A Systematic On-wafer Characterization Technique 
for Surface-Mounted Microwave and RF Packages

The rapid increase in processing speeds of today’s digital circuits demands effective, low-cost and more efficient electronic devices. One of the solutions for low-cost packaging is to use surface-mountable plastic packages. Such packages have inevitable parasitic elements. To ensure good packaging, the development of characterisation techniques for surface mounted packages is necessary so as to predict the parasitic behaviour at microwave and RF frequencies. There is very little information in the literature concerning characterising multi-conductor leads, most especially, the coupling effect between near neighbouring pins of microwave and RF packages. The conventional method of characterising coupled lines requires establishing a common voltage or ground. This common voltage is difficult to achieve for package characterisation at microwave frequencies. As such, a more reliable and systematic technique is required.

We have developed a new method for the characterisation of surface mountable plastic TSSOP packages, using air-coplanar probes and Through-Reflect-Line (TRL) multi-line calibration concepts. The method, which is more accurate than the conventional technique of odd and even measurement, is used to characterise the inter-couplings of the adjacent pins (near neighbouring pins) of the package. Applying this method, surface-mountable packages can be accurately characterised and modelled in the giga-hertz regime. This method was demonstrated by applying it to the characterisation of the TSSOP 16 package (16 pin RF package) at microwave frequencies. To perform this experiment we had to develop novel interconnects that interface the measurement instrumentation to the TSSOP 16 package.

(a)                                                                            (b)

Figure 1: The geometry of two variants of the proposed interconnects.

The novel interconnects we developed are shown in Figure 1. These enable a proper transition between the package pins and the wafer-probes. A Taconic substrate with a dielectric constant of 10.1 was used. The interconnects were designed to make a smooth transition between the pitch of 250 mm at wafer probe tips to a pitch of 11 mils at the package pins. These structures also provide a transition from coplanar waveguide (at the wafer-probe end) to microstripline (at the package pin end). The interconnect lengths are quarter-wavelength in the middle of the measurement band so as to avoid the unwanted variations in measurements. These structures consist of two parts; namely the coupled line interconnects, where the package pins sit, and the ground patch to which a common grounding of the neighbouring pins are provided. The ground patch also serves as a thermal path from the heat slug of the package to ground. All these structures were designed using the HP-MDS and IE3D, Zealand software for 2D planar structures.

To collect the scattering parameters, a HP-8510C Vector Network Analyser along with a probe-station and air-coplanar wafer probes were used. The network analyser was calibrated using the proposed characterisation algorithm over the measurement bandwidth of 1 GHz to 6 GHz. The electrical standards were fabricated using photolithography techniques so as to remove repeatable errors caused by the connectors and the probing contact. The package to be characterised was mounted on the designed interconnects using solder cream. The consistency of the algorithm was verified by performing measurements on a known load.