Electrical performance evolution and fatigue mechanisms of silver-filled polymer ink under uniaxial cyclic stretch

Abstract

Flexible hybrid electronics rely upon compliant interconnects in order to maintain performance integrity in cases that require repeated elongation, including repeated stretching. A class of such conductive interconnects are composites of polymer with conductive particles that can be stretched at high strains without circuit failure. However, their fatigue response has so far remained largely unexplored and is essential prior to using in health monitoring applications. In this research, a stretchable silver-filled conductor is evaluated under high-strain cycling. In-situ techniques, including four-point resistance measurement and laser profilometry, are used to correlate changes in electrical performance to the fatigue response. Surface crack formation is extensive upon stretching during the first loading cycle, forming a heavily interconnected crack network at higher strains that does not immediately result in open circuit failure. Resistance increase with cycling is attributed to a gradual deepening of these cracks until their depths approach the film thickness, eventually leading to electrical failure. Fatigue life, the number of cycles required to reach a predetermined electrical performance limit, is shown to be most influenced by the applied strain amplitude. Using a normalized resistance increase limit of R/R 0 = 500, it is found that 500 µm wide conductive lines endure 23 cycles at 35% strain amplitude, but this becomes over 500 cycles when the amplitude is dropped to 5%. Sensitivity to mean strain, ε m , is relevant to strain amplitudes below 15%. In this manner, a composite conductor was shown to exhibit crack evolution behavior distinctly different from homogeneous metallic films.

Understanding resistance increase in composite inks under monotonic and cyclic stretching

“…Therefore, we define the fatigue life N f by the number of cycles until a R/R 0 of 100 is reached. In a previous work by Cahn et al [47], fatigue life was instead defined by the number of cycles until a R/R 0 of 500 is reached.…”

Modeling resistance increase in a composite ink under cyclic loading

“…In wearable device applications, repeated elongation (stretching) of up to 30% strain is typically expected [12]. Various conductive interconnect materials have been studied for their electrical performance under repeated deformation, including thin metal films [13][14][15][16][17], metal nanoparticle inks [18][19][20][21], and metalpolymer composite inks [22][23][24][25][26][27][28]. One class of metalpolymer composite ink (conductive ink) consists of micron-sized metal flakes embedded in a compliant polymer binder material.…”

“…One way to characterize fatigue of a conductive ink specimen is by defining a fatigue life N f as the number of cycles until a specified normalized resistance R/R 0 (with respect to the initial resistance R 0 ) is reached, and measuring N f for different strain amplitudes. Previous work on the PE 874 ink employed critical normalized values of 500 [22] or 100 [28] to define N f . In each case, a strain amplitude ε a − fatigue life N f curve is obtained [22,28].…”

“…The model is first applied to the 2 mm-widespecimens fatigue data from Li et al [28]. Additional fatigue data are also presented and modeled for a narrower, 1 mm-ink trace width, in order to further assess the effect of trace width on the fatigue behavior [22,28]. An experimental procedure for obtaining the (R/R 0 ) ′ -ε a curve using only one specimen (by performing a series of interrupted cyclic tests at progressively higher ε a ) is also investigated and compared to the procedure where a different specimen is used for each ε a .…”

“…Previous work on the PE 874 ink employed critical normalized values of 500 [22] or 100 [28] to define N f . In each case, a strain amplitude ε a − fatigue life N f curve is obtained [22,28]. The advantage of these curves is that they provide estimates N f for any given ε a , using empirical fits relating N f to ε a from discrete experimental datasets.…”