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Adopt A New Approach To RF SIP Design

Sept. 10, 2009
Thanks to advances in circuit miniaturization and systemin- package (SiP) technology, the goals of smaller size and more functionality have become attainable.

By Diana Moncoqut and Chris Barratt

Thanks to advances in circuit miniaturization and systemin- package (SiP) technology, the goals of smaller size and more functionality have become attainable. Still, broader standardisation of the the SiP approach is stymied by the lack of design automation and portability.

A methodology developed by Insight SiP help solve those portability issues while allowing for greater integration. The methodology shows that SiPs scale and optimize, enabling aggressive design schedules from system definition down to fully tested layout.

Insight SiP’s method uses a combination of circuit and electromagnetic simulation tools to create a design progressively from basic schematic representation to a complete 3D electromagnetic representation of the layout. Manufacture is only carried out on a design for which the completed layout has been fully simulated.

Passive integrated functions (laminate, LTCC, IPD) use 2.5D or 3D electromagnetic simulations, while active circuits go through harmonic balance or device modelling. The functionality contained within buried functions inside the substrate is created by going through an iterative process.

The first step in the design methodology is to develop a range of parameterized mechanical objects for a given technology. With these objects, simple RF functions, such as capacitors, inductors and resonators, can be created.

The second step in the design process is to couple the technology file for the target process to the mechanical objects. A series of batch-based electromagnetic simulations of the mechanical objects within the desired technology file framework creates data for a lookup-table-based model for each component (L, C, or more complex resonator element).

The process described above makes it possible to create a set of project- and technology-related schematic objects that can be optimized to produce the required RF functionality. Simulations using these models can be carried out in both the frequency and time domains. At this level of the design, circuit optimization is carried out in order to determine the parameters of the schematic/ mechanical object.

The third step of the process is to create complete sections of physical layout with the mechanical objects using the circuit optimisation parameters.

A closed-loop iterative process is used to obtain final layout with the same electrical performance as the sum of the modelled portions. Coupling effects between blocks are compensated for at this stage. As a result, the mechanical objects can be placed close together without any risk of causing unseen effects. Designs created by this method are more compact than those that utilise a “P Cell” approach with large keep-out zones to avoid coupling.

RF SiP can be realized using a multitude of technologies. For each technology, different suppliers offer different materials, physical dispositions, and properties that require any design to be matched to the particular supplier.

Each technology and each supplier may be characterized by a technology file that describes the material parameters and physical disposition between the dielectric and metallic layers. In the case of organic and ceramic laminates, each supplier has a range of materials and layer structures that may be used. Figure 1 shows a typical technology file for LTCC.

Most SiP design methodologies that include integrated passive components rely on fixed libraries of components that are locked to a particular substrate supplier and stack-up. For high-volume consumer devices, it’s increasingly important to ensure that any given SiP can be sourced from at least two independent manufacturers.

Insight SiP’s methodology is based on a user-extendable library of mechanical objects; electrical models are created automatically for a given stack-up and/or technology. Thus, any design that’s initially made for a particular supplier can easily be re-tuned for an alternative source. The second manufacturer can have a different set of electromechanical parameters (stack-up, dielectric constant, layer thickness, loss factors, metal types) and may even use an alternative technology.

Once created, the mechanical objects of the first step above don’t have a direct link to the material properties, nor to the vertical stack of the particular technology and supplier that’s to be used. These objects can therefore be reused if a design is transferred between two suppliers. Figure 2 shows a typical LTCC mechanical object.

The second step of the methodology presented above is very easy to repeat with a new technology file, facilitating design transfer between suppliers and technologies. Figure 3 shows the schematic image of the LTCC coupled line resonator shown in Figure 2.

To re-design for an alternative supplier using the same technology, the process employs the same mechanical objects with a new set of technology values.

This process starts from the same basic schematic and simply re-optimizes the parameters of the objects to compensate for the new physical and mechanical parameters. The final EM closed-loop process, described in step three above, is then carried out to create new layout for the new supplier.

To change from one technology to another, equivalent mechanical objects with similar functions in both technologies must be created. In this case, the same basic schematic is used and the schematic/ mechanical objects are swapped. Thereafter, the process is similar to the normal design flow.

To illustrate how the methodology works, consider the transfer of a Bluetooth filter design between LTCC suppliers. A three-pole, one zero filter, designed for a Bluetooth module to fit under the active components, was developed using the methodology to operate with one LTTC foundry. The filter is designed using semi-distributed transmission line resonators with resonator coupling to create a suitable frequency zero in the response. Figure 4 compares the two LTCC stacks.

As seen, the stacks and material properties of the two suppliers are quite unalike. Despite this, the mechanical objects were converted from one stack to the other and the process completed rapidly. Figure 5 compares the completed EM-tuned filters for both stacks.

It can be noted that the two filters are quite similar but the physical dimensions of the filters are slightly different to compensate for the LTCC stack differences. Electrical performance for both realisations was similar.

The integrated antenna design uses the same methodology of combining electromagnetic simulations together with circuit-level simulation and optimization. A portion of the antenna has a schematic representation that can be simulated and optimized at the circuit level. This process helps circumvent lengthy parametric studies and significantly shortens the design cycle.

The methodology is used by Insight SiP to design antennas for various applications, including a quad-band GSM antenna as well as a 2.4GHz ISM band antenna. The methodology was also extended to implement a challenging UHF band antenna.

In the case of the 2.4GHz ISM module, the antenna was originally a printed trace wiggle antenna on a printed-circuit board. The antenna and ground plane measured 28.5 x 15 mm².

By applying the aforementioned technology, Insight SiP was able to integrate the module and antenna into an 8 x 12 mm² QFN-type package. In this case, the antenna takes advantage of the multilayered structure of the substrate in which it’s embedded; it has at least a top and a bottom wiring layer. A portion of the surface is reserved for the antenna, while the remaining area is used for active and passive component placement/routing and ground plane.

The most challenging parameter in designing integrated antennas is the environmental impact. The operating environment parameters include indoor, outdoor, building materials, high-rise buildings, factories, and major highways. So, a good amount of simulations and validations must be completed to optimize the antenna design.

In terms of wideband response for the antenna developed using the above methodology, the ground plane and manufacturing variations don’t significantly affect antenna performance. Figure 6 compares the simulated and measured S11 parameter for the 2.4GHz antenna-in-package.

Figure 7 shows the radiation pattern obtained with the antenna. The diagram, which plots the gain in dB of the antenna, illustrates the fact that antenna is indeed radiating in all directions of all planes.


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