Who cares about Coefficient of Hygroscopic Expansion?


    The electronics component industry is well aware of the damaging effects that differences in the Coefficient of Thermal Expansion (CTE) of materials can have on the performance and reliability of components. Differences in thermal expansion can lead to delamination of circuit layers and solder joint failure in electronic devices. On a macro level, We are all familiar with “road buckling” caused by thermal expansion of blacktop roads in the hot summer sun. What are much less known is the impacts that differences in the Coefficient of Hygroscopic Expansion (CHE) can have on electrical components.

    The Hard Disk Drive (HDD) industry for years has utilized a flex circuit mounted to a metal beam to position the read-write head above the spinning media. This system has to provide both the electrical connection and precision mechanical positioning of the head. As with many consumer electronics, the HDD has a large specified operating range to cover people utilizing it in the hot arid climate of the Southwest to the humid Southeast. What we found during development was that commercially available polymers that could be used for insulating layers on the flexible circuit had large CHE. This caused them to expand sufficiently during humidity changes to actually change the angle that the read-write head is held in the HDD, leading to reduced drive performance.

    The challenge on the development side was twofold: 1) how do we measure and characterize CHE of polymers and 2) how do we find a material that has a low CHE and still meets all the other performance requirements. First, our team worked directly with analytical equipment suppliers to develop new capabilities and then helped to beta test humidity controlled Dynamic Mechanical Analysis (DMA) equipment. This new internal capability allowed us to test the performance of new materials and rapidly screen them. Second, we partnered with material suppliers to jointly develop a low CHE material that meets the stringent requirements of the HDD industry. This custom photosensitive polyimide allowed us to both match CTE to our stainless steel substrate but also eliminate the design constraints that having a high CHE material would induce.

    Adoption of new MEMS (Micro Electrical Mechanical Systems) that blend conventional electronic component requirements with mechanical requirements will continue to push the need for low CHE materials.