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Now that you know how to design a proper compensated wirebonds interface, it is time to discuss the "V-bond" which is often used to reduce inductance.  By placing two wires at the interface and spreading them a little, the inductance per unit length can be reduced.  Note that in this case, while you are placing two inductors in parallel, the net value does not decrease 50% due to mutual coupling.  The best you might accomplish is perhaps 0.75 pH/um as opposed to 1 pH/um for a single wire.

The image below shows the correct placement of V-bonds. The two wires will share the RF pad on the MMIC in close proximity, as it is a small pad.  When they get to the flare, they are bonded close to where the 50 ohm line meets the flare, spreading them out into a vee-shape. The chip can be mounted in a cut-out in the thin-film network (TFN) or between two TFNs. You could make a ground pad on top of the TFN using via holes, but that would add inductance in the ground plane that adds directly to the wirebond inductance, so don't try that above X-band.

V bond correct2

Here's an example of what not to do,  Yes, the wires are spread farther apart and will have less mutual coupling.  They are also longer (not at all what you want), and the way they touch down on the flares defeats some of the capacitance you were carefully adding.  Your flare now behaves more like a tee intersection of three transmission lines. Take that assembler out to the woodshed...

V bond incorrect2

Let's look at some dimensions and try to add up the length of the wires.

A: distance inboard of flares where wires are attached: 75um

B: edge of flare to edge of substrate: 50um

C: gap between chip and substrate: 50um

D: distance between edge of chip and RF bondpad: 25um

E: distance to middle of RF bondpad: 50um

Total: 250um

Note that dimension F is a don't care, you should always oversize the ground pad or cutout in the Y-dimension as a good engineering practice, it will come in handy for scrubbing the chip during attachment, or for tweezer clearance.

Next, you will have to draw the side view and take into account any height discontinuities between the thin-film and the MMIC.  This might be 125um if a 100um chip is mounted on a ground pad with a 25um bond line, it could be close to zero if you mount the chip on a heat spreader and drop it into a hole in the substrate.  Oops, that would add another length: the heat spreader will be likely be larger than the MMIC in the X-dimension...

V bond dimensions2

In the end, you will soon see that 400um a practical lower limit for wirebond lengths, unless you do some very tight tolerancing on TFNs and heat spreaders which will come at considerable expense.  How's that E-band radio dream looking now?

One path toward millimeterwave frequency is to reduce tolerances of the entire geometry.  Have you considered PolyStrata micro-coax?





Author : Unknown Editor