Printed circuit board design

For the first prototype, a plug and measure configuration has been chosen, considering that is not so comfortable for the end user to perform some bonding (or many, depending on the size of the array) between the sample and a carrier in a user friendly device.

Some possible layouts have been studied and evaluated in terms of number of connections available, reliability during usage and the eventual cross talk between the conductive components during voltage or current applications.

Figure 5.1: Example of a layout made in Eagle CAD and the corresponding PCB already realized to fit a Hirose 20 pins connector.

Considering the past experiences inside the research group, first layout designed through the software Autodesk Eagle CAD and bought from the company Eurocircuit was based on the connector Hirose FX2-40S-1.27DSL (71), which has 20 pins for side and 1.27 𝑚𝑚 of pitch as shown in Figure 5.1. The board has a size of 67 𝑚𝑚 (width) x 70 𝑚𝑚 (height) x 1.5 𝑚𝑚 (thickness) and consists of two copper layers (top-red and bottom-blue) for the 400 𝜇𝑚 thick wires, obtained through etching by conductive plates and insulated by FR4, which is present between the conductive layers but also covers the system. The internal connections are made through vias, circular holes encased with copper, where it’s possible to solder electrical components on the board. 20 vias with internal diameter of 600 𝜇𝑚 are employed for the

81 through hole mounting of Hirose FX2-40S-1.27DSL (71), disposed according to its datasheet, followed by its soldering to fix the component, while two arrays of vias’ pair with a pitch of 6 mm running near the board edges are employed for mounting 20 female connectors to put in electrical contact Hirose pins with 2 mm plugs cables, able to interface the system with most of measurement instrumentation through some adapters (4 mm plugs, BNC, SMA etc.). The electrical resistance of any wire is similar (0.3 − 0.5 Ω), so they should not affect the signal transmission.

Thus, the sample should show pads with same pitch and width (635 𝜇𝑚) to properly interface with the printed circuit board. First characterizations were performed with this kind of interface, which demonstrated good versatility since it was used by the group also for other experiments with different transducers, for example based on Electrochemical Impedance Spectroscopy.

With the aim of optimizing and increasing the sensors density, after a deep market exploitation it was decided to develop a further interface based on the connector Samtec ERF5-050-01-L-D-RA-TR, which has the advantage of a smaller pitch (10 𝜇𝑚) and pads width (500 𝜇𝑚);

furthermore it has 50 pins for side. The disadvantages of this connector were the poor robustness with respect to the Hirose and connection to the PCB as Surface Mount Device (SMD), which imply a more complex procedure of bonding than the through hole configuration, since it was necessary to perform 50 micro soldering to fix properly the connector, avoiding contact vibrating noise that should affect the measurements in case of absence of mechanical stability of the system.

Figure 5.2: Electric field (a) and capacitive matrix (b) for the PCB fitting the connector Samtec ERF5.

Color scale reported on the right side is in pF and it’s possible to notice the very poor interaction between wires farer away than the first neighbors.

(a) (b)

82 Since in this case it should be necessary to design longer and closer wires on the PCB, considering the number of contacts, preliminary calculations were performed before sending the layout shown in Figure 5.2 (a) to Eurocircuits for fabrication. Specifically, the planar design consists of wires with 254 𝜇𝑚 of width and 12 𝜇𝑚 of thickness which run parallel one to the other along the longer dimension, having the distance of 500 𝜇𝑚 center-to-center, before deviating to perpendicular direction through two 45° meanders.

Regarding calculations, they were performed in a Finite Elements Method framework by employing the software COMSOL Multiphysics, evaluating the capacitive coupling between adjacent wires with excitation comparable with the measurement parameters, namely an applied voltage of several Volts between the investigated pad and ground.

The results are summarized in the capacitance matrix reported in Figure 5.2 (b). Notably, only the first neighbors and partially the second ones show a capacitive interaction but, in any case the coupling between any pair of electrodes should be at most of few 𝑝𝐹, so the design was ordered and fabricated. To confirm the calculations, a series of measurements were performed with the LCR meter Agilent E4980A on the purchased board in similar conditions for the excitation to the ones employed during calculation, with the only difference of the bonded Samtec connector on the pads in the designated area, obtaining capacitances of the same order of magnitude with exact values depending on pad’s length as expected.

Figure 5.3: Capacitance Vs frequency measurements performed on the PCB interface after bonding Samtec ERF5 connector letting the unused pad to be floating.

83 The measurement results of capacitance Vs frequency are shown in Figure 5.3, where it’s possible to notice the capacitance sharp decreasing starting from low frequencies and until several hundreds of Hz and the following slow change or saturation moving up to 10 𝑘𝐻𝑧. The points were chosen in logarithmic progression to be sure that any order of magnitude investigated has the same number of measurements. For lower frequencies, the environmental noise became not negligible, leading us to obtain non reproducible capacitance measurements.

To mimic more precisely the experimental conditions planned, we prepared a mock sample with 50 pads made by a bilayer 𝐶𝑟/𝐴u, which start from the edge and arrive, after several shrinking and 45° deviation, at a distance of 20 𝜇𝑚 between first neighbors, as it could be for the connection of PHE based sensors. The results, reported in Figure 5.4, show a two order of magnitude increasing in the capacitive coupling between adjacent pads but similar behavior with respect to frequency change.

Figure 5.4: Capacitance Vs frequency measurements performed on the PCB interface after bonding Samtec ERF5 connector and plugging a mock sample to mimic the real device coupling.

Then, we considered the opportunity to design an array of magnetoresistive structures linked one to the other for three aims. The first one regarded the goal of measuring the diffusion profile of magnetic particles in solution above the MR structures with as higher resolution as possible and the second one to feed them with a unique current coming from the same power supply.

The third consequence is the ohmic connection between the wires at one extremal, reducing the capacitive behavior. So, we performed another mock sample with a 20 𝜇𝑚 width wire

84 connecting the terminals with same pitch as in the previous case and measured the phase shift between the excitation voltage and the resulting current, which show a pure resistive behavior with a magnitude of several Ω. This value is negligible respect to the typical resistance of GMR and TMR based sensors, but it can affect PHE measurements with a shift in the signal to be measured depending also upon the different length of the connecting on-chip terminals.

After defining the interface, some samples were realized with the pads layout shown in Figure 5.5 (a), ready to be plugged and measured with this kind of PCB. In this way, we obtained good results in term of stability and reproducibility of the measurements.

Figure 5.5: Sample with an array of PHE based sensors (a) and interface with the chip plugged in and ready to be measured (b)

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