Automatic Finger Synthesis and Placement

Once you have imported or synthesized your die, FA4ST can create the bond fingers needed to connect from die to package. FA4ST helps you quickly decide on finger spacing, number of tiers and distance from the rings.

One of FA4ST's nicest feature is that it quickly tells you how many IO's are on each side of the die - you specify which nets are downbonded (we assume these are power and grounds) and the rest are IO.

The user can have LTGen solve for the number of fingers, the finger spacing or the distance (i.e. wire length) given the other parameters.

Ring - identify the ring layer that the bond fingers follow. Clearances are measured from the specified ring.

Distribution - shows how many pads are on each side of the die.

Assign from Die Pad - opens a dialog that enables you to specify nets for ring attach - effectively removing them from the IO count.

Allow 2 Tiers ... - tells LTGen it can use two tiers of bond fingers if needed to accomodate all of the fingers.

Tier-Tier Gap - space between the bondfinger tiers.

In the example we have a relatively simple die with 79 die pads on each side. We've asked LTGen to solve for the "ring-bondfinger" gap (which is essentially the wire length) given our finger width/spacing and the need for 79 bond fingers on each side of the die.

We had already synthesized the ground and power rings based on our standard design rules. The results are shown below:

autocad screen shot of rings and fingers.

I measured the corner wire length and immediately noticed that this wire is 5800 um when in fact my design rules state no more than 4500 um. Clearly the current layout is not going to work.

Reducing the Max Wire Length

The only way to reduce the maximum wire length is to pack more fingers together by making the finger a little narrower and the spacing a little tighter. I'll probably pay a little more for the substrate and assembly but I won't have to go to an exotic package or suffer low yields due to wirebonding problems.

reduce the finger
width to 90 and gap to 80 um.

Finger width reduced to 110 um and finger spacing reduced to 80 um.

Just click on Apply and immediately the new layout appears. See it below

finger layout with shorter corner wires.

I measure the corner wires (these are always the longest...) and see that they are now about 4380 um - under my 4500 um maximum.

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