Today’s fast-growing electronic market demands the development of cost-efficient, customizable yet mass-producible and environmentally friendly electrical components. Most additive manufacturing methods, particularly inkjet printing, have the potential to meet these requirements, provided that eco-friendly materials are used [1
]. The additive manufacturing of 2.5D (the expansion in one of three main spatial directions is much smaller) electronics structures is commonly referred to as printed electronics (PE). Such PE components, in particular sensors, are often low-cost, flexible and planar, which favours them for the integration in different types of materials and wearables [2
]. Owing to the special nature of PE, a multitude of new application possibilities in a variety of different industrial sectors have arisen. Amongst others, sporting goods manufacturers have started integrating printed sensors in products like clothes, shoes and others, wherein the main focus lies on the live recording of vital parameters, such as the heart rate and body temperature [7
], as well as antennas for wireless readout [9
]. Flexible sensors are also particularly suitable for integration into various components during manufacturing in order to be able to monitor fabrication process conditions [12
] and for structural health monitoring [14
Even though printed electronics technologies bear huge potential for various applications, it is neither feasible nor reasonable to replace all components of an entire silicon-based electronic system with printed parts. As for today, many fully printed electrical devices, such as thin film transistors [17
], inductors [18
], capacitors [19
], memory [20
] and batteries [21
], cannot yet keep up with conventional silicon components regarding performance and yield. Furthermore, the fully additive manufacturing of complete microcontrollers providing data read outs, analysis and transmission will neither be feasible nor reasonable in the foreseeable future. Although the manufacturing of printed electronics is considered as cost-efficient, the cost per function is higher compared to silicon electronics [22
]. Hence, printed components need to be connected and wired to external electronic parts and microcontrollers, resulting in hybrid electronics systems [23
]. Additionally, hybrid integration involving different 3D-printed materials might be favored [28
]. Consequently, there are various challenges regarding the electrical connection strategies. As an example, the connection needs to be established without damaging the printed layer or the substrate, while the contact resistance needs to be low and reproducible. For flexible substrates, the overall flexibility must be retained without any performance loss. Furthermore, mechanical stability and reliability is required, as well as a low thickness. In the research, printed and flexible electronic components are often mechanically connected using conventional crocodile clamps for laboratory tests, crimp connectors [32
] and low/zero insertion force (LIF/ZIF) connectors [34
]. For the hybrid integration of SMDs, classic lead-based soldering or bonding with conductive adhesives has been reported. As an example, Li et al. [36
] demonstrated the successful direct soldering of SMDs (surface mound devices) in screen-printed silver patterns on paper substrate using Sn42/Bi57.6/Ag0.4 low-temperature solder paste. On the other hand, solder bonding on polyimide was shown to be insufficient. Commercially available conductive adhesives are commonly applied [37
] and considered as quick and easy alternatives to soldering for low current devices on sensitive substrates, as they can be deposited and cured at room temperature [39
]. They typically consist of a conductive component, such as silver, nickel or copper, and an adhesive, like varnish, resin or silicone [41
One challenge when connecting PE applications arises from the large variety of substrate materials used. Consequently, there are many different requirements for the electrical bonding method, such as the process temperature, the desired flexibility, the conductivity and the adhesive strength. For many applications, such as resistive sensing elements, it is crucial that the connection does not only provide good conductivity but also has high reproducibility [43
]. Furthermore, especially in inkjet printing, the resulting layer is only a few µm thick, or even lies in the sub-µm range [1
], which can be considered as quite thin compared to other frequently employed printing techniques, such as screen printing (typical layer thickness: beyond 10 µm [47
]). This fact creates new challenges, as the printed layer might be easily damaged during contacting, either due to the high temperature when soldering or due to mechanical forces. Consequently, it is essential to investigate the applicability of different connection strategies on various substrates. The relevance of this topic in the context of flexible and printed electronics is emphasized by a large amount of scientific work having been conducted in this field for more than a decade, mainly focussing on conductive adhesives and soldering [48
]. The particular focus of the present work lies on providing a practical guide comprising standard techniques that are cost-efficient, easily implementable and frequently used. For this purpose, the electrical and mechanical properties of a set of printed and electrically bonded test structures were determined to provide an overview of possible integration methods. The samples were characterized before and after contacting. Subsequently, the performance of the junctions and possible quality degradations under material-stressing ambient conditions were investigated. More specifically, damp-heat testing was applied by storing the samples at 85 °C and 85% relative humidity (rH) for 140 h. This damp-heat test has a long tradition in the reliability testing of silicon electronics, such as photovoltaics [51
], and electrically conductive adhesive joints [53
]. For the evaluation of the mechanical stability, a destructive tensile test was performed. Therefore, the maximum applicable force until the samples’ connections failed before and after damp-heat treatment was determined using a tensile test device. In a recent study, Neff et al. [55
] thoroughly studied the performance of conductive epoxy and low temperature solder for hybrid electronics in harsh environments, employing wire bond pull testing as well as high acceleration drop tower testing. In contrast to the present work, they focussed on the evaluation of one type of substrate while their micro-dispensed layer was notably thicker than the inkjet-printed structures. Hence, one distinctive feature of the present work is the investigation of the performance on several different plastic and paper-based substrates. In addition, damp-heat testing is presented as a new approach for the assessment of the reliability of bonding strategies for hybrid electronics. Generally, much of the previous work in this field has focussed on adhesive bonding and soldering. As part of this paper, the properties of purely mechanical contacting, namely crimping, and its performance compared to more established contacting methods are evaluated for the first time.
