**4.4 Assembling and weldering**

One of the easiest and better ways to ensemble properly the layers of the prototype is by using alignment rivets, screws, or dowel pins. The position of the alignment holes to pass the pins through is arbitrary as long as they are outside the sensible areas of the circuit, but need to be the same in all the circuit layers [13].

Taking the former into account, the process consists of stacking all the circuit layers and roll out soldering paste between layers. Once all the layers have been stacked, a heating process must be done to melt the paste and weld the different layers of the prototype.

The soldering paste can be rolled out manually, but an automatic dispenser will provide more repeatable results. The soldering paste must be set by small dots and evenly separated among them. If there is too much paste, it will overflow inside the cavity, increasing the losses of the prototype, and producing missmatches and undesired frequency shifts. On the other hand, if there is not enough soldering paste, this will produce a poorly welded device with a highly degraded performance. After having manufactured several prototypes, the technician will have the experience to know almost the exact amount of paste to be used for each kind of circuit.

The first option to align the layers could be to use alignment screws with nuts properly tightened to make the needed pressure all around the prototype and to weld the soldering paste in a reflow oven. However, pressure could be not evenly distributed on the welding area, as it depends on the number and distribution of the screws. Moreover, the pressure done by the screws directly over the prototype layer can externally damage the structure. To prevent this, press covers can be used.

Press covers consist of some additional non-metallized and non-adherent layers that are located above the top cover and below the bottom one to make a more uniform pressure along the prototype. If covers were metallized or adherented, they could be welded to the prototype due to an eventual overflow of soldering paste. Once the prototype has been properly soldered, press covers should be removed, as well as screws and dowel pins.

When the welding among layers is not good enough, the discontinuities between successive layers can result in small cavities. This effect, for structures such as ESIW, will cause the magnetic currents to modify their path by coinciding these cavities with their direction due to the TEM mode (Transverse Electro-Magnetic), degrading the response of the device and causing anomalies. In ESICL lines, however, the magnetic currents do not coincide with the possible discontinuities (cavities) produced; for this reason, welding is not necessary in these prototypes, and screws and nuts can assure a proper performance of the device, this assembling process being quicker due to the lack of welding process.

Rivets work similar to screws for alignment, being the pressure along the prototype uneven and not as tight as screws.

Dowel pins combined with a multipress usually produce better results as pressing machines press evenly the circuit. For this process, press covers (additional layers similar to those described to be used with screws) must be used to protect the circuit. If pressing machines do too much pressure on the circuit, this can lead to a bending of the covers deforming the inner cavities. To prevent this, different press covers adapted to the geometry of the welded area must be designed [14]. The same idea improves the results using screws and a reflow oven; however, whenever possible, press machines and dowel pins give much better results than screws and reflow oven.

Another thing to be taken into account is that the presence of soldering paste produces a little layer of a few microns that can enlarge/enheight the waveguide, with enough pressure while the paste melts this layer being reduced, and this effect can be negligible.
