In spite of the availability of exotic materials such as sapphire glass and ceramics, it seems that metal is still, far and away, the most desired material for a flagship phone to utilize. This data is according to a GSMArena weekly poll, in which around 12,000 voters participated.
The poll results showed metals were most popular, with a 42 percent chunk of the vote. Ceramics, sapphire glass, or other exotics received 26 percent, glass with 19 percent, and plastic and rubber trailed with 13 percent. The results (to the degree they are generalizable) are probably not a surprise to many, and the same question was also asked by the outlet two years prior with nearly the same result. The main difference between the two polls is that exotic materials have increased in popularity. Why is metal so popular? There are good scientific reasons for it, but actually all materials used have both pros and cons:
Metals used for smartphones (typically aluminum or magnesium) are relatively soft, meaning they are easy to mold into interesting designs and shapes. Metal also happens to be an excellent conductor of heat, meaning the casing allows the phone to dissipate heat from the SoC better. Of course, the conductive properties of metal also mean that if managed poorly, the heat will make the phone uncomfortable to handle. An additional issue is that metals tend to disrupt radio waves, which means that there are often plastic channels in the chassis to allow for reception.
Glass (particularly speaking about Corning Gorilla Glass) is actually quite durable, and as a result has been used in the construction of over 3 billion smartphones to date. It is also easy to produce in varying quantities, making it more economical. The downside is that glass is still fragile when hit from certain angles (as many Galaxy S8 and iPhone owners can attest to); it is also very difficult to sculpt glass, and so glass phones are typically very simple in design.
Plastic and rubber are very handy due to their flexibility, durability, low cost, and ability to conduct wireless signals. Unfortunately, they don't feel great to most people (as the result of the poll attest) and are also poor at dissipating heat.
Exotic materials, such as ceramic, sapphire glass, or leather, all have their own unique issues: Ceramic is incredibly fragile; sapphire glass is prohibitively expensive and difficult to manufacture; leather is porous, weak to the elements, and will wear and tear easily.
So, it looks like metal will continue to be the material of choice for our smartphones — for the time being, at least.