you want the fine-pitch or the BGA devices to get the correct amount of solder volume.Good Stuff! Hope that's enough,

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#3381

Re: Step Stencil | 5 August, 2000

Stencil, Step Down. A stencil of varying thickness that allows printing different amounts of solder paste or adhesive.

Most stencil suppliers make stepped stencils

Step stencils are used when there are fine pitched device amoung coarse pitched devices and it's difficult to compensate for different paste volume requirements of the device pitches by other means, such as pinching.

Stepped portions of stencils are larger than the aperture courtyard to provide for good squeegee contact. Some applications have component spacing that do not allow stepping of stencils.

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fraser

#3382

Re: Step Stencil | 6 August, 2000

1. A step stencil is a waste of perfectly good stainless steel! 2. They are characterised by lousy prints on the very parts that are most critical in your process - whilst leaving good prints on the easy stuff. 3. MMM.... nasty chemicals! 4. Exactly!! 5. Advantages - er.. Limitations - see No2

Felix.

I have found that there are very few processes where the need for a step is justified - you can generally design them out. Tannlin make laser cut stencils for most of the most advanced OEMs and CEMs in the world and we have designed stencils for intrusive reflow and coplanarity failure parts where we have overprinted by 2 and 3 times the area of the target pad. this is always a far better solution than a step - unless you need a ridiculous amount of paste in which case you will end up overprinting and stepping, and generally a laminate step is the only way of adding enough paste volume. If you don't believe me do the maths - most steps add 2mil or 3 at most. so to step from 6 to 8 will increase your volume by a third. On for example a 20x20 pad, if you over print by only 2mils all round you will increase your volume by 40%!!

You did ask!!

Cheers

Fraser

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