Thermal expansion is another important temperature effect which must be taken into account when modules are designed.
The spacing between cells tries to increase an amount δ given by:
αG and αC are the expansion coefficients of the glass and the cell respectively;
D is the cell width; and
C is the cell centre to centre distance.
Typically, interconnections between cells are looped, as shown, to minimise cyclic stress, though the loop may not be very obvious and should not include any bend points that could turn into failure points. The connection to the cell is usually set back from the edge of the cell to allow a longer piece of wire to accommodate the straing.
Double interconnects were introduced to protect against the probability of fatigue failure caused by such stress. Modern cells use many interconnects as the number of interconnects increased from 2, to 3, to 4, to 5, to 12, etc. Using a larger number of thinner wires reduces the stress at each connection point, reducing the probability of failure.
In addition to interconnect stresses, all module interfaces are subject to temperature-related cyclic stress which may eventually lead to delamination.