On the integration of flexible circuit boards with hot embossed thermoplastic structures for actuator purposes

Abstract

In this work, the integration of flexible printed circuit boards (FPCB) on thermoplastic structures has been studied for the first time. The process was developed with the objective to provide thermal paraffin-based microactuators with local heaters. Essentially, such actuators consist of rigid cavities sealed with flexible membranes deflecting on the melting of enclosed paraffin. Following this concept, 100 μm deep and 2.5 mm wide, circular cavities were fabricated by means of hot embossing of polycarbonate (PC) and joined thermally and by gluing to FPCB or blank polyimide (PI). The bond strength was measured by cavity bursting. The adhesion between PC and PI with thermal bonding was too low to allow any mechanical post processing, whereas gluing facilitated satisfactory joining. Here, the bond strength, measured with cavity bursting was found to depend heavily on the curing conditions. For instance, the use of an intensive UV-exposure could increase the adhesion with a 100% compared to curing with an ordinary UV-lamp. Investigation of channel cross-sections revealed an overall glue thickness of 2–15 μm and that only a minor amount of glue migrated into the channels.

Two embossing tools of different resilience were investigated for the embossing of the microstructures. A polydimetylsiloxane (PDMS) mould replicated from SU-8-patterned silicon was compared to a more conventional nickel mould replicated from dry-etched silicon. The embossed samples were inspected in polarised light and it was found that PDMS embossed samples contained no interference fringes. This indicated that ridges, otherwise occurring just outside the cavities, were eliminated in these samples. Electron spectroscopy for chemical analysis (ESCA) revealed a slight difference in silicon contents between surfaces resulting from the two moulds. The nickel embossed surfaces were essentially free from silicon, whereas the PDMS embossed surfaces typically contained a significant concentration of silicon.

A couple of actuators with FPCB joined to PDMS embossed PC cavities were fabricated using the developed process. These devices facilitated heating far beyond the melting point of paraffin but failed through paraffin leakage at the bond interface after a small number of activation cycles.

Introduction

The interest for chemical and biological processes on microfluidic chips has increased in recent years. For such devices, polymers materials offer several advantages. One is the low material cost, especially in the case of thermoplastics. Another important advantage is the possibility of inexpensive replication of microstructures, allowing large-scale fabrication of disposable microchips, for e.g. point-of-care applications [1].

The ability to control liquid flows by means of pumps and valves is essential in many of these applications, and it is particularly advantageous if this control can be performed integrated, i.e. on chip. It is also preferable if the flow-controlling units can be fabricated in the same steps as the flow channels and other passive structures. Thermal actuators based on the expansion of paraffin seem a candidate in this respect. Paraffins undergo a large volume expansion upon melting, typically higher than 10%. They are inexpensive and easily tailored for various temperatures of operation. Also, paraffins are chemically inert and non-toxic.

In a previous work [2], a paraffin actuator was made through hot embossing of polycarbonate (PC)—a simple and inexpensive replication method suitable for small series. This actuator was filled with paraffin after covering the reservoirs with a thermally bonded PC membrane. It was shown that paraffin actuators could be made from thermoplastics with the same process used for the fabrication of microfluidic channels. However, for thermal actuators integrated in a microchip, it is vital that the paraffin containers can be heated locally. Also, it is favourable if the heaters can be made with lithography-based methods from easily available materials.

A group of materials that fulfils these requirements is the flexible printed circuit boards—in this work referred to as FPCBs. An FPCB consists of a thin foil of polyester or polyimide (PI) covered with a metal film (usually copper) on one or both sides. The good adhesion between the metallic layer and the substrate in combination with the small foil thickness make FPCBs mechanically flexible. In addition, if PI is used for substrate material, the circuit board can operate at high temperatures. Methods and chemicals for patterning of FPCBs are industrially available and widely employed in consumer electronics [3], [4]. The use of FPCBs as a membrane material in paraffin-based actuators has been demonstrated elsewhere [5] but not together with PC or true replication technology.

A method for replication of plastics [6], [7] suitable for relatively small series is hot embossing. With this, microstructures have been replicated to a large extent in poly(methyl methacrylate) (PMMA) [8], [9], [10], but also in e.g. PC [2], [10], [11], [12], [13], polyethylene (PE) [13], polystyrene (PS) [14], [15], copolyester [15], [16], Zeonor [17], styrenebutadien-block-copolymers [18] and poly(ether-ester) [19]. The moulds have been fabricated in various ways. The perhaps most straightforward method is micromilling of steel [20], aluminium [20] and brass [21]. This, however, drastically limits the feature size.

