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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Re: Paste Question!

Josue Jacquez

#7975

Paste Question! | 4 January, 2000

Does somebody knows what should be the max acceptable height for the paste printing process using a 6 mils stencil??

I'll appreciate your help!!

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#7976

Re: Paste Question! | 5 January, 2000

Josue: Many variables, which you control affect paste height, but paste height is not (or correctly shouldn't be) the issue. Paste volume is where you should focus your attention. Check the archives Good luck. Dave F

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Roni Haviv

#7977

Re: Paste Question! | 6 January, 2000

From my experience:

It's depends on your messurment tools and the component:

- Average high (at least 3 messurment points) should be 6 mils max. (you'll always get pick points of 4-7 mils!). - It depends on component tech./pitch: in pitches greater than 20mil you have no problem of exess solder paste, But missing solder paste can cause lead opens after reflow.

* I didn't mentioned the opens in the screen - It can change/effect the issue.

I hope it helps,

Roni

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Brian W.

#7978

Re: Paste Question! | 6 January, 2000

I agree with Dave that passte volume is the most preferable criteria. However, you need to determine what you will accept and what your process is capable of. You need to define the process parameters, put procedures in place to control them, reduce variation in the process as much as possible, and then determine your process capabilities (Cp and Cpk), and set CONTROL limits for the process based on the processes capabilities. Your process with your equipment is not going to be the same as someone else's. As Dave said, there are too many factors which change from process to process.

For example: In a previous incarnation, we were buying 6 mil screens. We had already done our process analysis by creating cause and effect process diagrams. We had in place our policies and procedures to control the process variables. once we had those factors in control (with mathematical calcualtions to prove it), we calculated our process capabilities. Based on the experience of our process engineers, we set some arbitrary spec limits to do the calculations. I have never seen any industry standard specification limits for stencils. We then calculated CONTROL limits to run the process to. If we violated the CONTROL limits, we stopped the process.....period! We then fixed the problem. If you run to spec liimits, you are sure to run with defects. By doing this through all the parts of the processes, we were able to reduce our SMT process ppm to less than 50ppm. Most of those were attributable to special occurences or to design issues that could not be overcome without customer design changes.

Hope this helps. Feel free to contact me with any questions about the above.

Brian

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Mark Alder

#7979

Re: Paste Question! | 7 January, 2000

Josue volume of paste will differ depending on whether you are using metal or rubber squeegees. Regards Mark

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