Printed circuit board (PCB) manufacturing involves many steps, one of which includes coating microvias and through holes with a thin layer of copper (Cu) to provide sufficient electrical conductivity between layers prior to applying the final surface finish to the board. The copper metallization will only create reliable electrical connections if the underlying dielectric layer has been properly prepared, and this is where silver catalysts can play an important role in making holes conductive.
The PCB industry has long relied on metallic catalysts to activate the dielectric layer surface before depositing electroless copper metallization. Metallic catalysts may either be ionic—relying on the electrostatic attraction of charged metallic ions to a surface—or colloidal.
Colloidal catalysts consist of nanoparticles of atomic metals suspended in an aqueous solution.
Several metals have been considered as potential catalysts for PCB applications. Palladium (Pd) is the traditional choice, and has been used in both ionic and colloidal forms. Ionic Pd catalysts are especially stable in highly oxidizing environments, such as those in horizontal plating lines. Colloidal Pd-based catalysts, which are popular for many applications, typically also contain tin.
Although Pd catalysts are well-established and effective, they come with some inherent drawbacks. The most critical one is cost. Pd is expensive, with highly fluctuating prices that have ranged from $400 to $800 or more per troy ounce over the past few years.
The presence of excess Pd catalyst in through holes can induce metal plating in non-plated through holes (NPTH). This can especially be a problem with electroless nickel immersion gold (ENIG) surface finishes. Pd catalyst adsorbed on the walls of through holes catalyzes nickel atoms, which may deposit indiscriminately on hole walls, including those of NPTH.
Cu might be appealing as a catalyst because of its low cost, high electrical conductivity, and theoretically high catalytic activity. Cu is easily oxidized in air, however, especially in an aqueous environment, making it difficult to use in practice. Gold and platinum can be effective catalysts, but they are expensive metals and therefore do not solve the problem of the high cost of Pd.
Silver (Ag) provides a balance of desirable features: good catalytic activity, high electrical conductivity, modest cost, and good process stability. Ag-based catalysts are common in many applications, such as water purification and medicine, where the antimicrobial properties of Ag are especially desirable. Such catalysts do not have a history in the PCB industry, but they can easily be dropped into the PCB manufacturing process without requiring any new equipment.
Dow has developed a new silver colloidal catalyst specifically for the PCB market. The catalyst consists of spherical Ag nanoparticles averaging under 10 nm in diameter. The zeta potential, a measure of effective electric charge on the surface of the particles, is 30 mV. This is a relatively large value, resulting in a catalyst with excellent long-term stability.
Figure 1: Comparison of colloid palladium and silver catalysts
Dow’s SILVRKAT™ system consists of a family of products that cover the entire process flow from de-smearing drilled holes through plating electroless copper on the PCB surface. The conditioner, pre-dip, post-dip, and electroless copper chemistries are all designed to optimize the performance of the Ag catalyst. The process flow is very similar to that used with Pd catalysts. No equipment modification is required to switch catalyst chemistry.
The SILVRKAT™ Ag catalyst provides several advantages that differentiate it from Pd catalysts, beyond the obvious cost difference between the two metals.
- No NPTH plating: Ag catalysts are not active in an electroless Ni bath, eliminating the possibility of unwanted plating of Ni in NPTH during the final finish process for ENIG finishes.
- No tin sludge: Unlike Pd/Sn colloidal catalysts, the catalyst is tin-free, avoiding the problem of tin residue and making the system easier to maintain.
- Easier rinsing: Lower flow rates are required for rinsing, reducing the volume of DI water needed and minimizing wastewater.
- Neutral pH: The pH ranges from 4 to 5, compared to less than 2 for Pd catalysts, which minimizes the risk of corrosion present in a highly acidic environment.
- Bath stability: Stable chemistry allows a wider process window with higher working bath stability under abnormal air agitation.
The performance of the new catalyst is similar to that of Pd catalysts. Backlight tests show consistent electroless copper coverage in conductive through holes. Reliability testing demonstrates excellent via integrity after 6 cycles of thermal stress test, and no detectable Ag residues on any surface.
Although the PCB industry has not typically considered Ag catalysts, they provide an excellent alternative to Pd. Ag catalysts allow PCB manufacturers to avoid the risk of volatile Pd metal prices with a drop-in replacement chemistry that outperforms Pd catalysts by creating lower chemical residues, greater bath stability, and a wider process window than conventional catalysts.