Chris Li is a former Product Design Manager at Amazon, where he led multiple generations of Amazon Kindle products.
As a product designer, one of the biggest compliments you can get is when the end user has no idea how the product was put together. A bigger compliment is when fellow designers can’t figure it out.
More and more products these days have few or no visible fasteners, making it seem like they were put together by magic. That magical experience is often the result of countless hours of clever engineering and development work, starting from the earliest architectural design decisions like choosing between screws and glue.
In recent years, many consumer electronics engineers have turned to glues and tapes to make their products sleeker. These adhesives are thin and versatile, helping disguise how a product was made but also making it smaller, more durable, and waterproof. Using adhesives is often justified by the increased durability of the products, which translates to fewer product returns and repairs. However, critical components like the battery and display often fail within 1-2 years of heavy use, incentivizing users to buy the latest product and discard the old devices. To combat this growing e-waste problem, the European Union has passed laws codifying the right to repair -- essentially outlawing the "glue it together" assembly method. Engineers will need to dig deep again to determine how to strike the right balance between form, function, reliability, and repairability.
Design considerations
Screws offer designers a straightforward method of assembling parts. The way they translate rotational torque to axial force allows them to sandwich components between the screw and the mating nut or tapped hole. This is useful for clamping parts against a design datum and resisting oppositional forces from spring contacts, buttons, or foams. Often charging connectors are screwed in place to help withstand the loads from repeated insertions that might otherwise weaken nearby solder joints.
Photo Credit:Fixit Galaxy S20 Ultra teardown . The purple circles indicate spring contacts to the speaker, and the red circle indicates a screw through-hole to engage the contacts. The green circles indicate screws on either side of the charging connector.
Because screws are usually made from plated steel or other metal, they often serve dual purposes with printed circuit boards. Not only are they used to fasten the PCB to the housing, but they can also provide electrical ground connections helping to reduce interference with the ever-increasing number of antennas in consumer products.
While screws can be easily customized to fit your desired length, diameter, pitch, and drive type, sometimes there is not enough space in the X, Y, or Z axis to accommodate the screw head or the mating screw boss. For example, an edge-to-edge display module with 2mm perimeter bezels does not have enough space to hold a screw boss for some of the smallest screws in use today. Instead, most display modules and cover lenses use liquid glues and adhesive tapes to bond them to the housing. 
An example cross-section through an edge-to-edge display. The image on the left shows the narrow bonding area for the red adhesive strip while the image on the right shows that there is not enough room to attach a screw boss to the cover lens without interfering with the display.
Adhesives are often used in areas with limited space and complex geometries. These materials leverage a large surface area to create a strong bond. Even in a narrow opening, liquid glue can be dispensed as a continuous bead, and tapes can be die-cut as solid rings. This makes both types of adhesive ideal choices for preventing water or dust ingress into an assembly. Liquid glues or foam-backed adhesives are more compressible than screws and can accommodate greater tolerance variation in the mating parts. While liquid glues might cure or harden over time, they can flow and fill the gap when initially manufactured.
Battery cells in a consumer device are often some of the largest and thickest components -- and since battery energy density in lithium polymer jellyroll batteries is most influenced by thickness, every 0.1mm counts. This means products with thin sheets of adhesive to affix the battery in place can have more energy capacity than those that enclose the battery and then screw or snap that into place. Using adhesive instead of other fastening methods also reduces the risk of puncture from an errant screw (but does not eliminate it).
Assembly considerations
On the assembly line, screws are generally easy to manage. They store well in moderately controlled warehouses and don’t go bad unless exposed to extremely corrosive conditions. If there are multiple types of screws to track, it becomes a bit more challenging to manage inventory and line stations. Processes must be implemented to ensure no cross-contamination between different screws. To combat this, some brands use different drive types, plating colors, or sizes for different applications to identify when the wrong screw is used.
Screws do have some process requirements for use on the line. This includes specifying and calibrating the torque of the drivers and having protocols to deal with the rare situations of bare or broken heads and stripped threads on the mating component. If there are a lot of screws in an assembly, it is quite easy for an operator to miss one or more of them, so having an automated visual inspection system to check that screws are in place is quite useful.
On the other hand, adhesives tend to have many more challenges in processing and storage at the factory. They are more sensitive to environmental conditions like temperature and humidity, which can affect the chemical composition of the bonding agents. Adhesive tapes can be bent or folded easily, which could cause the adhesive to bond with itself or become exposed to the air and particles.
