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APRIL 2025 I PCB007 MAGAZINE 87 1. e textured surface promotes palladium catalyst adsorption which in turn promotes electroless copper deposition and coverage. 2. Surface improves copper adhesion. is promotes improved solderability by minimizing or eliminating the occurrence of blowholes seen in the wave soldering opera- tion. I will explain more about blowholes in a future column. Regardless, if the texturing is inadequate, the possibility of voids exists. With insufficient catalyst anywhere on the via wall, the electro- less copper deposition will fail to initiate, thus leading to a void in that area. Electroless cop- per processes require an adequate amount of catalyst on the hole wall in order to promote electroless copper deposition. Key questions to ask in the brainstorming session: 1. Is the solvent conditioner (swellant) mak- ing sufficient penetration into the resin matrix and is the solvent system compat- ible with the resin? Note that some of the higher Tg resins do not react the same as standard FR-4. e higher degree of cross- linking of the polymer resin makes it more difficult for the solvent to penetrate and thus weaken the polymer-polymer bonds in the resin. is, in turn, reduces the action of the permanganate solution in terms of resin removal and texturing. 2. Check the operating temperature of the alkaline permanganate solution, as well as the hydroxide and actual permanganate content. e hydroxide content aids in pro- moting the aggressiveness of the perman- ganate attack on the resin. e permanga- nate in the +7 oxidation state will perform the actual oxidization and breakdown of the resin. Manganate (+4) is a by-prod- uct of this reaction and will not partici- pate in the resin oxidation. Manganate res- idue should be controlled and maintained below 20-25 grams/liter. Otherwise, as manganate builds up over time, the rate of resin removal and texturing will diminish. If solvent compatibility and degree of pen- etration is sufficient, then consider utiliz- ing a higher concentration of permanga- nate. Sodium permanganate will allow for more actual permanganate in solution over the corresponding potassium salt. Finally, review the neutralizer and glass-etch. A major cause of voiding is due to manganate residues remaining on the hole wall resin and glass. One should frequently renew this solu- tion to ensure fresh working chemistry. Also, check the acid concentration. Lower concentra- tion reduces the effectiveness of neutralization. Oen, in order to reduce process steps, fabrica- tors will combine the neutralizer and glass etch in the same process tank. (I prefer separate steps as the process is more effective). As the glass etch weakens due to continued use, the ability of the glass etch to roughen the glass fibers (referred to as "frosting") is reduced. is reduces the ability of the catalyst to adhere to glass. When this hap- pens, voids are the result. PCB007 Michael Carano brings over 40 years of electronics industry expe- rience with special expertise in manufacturing, performance chem- icals, metals, semiconductors, medical devices, and advanced packaging. To read past columns, click here. Figure 3: Preferred surface texture of resin after treatment with permanganate.