For prototypes or really small series, microchannels with semicircular cross-sections have been made through embossing of the plastic with thin metal wires [8], [22] or fused silica capillaries [23]. Unfortunately, even modestly complex structures, such as channel crossings are difficult to fabricate, since wires on top of each other are required [22].

For larger series, higher resolution or precision and full in-plane geometrical freedom, moulds may also be fabricated using lithographic techniques. Wafers of wet-etched silicon [8], dry-etched silicon [17] and wet-etched quartz [24], as well as SU-8 spun on silicon [12] have been used as moulds.

It is also possible to use the microstructured wafer as a master for the embossing tool. Nickel moulds, for instance, have been made with electroplating from different substrates, such as silicon [2], [10], [25], glass [26] and poly(dimethylsiloxane) (PDMS) [27]. Using PMMA patterned with X-ray lithography as a master [28], smooth channel walls simplifying the de-embossing step are achieved. However, there is limited access to the synchrotron radiation facilities required. Moulds in other materials, such as PDMS [29], polyester [9] and films of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) [18] have been made through casting or compression moulding from SU-8 [29], PDMS [9] and silicon [18].

In order to seal the channels or cavities, the chip must be joined to a coverlid or a film. In some fluidic applications, mechanical clamping and a gasket is sufficient for sealing [11], [30]. In low-pressure devices when chip and/or coverlid are made of PDMS, clamping may not even be necessary [16], [20]. An advantage with reversible sealings is the possibility of cleaning of the microstructures between experiments.

In many applications (e.g. high pressure ones), however, a more permanent sealing is necessary. Thermoplastic microstructures can be sealed with thermal bonding, where the chip and film/lid are subjected to pressure and heated to above their glass transition temperature. The same method can be applied to microchips in semi-cured thermo-sets, allowing chip and lid to fully cure in contact with each other [9]. Thermal bonding is most commonly used with PMMA [31], [32], but microchips in PC [2], [13], [33], [34], Zeonor [17], acrylic [15], PS [15], PE [13] and copolyester [15] have also been sealed in this way. A drawback with thermal bonding is that the microstructures are easily deformed [6]. In addition, thermal bonding of plastic films easily results in bagging of the films above the cavities [2].

A much less studied method at the microscale is gluing, potentially allowing for materials other than thermoplastics to be joined. The difficulty is to find an adhesive that has the same qualities as the plastics in terms of, e.g. optical properties and biocompatibility, as well as good joining properties. Among the successfully glued devices are a PMMA microfluidic chip with metal electrodes [21], an epoxy chip joined to a soft PDMS layer with electrodes of conducting epoxy [35] and a poly(ethylene thereftatalate) (PET) film glued to silicon [36]. An apparent difficulty with gluing of structured surfaces is that the adhesive easily moves into the channels causing blocking [37].

Plastic films with pre-deposited, highly viscous or passivated adhesives can be used for sealing. Such films of e.g. PET or PI, are commercially available. The adhesive can be activated thermally or by pressure. Channels and cavities in PMMA [38], [39], PC [40], [41], [42], acrylic copolymer resin [43], PET [44], [45], PI [46] and PS [42] have been sealed with adhesive films. As with gluing, excess adhesive blocking the channels may be a problem. As a solution, microchips with sacrificial channels outside the fluidic channels have been demonstrated [38]. During sealing, excess glue, air bubbles, etc., can escape into these channels.

In this work, the joining of Pi-based adhesiveless FPCBs to hot embossed PC microstructures is studied with the long-term objective of integrating FPCB heaters in paraffin-based microactuators made of thermoplastics. The process compatibility between hot embossing and joining by gluing with an UV-curing adhesive or thermal bonding is investigated. More specifically, efforts are made to correlate embossing artefacts with the joining performance. A method recently demonstrated for embossing of PMMA with poly(dimethylsiloxane) as a tool [29], is evaluated for the embossing of PC and as an alternative to embossing with nickel. A first attempt to make a prototype actuator is made based on the findings from the process investigation.