Issue link: https://iconnect007.uberflip.com/i/1437606
120 PCB007 MAGAZINE I DECEMBER 2021 drill dust due to poor vacuum draw in the drill- ing operation. e cross-section in Figure 2 depicts another issue. Is this a plating blister or separation? Or another type of defect? Is the genesis of the bulging copper anything to be concerned with? inking this through a bit, there are a few points to consider. First, it is possible that during the electroless copper process, the copper deposit blistered or sepa- rated in mid-cycle. is can happen for several reasons: • Excessive plating rate of the electroless copper • Highly stressed copper deposit • Laminate/resin either has smear remaining or hole wall not sufficiently micro-roughened from the desmear/ etchback process • Possibility of drill dust/debris in via Now the question is, where is the origin of the blister? e electrolytic copper has been applied, apparently encapsulating the blistered deposit. ere have been past occasions such as this described above whereby the fabricator will say all is okay with this since the blister has been covered by the electrodeposited copper. is, of course, is wrong. A blister can fracture or break off in mid-stream plating and cause at the very least a voided via. So, don't confuse a blistered deposit that fractures or breaks away with another defect known as hole wall pullaway (HWPA). Hole Wall Pullaway (HWPA) While several of the root causes of HWPA apply as well to a blistered copper deposit, there are significant differences. HWPA is a loss of adhesion of the plated deposit from the hole wall surfaces (Figure 3). A more typical example of HWPA is depicted in Figure 3. e deposit pulled away from the hole wall but did not fracture or flake off. Nonetheless, this is a defect and must be rem- edied. e root cause of HWPA is very simi- lar to those that lead to flaking and blistering. I consider this at the very least a "process indi- cator." is means that the desmear and metal- lization processes need attention. Of course, if the hole wall pullaway causes a violation of hole wall minimum diameter or leads to other vio- lations in hole diameter dimensional require- ments, then this is a cause of concern and will result in non-conformance. So, one is dealing with the origin of the blis- ter or peeling deposit prior to electrolytic cop- per plating. Table 1 lists the most common causes for this type of defect. A precautionary note: Don't confuse HWPA with resin recession (Figure 4). Resin reces- Figure 2: Arrow pointing to a bulge in the plated copper. Figure 3: Example of HWPA.