Issue link: https://iconnect007.uberflip.com/i/1537616
72 PCB007 MAGAZINE I JULY 2025 Metallizing Flexible Circuit Materials: Mitigating Deposit Stress by Michael Carano, C onsultant, Global Ele ctronics Asso ciation Metallizing materials, such as polyimide used for flexible circuitry and high-reliability multilayer printed wiring boards, provides a significant chal- lenge for process engineers. Conventional electro- less copper systems often require pre-treatments with hazardous chemicals or have a small process window to achieve uniform coverage without blis- tering. It all boils down to enhancing the adhe- sion of the thin film of electroless copper to these smooth surfaces. Internal stress in the copper deposit is a signifi- cant factor with respect to adhesion of the plated metal to the substrate. This process of plating on polyimide flexible materials is very much plating on plastics (POP). Electroless copper has been adapted for metallization of difficult-to-plate sub- strates and materials such as polyimide, POP, and molded interconnect devices (MID). As the tech- nology advances toward higher frequencies and faster data transfer rates, highly engineered mate- rials further complicate the plating process. The Challenge of Metallizing Smooth Surfaces One significant concern with metallizing thin films over substrates is the concept of deposit stress. Hydrogen gas is a by-product of the electroless copper deposition process. Consequently, hydro- ▼ F i g u re 1 : H o l e wa l l af te r d e s m e a r w i t h a l ka l i n e p e r m a n g a n ate ( r i g i d c i rc u i t b o a rd w i t h e p ox y re s i n , h i g h Tg 1 70 ° C m ate r i a l ) . ( S o u rc e : I P C - 91 2 1 ) ▼ F i g u re 1 : H o l e wa l l af te r d e s m e a r w i t h a l ka l i n e p e r m a n g a n ate ( r i g i d c i rc u i t b o a rd w i t h e p ox y re s i n , h i g h Tg 1 70 ° C m ate r i a l ) . ( S o u rc e : I P C - 91 2 1 ) ▼ F i g u re 1 : H o l e wa l l af te r d e s m e a r w i t h a l ka l i n e p e r m a n g a n ate ( r i g i d c i rc u i t b o a rd w i t h e p ox y re s i n , h i g h Tg 1 70 ° C m ate r i a l ) . ( S o u rc e : I P C - 91 2 1 ) T RO U B L E I N YO U R TA N K