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E-band cost, reliability concerns in MMIC packaging

Posted: 15 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:RF systems? semiconductor packaging? E band? millimeter-wave? microstrip?

But this is changing, making SMT plastic packages practical for millimeter-wave devices. A new commercial manufacturing process is becoming available that can transform bond wires into coaxial connections with tightly-controlled impedance characteristics matching those of the customised frame leads and contacts to printed circuit boards. The process sheaths bond wires with a dielectric material of the precise, uniform thickness needed to achieve a desired impedance value, generally 50?, with respect to the bond-wire diameter. A metallisation step in turn surrounds the dielectric with a ground shield, making the completed wire resemble an impedance-controlled coaxial cable.

Package I/O design
Bond wires are only one step. To ensure signal integrity from the PCB, into the package, through the internal connections, and to the chip the whole signal path should be viewed as a waveguide. Fortunately, lead frames, if properly architected, can behave as impedance-controlled waveguide structures, and waveguide structures can also be built on the substrate.

Several different waveguide forms can be employed. Substrate waveguides fall into three categories: microstrip, stripline, and coplanar waveguide (CPW), with signal and ground conductors placed in specific geometric relationships such that their impedance is controlled. A CPW, for instance, is implemented by arranging three leads in a ground-signal-ground configuration on a single plane. Likewise, a stripline has a ground-signal-ground configuration but in stacked planes, making it slightly more complicated for attachment to a PCB.

Lead frames can also be configured as quasi-CPW or quasi-stripline waveguides. Three adjacent leads on the lead frame can be configured in a ground-signal-ground arrangement to achieve a waveguide-like structure.

A standard QFN cavity package can range in lateral size from 3.0 to 12.0 mm in 0.5-mm increments. Terminals range in width from 0.15 to 0.5 mm, range in pitch from 0.4 to 1.27 mm, and have a thickness from seating plane to upper terminal surface of 0.2 mm with an overall height of 0.3 to 1.0 mm. The plastic from which the QFN is fabricated has a dielectric constant of between 3.5 and 4.0. Given the wide range of physical dimensions available in industry standard QFNs, any desired impedance value can be achieved for package lead-frame conductors in ground-signal-ground configurations.

Simply matching the impedance of two structures is not enough to ensure efficient energy transfer in waveguides, though. In a CPW, energy flows mostly in the two gaps between the signal conductor and the ground electrodes adjacent to it. Similarly, in a coaxial link, in its lowest-order mode, the energy fills the entire space between the centre conductor and the outer ground shield. Such geometric distribution of the electromagnetic energy is referred to as the mode shape. The fundamental modes in the respective waveguides are much different in shape; therefore it is important to blend the energy distribution shapes smoothly from one type of waveguide to another. The blend region is known as a transition.

JEDEC does not define the dimensions and spacing internal to the QFN package, merely the external elements, so package developers have the freedom they need. Complex three-dimensional shapes created using varied front and back photolithographic patterns allow the sculpting of signal lead and ground lead shapes so structures can be smoothly transformed from CPW, with its planar ground-signal-ground configuration, to a coaxial bond wire, with its concentric signal-insulator-ground structure. Thus, two-sided forming can create 3D mode-transforming shapes in the metal lead frame. Selective metallisation during deposition of the ground jacket around the coaxial bond wires can tie the jacket to lead-frame grounds.

Chip packaging is hardly the whole story
Thus, it's now becoming possible to use standard, relatively inexpensive, plastic QFN packaging for millimeter-wave devices. Although the technology is a breakthrough, however, it has no value unless the impedance of the corresponding signal paths throughout the electronic system is tightly controlled. What's more, the economy that can be achieved with those packages is worthless for such high-volume consumer applications as automotive radar, if the system's production yield is low. There are several areas of concern.

For one, wet-chemistry processes for fabricating printed circuits boards, the processes in place at virtually all PCB manufacturers, are inherently incapable of holding the geometry of fine traces within tolerances tight enough for economic production of mass-market millimeter-wave systems. Ideally, traces would have vertical walls, would not vary from as-designed width and position, and would remain true to those criteria from board to board across a panel and from panel to panel and batch to batch. The reality is far from ideal, though. Etch chemistry is to blame and there is no way to improve it.

Conventional PCB fabrication, in other words subtractive fabrication, starts with a cured dielectric laminate that has copper bonded to it on one or both sides. The copper comes in a range of weights (thicknesses) and finishes. There are hundreds and hundreds of different PCB laminates, many of them composed of woven glass fibre impregnated with many, many different thermoset resins. There are also, fewer, laminates composed of PTFE or other high-performance materials. PCB manufactures also create laminates by bonding copper foils to uncured substrate materials (called prepregs) in a press. In any case, though, the same basic subtractive process is employed to make boards, whatever their constituents.

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