Overcoming Smart Bracelet Screw Corrosion: A Targeted Solution

 Recently, a smart bracelet designed and manufactured for an Italian client with stringent quality requirements underwent a critical reliability test—metal screws embedded within the plastic housing exhibited signs of corrosion during actual use. Although seemingly minor, this defect not only compromises the product’s aesthetic integrity but also raises concerns regarding its long-term durability and functional reliability.

Screw corrosion is not an isolated issue; it represents a common challenge in plastic-encapsulated electronic devices, particularly wearable consumer electronics exposed to complex environmental conditions. The root cause typically stems from the interplay of multiple factors:

First, as a wearable device in constant contact with human skin, smart bracelets are inevitably exposed to sweat. Sweat contains water, salts, and weakly acidic components that continuously attack the most vulnerable points of metallic fasteners.

Second, early design iterations often employed carbon steel screws with electro-galvanized coatings for protection. However, if the plating layer contains micro-pores, exhibits uneven thickness, or sustains minor damage during manufacturing or assembly, moisture and electrolytes can penetrate through these defects, initiating localized corrosion.

Third, during the insert molding process, molten plastic at high temperature encapsulates pre-placed metal screws. Due to the mismatch in thermal expansion coefficients between metal and polymer, residual internal stresses develop upon cooling. These stresses can degrade the inherent corrosion resistance of the metal components.

Our Solution

1. Mold Structure Optimization  

The mold design was refined by improving the positioning mechanism and encapsulation method of the inserts. This ensures more uniform and complete coverage of the screws by the surrounding plastic, thereby minimizing the surface area exposed to humid environments.

2. Material Upgrade to AISI 304 Stainless Steel  

Superior Screw Material: We replaced the carbon steel with AISI 304 stainless steel, which has a self-repairing protective oxide layer that resists sweat and corrosion.

3. Selection of Enhanced Engineering Plastics  

Concurrently, we evaluated and implemented engineering plastics with superior chemical stability, lower moisture absorption, and reduced tendency to decompose into corrosive byproducts. This mitigates one of the key environmental contributors to metal degradation.

We thank our client for their valuable feedback. This solution underscores our commitment to continuous improvement and delivering durable, high-quality products.