2. Materials and Methods
A total of 15 samples on 5 different substrates were prepared using inkjet printing of silver (Ag) nanoparticle ink (Sicrys 115-TM119, PV Nanocell, Migdal HaEmek, Israel) which is composed of 38–52% silver nanoparticles (average particle size ~100 nm) dissolved in Triethylene glycol monomethyl ether (TGME). The substrates are (1) commercially available uncoated paper (in the following referred to as type 4 paper substrate, Mondi AG, Austria), (2) Mylar® A FI13010 PET electric insulating foil, (3) Kapton® HN300 polyimide foil (DuPont, USA), (4) p_e:smart® type 2 (Felix Schoeller Group, Osnabrück, Germany) and (5) Kemafoil® white PET (HSPL 80 W 75, Coveme, Bologna, Italy).
The test structures consist of simple rectangular strips with a dimension of 3 × 30 mm, as illustrated in Figure 1
Two layers of ink were printed on each sample using a PIXDRO LP50 (Meyer Burger Technology AG, Thun, Netherlands) system with a Spectra SE-128 AA 128 (Fujifilm Dimatix Inc., Santa Clara, CA, USA) 30 pL print head at a resolution of 600 × 600 dpi. Before printing the second layer the first had to sufficiently dry to obtain a homogenous surface and avoid spreading of ink (which depends on the cohesive forces of the ink and the adhesive forces between the ink and the substrate) [56
]. To promote the evaporation of solvents and hence accelerate the drying process, the substrate table was heated to 57 °C. Subsequently, the samples were sintered in order to achieve formation of a compact, homogeneous layer. For the type 4 and p_e:smart®
paper substrates, photonic curing (PulseForge 1200, Novacentrix, Austin, TX, USA) with an overall sintering energy of 2.1 J/cm2
and 1 J/cm2
was employed, respectively. The used photonic curing parameters were obtained by iteratively approaching optimum values. The other samples were sintered thermally in an oven. The employed sintering parameters are summarized in Table 1
The electrical properties of the samples were determined using an LCR meter (measurement device for measuring inductance L, capacitance C, resistance R) (1V DC, Wayne Kerr (Iserlohn, Germany) Precision Analyzer 6440B) and statistically analyzed before and after contacting the samples with enamelled copper wires (Ø 120 µm). In this way the influence of the junctions on the electrical properties, such as the conductivity, could be determined.
For the contacting, four different possibilities have been investigated: (a) direct soldering on the inkjet-printed structures using standard solder (Sn62Pb36Ag2, 2.5% 1.1.2.B flux, Stannol, Velbert, Germany) at a temperature of 265 °C, (b) screen printing Ag pads (Novacentrix HPS-FG32 Ag screen ink) and subsequent soldering using commercial standard solder (Sn62Pb36Ag2, 2.5% 1.1.2.B flux, Stannol) at 265 °C, (c) adhesive bonding with a commercially available conductive polyurethane glue (Polytec PT (Karlsbad, Germany) PU 1000) then cured at 60 °C for 1 h in an oven and (d) mechanical connection using customary crimps for flexible printed electronics (Memcon (Kiel, Germany) Short Male Contacts).
For the damp-heat the samples were exposed to 85% relative humidity at 85 °C for 140 h inside of a climate chamber. Again, the junctions’ conductivities were measured before and after the damp-heat test.
To determine the mechanical stability of the individual joints, destructive tensile testing using a digital pressure and tensile force measurement device (Sauter (Metzingen, Germany) FK50) with peak-hold-function was conducted. The measurement device was designed for measuring forces up to 50 N at a resolution of 0.02 N. For the tensile testing, the sample was fastened to the rigidly attached measurement device, as illustrated in Figure 2
. The sample was pulled downwards vertically with continuously increasing force until the connection broke. The maximum applied force before the connection failed was recorded by the device’s peak-hold function. One sample set was tested right after contacting while the second set consisted of the samples that were exposed to the damp-heat test before.
