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84 The PCB Magazine • February 2017 Now, one remedy to minimize hydrogen gas effects on the deposit is to lower the sur- face tension of the electroless copper electro- lyte. By lowering surface tension using specific wetting agents, the hydrogen gas bubbles are less likely to remain on the plated surface. The hydrogen gas issue not withstanding, one must also be concerned with electroless copper de- posit stress. Excessive deposit stress will cause the copper plated deposit to blister or pull away from the substrate. Essentially this is a stress re- lief phenomenon. It is well known that polyimide materials for flexible circuit fabrication have a rather smooth texture even after plasma desmear. Unlike most of the epoxy-based resin systems, polyimide for flex remains void of the micro-roughened sur- face texture generally experienced with alkaline permanganate chemical desmear. With these "anchoring" sites (Figure 1), one can see that there is ample surface area for which the thin film of electroless copper can adhere to epoxy- based resin systems. Another limiting factor with respect to ad- hesion of copper to flexible polyimide is the low surface energy of polyimide films. Surfaces with low surface energy tend to repel chemical interactions, making it more difficult for pro- cess chemistry to effect good adhesion proper- ties. In addition, flexible circuits are by design flexed and bent during their useful service life. Therefore, creating excellent adhesion of the plated copper to the substrate is critical to the functioning of the circuit. Certainly, plasma treatment of the polyimide material provides an improvement to the low surface energy of polyimide. However, such treatments often do not go far enough to insure 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 ten- sile stress, or compressed, which is compressive stress. Compressively stressed copper deposits lift or blister off smooth surfaces that lack an- choring sites such as adhesive-less polyimide flexible materials. The spiral contractometer as shown in 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 compressively stressed, the coil will tighten. If the stress is tensile, the coil will expand. The contractometer is equipped with a gauge to measure quantitatively whether the stress is compressive or tensile. Figure 1: Hole wall after desmear with alkaline permanganate (rigid circuit board with epoxy resin-high Tg 170°C material). Source: RBP Chemical Technology Figure 2: Polyimide flexible circuit after plasma desmear—note the smooth topography. (Source IPC 9121 Process Effects Handbook) METALIZING DIFFICULT-TO-PLATE SUBSTRATES

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