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Hardware Requirements for Super Fine Pitch Screen Printing

IP.com Disclosure Number: IPCOM000112976D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 77K

Publishing Venue

IBM

Related People

Ferguson, M: AUTHOR

Abstract

There are two main defects in the fine pitch screening process; bridging and insufficient solder. Bridging is usually caused by either depositing too much paste due to an incorrectly sized stencil or from a paste that is too low in viscosity. Insufficient solder is the tradeoff from bridging and is almost always caused by clogging of the stencil apertures. As the stencil aperture widths and the stencil thickness are reduced in order to avoid bridging, the number of insufficient solder defects usually increases. There are also a number of manufacturing defects down the line that can be traced back (to some extent) to insufficient solder such as dewets, skewed components, etc. As a general rule the more solder that is deposited on the pad without causing bridging, the more robust the process is.

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Hardware Requirements for Super Fine Pitch Screen Printing

      There are two main defects in the fine pitch screening process;
bridging and insufficient solder.  Bridging is usually caused by
either depositing too much paste due to an incorrectly sized stencil
or from a paste that is too low in viscosity.  Insufficient solder is
the tradeoff from bridging and is almost always caused by clogging of
the stencil apertures.  As the stencil aperture widths and the
stencil thickness are reduced in order to avoid bridging, the number
of insufficient solder defects usually increases.  There are also a
number of manufacturing defects down the line that can be traced back
(to some extent) to insufficient solder such as dewets, skewed
components, etc.  As a general rule the more solder that is deposited
on the pad without causing bridging, the more robust the process is.

      A commonly used term in the fine pitch screening industry is
the aspect ratio which is the ratio between the width of the stencil
aperture and the stencil thickness.  Studies have shown that the
aspect ratio must be above some minimum number in order to obtain a
proper print and to avoid clogging the stencil apertures leading to
insufficient solder defects.  Following this logic, as finer pitch
components are introduced into the process, the stencil aperture
widths must decrease which corresponds to a similar reduction in the
stencil thickness in order to maintain the appropriate aspect ratio.
Today's 16 mil pitch boards require stencil thicknesses of 5 to 6
mils compared to 10 mil thick stencils that were used back when the
closest lead spacing was only 50 mils apart.  The problem arises when
50 mil components are combined on the same board with the fine pitch
components.  A 5 mil stencil deposits the very minimum paste
necessary to maintain a low defect rate manufacturing proc...