Spurious Response wrote:

On Thu, 26 Jul 2007 19:56:31 -0400, PeteS wrote:

Glen Walpert wrote:
On Tue, 24 Jul 2007 23:42:47 +0100, Eeyore
wrote:

The debate about lead free solders seem to be nearly as politically
charged as that about anthropogenic global warming and a casualty seems
to be useful data.

I've read plenty of comments to the effect that lead-free is less
reliable in the long term (vibration seems to be a key weakness AIUI -
maybe also thermal cycling) which presumably explains the exemptions
for certain categories, yet I've also seen some studies that claim it
can out-perform lead containing solders.

Is there any real hard and fast information out there that one can rely
on ?

Graham

Don't look to this newsgroup for factual info on lead free! Instead
look at actual test results in the trade publications such as SMT
magazine:
http://smt.pennnet.com/home.cfm
They have published numerous tests comparing various lead free
materials and processes with tin-lead. Some lead free materials and
processes are better than others (no surprise) and picking the best
one for your situation is non-trivial.

My nutshell summary of the published test results is that lead free is
significantly harder to do right than tin-lead, requiring tighter
process controls, but if done right it can be more reliable than
tin-lead for non-shock situations. Lead free is harder, stronger and
more brittle than tin-lead so tin-lead will deform plastically under
high shock when lead free will break, however lead free will withstand
more hot-cold cycles than before failure than tin-lead (better fatigue
resistance). So you need to know what the significant failure
mechanisms are in your design to pick the most reliable materials.

Glen

Another major issue is tin whiskers. We have hard evidence at
$WeBuildAvionics (where I am currently consluting) that the current Pb
Free / RoHS solder mixes have significant problems with growing
whiskers, leading to wonderful issues such as short circuits developing
under BGAs a few months after production. In a Flight control computer
(don't laugh - in a fly by wire environment it's the ONLY flight control
and virtually all late model airliners use it) this is Not a Good Thing
[tm].

The whole RoHS / Pb free thing is a political issue - the processes for
Pb Free use more hazardous substances than they get rid of.

Typical EU beauraucrats - unelected, overpaid and have to find something
to regulate to justify their existence. [1]

Cheers

PeteS

[1] Their existence, even from birth, might not be justifiable.


Damn. You actually made a post that I agree with 100%.

Seems one must fully encapsulate a finished assembly in transformer
varnish under vacuum ....

Sorry, I wasn't concentrating and I thought you were still talking about the
bureaucrats, I thought that would be a very effective solution.

to lock out the whisker growth.
Oh.. well can we still do the bureaucrats afterwards? ??

Chris