Bill:
What process changes are you making when you change substrate layer count?
Our design guys are considering 4 layer 388 & 456 ball 35x35 packages at
1.27 pitch instead of the 2 layer 388 style we currently use.  I not sure
whether the extra ground layer in the 4 layer is good news or bad news to
manufacturability/reworkability.

Larry Jindra
TRW/Avionics Systems Division
[log in to unmask]
(619) 592-3424

> -----Original Message-----
> From: Bill Davis [mailto:[log in to unmask]]
> Sent: Monday, November 16, 1998 8:31 AM
> To: [log in to unmask]
> Subject: Re: [TN] 1mm PBGA Assembly
>
>
> Yves:
> We have just started into the 1.0mm BGA's ourselves. Our PCB Assembly
> yields have been identical (actually slightly higher) than
> the 1.27. Of
> course we are in the <300 ball regime for the 1.00, whereas we were in
> the >400 ball range for the 1.27. Package size appears to have a great
> deal to do with our yields. If we use a 1.00 27X27 at 196
> balls, we get
> better yeilds than a 35X35 1.27 at 196 balls. Likewise a 452
> ball 35X35
> 1.00 yielded better than a 40X40 452 1.27mm package. Most of the
> theories here have something to do with laminar coplanarity. The BT
> substrate moves considerably (2~3X) in the Z axis during reflow (when
> compared to all of the other materials).  Constrictions to
> this movement
> and levers exist where the overmold package body meets the
> BT; likewise,
> the center configuration will also be a localized constrictor.
>
> Finally, we have also seen a vriation in yields when considering the
> number of layers in the BT substrate as well. It may be where our
> process is optimized (we originally did it for 4 layers). With 2 layer
> or 6 layer subs we tend to get lower yields. So what we have don is
> required all substrates to be at the same layer count on a given
> assembly (i.e., no mixing of # of layers within a given assembly).
>
> Hope this helps...
>
> > Bill Davis, Ph.D.
> > Diamond Multimedia Systems
> > Senior Scientist
> > Tel. 408.325.7868
> > Cell. 408.888.5650
> > e-mail: [log in to unmask]
> >

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