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40 PCB007 MAGAZINE I AUGUST 2024 C o n s i d e r i s s u e s related to the electro- less copper metaliza- tion process. Poor rins- ing aer the micro- etch step will lead to copper ion drag-in to the palladium-based catalyst. It is well docu- mented that high cop- per levels in this pro- cess will act as a cata- lytic poison, leading to possible voids in the copper deposit. Conductivity Probes One of the least expensive things you can purchase for pro- cess control is a con- ductivity probe. is little unit (which looks like a ballpoint pen) costs about $200. It is a great way to monitor the rinse water cleanli- ness and the overall effectiveness of the rinsing operation. ese probes are also an excellent method of measuring contaminant build-up in certain chemical processes. Filtration e need for filtration must be emphasized, especially when plating through-holes and blind vias. Any void in the plating in the hole caused by small pieces of contaminant reduces the area available to carry the electric current. Rejects can also occur when insoluble debris is co-deposited on the surface or in the hole. Flow rates are the only means of carrying solids to a filter or bringing fresh solution into contact with the particulate matter. e rate of flow is referred to as the turnover—total gal- lons pumped per hour in relation to the size of the tank (for example, 200 gallons per hour on a 100-gallon tank is two turnovers per hour). Dirt holding capacity is essential and can be attained with throw-away filter cartridges of different porosities, or filter surfaces coated with a filter aid. Porosities of 100 microns down to less than 1 micron are typical. In prac- tice, the average plating solution is turned over once per hour. e recommended flow rates should provide at least two complete tank vol- ume turnovers per hour. However, to achieve the ultimate in clarity, turnovers of up to 10X per hour may be necessary. Keep in mind that the initial flow rate is not the average flow rate. In other words, if one started at 1,000 gallons per hour, and cleaned or replaced the filter when the flow was reduced to 200 gallons per hour, the actual average flow would probably be about 600 gallons per hour, depending upon the type of filter media used. Ineffective filtra- tion leaves debris in rinse and process tanks (Figure 5). Figure 4: Conductivity probe. (Source: Myron L Company) Figure 5: Sediment remaining in a process tank that used no filtration.