> -----Original Message-----
> From: Tom Robinson [mailto:[EMAIL PROTECTED]
> Sent: Tuesday, December 16, 2003 9:26 PM
> To: Protel EDA Forum
> Subject: [PEDA] Solder mask...
> 
> 
> Is it ok to block open the solder mask on gull-wing i.c.'s with .4mm 
> AND .5mm pitches?
> 
> It seems to get enough web in between the pads to stick will cause a 
> LPI registration thats too tight, with possible smearing. And if you 
> make the pads themselves too narrow -- thats another problem.

Tom

I always block open the pads on these pitches. 

It is not so easy to get a manufacturer to get SM registration right at
these tolerances although it is possible.

Most quick turn boards I have had to take as 'free issue' parts to assemble,
which have not blocked off these areas, I have had quality issues with the
finished joints around these fine pitch pads.  

I have never had an issue with blocking them open, biggest solder mask issue
I have ever had was when the SM thickness to finished pad height
differential was too much (SM height was bigger than finished pad by a lot)
and we had gasketing issues from the paste screen to the pad, causing
problems like, paste bleeding out on the PCB side of the screen, some
shorts, failed 3d inspection after print cycle, loose solder ball symptoms
and aperture blockages, with our MPM printer having to make undue clean
cycles between prints. Also undue print down force had to be applied to
squeegee blades, increasing wear and decreasing accuracy.

Never had solder ball or short issues as long as the board is printed
correctly and the oven specifically profiled to suit your board. (also
optimise paste volumee, height, reductions...) 

John



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