Editor’s Note: This is the first in a series of articles to appear in Electronics Cooling addressing emerging areas for electronics thermal management.
Printed electronics (PE) is one of the key enabling technologies of many electronic products that we take for granted today, not just in functional feasibility but also in cost that seems to be on a continuous downward trend. PE products today are thin, flexible, wearable, stretchable, lightweight, and cost-effective and are environmentally friendly. PE encompasses many electronics manufacturing processes starting with deposition techniques which initially began with screen printing with stencils for circuit boards to inkjet printing, gravure printing, roll-to-roll (R2R) printing, direct ink transfer and of course, 3D printing of electronics! Complementing these deposition method processes for curing, drying, passivating, painting and finishing make up more than 99% of the manufacturing flow unique to PE.
This blog post presents a high level overview of thermal management problems design engineers encounter as PE rapidly approaches process maturity and the breadth of product offering grows. Note that the discussion in this blog post will also encompass flexible hybrid electronics (FHE) products which combine ‘printed electronics’ with conventional electronics. Furthermore, we will limit our discussions to ‘flat’ PE –the fast-growing technology like 3D PE is definitely in our editorial strategy and will be addressed in another blog post (here is a good intro presentation on 3D PE in the interim!).
This blog is the first in a series of articles to be featured in the coming weeks and months. As outlined in Electronics Cooling’ editorial plan, we will be presenting articles on printed electronics topics that cover material properties and design attributes addressing thermal management. The article Low Cost Plastic Films Lower Exposed Surface Temperatures by 10°C published earlier in Electronics Cooling addressed the topic of thermal properties that benefit PE applications.
The technology of printing “electronics” is more than a half-century old with its roots in the 1950s and the early days of printed wiring boards. Researchers at Nippon Telegraph and Telephone (NTT) found gravure printing (which is a rotary printing press) to be one of the promising printing methods for fine pitch accuracy. But printed wiring boards made better progress using the lithography process on copper films laminated on glass fiber reinforced substrates, known today as printed circuit boards. These days many billions of tiny ceramic passive components are roll-to-roll printed with Ni nanoparticle ink on ceramic green sheets. Figure 1 below shows one of the early versions of gravure printing at NTT.
Printed Electronics Products
Before we delve into the details of thermal management of products produced using PE processes, the products can be categorized into the following groups:
- Lighting (Organic Light Emitting Diodes, OLED)
- Photovoltaics (PV): Organic / Inorganic
- Displays (OLED-based, e-paper, electrochromic, etc.)
- Electronic Components (memories, antennas, batteries, wiring, interconnects, etc.)
- Integrated Smart Systems (RFID, sports fitness/healthcare devices, smart cards, sensors, and smart textiles, etc.)
All of the above product categories have unique challenges which include design topologies meeting product requirements and industrial design, material sets, processing parameters and operating / performance requirements. As opposed to conventional electronic assemblies, one needs to approach the problem holistically and address thermal management as an essential requirement at multiple junctures of product development, manufacturing and operation. In that exercise, thermal endurance and thermal durability must be maintained without compromising the product’s integrity at any stage.
Thermal Management Considerations in Printed Electronics
To thermal management of PE products in a holistic approach, one has to analyze the following (in addition to the design topology) for a given product category:
- Choice of conducting materials for printing
- Choice of semiconductor materials
- Choice of substrate and barrier film
- Interconnection technology
- Type of printing technology used
- Post-printing treatment process
As far as conducting materials are concerned, there are many options for PE which include metallic materials such as Ag, Cu, and Au, organic molecules such as PEDOT/PSS (poly ((3,4-ethylenedioxy-thiophene)) poly ((styrene sulfonate)), a polymer mixture of two ionomers), and conductive ceramics such as oxides and carbon nanomaterials. Metallic Nanoparticles have higher surface energy and experiments have shown that melting temperature decreases as the diameter of particles decreases. For Au particles as an example, it has been shown that the diameter needed for a melting temperature of 200°C is very small, approximately 2 nm.
Regarding semiconductor materials for PE, the basic requirement is that Si semiconductors will need to be replaced by new materials that can be formulated into inks or modified into Si inks with low temperature manufacturing process. Choices here include thin film transistors (TFTs), organic thin film transistors (OTFTs), Oxide semiconductors (ZnO), etc. One of the primary considerations for selecting semiconductors for PE is the charge mobility –any respectable number that makes it worthy of a Si substitute! In reality, many of the foregoing PE semiconductors have two or more orders of magnitude lower charge mobility in comparison to Si.
Substrates are an active area of technology advancement for PE and there are continued advances (the reader is referred to the article Low Cost Plastic Films Lower Exposed Surface Temperatures by 10°C published earlier in Electronics Cooling). PET (polyethylene terephthalate) film is the most popular and widely used plastic film in PE applications. Due to its poor heat capability, the printing process using PET films must be performed at temperatures below 130°C and under low tension. The heat resistance gets better for polyethylene naphthalate (PEN) and even better for polyimide (PI) while transparency decreases and cost increases. Though polymeric films are the first choice for PE products, several things must be carefully controlled to minimize distortion, especially in roll-to-roll printing. Some of the dominant causes for distortions are the temperature and the tension between the rollers. Glass is another choice as substrate in PE applications but requires higher processing temperature.
From a thermal management perspective, as the process thermal energy is transported to the substrate, the transient temperature response in the substrate is a very important characteristic to estimate as it determines not only the integrity of the process but also the energy efficiency. The thermally conducting / insulating properties of each layer in the substrate determine the rate of temperature rise on the surface. These calculations must be performed to select the appropriate and process-suitable interconnects.
The choice of interconnection technology depends on the scale of integration, i.e., at component- or system-level and is one of the most essential technologies for PE products. The basic requirements for PE interconnection technology are:
- Low process temperature, ideally ≤80–130°C
- Task time, less than a few seconds to a few minutes
- Resistance, approx. 10 −5 to 10 −4 Ω cm
- Thermal conductivity, as better as possible
Depending on the temperature range, the choices for interconnection in PE are Ag nanoparticles (room temperature and high temperature bonding), sintered Ag flakes, isotropic conductive adhesives, an isotropic conductive films, Lead-free solders, etc. Their processing temperatures range from the low 100°C to upwards of 300°C depending on the material choice.
Post-printing treatment process includes multiple steps depending on the printing method and product, for example ink drying, encapsulation, etc. For ink drying, the dryer transfers the thermal energy to the substrate and its energy consumption depends on the heat transfer coefficient of the dryer. Nozzle design and its heat transfer coefficient in the dryer is one of the important exercises driven primarily by heat transfer simulation. On the receiving end, the drying of thin films of ink is best modeled and simulated by coupled effects of heat and mass transfer taking into account the jet velocity and the temperature.
In closing, the implications for thermal management professionals working on PE products are quite unlike the conventional electronics thermal management. It is a multi-objective optimization issue centered on the thermal behavior of various components at multiple stages!
In the coming months of 2017, we will attempt to provide more in-depth and topical coverage of thermal management in PE. Your comments in the meantime are most welcome.