In this example we are going to take the Gerber data from a two layer BGA and use it to show what the different extraction directives do. In the illustration above, you can see a quadrant of the BGA.

Nodes - I started by first defining 7 nodes on the top (red) side of the BGA. These correspond to bond fingers 1-7 and are labeled BF1 ... BF7. On the bottom side of the board I labeled 7 nodes using the JEDEC nomenclature. You can see the node coordinates in the table below:

B_NODES
BF1     7.00 17.70 1
BF2     6.89 17.56 1
BF3     6.82 17.42 1
BF4     6.75 17.27 1
BF5     6.67 17.14 1
BF6     6.60 17.00 1
BF7     6.55 16.86 1
A1      2.23 23.26 3
B2      3.24 22.27 3
C3      4.24 21.26 3
D4      5.23 20.25 3
B1      2.23 22.27 3
G4      5.23 17.25 3
C2      3.24 21.26 3
E_NODES

The 1 at the end of the first 7 lines indicates that NETEX-G should look on stackup layer 1 (the top layer) and the 3 indicates that NETEX-G should look on stackup layer 3 (the bottom of the board). This node information was entered into the NETEX-G dialog or it could have been pasted into the NETEX-G job file if you are running NETEX in batch mode.

Net to Extract

Our goal is to extract the net defined by the node BF3. (Let's suppose we wish to create a Spice model from the bond wire attach point to the ball pad on the bottom of the board.) To build an accurate model we need more than just the net in isolation. It is clear that this net will be influenced by the adjacent nets and also by conductors that pass under it and over it. The extract directive will enable us to get not only the net BF3 but also the surrounding metal that couples to that net.

The Expansion Distance The first decision to make is: How far away from the net need we look to accurately calculate the net's impedance and response? If we attempt to grab everything on this package our field analysis program will have to deal with 300 nets -- but most of them are far enough away that their contribution is "neglible." In fact, one could use a field analysis tool to examine the field around a single conductor and then use that information to decide how far to look. Let's skip that and assume that we've decided to look about 250 um on each side of the BF3 net.

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