LEADFREE Archives

November 2005

Leadfree@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"Davy, Gordon" <[log in to unmask]>
Reply To:
(Leadfree Electronics Assembly Forum)
Date:
Thu, 17 Nov 2005 11:49:53 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (65 lines)
DDave has suggested that I might have some comment on John Burke's use of J-STD-020 for testing bare boards for delamination. Here is my response. Below that I provide a discussion of, and a link to, a recent presentation on disposal of electronic waste that I prepared based on my postings to this forum.

Application of J-STD-020 to items other than PEMs. It's true of course that J-STD-020 is intended for plastic-encapsulated microcircuits, but I assume that what Mr. Burke means is that he is using the lead-free reflow profile of this standard and looking for delamination. There might be an opportunity here for bare board manufacturers (though I doubt that many subscribe to this forum) to use the standard as a template and develop a means to measure the moisture sensitivity of, and assign moisture sensitivity levels to, boards during a simulated lead-free reflow.

A story worth telling is that the lead-free reflow profile peak temperatures for microcircuits that appear in the standard took years to become established. Users said that they needed a safety margin (to cover for unavoidable temperature differences and thermocouple inaccuracies) above the minimum practical reflow temperature (somewhere in the range of 230-235°C) at the coldest connection on an assembly. When they first heard these claims, component manufacturers responded that they did not know how to make components that would survive the temperatures the users insisted they needed. The manufacturers argued for lower temperatures. A five-degree difference divided the sides for years. While the arguments played out, polymer chemists came to the rescue with encapsulant materials that can survive pizza-oven temperatures, even if they contain some dissolved water. Eventually the nine-category thickness-volume table with three temperatures was hammered out, with neither side really happy with the compromise - yet another problem caused by RoHS.

Jack McCullen of Intel, the co-chairman of the -020 committee (and sometime contributor to this forum), came up with the idea of specifying for a component with a given thickness and volume a single temperature that is the lowest that makers can test at and the highest that users can test at - supplier minimum, user maximum. This approach avoids all kinds of confusion associated with assigning a tolerance to the tabulated temperature.

Mr. Burke didn't say whether he was using acoustic microscopy for detecting the delamination, or just visual examination. I guess it doesn't really matter - the concern is that lead-free reflow not damage the board. A method that I have wanted for years to explore but have not been able to (too busy posting to Leadfree!), is acoustic emission, in which an ultrasonic transducer is attached to the specimen and is used to "listen" for "events" that indicate damage during simulated reflow. This method would, I think, be great for process development, as it seems likely to be able to detect incipient damage. Again, it is unlikely that many component or board manufacturers subscribe, but maybe there's an enterprising engineer out there who would be interested in trying this. For those interested in publishing, this would definitely qualify. (Incidentally, the transducer doesn't have to be exposed to reflow conditions, as the connection to the specimen can be through a quartz rod that extends outside the oven.) 

Also, I'd like to comment on a related issue that came up in Technet last June in which Steve Gregory raised a question about moisture sensitivity of tantalum capacitors. I don't subscribe to Technet (no time!), but I came across it in trying to answer the same question: Vishay has identified their Ta caps as MSL 2a, but 

1)       doesn't say how they arrived at that rating (again, no provision in J-STD-020 for anything other than PEMs), and 

2)       provides a humidity indicator card with spots of 30-40-50 percent RH instead of the prescribed 5-10-15 percent.

Also, AVX says that its Ta caps are mostly MSL 1, but some are MSL level 2 - no reference to how this level was assigned (see page 14 of https://www.avx.com/docs/catalogs/RoHS_Status.pdf.) 

In my opinion the only substantive Technet response to Mr. Gregory's inquiry was from John Maxwell, who said that he had "a lovely photo of a tantalum cap that burst open during lead free reflow." He didn't say what he meant by "burst open". 

In correspondence with my company that has been going on for months, Vishay says that while humidity needs to be controlled during storage, there's no need for it to be as low as for a PEM. We have yet to get an answer to this basic question: what kind of damage occurs when a  "too wet" Ta cap is exposed to reflow conditions? There are two possibilities:

1)       The damage is confined to the epoxy coating - moisture bursts out suddenly, possibly scattering nearby chip capacitors and producing bridging and opens under a nearby BGA (see http://www.smta.org/files/SMTAI00-Amir.PDF and Daan Terstegge, 17 Dec. 2003 Technet), or

2)       The damage is to the capacitor itself, resulting in a significant change in electrical performance (e.g., capacitance). 

