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How to define solder paste printing height tolerance

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#66275

How to define solder paste printing height tolerance | 7 May, 2012

Hi,

I need one clarification about solder paste printing height tolerance. How to define and fix the solder paste height tolerance after screen printing for inspection purpose. Currently we are using 5mils thickness of stencil, in that we are gave tolerance as 4 to 7mils. Please help to provide the exact tolerance for the above.

And also please let me know, Is there any specification given in IPC Std.

Thanks & regards, Saju.A

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#66276

How to define solder paste printing height tolerance | 7 May, 2012

Start with targets based on the stencil thickness (for height) and stencil thickness times the aperture size (for volume). * Set reasonable upper and lower control limits (+ 25% and –25 % of the targets, for example). * Next, check the printer set-up by printing and measuring a test print on a flat sample surface. A piece of white poster board will work. ** Take sample measurements at critical locations (the corners of fine pitch sites, BGA and ?BGA sites, etc.) ** Gather readings at the corners and the middle of the complete print area, checking for parallelism. * Once you are confident in the printer set-up, record measurements on every PCB in the pilot lot. * If a print failure is found (i.e. exceeding your selected control limits) wash the sample, adjust the printer and reprint. * Process the boards through reflow. * After reflow, inspect and calculate the yield. * Adjust your process limits based on the defect analysis. If post reflow inspection identifies: ** Shorts, lower the upper control limit. ** Opens, consider increasing your targets.

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