Going glass: the next generation of smartphone casing
An all-glass smartphone housing may be an industrial designer’s ambition – at least aesthetically. Yet it may not have the required mechanical integrity needed to pass product tests, such as drop and tumble, necessary to ensure reliability during the course of its day-to-day use. As a result of this and the ongoing thinnovation trend, we can expect to see high-end models with all-glass designs continue to use metallic frames, so long as fully flexible displays are not implemented. These metal frames will continue to support the smartphone design, acting as an antenna amplifier that counteracts the RF attenuation linked to parts in the smartphone housings, such as printed circuit boards (PCBs), EMI shields, and metallic coatings.
The selection of metal used for the frame has implications for mechanical integrity, weight, durability and the achievable finishing processes and effects. Aluminum is an attractive choice due to its low density, recyclability and low raw material and fabrications costs. However, aluminum alloys exhibit corrosion over long-term usage in metal frames. Stainless steel alloys are capable of a wider selection of finishes than aluminum alloys, including finishes from matte to high gloss, from polished to brushed metal, and even fine hairline finishes. Its surface can be treated with Vacuum Metallization (VM), Physical Vapor Deposition (PVD) or Electrodeposition (ED). PVD can also be considered as an alternative to nickel plating, as some consumers are sensitive to nickel, and the waste treatment has environmental restrictions.
Increased weight, due to its higher density, may be a concern, however its higher mechanical properties also allowed for reduced wall thickness, possibly making the increase in device weight negligible. If weight is a concern, titanium can also be considered. The stainless steel alloy SUS 304 is the most widely used. New demands for metal-to-polymer bonding Nano Mold Technology (NMT) provides direct metal-to-polymer bonding, and is an important secondary process that happens prior to decoration processes such as anodization or PVD. NMT overmolds a specially textured metal surface with a polymer to generate a strong bond. The bond’s strength may be influenced by the metal surface, the processing conditions, or the properties of the compound. Different substrates require different treatments to achieve a suitable surface texture, but the key characteristic of the surface texture is a multi-scale roughness that contains micro and nano features to allow for optimum polymer infiltration and interlock.
An overview of the NMT process is shown below, as well as examples of suitably textured substrate surfaces. It is important to achieve strong bonding that avoids trapped air gaps, and that prevents delaminating under stress or poor conditions. The selection of stainless steel over aluminum poses additional challenges in selecting polymers for NMT.
Aluminum is commonly anodized to protect the soft aluminum surface from scratches and corrosion and to add colour, depending on the design. Since anodization occurs after NMT, polymers that can survive the anodization process must be used, such as polybutylene terephthalate (PBT), PBT/polyethylene terephthalate (PET) blends, or polysulfone (PSU). While anodization is ideal for aluminum, it is not suitable for other metals used for their higher mechanical strength, such as stainless steel and titanium.
These alternative alloys are prime candidates to provide structural integrity to next-generation smartphones, yet they require polymers for NMT that can withstand the higher temperature of processing techniques such as PVD. PVD processing requires three hours at temperatures of 150°C to 180°C, making the polymers commonly used for NMT unsuitable. PPS is not an ideal material for external parts that need to meet high aesthetic requirements, due to its poor UV resistance and colourability. OEMs must therefore select from higher temperature plastics, such as polyether ether ketone (PEEK), and the new and specialized high glass transition temperature (Tg) and heat deflection temperature (HDT) PPAs, which demonstrate performance competitive with PEEK.
The NMT process can be described using the following steps:
- After cleaning the substrate, micro and nano-scale porosity are generated on the metal surface. For aluminum, a wet, multi-step chemical etching process is used.
- Once the metal is suitably etched and cleaned, it is overloaded with a plastic compound. This results in a strong, metal-to-polymer bond due to the polymer filing of the surface texture.
- After overmolding, the metallic layers of the part may undergo decoration processes such as PVD.
New high-temperature materials for NMT DSM has developed a new material, based on its high-performance ForTii Ace polymer, that is suitable for NMT. ForTii Ace has the highest Tg of all polyphthalamides (PPAs). Its uniqueness lies in the chemistry of the C4 molecules, which enable superior crystallization behavior to other PPAs. Additionally, its high Tg, driven by aromatic content, brings the temperature and chemical resistance in line to that of PEEK, while outperforming PEEK in stiffness at high processing temperatures. ForTii Ace’s high polymer/molecular strength gives it the highest mechanical strength of all PPAs.
The material is able to achieve exceptionally high NMT bonding forces with titanium and stainless steel allows. This is due to its high polymer strength and excellent processability. Control over flow and crystallization is essential to good bonding via NMT, to allow sufficient filling of the micro/nano pores. Once the metal surface structure is sufficiently filled, crystallization ensures excellent bond strength, and high stiffness and strength of the compound.
For bonding to titanium and stainless steel alloys, DSM has developed two dedicated NMT grades, ForTii NMX33 and ForTii Ace NMX5. For applications where dimensional stability, resistance to heat and UV for light colors, or dielectrics play an essential role, the ForTii Ace polymer is an ideal material. Both grades are commercially available, enabling designers to realize next-generation all-glass casings or other creative metal-to-plastic bonds that require the highest aesthetics and material performance. NMT is an elegant technology for bonding metal to plastic, and is used in various applications and industries, including the industrial, aerospace and automotive sectors.
Our complete NMT portfolio includes PBT and PPS compounds. This broad portfolio enables manufacturers to obtain material and application support for a variety of designs and industries via one focused point of contact. ForTii NMX33 and ForTii Ace NMX5 show the highest performance in bonding strength to both stainless steel and titanium. With a melt temperature of 320°C and 340°C, respectively, these compounds show the highest temperature resistance, making them fully compatible with the extreme temperatures reached during PVD processing.
Realizing the next jump in smartphone innovation
As the smartphone matures as an application, the push to innovate its design and features increases rapidly. Both small startups and global multinationals are putting huge effort into new design concepts that allow differentiation in a lucrative multibillion dollar market.
The clear trend in smartphones is moving away from aluminum enclosures to glass or ceramic bodies with metal frames made from materials with higher mechanical strength, such as stainless steel or titanium. These designs will rely on proven plastic-to-metal bonding techniques based on high-performance engineering plastics, such as those in DSM’s portfolio, which can be seen in the next-generation concepts launched in 2017. While standard NMT technology is widely used with PBT, PBT/PET, and PPS polymers, DSM’s new ForTii Ace compounds raise performance to the levels only seen with PEEK so far.
These new materials are comparable with PEEK in terms of chemical, mechanical and thermal resistance, while offering very good dimensional stability and excellent bonding strength to metals with more mechanical strength. ForTii Ace enables designers to move beyond the current materials’ capabilities to realize the next big jump in smartphone innovation.