Technet:

	The problem that I am having with pins breaking off the pads is
actually ocurring after assembly and durring handling of the board in our
lab (inserting the card into the PC, probing with the scope and logic
analyzer, people drooling over it, etc).  I also expect the same
treatment of these boards from our customers so they need to be able to
withstand a little torture.  

	The aspect ratio of the board does allow the board to be
flexable, but I have handled other boards with the same dimensions and
they seem to be a lot more rigid.

	I spoke to our assembly house the other day and they reccommended
specifyng a high quality SMT grade FR4.  This is the first I am hearing
about different types of FR4.  He said that it is more expensive and many
board houses won't use it unless it is explicitly asked for, even though
fine pitch parts are clearly used on the board.  He also told me to
specify compliance to IPC RB-276, IPC A-600, and J-STD-003.

	As far as pad size goes on the .5mm pitch devices, I was told
that a wider pad may actually be better than a thinner one, because it
would be easier for solder to wick up onto the leads than to form a
solder ball and stay in between the pads.  My thinking before was to 
make the pads as thin as possible to widen the gap between them.  What
are
your experiences with this?

Thanks,

Charles Wills
[log in to unmask]



On Tue, 12 Nov 1996 21:49:24 EST [log in to unmask] (Charles J Wills)
writes:
>Technet:
>
>	I have a board about 13.5" x 4.5" (PC plug in card), .062" 
>thick, 8 layer, FR4, 1 oz copper, 2 power planes, with 16  100 pin  
>TQFP packages (0.5mm lead pitch) spread across the board (2 rows of 
>8), as well as many other PLCC and SOIC devices.
>
>	The problem we are having with these prototypes is that some 
>pins on the TQFP's pop up durring handling.  The boards seem to be 
too 
>flexable, which probably causes the pins to break off the pads.  Is 
>there a way to make the board more rigid besides using board 
>stiffiners?  In other words can anything be done as far as changing 
>the board stack up, or will using thicker copper help this situation? 
 
>Or are there different types of FR4 available?  Or is there something 

>I am not looking at?
>
>	Also, what would be a good pad size for the 100 pin TQFP 
chip? 
> Currently I am using 1.2mm x .25mm, but I think the IPC recommends 
>1.6mm x .35mm, but it doesn't look like it would allow any soldermask 

>between pads.  Or does anybody even bother with soldermask between 
>pads at .5mm pitch?  What is the middle ground?
>
>Thanks in advance!
>
>Charles Wills
>[log in to unmask]


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