My recommendation would be to establish a chart for different stencil
thicknesses.  Allow your own process capability to establish upper and
lower control limits for those charts either by printing and measuring a
number of samples or, just taking measurements from production (30 may
be considered statistically sound).  Of course, these UCL and LCL's
should be within the upper and lower specification limits.  The question
as to WHAT the upper and lower spec limits are depends on how much paste
each joint really needs to ensure a reliable joint for your specific
desings.  That can depend on the individual pad and component design.
Sometimes .005" will be just barely enough.  Sometimes .008" will cause
bridging.  If you feel you MUST establish and upper and lower
specification limit, my recommendation would be .005"-.008" for 5 mil
stencils and .006"-.009" for 6 mil stencils.  IF you go outside the
limits, the procedure should state that a Tech or Engineer should be
called to look into the situation and identify the root cause.  

-Carrie 



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Wednesday, April 21, 2004 10:46 AM
To: [log in to unmask]
Subject: [TN] IPC standard on paste height?

Hi Technos,

the fact that paste height is maybe not a relevant and worthwile factor
to measure has been brought up several times on this forum. My
experience also says that I will have the same acceptable joints whether
using a 6 mil stencil or a 5 mil one, if the paste deposits are uniform
(nice bricks). But if I write a procedure setting the control zone
between 5.5 to 8 mils I will get flamed by both the customers auditing
the process and my bosses.

Is there any standard/handbook/specification that sets control limits
for the height of the paste after stencil printing? Or which prescribes
other measurable indicators for good printing?

I must mention that I only have an LSM that can measure the height, no
volume checking available.

Thanks,
Ioan

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