PCB007 Magazine

PCB-Apr2017

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66 The PCB Magazine • April 2017 prior to the electroless copper deposition pro- cess. Some of these chemicals, while helping to promote strong palladium catalyst adsorption to the surface, may also leave a surfactant-like film on the copper. This film in turn will act as an ad - hesion barrier, causing the copper plated deposit to peel from the foil copper. One suggestion is to carefully evaluate any cleaner-conditioner to ensure compatibility with the electroless copper. There are other possibilities that would lead to this type of adhesion failure. One such fail- ure mechanism is related to resin smear remain- ing on the capture pad of a blind via. Figure 1 shows significant blistering of the copper from the blind via capture pad most likely due to resin. One way to fix this issue is to design the laser process to ablate the resin material down to the capture pad. Even with this scenario, the concern with UV YAG is that once the beam reaches the copper pad, the energy is "stepped down" to minimize further attack on the cop- per pad. This can lead to resin and ablated by- products to remain on parts of the pad. In turn this provides a mechanism for adhesion failure. Any remaining resin material can act as a ful- crum, forcing the plated copper to peel. Pattern Plating Copper-to-Copper Peeling There are many reasons for electrolytic cop- per to peel or fail to adhere to the electroless copper deposit. However, one must understand the process in general. For example, is this a panel plating process or pattern plating? Are we looking at a flash (low thickness electrolytic) copper separation or is the issue related to a full thickness of plated copper? This is important to keep in perspective as one troubleshoots the defect. With respect to pattern plating, one of the most common reasons for copper-to-copper peel is photoresist residues that were not completely removed during the developing process step. There are many reasons for this—but none of them are excuses! If the defect is clearly elec- trolytic copper peeling from electroless copper, and the fabricator is employing pattern plat- ing, the first order of business is to examine the surface preparation and developing operations. This includes surface prep, resist lamination, exposure and development. Surface prep At this point, with thin coating of fragile electroless copper on the surface and in the via, one must not over-prep the surface for fear of creating a void in the thin deposit. It is sug- gested that to widen the process window to prevent voiding from etching, a very thin elec- troless copper deposit can be followed with an electrolytic copper flash plate. Typical thickness of 100−150 microinches would add protection from the possibility of etch voids. This step is performed immediately after electroless plating and prior to resist coating and imaging. As an aside, this author prefers an electroless copper process that delivers 40−60 microinches. In this case, there is no need for flash plate. In addition, one can be more aggressive in the pat- COPPER-TO-COPPER PEELING Figure 1: Plated copper blistering from capture pad. (Source IPC-9121) Figure 2: Copper-to-copper peelers.

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