**4.7 External elements**

External elements, such as wires and rods, are very useful in order to design transitions [10, 17] or filters [18]. In the case of transitions, wires are used to connect the layers of the prototype (rods can also be used if they are small enough). On the other hand, rods are used as filter resonators.

Two factors are essential when wires and rods are used in the prototypes: their position and shape. Wires must be stretched as much as possible in order to reduce deformations and foldings of itself. On the other hand, the easiest way to ensure the position is through a hole. To assure a good electrical contact in the weldering, the hole must be fully metallized, a galvanic or a through-hole plating metallization is recommended for that. The metallization must be taking into account, as the hole will narrow some microns. If a laser cutting machine is used, the diameter of this hole must be found out by testing different diameters in order to take into account the laser error due to the non-perpendicular hole and the reduction of the hole due to the metallization process, as it was explained in Section 4.3. **Figure 20** shows an example of this testing; in each column each hole is increased by a few microns regarding the previous one.

After the wire is crossed through the hole, it has to be welded to the hole with soldering paste. It must be done trying to minimize the residues of this paste. Once

**Figure 20.** *Swept hole diameter.*

this wire is welded, the spare wire must be removed by cutting it. Then, depending on the structure of the prototype, the other end of the wire can be welded to its own hole. While crossing the wire through the whole, if the hole is very tight, metallization can be detached as the wire pushes it away. Therefore, electrical contact would be spoiled, so that it must be done carefully. If metallization is taken away, performance of the prototype will degrade.

There are other considerations to take into account. If there are more welding phases after weldering the wire, a soldering paste with higher melting point must be used to weld the wire to avoid de-soldering in the following processes which will have lower melting points.

The higher the conductivity of the wire is, the better the results will be. However, due to the small size of these elements in microwave devices, conductors such as brass or tinned copper are enough to achieve good results.

When possible, wires should be replaced by rods. In case wires were used, they must be stretched in order to reduce foldings and deformations.
