Issue link: https://iconnect007.uberflip.com/i/1537616
74 PCB007 MAGAZINE I JULY 2025 gen may be incorporated into the thin deposit, which may have a negative influence on internal stress. The hydrogen gas issue has been attrib- uted to blister formation in the copper deposit. A challenge typically encountered on smooth sur- faces, such as polyimide-based flexible circuit materials, is ensuring good adhesion of the elec- troless copper to the substrate. Adhesion of a thin film deposit to a substrate that lacks sufficient microroughness is compromised. To mitigate this issue, one solution is to reduce the internal stress of the copper deposit as much as possible. Now, one remedy to minimize hydrogen gas effects on the deposit is to lower the surface ten- sion of the electroless copper electrolyte. By low- ering surface tension through the use of specific wetting agents, the hydrogen gas bubbles are less likely to remain on the plated surface. The hydro- gen gas issue notwithstanding, there is a concern with electroless copper deposit stress. Excessive deposit stress will cause the copper-plated deposit to blister or pull away from the substrate. This is, essentially, a stress relief phenomenon. It is well known that polyimide materials for flex- ible circuit fabrication have a rather smooth tex- ture even after plasma desmear. Unlike most epoxy-based resin systems, polyimide for flex does not have the micro-roughened surface tex- ture generally experienced with alkaline perman- ganate chemical desmear. With these "anchoring sites (Figure 1), there is ample surface area for the thin film of electroless copper to adhere to epoxy- based resin systems. Another limiting factor with respect to adhesion of copper to flexible polyimide is the low surface energy of polyimide films. Surfaces with low sur- face energy tend to repel chemical interactions, making it more difficult for process chemistry to effect good adhesion properties. In addition, flex- ible circuits are, by design, flexed and bent dur- ing their useful service life. Thus, creating excel- lent adhesion of the plated copper to the substrate is critical to the life function of the circuit. Certainly, plasma treatment of the polyimide material pro- vides an improvement to the low surface energy of polyimide. However, such treatments often don't go far enough to ensure long-term adhesion by relieving internal stress. The Influence of Stress Stress in the electroless copper deposit can be either compressive or tensile. The copper deposit can be thought of as a spring that is either under tension (stretched) which is tensile stress or com- pressed which is compressive stress. Compres- sively stressed copper deposits lift or blister off smooth surfaces that lack anchoring sites such as adhesive-less polyimide flexible materials. The spiral contractometer (Figure 3) utilizes a strip of metal wound into a coil or helix. The unit is attached to the plating cell and the metal is then deposited on one side of the coil only. Depending on the type and extent of the internal stress, the coil will either expand or contract. If the deposit is com- pressively stressed, the coil will tighten. If the stress is ten- sile, the coil will expand. The contractometer is equipped T RO U B L E I N YO U R TA N K ▼ F i g u re 2 : Po l y i m i d e f l ex i b l e c i rc u i t af te r p l a s m a d e s m e a r ( n ote t h e s m o ot h to p o g ra p hy ) . ( S o u rc e : I P C 91 2 1 , P ro c e s s Ef fe ct s H a n d b o o k )