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Importance of Net Names

When designing a modern package it is critical to know the net of each die pad. Many of the routing rules are net based. Power and grounds are of course treated much differently than IOs. Yet many IC designers persist in sending the packaging designer the pad information without the proper net names.

At right you can see a typical pad coordinate table provided by an IC desginer. In fact, sometimes the die pads don't even have numbers -- all that is provided is the center coordinates of the pad!

 
1   -685  420
2   -685  300
3   -685  180
4   -685  60
5   -685 -60
6   -685 -180
7   -685 -300
8   -685 -420
9   -420 -685
10  -300 -685
11  -180 -685
.
.
.

28   60  685
29  -60  685
30  -180 685
31  -300 685
32  -420 685


Power & Ground Nets are Crucial!

If nothing else, the IC designer should assign the power and ground nets. Give all nets that are to be connected to a common metal ring the same net name.

For the rest of the IO pads, give each die pad a unique name. The package design software assumes that any pads that share a common net name will be connected together!

 
vss    1   -685  420
io1    2   -685  300
io2    3   -685  180
io3    4   -685  60
vdd    5   -685 -60
io4    6   -685 -180
io5    7   -685 -300
vddint 8   -685 -420
io6    9   -420 -685
io7    10  -300 -685
vss    11  -180 -685
.
.
.

io8    28   60  685
nc     29  -60  685
vdd    30  -180 685
io10   31  -300 685
io11   32  -420 685
Verification using Connectivity

If a proper name is assigned to each die pad, upon completion of the package, the package designer can create a netlist derived from the actual connectivity that matches up each die pad with the ball it is attached to.

This makes verifying that the package is correctly designed more accurate. Many companies have found to their chagrin, that a completed package connects to the wrong ball pads. The error was not caught during package design because the designer had to hand edit the netlist provided by the IC designer to add in the signal names.

 


vss    1   -
io1    2   A5
io2    3   B4  
io3    4   C4 
vdd    5   -
io4    6   E5
io5    7   F4 
vddint 8   -
io6    9   G3
io7    10  G4 
vss    11  -
.
.
.

io8    28  H3 
nc     29  -
vdd    30  -
io10   31  J4 
io11   32  J5  


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