Final Finishes: Taking Gold Thickness into Account

Estimated reading time: 9 minutes

Bunce: Exactly, and we’ve been helping our customers understand these challenges over the last six months as the specification neared completion.

Goldman: How about educating the end-user who's specifying that they want this extra gold thickness?

Bunce: If you look at the participation in 4-14 Plating subcommittee, I'm estimating that OEMs make up over 50% of the contribution so you could say that the new specification has been driven by end user desires. However, there are OEMs, PCB fabricators and even chemistry suppliers who do not participate and some of the changes are likely to cause some shock.

Goldman: Okay, so the OEMs don't have a problem with this.

Bunce: From the end user point of view, if you're buying a more consistent product regarding gold thickness and corrosion, I think you're going to be a lot happier. From a PCB fabrication standpoint, there may be some further scrutiny of existing ENIG capability needed, but it should be achievable, maybe at a slightly higher cost.

Goldman: It sounds like, if they don't have the right measuring equipment, they can't measure their gold thickness properly?

Bunce: Measurement system evaluation for gold thickness is the starting point. A PCB fab first needs to understand their current capability and what it means in the number of required XRF measurements or a reduction in the gold thickness tolerance. If the number of measurements required (to conform to IPC-4552A) becomes excessive, and Capex for new instruments is not available, then the reduction in gold thickness tolerance may require adjustment to the ENIG process operation (or a change of process chemistry) to allow suitable gold thickness control.

Goldman: And it's almost too wide a range for what you're measuring?

Bunce: You can take up some of the gold thickness specification tolerance with measurement system variation. You can take up some of the tolerance with process variation. The more you take up with your measurement system variation, the more tightly you must control your ENIG process. That's something that we've addressed with development of the new MacDermid Enthone ENIG system—the ability to produce consistent gold thickness, basket to basket and pad to pad.

Producing an electroless nickel deposit with stable phosphorous content is key to meeting this challenge. Electroless nickel phosphorous content dictates the deposit’s corrosion resistance, so if we see significant variation in the phosphorous content then we see variation in corrosion resistance and thus variable gold thickness. We aim to produce a consistent 9% phosphorous over the electroless nickel chemical life to provide corrosion resistance stability and resultant gold thickness consistency.

Goldman: How do you maintain a 9% phosphorous in the nickel? How do you measure that?

Bunce: This is something the IPC has worked on. We've done a lot of round-robin testing. Traditionally, people used to use EDS to measure the phosphorous content of EN. If you look at some of the data produced by the committee, the amount of variation is quite frightening. Some XRF suppliers have supplied a simple-to-use XRF method that's fairly reliable—again discussed in the paper. And you can also strip the nickel and analyze the amount of phosphorous by a wet chemistry analysis but that's a real nuisance. The XRF method has been a bit of a godsend; maybe it's not as accurate as the wet analysis, but it's a hell of a lot quicker and non-destructive.

Goldman: That's important, too. I hope we get an opportunity to publish your paper in our magazine. Thank you very much for your time. I appreciate it.

Bunce: You’re welcome.

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