I do know that conventional Ta caps use manganese dioxide as the cathode, and that some new caps use a conductive polymer instead. I'm guessing that manganese dioxide, being an inorganic compound, is not much affected by ambient moisture and hence is not much affected by reflow conditions even after exposure to high humidity. 

Polymers are another matter. All polymers absorb water to some degree, and it is certainly conceivable that caps using a polymer for the cathode could be damaged if they had been exposed to high humidity before reflow. Kemet says for its "organic" Ta capacitor they use as their basis of assigning MSL a thirty percent shift in capacitance - see item 17 of http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfilesname/F3102PolTaPerChar.pdf/$file/F3102PolTaPerChar.pdf. 

Perhaps someone will be able to add to this. 

Disposal of electronic waste. On a totally unrelated topic, I was recently asked to speak at the National Academies in Washington DC on the subject of disposal of electronic waste. The other speaker was Ted Smith of Silicon Valley Toxics Coalition, which he founded in 1982. As subscribers to this forum will know, I took the position that the best way of handling e-waste is to treat it as conventional municipal solid waste - i.e., to not coerce people to pay to recycle it.  (Voluntary recycling is fine, but activists shouldn't try to force their beliefs on unbelievers.)

Mr. Smith implies that e-waste is too dangerous to burn or bury (he says that electronic products contain a "witch's brew" of toxic chemicals). For years he and like-minded people have been as busy as termites in promoting government-coerced recycling in various states in the US to prevent ordinary disposal. 

Also, he expresses great concern about the way in which e-waste from the US and other countries is being recycled in third-world countries. He provides ample evidence that the people doing the recycling there are polluting their environment. He thinks that it is manifestly wrong for a rich country to contribute to pollution in poor countries by allowing its e-waste to be recycled there. (In effect, rich countries also pollute third-world countries by buying metals mined there, but he hasn't called for a ban on importing such metals - so far.) 

In spite of the obvious damage to the environment, it is not clear how many people - if any - are being poisoned. SVTC in 2002 and Greenpeace in 2005 visited some recycling sites and took lots of samples of the soil and water that showed horrible pollution. You can read all about it at the SVTC web site. Curiously, during neither visit were samples of recycler blood or urine taken, and so they are left warning us about poisoning "risk" instead of showing us poisoning victims. 

Mr. Smith's solution is to prohibit the export of e-waste from the US. He likes to say that the people in engaged in this recycling should not have to choose between poverty and poison. What he does not say is that since he can't get them to recycle responsibly he wants to choose for them - poverty. 

However, it is not clear is how much his solution would help. There is plenty of e-waste being produced (and recycled) in third-world countries, and it is not likely that a prohibition on export would stop smuggling. European Union countries have had such a ban ("Basel Convention") in place for nearly a decade, and yet SVTC acknowledges that that they found e-waste from those countries in China and India. In the US, how many Homeland Security officials are likely to be assigned to trying to catch people smuggling out e-waste instead of trying to catch people smuggling in narcotics and terrorist bombs and missiles? How many would you assign?

One is left with the suspicion Mr. Smith knows that his proposal is not likely to make much difference in third-world pollution (or poisonings), but he hopes that the foundations that provide SVTC with its major source of income will believe that the effort is worth contributing to. One also suspects that his real goal is to increase business for domestic e-waste recyclers. As reporters are admonished, "follow the money". How might e-waste recyclers express their gratitude for Mr. Smith's efforts? It isn't just industrialists that have to ensure a continuing cash flow to keep their organization in existence. 

A presentation based on my talk at the National Academies can be found at http://www7.nationalacademies.org/policyfellows/Events.html.

Gordon Davy 

Baltimore, MD 

[log in to unmask]

410-993-7399 


-------------------------------------------------------------------------------Leadfee Mail List provided as a service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Leadfree
To temporarily stop/(start) delivery of Leadree for vacation breaks send: SET Leadfree NOMAIL/(MAIL)
Search previous postings at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-------------------------------------------------------------------------------

ATOM RSS1 RSS2