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?
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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.
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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.
We’ve used OSP, IAg, ENIG, and ENIPIG across different products and environments. Here’s what we’ve learned:
- OSP can work well in controlled assembly lines, but the shelf life is limited, and once it oxidizes, reprocessing becomes a hassle. Re-OSP is possible but adds logistical overhead.
- Ag offers great flatness and solderability for fine-pitch parts, but in environments with hydrogen sulfide or high humidity, tarnishing can be an issue—especially if boards aren’t assembled promptly or sealed well. We haven’t seen whiskering ourselves, but it’s something we monitor.
- ENIPIG has become our go-to for applications where we want the reliability of ENIG but without nickel-related issues like black pad. Works well in harsh environments, especially with a conformal coat.
- We’ve moved most products to lead-free HASL, particularly those with less stringent pitch or coplanarity requirements. For marine or automotive environments, we either conformally coat or epoxy-encapsulate the whole board.
In our experience, the surface finish matters, but so does post-processing and protection strategy, especially when moisture, salt, or vibration is involved.
ENEPIG has largely superseded ENIG in many applications, especially where gold wire bonding or improved reliability is required. Compared to ENIG, it helps mitigate black pad issues and provides a more universal finish across assembly types.
If RF performance and environmental impact are key concerns, EPAG is worth a closer look. It eliminates nickel entirely, which improves signal integrity in high-frequency designs and avoids the brittleness and skin-effect drawbacks sometimes seen with high-phosphorus ENIG. It also generates less hazardous waste and consumes less energy during manufacturing.
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