(Although this may sound like an flippant response, it is not intended to
be...)

Ask your fabricator and your manufacturing group.

If this is your first venture into 20 mil pitch components, you will be facing
new challenges/opportunities in fabrication and assembly.  By far, the largest
assembly defect associated with 20 mil pitch components is solder bridging
(shorts).  There are several steps that can be taken during PCB design that
can help.  In short, here are four areas that should be considered for 20 mil
pitch applications.

*  Immersion gold over nickel or OSP over copper finishes are preferred over
HASL (so that you have flat pads on the PCB).  Which finish would your
manufacturing group prefer?  Which finishes can your fabricator provide?  Is
your solder paste compatible with the plating finish?  How will you clean bad
prints?  (Hint: do not clean OSP PCBs with alcohol.)  Should you do SIR
testing when introducing new platings or new solder pastes?

*  Typically, the width of the stencil aperture is reduced 1 mil per side
compared to the pad width.  (e.g. If the pad is 12 mils wide, the
corresponding stencil aperture would typically be 10 mils wide.)  When
dividing the smallest aperture width by the stencil thickness is the ratio
less than 1.5?  If so, you may have problems getting the solder paste to
release properly from the stencil.  You may need to consider using smaller
mesh solder in your paste or using a lower viscosity paste.  You should also
consider having 20 mil pitch apertures laser etched in the stencil.

*  Although there is some controversy on this subject, many companies require
the fabricator to provide a minimum 3 mil mesh of solder mask between pads to
help reduce solder bridges.  (The issue of controversy is whether this really
helps - you can ask this question back to TechNet and watch the sparks fly.)
If you choose to do this, you must use an LPI solder mask and you need to get
some guidance from your fabricator on applicable drawing notes.  Should you do
SIR testing when introducing new solder masks?

*  How will you rework 20 mil pitch components?  Will you use custom rework
tips (e.g. Metcal) or will you use a hot air rework system (e.g. AirVac, SRT,
etc.)?  Either way, you need to consider how close you can place other
components next to your TQFP so that you can safely remove the TQFP.  You
should also consider how much heat will be applied during rework and decide if
you need to use a higher temperature material.  (Some companies will build
prototype PCBs using a higher Tg material like multifunctional FR4 and then
switch back to normal FR4 for production.  Using a higher Tg material can help
extend the life of a board if your engineers are prone to replacing components
many times.)

Hope this helps,

Andy

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