Hi Valerie,
When specifying Tg you really want to use the lowest temperature that the
material is still stable at as the criteria for design... it's a 'worst
case' design approach and guarantees success by not allowing the tolerances
to go against you. The maximum operating temperature would be the point at
which the material just starts to change for the worse... According to the
IPC-4101A spec the Tg of FR4 (IPC-4101/21)is a range from 110 deg C to 150
deg C. Since it can be as low as 110 deg C, I would use that number in my
calculations for worst case environmental conditions... I cannot guarantee
that the FR4 material ordered under that spec would always be a 150 Deg C
unless I further defined the glass and resin content of the material...
which would drive up the price of the board if it wasn't readily available.
Your last comment actually causes me to want to define the temp evaluation
a little better. If the ambient temperature maxium is 120 deg C like he
indicated, the board temperature could be much higher if there are hotter
components on it... they would cause the temp at the board surface to rise
above ambient. The Tg needs to be above the temp the board can be exposed
to, not just the ambient temp the board is in...
So let's say you have a hot microprocessor on the board that can be giving
off 100 watts of heat while operating... then if the ambient temp is at 120C
the board may be experiencing much higher temperatures as would the chip...
you would have to calculate the amount of temperature rise above ambient at
the board surface to figure out weather or not you were exceeding the
material's Tg. Also take into account the heat generated by the currents
through the traces... this also would affect the temp rise above ambient...
there's more than just the ambient temp to consider when specifying the Tg
of the material.
Great comment Valerie,
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
-----Original Message-----
From: Valerie St.Cyr [mailto:[log in to unmask]]
Sent: Thursday, April 21, 2005 3:00 PM
To: [log in to unmask]
Subject: Re: [DC] High Temperature Environment
The Maximum Operating Temperature of FR4 is 130 to 135C (depends on the
specific material and fabricator). I can't comment more than that, but I
am sure others on this forum can add to that thought - I throw it out
there to stimulate the conversation.
The Tg really relates to what happens at the higher assembly temperatures
... not so much the working environment.
Valerie
Denis Lefebvre <[log in to unmask]>
Sent by: DesignerCouncil <[log in to unmask]>
04/21/2005 05:11 PM
Please respond to "(Designers Council Forum)"; Please respond to Denis
Lefebvre
To: [log in to unmask]
cc:
Subject: Re: [DC] High Temperature Environment
I only mentioned the Isola FR408 because I know it's readily available and
not too pricey. The intent, obviously, is to use a material with a higher
Tg than standard FR4. I believe FR4 is rated at 135C. (don't have any data
handy)
-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Thursday, April 21, 2005 2:01 PM
To: [log in to unmask]
Subject: Re: [DC] High Temperature Environment
Dennis,
That seems to be a great choice based on the limited information
supplied...
If more margin is needed for unknowns you could try the FR406 IPC-4101/26
which has a minimum Tg of 170 deg C
http://www.isola-usa.com/images/uploaded/FR406Bulletin-1.pdf
FR408 IPC-4101/24 has a minimum Tg of 150 deg C although ISOLA brand
material claims 180 deg C. But I am basing that off the IPC-4101 spec.
http://www.isola-usa.com/images/uploaded/FR408Bulletin2.pdf
I would specify the IPC-4101 spec because that is not vendor specific.
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
-----Original Message-----
From: Denis Lefebvre [mailto:[log in to unmask]]
Sent: Thursday, April 21, 2005 1:19 PM
To: [log in to unmask]
Subject: Re: [DC] High Temperature Environment
I think I'd be inclined to go with FR408 or any IPC4101/24 grade material.
-----Original Message-----
From: Don Larisey [mailto:[log in to unmask]]
Sent: Thursday, April 21, 2005 1:13 PM
To: [log in to unmask]
Subject: [DC] High Temperature Environment
We are looking at putting a small board with all components rated to
125C into a sustained working environment of 120C. Should we be
specifying a different board material other than FR4. This is a small
double sided board about 25mm X 40mm with components both sides.
Any comments???
Don
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