My designs are mostly small, low-voltage, sub-GHz embedded systems, with some non-safety-critical automotive products. Operating temperatures range from -40°C to 85°C. Most components are 0.5 mm pitch or smaller. Occasionally, there are PTHs and plugged thermal vias, but blind and buried vias are rare. Routing is rarely below 0.2/0.2 mm (8/8 mils).
Are IAg, ISn, or OSP viable options? Do assemblers typically object to them? Any issues with rework?
1 Like
There’s no one-size-fits-all lead-free surface finish, each option has trade-offs. What works best often depends on the design, rework needs, and your assembler’s preferences. That said, all three finishes you mentioned IAg, ISn, and OSP, are commonly used and technically viable for the kind of designs you described. OSP is typically the most cost-effective but less robust for multiple reflow cycles or rework. IAg offers good solderability and is fine-pitch friendly but can be more sensitive to handling and tarnish. ISn is decent for rework but may not be ideal for dense or fine-pitch assemblies.
It’s always worth checking with your fab and especially your assembler, some have strong preferences based on their processes or rework experience. Also, many PCB fabs publish helpful selection guides and relative cost breakdowns for surface finishes.
1 Like
We’ve had consistent results with Immersion Silver (IAg) across low-voltage, fine-pitch embedded designs. It’s a good fit for 0.5 mm pitch and has great solderability out of the box. The flatness helps with stencil printing and component alignment. IAg boards need to be vacuum-sealed and kept away from humidity, once the packaging is opened, it must be assembled within a few days to avoid tarnishing. If your assembler has good process control and moves boards quickly into production, it’s usually not an issue. For builds that sit on the shelf or go through multiple reflow cycles, consider other options.
I’ve heard EPAG (Electroless Palladium, Autocatalytic Gold) is a good alternative to ENIG or IAg, especially for fine-pitch and cost-sensitive designs. It avoids the reliability concerns often associated with nickel in ENIG/ENEPIG, such as brittleness or black pad, and can improve signal integrity at higher frequencies.
Since there’s no nickel, you also avoid the skin effect issues or brittleness issues often associated with high-phosphorus ENIG in RF or fast-edge applications. From what I understand, EPAG also has a lower environmental and energy footprint, which some customers are starting to care about more.
If you’re using high-silver content solder, it’s worth checking compatibility, as palladium can behave differently during reflow.