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90 PCB007 MAGAZINE I JANUARY 2018 CASE STUDY: Pits and Mouse Bite Issues, Part 2 Trouble in Your Tank, a column by Michael Carano RBP CHEMICAL TECHNOLOGY Introduction In last month's column, I introduced a case study that centered on plating pits and mouse bites. There were three areas in the process that raised concern as to the potential root cause of the defect. Of course, as with the case in all troubleshooting situations, it is best to look at the problem with wide open eyes. Just because one is looking at an issue that is visible after copper plating, this should not mean that is the only place to look. And this case study il- lustrates that point quite clearly. The Problem Again, keep in mind that there seems to be two very different types of plating defects. One par- ticular pit seems to be random, and that there is no pattern that can be detected (Figure 1). In this case, these pits can be seen practically anywhere on the board. In addition, the char- acteristic of this type of pit essentially is found to go down to the base metal. In Figure 1, the reader can easily see that the plated copper is evident around the pitted area. The pit itself can be described as irregular in shape. Obvi- ously not the same shape as a pit caused by an air bubble. There can be several possibilities here. One issue may relate to organic contamination of the plating solution itself. Or, is it possible that the primary photoresist was improperly ex- posed and that allowed resist materials to leach onto the copper surface? In general, interactions between the photore- sist and plating process are quite subtle. Resists have been associated with a variety of plating failures. These failures increase in severity as the technology shifts to higher cir- cuit densities and smaller vias and pads. There are some reasonably clear ties between plating problems and photoresist, as in the case of re- sist development residues causing copper-to-copper peelers, or strip- ping residues causing ragged plated lines after etching. One should ask if the pre-plate cleaning is effective- ly cleaning the copper surface (the area that is exposed after develop- ing) prior to the electrodeposition of copper. Now, remember the fabricator is also dealing with another type of pit Figure 1: Pit down to base metal indicating residue inhibiting the electroplating of copper. Note the residue on the copper substrate surface which is most likely dry film and/or developer residues.

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