Jack,
Duty cycle does affect the total amount of heat energy generated. You need
to know what that duty cycle is and over how much time... is it on for 1 sec
out of every 2 seconds? or 10 sec for every 20 seconds or longer? Perhaps
it's on for 6 hours and off for 6 hours... that is also a 50% duty cycle
'per day'... so you need to specify just what the duty cycle measurement
is...
If it's longer than the time it takes to reach the Tg of the laminate, you
may disregard it. It might as well be on all the time. You need to know how
long it takes for your project to reach thermal equilibrium in the highest
ambient air temperature the unit will be required to operate in.
The time to reach critical temperature failure is a function of the heat
generated, the thermal conductivity of the medium through which it must pass
to the ambient air and the temperature of the ambient air and it's movement
characteristics under forced convection if you have that option...
The Mass of the heatsink (or copper trace or plane) will also affect the
'dwell time' before reaching thermal breakdown of the board material... If
it's a small mass the board temperature will rise quicker. If it's a large
mass the board temp will rise more slowly... but it will eventually reach
thermal equilibrium either way.
Thermal equilibrium is reached when the rate of heat generated at the source
is equal to the rate of heat escaping to the ambient air... and the
temperature of the board material, heatsink, components, etc. will have
stabilized at the temperature level that it will for that specific
combination of conditions. You might want to measure that and plot a graph
of the temperature over time of the critical component or board material...
Then do a 50% duty cycle measurement and see how much longer it takes for
assembly or component to reach thermal equilibrium... then you can make some
judgments based on the empirical data. The key to this is what device or
item will fail first at the lowest temperature or highest temperature that
it will see during operation or storage for the expected life of the
product.
Best regards,
Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
http://pcbwizards.com
-----Original Message-----
From: Brand, Rodney [mailto:[log in to unmask]]
Sent: Thursday, October 07, 2004 12:01 PM
To: [log in to unmask]
Subject: Re: [DC] Trace Heating
Jack the heat generated is almost instantaneous, heat transfer takes
time to reach equilibrium. A 50% duty cycle will heat the trace
instantly but the system will reach a different equilibrium temperature.
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
C. Olson
Sent: Thursday, October 07, 2004 2:35 PM
To: [log in to unmask]
Subject: [DC] Trace Heating
It seems like everyone is asking me questions I can't answer today.
We have been using the thermal trace calculator from UltraCAD
( http://www.ultracad.com/calc.htm )
but today someone asked me how time affects the temperature.
For example, what if the trace is only energized for half a second?
How hot will it get? Or how about a 50% duty cycle?
I can't seem to find any info about it. Anyone have references?
Jack (aka "one hot board designer")
------------------------------------------------------------------------
---------
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
------------------------------------------------------------------------
---------
----------------------------------------------------------------------------
-----
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
----------------------------------------------------------------------------
-----
---------------------------------------------------------------------------------
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
---------------------------------------------------------------------------------
|
