RF wirebonds

Click here to go to our main page on wirebonds

Click here to learn about wirebond inductance, resistance and attenuation

Click here to go to our page on chip-and-wire construction

Here's a video of auto-wirebonder in action, in slow motion.  Check out the complex trajectory that was programmed to make a loop for stress relief.

RF wirebonds (mainly from interconnect substrates to MMICs and RFICs) are a key part of microwave engineering.  Unfortunately, they never seem to get the worthwhile treatment in coursework or in actual designs.  RF wirebonds present a problem that is going to get more attention in the future: wirebonding above 40 GHz comes with a lot of baggage that gets heavier and heavier with frequency, and is too hard to lift at 200 GHz.  How many times have you read a paper or watched a presentation on an amplifier that operates above 100 GHz, and the author or presenter provides only RF probe data and does not consider how the circuit would be used in the real world?  Too many to count, it you attend IEEE IMS, RFIC or CSICS conferences.

On this page, when we speak of RF wirebonds, we refer to connections that are supposed to maintain system impedance, for example in a 50-ohm system.  Other types of RF wirebonds include wires used to connect to chicken dots.

Before we get started, let's review a rule of thumb that is posted elsewhere on the site:

thumb Rule of thumb #11

For 25um diameter gold wire, the inductance will be approximately 1 pH for every micron of length.  For example, a 200um wire will have 200 pH inductance. If you used thinner or thicker wire (or ribbon), the inductance will have to be scaled slightly.  If someone wants to provide some input on scaling the rule of thumb, please send it our way.

Resonating out a wirebond

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RF probing versus embedded performance

When RF probing MMICs, you will get different data that when they are wirebonded in an assembly, because the low-pass filter structure is incomplete.  It is not considered cheating to massage the RF probe data to add in wirebond inductance and compensation capacitance. However, if you are RF probing large signal data such as output power and efficiency, there is no easy way to correct the data and you should just accept that it will be slightly off and (hopefully) will improve when assembled.

You will have to create a "correction circuit" in a linear simulator, that models the missing wirebond and flare or its equivalent capacitance.  Then you will need to cascade it with all of the RF data you gathered.  This is not usually a simple task, but there are people out there that are good at data manipulation, you should hire one. RF designers have enough to do already!


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Author : Unknown Editor