What happens when you clean a solder misprint with alcohol on OSP
pads??? If not alcohol, what else can you use instead?  As long as you
are re-stenciling the solder paste within an hour  or so, I have not
seen any problems....

Marcelo

> -----Original Message-----
> From: Electr1998 [SMTP:[log in to unmask]]
> Sent: Monday, January 12, 1998 10:39 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Landpatterns for TQFP devices
>
> (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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