A question many of our customers often ask us is, “How do we improve the thermal performance of high-power RF Parts?” RF Power parts are often the critical part in the RF signal chain impacting the fidelity of the RF Signal and the heat dissipation of the subsystem they drive. The thermal dissipation is greatly impacted by the die attach mechanism. Excessive voiding is bad news when it comes to die attach, not only because voids increase the likelihood and location of cracks, but also because they impact the thermal and electrical conductivity of devices. Eutectic bonding of high-temperature power amplifiers presents a unique challenge due to the thin, brittle nature of the devices and the need for a virtually void free attachment of the backside of the device to CTE matched thermal spreader.
To solve this problem and to maximize the full potential of power densities from space qualified devices, we implemented a high yielding void free manufacturing process that limits void sizes to less than 0.15mm during eutectic soldering. Die can handle power densities of 1.0 watt / mm while new devices push this power density to 5.0 watts / mm or greater. Traditional assembly methods for attaching large area die to thermal spreaders using Au-20Sn preforms are typically poor yielding due to significant voiding under power FETs and bond pads. Thermal simulations show that voiding greater than 0.15mm under power FET’s can seriously reduce performance or in some cases cause catastrophic device failure. Without void size process control, assembly yields are significantly compromised.
Void control can be significantly improved when using vacuum reflow equipment and a technique to reduce chamber pressure after the solder has transitioned from solid to liquid. Void size expansion is inhibited by a Void Expansion Slip (VES) factor, creating smaller voids and allows large compression ratios at the completion of the solder process. Adding this technique to a vacuum reflow process will produce void free solder yields greater than 98% on very large die.
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