4 of the top 10 problems in production are related to the usage of glue or adhesive-backed tapes, foams, and labels.
InstrumentalTop 10 Issues in Mass Production
Adhesive tapes are often pressure-sensitive, requiring force to activate fully. Special fixtures are often needed to assemble and apply the necessary pressure and time to ensure consistency in manufacturing. Adhesives are difficult to place without alignment fixtures. Sometimes vacuum fixtures are used to hold the liners in place. Once applied, adhesives are difficult to rework or remove, especially if designed to be aggressive. This means scrap and rework costs increase if an assembly has a problem.
In a recent survey of our Instrumental customer data, 4 of the top 10 problems in production are related to the usage of glue or adhesive-backed tapes, foams, and labels. Too much glue can cause cosmetic problems or fit issues as the adhesive flows into areas it is not supposed to be (overflow). Too little glue and the required strength or sealing may not be achieved (underfill). Adhesive-backed materials like tapes and foams can also cause problems if installed improperly. Wrinkles in adhesive mean reduced surface area for bonding and potential tolerance problems as the adhesive takes up more space than designed.
Repair and Recycling with Screws and Glue
Once a product leaves the factory, it will be subjected to the wear and tear of the real world. Parts break and wear out, or a new generation is released, allowing customers to repair or recycle their old devices.
Repairing a device with many screws is generally much easier than one that is glued shut. Even with screws, repair can be a little tricky. Some devices use special security screws requiring customized drivers. By now, everyone knows that iPhones use pentalobe screws on the outside and sometimes tri-wing screws on the inside. Often devices use more than one type of screw. If consumers are not careful, they could inadvertently use the wrong screw when repairing or reassembling their devices. A long screw in the wrong place could puncture through a sensitive component like the display or battery. With small screws, the wrong torque could cause the screws to back out or break the head off. Recently, Amazon’s Ring Doorbell was recalled when customers mistakenly used a long mounting screw in the wrong place, damaging the rechargeable battery and potentially starting fires.
Devices like phones and smart watches will likely continue to use adhesives as a physical barrier for waterproofing and other durability reasons. However, the European push for the right to repair will encourage brands to innovate and find ways to make it easier for consumers to repair devices themselves.
While many devices use adhesive around the perimeter, that adhesive may not need to be as aggressive if the key modules are all waterproof. Companies like P2i deposit a nanocoating over the entire surface of the board, protecting all exposed leads and solder joints from shorting when exposed to water. We are also seeing increased adoption of weak-shear adhesives underneath batteries. These adhesives are similar to those popularized by the 3M Command Strips, which are quite strong in the perpendicular directions, but weak in the parallel direction. This allows the adhesive to be pulled off at a low angle, leaving little residue behind. Sometimes, aging can cause pull tabs to break earlier than expected, but this is an exceptional example of companies adopting new technologies to improve repairs.
As the right-to-repair movement gains momentum, questions will arise on who bears responsibility for repaired products that fail. Brands spend a lot of time and effort to make their products meet high standards for consistency and reliability. But once a 3rd party opens the device, manufacturer warranties are often voided.
Making the right choice
Regardless of whether the new European regulations cause you to rethink how you put your products together, Instrumental's AI manufacturing solutions can help you find and fix problems that distract you from implementing the right fastener.
By identifying issues in NPI as they happen, instead of waiting for problems to be discovered during reliability testing, Instrumental has helped teams reduce wasted effort and materials in development and production. Automated visual inspection systems help earlier discovery of anomalies in glues, adhesives, and screws so the engineering team can dial in process parameters and adapt on the fly instead of waiting for test results weeks later. This can be especially helpful for glues because the team can screen common issues and leverage manufacturing traceability to track progress across builds and into production. This allows the team to focus on continuously improving their designs and processes during the build instead of waiting for results from reliability tests to make crucial decisions.
For screws, a machine vision system can check not only for their presence, but also the condition of the head and driving recess, to catch operator errors and process parameter problems before the product gets fully assembled, tested, and shipped out. In production, this ability to detect and monitor in real-time translates to higher yields and fewer repairs when issues with adhesive and screws are discovered before expensive components are attached. Instead of costly repairs or customer escapes, many of these units can be reworked and recovered.
As consumers and governing bodies become more aware of the entire lifecycle of a product and demand the ability to repair products, our job as designers and builders will be to strike the right balance between how the product performs in use and how to enable users to make repairs after the fact. Using new tools like Instrumental's quality management software can make those decisions faster and easier.