In a module the cells are usually arranged in series. For that, tinned copper ribbons (tabs) are used to connect the front of one cell to the rear side of the adjacent one (Figure 7.16a). In such a way strings of typically 9-12 series-connected cells are formed.
It has to be noted that tabs must overlap a long distance along the busbar length since the conductance of the printed busbars is too low. Two (or three for larger cells) tabs per cell are employed, thus providing redundancy which allows current to flow in case electrical continuity is broken due to some failure . Besides, the effective length of grid fingers is one-fourth (or one-sixth) of the cell side, and series resistance is alleviated. Tabs provide a non-rigid link between cells, allowing thermal expansions to be accommodated.
In the past, stringing was done in a two-step process: tabs were first soldered to the front side, followed by the soldering to the rear side of another cell. However, it has to be taken into account that during the last few years wafer thickness has been decreased to below 200 |im. In this case a large bowing of the solar cells during the soldering due to the difference in thermal expansion coefficient between silicon and copper is produced (Figure 7.17a). This bowing would cause, above all, an increased breakage during the subsequent rear soldering and during lamination step. In order to overcome this problem the attachment of the tabs (front and rear) on the same cell is done simultaneously. Nevertheless, thermal stress induced during the heating and cooling would lead to microcracks (Figure 7.17b), which degrades module performance after some time in the field.
Conductive epoxies or low-temperature solder alloys  can replace conventional ones and illumination or induction is widely used instead of former iron heating, in order to reduce the stress during the stringing process.
Several strings are interconnected using auxiliary ribbons or a printed circuit board (PCB) to form the cell matrix. Typically it consists of a single series string (Figure 7.16b) although several strings internally paralleled is also a possibility.
Traditionally, a typical module configuration used 36 series-connected cells, which, under operating conditions, would produce around 15 V at maximum power, appropriate for 12 V battery charging . As grid-connected applications and, to a less extent building-integrated systems, grow, modules with different electrical configurations enter the market, so that the standard now is 72 cells of 125x125cm2 and 60 cells of 156 156cm2.