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64 The PCB Magazine • October 2014 fabricator can do to ensure proper solder mask adhesion. Of course, proper surface prep is one of them. However, these other critical success factors are important (and will be presented in a future column): • thoroughly pre-cleaning of the substrate • ink layer thickness • complete pre-drying of the LPISM • exposure energy • correct adjustment of the developing parameters • control of the corresponding final curing conditions Surface Prep of Copper Prior to Soldermask In Figure 1, the surface copper of the PWB was prepared with aluminum oxide. Even though the surface roughness appears suffi- cient, the fabricator experienced issues with LPI breakdown after ENIG. Figure 2 shows the mask peeling from the surface due to marginal adhe- sion. Most likely, even with less than adequate surface preparation, most surface finishes would not have had such an adverse effect as ENIG. While the above case is extreme, it none- theless illustrates that marginal mask adhesion leads to yield loss for fabricators. Surface preparation must be optimized to improve mask adhesion. One solution is to eliminate scrubbing (either pumice, AlOx) and utilize the oxide alternative (organo-me- tallic) adhesion promotion system. The sur- face topography that is achieved on plated copper outerlayers with this process is shown in Figure 3. The topography represented in Figure 3 is remarkable simply because the copper surface is electrodeposited copper (from acid copper electroplating process). The electrodeposited OxIDE ALTERNATIVES TO ENHANCE LPI ADHESION TO COPPER continues figure 2: severe peeling of lPi after enig. the above panel treated with aluminum oxide (figure 1). mask lifting noted after processing through enig. figure 3: oxide alternative process as an adhesion promoter for soldermask. figure 1: Copper surface outerlayer prepared with aluminum oxide prior to lPi.