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68 PCB007 MAGAZINE I SEPTEMBER 2021 innerlayer. Looking at sections of these panels under SEM yields additional information (Fig- ure 3). Here, one can see the debris in the hole and detrimental effects. Initially, the team considered that the elec- troless copper itself (plating rates, overall sta- bility, etc.) was the primary cause. And with the detection of some sort of debris, there was a high probability that the source of the debris may have originated with the electroless cop- per solution itself. e question is: Where did the debris originate? Certainly, there are sev- eral potential sources for insoluble material to lodge onto the hole wall or interconnect. Insoluble materials and particulate matter accumulate over time from various sources. is includes drag-in of chemicals from previ- ous process steps that lead to formation of in- soluable particles. Solutions—including the catalyst or the electroless copper solution it- self—are a main source of insoluble matter. One should maintain filtration on the catalyst and electroless copper solutions at the very least. Palladium drag-in to the electroless cop- per tank will destabilize the process solution. e palladium (part of the electroless cop- per catalyst) drag-in will lead to copper dust- ing. is dusting, or literally the formation of copper metallic particles, will cause deposit roughness. Figure 4 shows a simple overflow weir. is set-up helps greatly to remove par- ticulates and other materials that can cause rapid plate-out or roughness/blisters. Another source of debris is introduced from the drilling process. Drill debris entrap- ment is a major source of particulate matter. One of the causes of the debris is from drilling too deep into the back-up material. One only needs less than one drill bit diameter turn into the back-up material. Over-penetration into the back-up material leads to additional debris introduced into the vias and the potential for drill bit breakage. Poor quality back-up mate- rial will contaminate the via as well. Most fab- ricators have opted for aluminum-clad back-up materials. Although more expensive than the phenolic materials, the aluminum clad will not contaminate the holes. e vacuum system on the drilling machine is a very important facet in drilling vias. e vacuum cleans the drill bit flutes of chips and glass debris on each stroke. Low vacuum pow- er will result in the chips and debris remaining in the via, thus adversely affecting the quality of the plating operation. Follow the drill equip- ment supplier's guidelines on proper vacuum operation. In addition, inspect the drill area for dust. is could indicate there is static electricity in the room, allowing drill debris to settle on the equipment. Figure 3: After electroless copper plating. Plating formed over debris. Figure 4: Top view of electroless copper process tank. Note the overflow filtration weir on the right-hand side of the photo.