Ding Yandoc led the team who developed LFPAK packaging
Up until relatively recently, the packaging of power semiconductors was not high on the list of priorities when designing products. However, the continuing need to fit more power into smaller footprints has meant that situation has changed, and the specifications of the packaging have become almost as important as the design of the silicon inside. High power density means that the packaging has a more important role in taking heat away from the die. Wire-bonding was initially the preferred method of attaching the die to the outside world, but as density and currents increased, a single wire could no longer provide an efficient link and that was detrimental to the operation of the device. Wires were then made thicker to handle more current, and additional wires were added, but that didn’t completely solve the problem and the expected gains from new designs were still not being realized. Wires also expand and contract with heat, and each time that happens it places a strain on the bond and lowers the lifetime of the device.
In 2002, Nexperia engineers, while still part of Dutch multinational Philips, developed a new concept that eliminated the need for wires to be bonded to the die. Instead, they used large copper clip attached to the source metallization on the topside of the die to connect it to the outside circuit. Gull wing pins were also developed that could efficiently carry higher currents, while still being flexible enough to cater for heat expansion and contraction without damaging the connection to the die or board, and therefore increasing reliability. The new LFPAK package significantly lowered both the thermal and electrical on-resistance of the device, outperforming traditional Power-SO8 packages.
Chris Boyce, Senior Director, Marketing & Product Group, BG MOS Discretes at Nexperia explains how revolutionary the package was at the time, “It's easy to forget how much has changed in 20 years. Some of the innovations now seem obvious and have been replicated a number of different ways. However, at the time it was pioneering, and the LFPAK devices gained rapid market acceptance, with initial deployments in engine control systems, braking, ABS products and power steering”.
Boyce continues by describing how the LFPAK has increased reliability, “The gull wing pins act like a shock absorber and give excellent board level reliability, and the evidence of that is clear now that LFPAK devices have been in the field for 20 years. All of the theory has has played out in practice over the long term”.
Semiconductor technology has evolved over the last two decades and Nexperia engineers have continued to refine and improve the LFPAK concept to cope with its increasing demands. Each iteration has improved LFPAK’s performance with lower RDS(on), lighter-weight clips, reduced gate pads and thinner wafers.
Today, around 90% of the Nexperia product range is LFPAK-based, with more than 400 devices across 7 distinct design variants. The original LFPAK56, (named after its dimensions – 5 mm x 6 mm) was soon followed by the LFPAK33, and the LFPAK88, delivering the benefits of the latest device package technology in a wide range of voltage and current ratings. Today a single MOSFET in LFPAK88 boasts an RDS(on) of just 0.55 mΩ handling a maximum current of 500 A. Dual LFPAK devices are also available, containing two individual MOSFET dies in one package, offering additional space-saving. Nexperia has also produced an additional variant of the LFPAK56D containing a series of MOSFETs in half-bridge configuration, for space constrained motor applications. Some of the latest innovation in LFPAK-type packaging has come from the need to integrate wide bandgap materials. CCPAK is a copper clip cascode GaN HEMT package that brings all the benefits of copper clip, such as thermal performance and board level reliability to power GaN FETs.
Almost 10 billion LFPAK MOSFETs have been shipped since production began, with 1.7 billion in 2021 alone.
Boyce concludes by looking to the future, “Although it started with a 5 mm x 6 mm device twenty years ago, we've expanded the portfolio significantly over the years. Now, we have the CCPAK 1212, a new 12 mm x 12 mm package that we use for GaN devices. It's non trivial to make them work and be able to produce them in in huge volumes, and its taken a lot of learning and experience from working with LFPAK technology to accomplish that. There will be future generations of packages that will also build on the learning of the team, which includes 12 of the original LFPAK creators. We are not done yet, and hopefully we will come back in another 10 years with an update”.
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