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After going several years between updates, IPC has released IPC-2221B, the industry standard for designing printed circuit boards. PCD&F editor in chief Mike Buetow spoke in January with Task Group chairman Gary Ferrari about the critical changes to the spec.
[Disclosure: Buetow worked as an IPC staff member under Ferrari on the original IPC-2221.]
MB: It’s been a long road since the IPC-2221A was released. What’s new, and what’s changed?
GF: IPC-2221A didn’t cover anything on lead-free. We wanted to make sure that in the appropriate sections we included warnings or advice on as much of lead-free as we could – the materials, decomposition, and so on – that’s a big factor in the materials set. Then there are all the new surface finishes. For tin-lead, everyone used ENIG and HASL. Now we have nine or 10 finishes: ENEPIG, immersion silver, immersion tin, electroless gold, it goes on and on. IPC-2221B includes a chart that lists all the surface finishes, and what they are recommended for – e.g., keyboards, keypads, gold wirebond – what’s good for RF, what’s not good for RF. It’s all shown in a Consumer Reports type of layout.
We realized that there was no point for [the IPC-2221 task group] to try to re-describe all these finishes; that’s what the IPC Surface Finishes Task Group is better suited to do. So we went to [Surface Finishes Task Group chairmen] George Milad and Gerard O’Brien and asked for descriptive summaries for each of the most popular surface finishes. Information such as typical thicknesses, what applications they are good, and not good for, and shelf life, to name a few.
Then we conducted a big research project on the press-fit connectors. That one was tough. Phil Henault of Raytheon was in charge of that effort. He did a lot of research. At the end of the day, he had all this data. Each connector had its own hole size; you couldn’t really come up with a recommendation that was common to all. We finally came up with a tutorial about press-fit pins and what to look for, but could not come up with a table for recommended hole sizes because it was all over the map.
We expanded the sections on thermal management and metal core designs. This wasn’t really influenced by LEDs; 80% to 90% of the task group members come from the aerospace sector. We tried to glean information from wherever we could.
The biggest controversy was the development of a new quality conformance coupon set. We spun off a task group, chaired by Tim Estes of CAT to look at the coupons. In the past, we would develop all the coupons, and the IPC-6012 Task Group would pick them up and apply the appropriate testing, but the IPC-2221 had to invoke them and show the design in the design standard. This time, we spun it into an Appendix, which can be updated on a regular basis instead of waiting for a major revision. The instructions we gave Tim were simple: Look at the coupons and find out whether people were using them. If not, why not? And if they don’t apply anymore, get them out of the standard.
As an example, the registration coupon came from IBM years ago to measure annual ring and measure breakout using an x-ray. With all the copper planes, you couldn’t see anything. We scuttled that, and developed a new registration coupon that is mathematically developed. We modified or added new coupons – there are nine new coupon designs now.
This was a really big one. What we were afraid of is that, when a coupon says you need to take the worst-case land size and worst-case component, there’s no guarantee the board shop will use the right one. There’s no advice about which blind and buried vias to use, and today’s designs are usually so dense, the plating of holes is affected by the hole density in an area. So we now have a coupon that reflects the density we see today. The concern we had was, we wanted to develop a software package that could be distributed to the fabricators that would automatically extract from the native CAD files what data was needed to create the coupons. There would be no human intervention. We should have a software validator for the extraction program and generator. You can’t just go with Gerber; it needs to be the IPC-2581 [the data transfer standard]. That’s the primary route we’re going to go with.”
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