Bill,
You are not slow on the uptake; this is a new world and a new test.
TD, time to decomposition, is defined as the amount of time at
a specified temperature that it takes a material to lose 5% of its
weight. The working hypothesis is that at that time the material is
starting to come apart at the molecular level - delamination can't
be far behind; and, there is a loose correlation with other bad things
like via's being stretched out and pulled apart ...
It's new; not a lot of the material specs up until now have carried
a TD value; I am only just now starting to see it on new specs.
The laminators have been testing their materials, all of them, (at
least the laminators I know ...) for TD. There is work to try to
correlate a min TD to the ability to withstand so-many (to be
defined) lead-free assembly process exposures.
So far, the higher the TD the better. Early data seems to indicate
that the culprit in "regular FR4" that causes it to be not robust
at lead-free temperatures is the curing agent; new agents are
being developed. But I stress that this is an area with intense
investigation right now, and new information is coming along pretty
regularly (everyone needs to be ready for 2006) - the laminators who
want to stay in business will make darn sure that their newer
resin formulations can stand up to lead free temperatures. Right
now, that is not true for all laminates ... there will be some painful
transitions for those not watching the store. If an assembly house
moves to an all-lead-free process and the design hasn't called out
a material that is lead-free capable, it *might* be a problem. I say
might because it also depends how many reflow cycles any particular
board will see; how much copper is in the board which means how much
heat they will pump into it, etc... I am hoping that the assembly
shops are or will take a policing action and check for laminate
compatibility for awhile after they convert to lead-free ...
Anyway, a lot of people don't even know about TD yet, so ..
if you want to read up a little, go to CircuiTree's home page;
go to archives, then do a search - see if you can snag an
article or two
Regards,
Valerie
Bill wrote:
It's still not clear what the issue is with the Td for me... I guess I'm
just a little slow on the uptake...
The boards do get exposed to excursions of temperatures much higher than
the
Tg temp when soldering and it has always been a blurry concept to refer to
the point or range where the material changes... but it seems that if the
exposure is short, the change is not significant or can't react to the
fast
change in temperature fast enough to affect the board... repeated exposure
seems to add up to more stresses and help break down the bonds of the
epoxy/glass resin system... so maybe that's what they are trying to
quantify
by calling it Td...
Can anyone clarify that concept for me?
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free 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 DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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
---------------------------------------------------------------------------------
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
|