Direct soldering led to comparatively poor results on the polymer-based substrates, i.e., a conductive connection could only be established on the paper substrates (type 4 paper and p_e:smart®
), yet with some restrictions. The advantage of direct soldering is that no curing is required, and the connection can be established within one single processing step. However, inkjet-printed layers have a comparatively low thickness, in the range of a few micrometres, therefore the heat is basically directly transferred to the substrate leading to ablation of the printed layer. This might be attributed to the different thermal expansion coefficients between electrode and substrate. Upon rapid heating, the material expansions are different, resulting in stress that leads to delamination. Although a connection could be established for some samples, the direct soldering requires advanced soldering skills as the substrate should not be touched with the soldering iron for too long. Usually, in hand-soldering, temperatures above 350 °C and contact durations of a few seconds are desired to facilitate the evaporation and decomposition of the corrosive ingredients of the fluxing agent [59
], which cannot be done when working with sensitive specimens. Furthermore, it is not feasible to clean the residual soldering flux off without damaging the printed structure, hence no cleaning was done. The fluxing agent of the used solder contains halogen, which is known to promote corrosion [59
]. After the damp-heat test, dissolving of the printed layer could be observed in many cases; furthermore, the adhesion decreased. The associated visually observable discoloration of the residual flux indicates that chemical and structural changes have indeed taken place. The practicability and performance of flux-free soldering has to be investigated, as part of future works.
In contrast to that, the soldering on screen-printed Ag pads has proven to be comparatively stable on most substrates except for the uncoated type 4 paper substrate. This behavior was expected, as successful soldering on screen printed structures has been reported in the literature [36
]. The ablation of the soldered screen-printed contacts on the type 4 paper substrate after damp-heat testing might be explained by the poor adhesion of the screen-printed layer on the inkjet-printed layer, while a vast majority of the inkjet-printed pattern remains attached to the paper substrate, as illustrated in Figure 7
b. This can be due to fiber swelling effects [61
], where the uncoated fibrous substrate absorbs a lot of humidity, leading to an expansion of individual fibres. Again, this condition can lead to the alteration of the ink-substrate interplay and a drastic reduction in the adhesion to the substrate. Furthermore, at the edges of the screen-printed pads the corrosive nature of the soldering flux combined with high temperatures can lead to destruction of the inkjet-printed Ag layer (see Figure 7
b). On the other hand, considering the damp-heat as well as the tensile testing on p_e:smart®
PET, the best connection could be achieved with soldering on Ag pads. Here, the contact resistances were low, while providing superior mechanical durability.
Purely mechanical contacting using customary crimps for printed electronics applications led to the worst results on all substrates under test. As the crimp connections are established under application of comparatively high mechanical forces, the printed layers tend to be damaged during processing. Furthermore, the crimps resulted in rigid and relatively thick connections, which is unfavourable for flexible 2.5D electronics applications. The handling of the crimps turned out to be quite difficult from a usability point of view, yet special tools might simplify the use of this contacting method.
The highest versatility and overall stability could be achieved by employing adhesive bonding. Although the connections using conductive glue resulted in the greatest contact resistances, the connections remained stable after damp-heat testing and showed superior adhesion on all but one substrate under test. This exception is Mylar®
PET, where the mechanical stability was only mediocre from the beginning, and the adhesion dramatically decreased after the damp-heat testing as the subsequent tensile test revealed. The optical microscopy images indicate that before damp-heat testing, the printed layer remained adherent to the substrate (see Figure 7
c), while after the damp-heat test the contact ablated together with the printed Ag layer (see Figure 7
d). From a usability point of view, it is quite challenging to apply adhesive bonding without using specialized expensive tools. As a result, the joints differ in size and form, reducing the overall reproducibility of the process (see Figure 3
b). Additionally, to improve adhesion, the electrodes should be cleaned before applying the conductive glue. For printed electrodes this is not possible, as cleaning agents might damage the printed layers. On the paper-based substrates, cleaning liquids would be absorbed and consequently lead to fiber swelling. As cleaning is obviously not trivial in this case, careful handling of the samples and a clean working routine is required to avoid contamination in advance. Another point to consider is the adhesive bonding’s low electrical stability during movement. Even minor displacements of the wires resulted in non-negligible distortions of the measurement results, which needs to be taken into account when the flexibility and wearability of hybrid electronics are in demand.
The electrical characterization of the inkjet-printed samples revealed that for the polymer-based substrates post-curing or re-sintering effects during damp-heat testing occurred. As those samples were initially sintered at much higher temperatures than 85 °C, the decrease in resistance must rather be attributed to the storage at high humidity or to a certain combination of both conditions. Similarly, such a humidity sintering effect has been observed by Andersson et al. [62
], who exploited this effect for the development of a resistive humidity sensor with memory effect. In a further study, they observed that this behavior can most likely be attributed to certain ions in the substrate’s coating [63
]. The presence of silver halides on the nanoparticles’ surfaces might reduce the conductivity while increasing the required sintering energy of the printed layer. Chloride and bromide ions can precipitate silver ions, resulting in an overall decrease in the resistance, as also reported by Tang et al. [64
]. However, in the present paper the used polymer-based substrates were all uncoated. Even Mylar®
PET, on which this effect was most pronounced, was untreated. The climate chamber is operated with demineralized water; therefore, it is unlikely that this effect might be attributed to the presence of ions in the environment. Hence, the observed low temperature–high humidity sintering effect needs to be studied more thoroughly as part of future works. For the paper-based substrates, the median and mean resistance values remained stable, yet the scattering of the values increased. This can be attributed to the fiber swelling effect [61
]. As fibrous paper substrates consist of purely statistically oriented fibers, this effect varies to different degrees for the individual samples, resulting in a broader distribution of the resistance values [65