Issue link: https://iconnect007.uberflip.com/i/1156271
80 PCB007 MAGAZINE I AUGUST 2019 turers to use. The sputter- ing process is performed under a vacuum, and the sputtering chamber can take only one panel at a time. These factors limit equipment capacity and applicable panel size. The final step of the SAP—a quick etch—is not only to remove the thin base copper but al - so to etch the final circuit pattern. Minimiz- ing the duration of this quick etching process provides the best circuit conductor shape and accuracy. This means a minimum base copper thickness delivers the best result. For this reason, the sputtered copper technically promises the best result, but it is not econom - ically feasible (Table 1, #4). Approximately 40-micron trace and space feature size can be achieved using the etched copper foil process (Table 1, #1). This is used for major consumer applications, such as cellphones and mother - boards. For conductor widths <40 mm, the ultrathin copper foil process is used. This is used for today's package substrate manufac- turing. The electroless copper plate method is used for advanced package substrates and can produce near 20-micron trace and space feature size. Fundamentals of Current Challenges The electroless copper plate is a good solu- tion to reach beyond the copper foil method for a finer pitch design with SAP because it is possible to use a thinner base conductor. But the traditional tin-palladium colloidal catalyst sporadically deposits over the substrate surface and the distance between the particles is 10 or more nanometers. Also, the deposited catalyst is a tin-palladium alloy, and the catalytic active points are reduced compared to a pure palla- dium catalyst particle. The initial copper atom deposition starts, sporadically and discontinu- ously, then the copper atom deposition even- tually becomes aligned and densified when the deposited copper has accumulated enough (Figure 2). This deposition mechanism is undesirable for the SAP or microvia formation, which looks for a thinner base conductive layer. An ionic pal- ladium catalyst was developed for better cata- lyst coverage over the substrate surface. The ionic palladium catalyst has more active pal- ladium than a tin-palladium colloidal system and relatively higher covering density of the substrate surface than a tin-palladium colloi- dal system particle. However, the reduction of the copper ion to the metal atom preferentially occurs in the vicinity of its palladium neighbor, so the copper deposition of the ionic palladium system is still started sporadically. This means the minimum base copper layer thickness still has a limitation to getting enough conductivity over the panel surface. With manufacturability as the other aspect, the colloidal tin-palladium process is well-ma- tured with a long history. It has no major manu- facturing issues today, and it can be used in long duration without problems. The ionic palladium process is relatively new, and it is not as ma- ture as the tin-palladium system. The high activ- ity bath may be corrupted by some factors and control is not as easy as the matured colloidal tin-palladium system. The bath also has a rela- tively short life. This brings a major economic disadvantage, especially for smaller size or high- mix, low-volume production factories. The other possible issue is adhesion. Because of a porous boundary structure, it has limited chemical in - teraction between the substrate and deposited copper, and it is possible to intrude oxygen mol- ecules and moisture diffusion from the base sub- strate. These phenomena may ruin the copper- to-base substrate adhesion over time. Figure 2: Schematic electroless copper deposition using conventional tin-palladium or ionic palladium catalyst.