Wireless charging is become an increasing important and common method for charging electronic devices across aerospace, defence, healthcare, industrial, automotive and consumer electronics. Paul Gleeson, senior business development & global account manager – electronics, first covers technology and design issues and then discusses construction methods.
The wireless chargers used in the consumer electronics industry today vary in design, shape and size however typically have relatively consistent constituent parts. These applications typically use adhesives as the preferred method of joining technology and are usually performed as part of the wireless charger enclosure construction or during the FATP process.
The advantages of a wireless approach compared to more standard charging approach via an attached cable include the removal of large, bulky power cables, reducing the transfer of bacteria and other impurities in sterile environments, reduced fire risk associated with stray sparks that can occur during connection and disconnection of industrial power cables and the provision of waterproof, airtight sealed surfaces as opposed to the less-contained traditional plug and socket.
The three most common wireless charging technologies are:
- Inductive Charging - Using electromagnetic induction, the device can be placed near the charging station without the need to align precisely.
- Resonant Inductive Coupling – Coupling becomes stronger when the secondary side of the loosely coupled coil resonates.
- Radio Frequency (RF) -Uncoupled charging in which an antenna in an electronic device can pick-up low-level radio frequency waves from an external source and convert the wave energy to direct current (DC) voltage.
By far the largest market share for wireless charging is consumer electronics (~45%), mostly driven by product innovations with mobile devices (such as smartphones and tablets) as well as the rapid growth in wearable electronic devices. These consumer electronic devices are ‘low power’ devices e.g., <100 Watts, and as a result the dominant wireless technology used is inductive charging.
Other advantages include intermittent recharging, with frequent reconnections, without wearing out connectors; increased convenience and improved aesthetics.
Some of the negative aspects associated with inductive charging compared with other charging methods include slower charging (approximately 15% slower than plug to socket); increased manufacturing complexity, meaning increased cost; increased heat that can result in reduced battery life; inconvenience of being unable to use while charging.
The wireless chargers used in the consumer electronics industry today vary in design, shape and size however typically have relatively consistent constituent parts. These applications typically use adhesives as the preferred method of joining technology and are usually performed as part of the wireless charger enclosure construction or during the FATP (final assembly, test and pack) process.
The typical wireless charger will incorporate numerous magnets bonded into the device enclosure using an adhesive. The magnets are usually situated around the periphery of the device enclosure and need to be bonded to both a DC shield and magnet shim. The different bonding surfaces will often require the selection of multiple adhesive products to get the best reliability and performance. Typically, it is common for hot melt PUR (polyurethane reactive) adhesives to be the best adhesive chemistry choice for bonding these components into the enclosure casing.
Often the adhesives will have to provide good adhesive characteristics when boing the dissimilar substrates used; for example, ferrite to polycarbonate (PC) or ferrite to polyimide (PI). Depending on the product selected, the adhesive needs to provide the balanced adhesion to both bonding surfaces but also be compatible with the inherent sensitivity of the substrate materials. Magnet performance can be significantly impacted if exposed to elevated temperatures, which often degrade the plastics and impact aesthetic appearance. As such, heat cure adhesive solutions are not practical.
In addition, using two-part (2k) epoxy and acrylate chemistries would not provide sufficient impact performance that is required with this kind of electronic device accessory.
The appropriate polyurethane hot melt adhesives offers customers a RT (room-temperature) process that protects sensitive substrates but also offers a balanced adhesion between plastics and metal, while providing industry leading impact performance to ensure product reliability.
Final Assembly, Test and Pack (FATP)
After initial enclosure assembly and bonding, the next part of wireless charger assembly will normally proceed to the FATP line. FATP is the process of assembling, often by hand, a product from different parts. This process is often done in an assembly line, where at each station the operator has only one function; that is, adding one piece, gluing one component to another, etc. A significant part of the FATP wireless charger assembly process is the incorporation of the coil assembly, ring support sub assembly and AC sub-assembly. Again, it is often required to bond metals such as ferrite to high surface energy plastics, for example PC.
PUR hot melts continue to be the best choice of product class for this part of wireless charger construction process. The choice of product will be dictated by the substrates involved, but also its relative location to the wireless charger coil. PURs used to bond near the coil need to offer both high and low temperature resistance, because heat will be generated when charging is in process, but the assembly will cool when the charger is not in use. The adhesive needs to be able tolerate these heating and cooling cycles throughout the lifetime of the device, which could mean hundreds or thousands of cycles.
For this application, Chemence recommends the Krylex KH9000 series of polyurethane (PUR) reactive hot melt adhesives said to offer outstanding adhesive performance whilst also offering customers increased sustainability and end of life re-workability; Krylex KH9012 designed for high performance structural bonding of electronic components and enclosures; and Krylex KH9035 said to offer best-in-class jet dispense performance and high and rapid green strength development to reduce clamping times.
This article was first published on 16 December as a blog post.