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JULY 2025 I PCB007 MAGAZINE 75 with a gauge to measure quantitatively whether the stress is compressive or tensile. It is somewhat obvious that for a circuit expe- riencing the potential for many flexures through- out its life, excellent plating adhesion to the sub- strate is necessary. Low to no stress in the copper deposit is preferred. It is important to remember that adhesion of a thin film to a substrate is a com- T RO U B L E I N YO U R TA N K plex mechanism. Internal deposit stress is just one factor. Additional factors affecting adhesion are: • Ionic bonding between two surfaces • Adsorption: Adhesion is based on interatomic and intermolecular interactions such as van der Waals and perhaps Lewis Acid interactions • Mechanical interlocking Mechanical interlocking depends on the contri- bution from a roughened surface. In the case of flexible polyimide, a roughened topography is not achievable. With respect to flexible polyimide, optimum adhe- sion of the electroless copper deposit is heavily dependent on internal deposit stress and adsorp- tion of the palladium catalyst to a plasma desmear treated surface. Please keep these factors in mind when troubleshooting an adhesion issue. You might consider using a direct metallization process for flexible circuity and difficult to metallize substrate materials. PCB007 Michael Carano brings over 40 years of electronics industry experience with special expertise in manufacturing, performance chemicals, metals, semiconductors, medical devices, and advanced packaging. To read past columns, click here. ▼ F i g u re 3 : S p i ra l c o nt ra cto m ete r fo r m e a s u r i n g i nte r n a l s t r e s s i n t h e p l a t e d d e p o s i t .