What we wound up doing, since we know the industry won’t just move to something new, is to keep the legacy coupons in an Appendix B, as is, they won’t be supported moving forward. So the aerospace guys won’t have to go back and revise their drawings. But in the next revision, we’ll make the swap and say, “This is the one you must have.”

We also fine-tuned a bunch of other sections. Laminate materials; we added info about high-pot testing. We recommended  values  and ranges to test at. For plating, we had to include several of the new finishes. We addressed hole fill, adding a table for clarity.

IPC-4761 talks about the three ways vias are protected: partial fill, total fill and tenting. We took the IPC-4761 chart showing when to use and how to use each one, and added design criteria. For certain via protection, we said, “You just don’t do it.” Then we added limits for minimum and maximum hole size and tolerances for via protection.

We also extracted anything of value from the now defunct IPC-2224, the PCMCIA standard, and made sure it was included in our document. We went through the thin core spec (IPC-2224) because everyone is using those materials.

Finally, we addressed problems with microvias. The conventional definition said a microvia hole is 0.006" and below. The problem is the hole wall plating, which is typically 0.001" in the hole. Microvias plate very quickly and generally very small. So the spec allowed the hole wall plating to be half the amount a through hole wall plating would be. Companies are mechanically drilling holes at 0.006", and since the hole fits the definition of  0.006"  and below, they are providing half the plating and indicating that it meets the specification. So we redefined the microvia based on an aspect ratio, so someone drilling a hole at 0.005" would have to still follow drilled-hole plating specs if they were above the defined aspect ratio.

MB: Going back to when I worked on the standard there's been the question of how much tutorial to leave in. Given that the IPC-D-330 Design Guide is 20 years old, did you talk about moving the “how-to” to the Design Guide and leaving just the minimums and maximums in IPC-2221?
GF: The discussion came up, but we left the tutorial in, thinking that if the standard just gives a number, designers won’t know how it was derived.  Besides IPC-D-330 was nothing more than excerpts from the released standards and is in dire need for an update.

MB: You’ve been working on the industry design standards for 30 years. What did you do differently this time around?
GF: I think what we did very differently was to use sub-task groups, such as for press-fit components … we took out all the test coupons and made that effort a dedicated task group … going out to the Plating Subcommittee. That was a departure from the past; being able to get a group offline to put together what they think and let them present to us and kick the tires.

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