Great topic this - there's a lot of stuff unearthed in this thread that
rarely is understood by designers.
Just to clarify expansion a bit....
The Tg temperature is the point at which the resin in the resin plus glass
reinforcement mix starts to turn mushy. Board laminates are made up of one
or more layers of various types and thicknesses of glass weave, then
impregnated with a resin and cured to form the flat board that we all know
and love so well. Same in multilayer boards, except the extra layers are
added by fab operations using various types and thicknesses of partially
cured resin, called prepregs, becoming fully cured after the lamination
cycle.
The glass itself expands at only 3 parts per million and in itself
contributes virtually nothing to the expansion. The resin below Tg expands
at about 15 to 18 ppm and above Tg anything between 50 and 100ppm.
So at higher temperatures, imagine the resign turning mushy and starting to
expand much more (about three to six times more) than it did below Tg. The
expansion has to go somewhere, but the glass weave in the X and y axis stops
it from going that way to any great extent. The only place the resin can
expand into is the Z axis, and that's where via barrels start to take the
strain, especially on thicker boards where the absolute amount of Z axis
expansion can be enough to pull the barrel apart and create an open from one
side of the board to the other.
Different types of resins and different types of reinforcement have their
own characteristics, and picking laminates to overcome these perennial
temperature problems can be critical to design success.
Andy Kowalewski
13 McCready Place
Berowra. NSW. 2081.
Australia
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Valerie
St.Cyr
Sent: Saturday, 23 April 2005 08:06
To: [log in to unmask]
Subject: Re: [DC] IPC-4101
Bill,
I said in my intro that I made up the numbers just to show the point: that
the material CTE is
a lot higher above Tg compared to below Tg. My "numbers" are NTBTS (not to
be taken
seriously)... I didn't take the time to pull any specs ...
Sorry if I confuse that issue.
Rgds,
Valerie
Bill wrote:
I'm curious where you are getting your numbers from... as I have been
quoting IPC specs... I guess they may be slightly out of sync with your
information... the CTE or coefficient of Thermal Expansion numbers I used
came from page 127 of the CID+ study guide...
There is a chart there that I was getting the data from... but I did just
notice a note there, it's a reference that says "Z-axis expansion above Tg
can be as much as 4 times greater. For FR-4 it is 240-390 ppm. Contact
supplier for specific values of the other materials"
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
_______________________________________
Member of the San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com
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