Profiles in Standards Print E-mail
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Written by Mike Buetow   
Tuesday, 30 September 2008 19:00
Caveat Lector As standards go, so goes our industry, not that we talk about them much. But you don’t have to look much further than a master drawing or the program contract to know how vital a role standards play in our daily operations.

It should come as no surprise, then, that for four years running, each time we conduct a subscriber survey, coverage of standards ranks in the top two areas readers want to see (bested only by Pb-free manufacturing).

Personally, I take pleasure in that, as I spent four years at IPC working on new specs, and the poll results serve as validation I wasn’t wasting my youth (well, not in the workplace, anyway). The work did have its perks, however. For instance, committee members get to see the world. (If this is Tuesday, this must be Hartford!)

One of my favorite groups to work with was the Packaged Electronic Components Committee, in large part because they didn’t mess around: They came to work, and their main interest – component moisture sensitivity – was a hot (no pun intended) one.

Sometimes hotter than we wanted. I recall getting an urgent fax (there’s a contradiction) from a company that was threatened with a lawsuit over an alleged patent violation involving the firm’s practice of packing components in desiccant and storing them in moisture-sensitive barrier bags – a process that, as it happens, was precisely what IPC-SM-786 stipulated. Panic! A few phone calls and some digging in old conference proceedings later, and we produced a paper by Ray Prasad that predated the patent application by several months. (Whew.)

My old committee is still kicking, I’m happy to say, and this month is releasing its first effort at cracking (pun intended) the mystery of handling lead-free passives and connectors.

While IPC had previously published the related IPC-9501 series, they covered how to process components. J-STD-075, “Classification of Non-IC Electronic Components for Assembly Processes,” a joint ECA, IPC and JEDEC document, came about when “there was realization no one was working on the non-IC parts,” explains chair and old friend Steve Martell. “Once RoHS was pushed through, there was no standard, and old standards were no longer valid.”

Enter the Electronic Components, Assemblies and Materials Association (ec-central.org). Using a draft submitted by IBM, which was independently trying to tackle the problem, a consensus was formed to get IPC, Jedec and ECA together to do a joint standard under the ECA umbrella.

In lieu of anything else, many companies have been using the profiles set forth in J-STD-020 for non-IC parts. The problem, says IBM engineer Paul Krystek, who developed the draft, “was the assumption passives pose no problem.” As it turned out, in the Pb-free era, heavier modules do well at higher soldering temperatures. But passives do not, especially when situated near a large part. “Some passives barely made it with SnPb. With Pb-free, they didn’t. Passives sitting next to large mass devices perform differently than passives next to low mass devices,” says Krystek.

“And here’s the big scary part: When passives are over-thermally processed, the defect mechanisms are field defects. The devices will fail in the warranty period. No one would have found this at the contract assembly level. By now, these probably number in the hundreds of millions or billions,” Krystek says.

Given the number of soldering operations available – reflow, wave, manual and selective – what J-STD-075 sets forth is PSL, or process sensitivity level. Unlike J-STD-020, the standard foregoes accept/reject criteria for all these components. Parts that don't have established solder process conditions are classified per their PSL, and certain soldering parameters covering ramp rate and maximum temperature kick in. Then there’s what Martell and Krytek call the “exceptions table,” which includes devices that cannot live with the standard profile.

“We share this info with our designers so they understand these devices are process-sensitive, Krystek says. “We also worked with our suppliers, who need to improve their process capabilities if they want to keep being designed in. But that doesn’t happen overnight.”

Plenty of work remains. Rework, for instance was not covered because it’s a subject in itself. J-STD-075 could grow to include modules and bare boards (because they undergo multiple reflow profiles), says Martell. And component packages covered in the standard don’t go below 0603s because of a lack of data and because those parts aren’t recommended for wave soldering. “There are other conditions we need to talk about and we’re not ready to deal with deal with now," he says.

“We won’t be able to boil the whole ocean all at once,” Krystek adds.

Perhaps not. But he, Martell and the rest of the committee deserve kudos for trying.

November calling. For those who hadn’t heard, the US will elect a new president next month. It says here the past eight years have been an unmitigated disaster, at home and abroad. Let’s hope the next person we elect is concerned with fighting the battles of tomorrow, not yesterday.

 

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