“Thin is in” as RF-module shrinkage also enhances thermal performance

Packaging innovation has always been critical to the cooling of components, especially for power-switching devices such as MOSFETs and IGBTs. The non-stop demand to make these devices smaller and lighter also applies to RF PA modules, even though the inner workings of these analog modules are very different than those of on/off switching of power devices. This need for “make it less, but also do more” is especially intensified due to the multichannel requirements of massive MIMO 5G systems.

Nonetheless, when it comes to packaging details, the primary concern of designers is “What does it do for me?” more than “How did you do it?” Yet the “how” part is important, as it defines the capabilities if newer parts and sets the groundwork for future innovations which build on it.

A good example is the top-side cooling (TSC) for RF power amplifier (PA) modules introduced by NXP Semiconductors in 2023. This advance was not “hey, we’ve got a new package in the works” but it was coupled with deliverable parts—always a big plus a world where pre-release hype and promotion are considered normal (thankfully, not so much in the no-nonsense “analog” world from DC to RF).

NXP’s packaging results in an RF PA module which is thinner and lighter than existing designs, with a better thermal path as well. This top-side cooling contrasts with conventional bottom-side cooling (BSC), where the thermally conductive paths transfer heat from the package components—primarily the PA itself—to the PCB, which is thermally bonded to a cold plate or heat sink. While TSC is not unique to NXP (other vendors have somewhat different implementations), the NXP approach is illustrative, Figure 1.

Figure 1 Compared to the bottom-side cooling approach (left), NXP’s top-side cooling (right) flips the placement of the thermal coin as well as active and passive components, for a thinner and more thermally conductive package. Source: NXP

In a usual BSC approach, the dissipation of the PA is conducted through a metallic “coin” in the PCB and then to a heat sink on the underside of the board. The associated module components, including the PA, circulator, and filter, are mounted on the top side of the board, and all are covered by an RF electromagnetic (EM) shield. To complete the signal path, the antenna array is connected to the board.

In contrast, with TSC, the PA chip is connected to a direct-bonded copper-ceramic substate on the top side of the package. The chip is mounted on the surface of the board, thus making direct contact with the external heat sink. The benefit of this arrangement is that it maximizes dissipation and thermal performance, while yielding a smaller package which increases functional density.

Specifically, in the TSC arrangement, the coin is connected instead to the other side of the board and directly to the heat sink, while the circulator and dielectric filter are also mounted there. As a result, all the RF components are on one side of the PCB. At the same time, the shield is integrated into the heat sink rather than on top side of the PCB, which puts the antenna closer to the board with a clean separation of thermal and RF paths. The overall design shortens the connectors, improving RF performance while reducing thickness and weight of the overall assembly.

In contrast, bottom-side cooling is a compromise between thermal performance and use of the board’s real estate since module components can be placed on one side only. The result is lower functional density of the board while it is being challenged to support multiple RF channels.

TSC is not just a preliminary investigation or available as sampling prototypes. Off-the-shelf RF power modules such as the A5M35TG140-TC are available for 32T32R-class, 200-W 5G radios covering 3.3 GHz to 3.8 GHz. The devices combine LDMOS and GaN semiconductor technologies to create 10.5 W (average) fully integrated Doherty PAs to with ~30 dB gain and 46 percent efficiency along with 400 MHz of instantaneous bandwidth—all in a package measuring just 14mm × 10mm × 2 mm thick, Figure 2.

Figure 2 The A5M35TG140-TC is one of three similar multi-GHz PA modules, each with a simple schematic which does not begin to indicate their sophisticated underlying processes or advanced package implementation. Source: NXP

There are also evaluation boards which ease the design task of assessing the PA module performance and characteristics without having to “reinvent the wheel” of a relatively simple-looking schematic and layout which inevitably has its RF subtleties, Figure 3.

Figure 3 Vendor-supplied evaluation boards are essential to speeding up the assessment and design-in process. Source: NXP

All these substantive improvements in packaging still leaves one evasive cooling question: where is this mythical, wonderful place called “away” to which all the dissipated heat is being conveyed? By doing a better job of getting heat away from the package, in addition to shrinking the package itself, are you making your previous thermal problem into someone else’s headache, as they now must contend with heat you toss off? Or would you have had that total amount of heat anyway, but with a different distribution across the PCB and within the chassis? Have you seen any other power-package developments for non-switching devices with which you were impressed?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

References (all from NXP)

5G Radios Shrink With NXP’s New Top-Side Cooling For RF Power
Fact sheet: Top-Side Cooling RF Power Modules for 5G Infrastructure
A5M36TG140-TC Top-Side Cooling Evaluation Board
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The post “Thin is in” as RF-module shrinkage also enhances thermal performance appeared first on EDN.

“Thin is in” as RF-module shrinkage also enhances thermal performance

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