Hi Ted,
Your understanding of UL requirements is in sync with my own personal
experience... When Underwriters Laboratory or TUV or CE certifies a product
with their label it is for product user SAFETY reasons... certainly not to
evaluate the electrical or mechanical performance of the product. There are
other agencies that provide that kind of performance related evaluation
service.
The primary certifications we applied for when developing commercial
electrical instruments and equipment were shock and fire hazards. There are
two methods to get safety agency certified... one is by demonstrating
compliance with the design guidelines they publish for clearance and creep
distance for electrical voltage breakdown and using parts and materials that
have already been certified or the other method was through demonstration by
actual passing under rigorous destructive testing... a more expensive route
to take but still as valid because they test multiple units under the
extreme limits and observe and document the results or a number of
statistical samples.
I moved our standard choice here for fiberglass/epoxy boards from the old
110C minimum Tg FR4 material to the IPC-4101/26 (170 Tg) material a few
years ago basically to add reliability to the product and avoid material
fatigue or failure either through process exposure or in the final product
under extreme environmental exposure.
Many of the circuits we design have thermal challenges they must survive and
the extra 'designed in' margin buys you a good night's sleep when someone is
depending on the performance of your product.
It is an accepted 'good design' practice to 'de-rate' or increase your
product's tolerance for survivability and reliability by choosing components
that are not being stressed to their published limits and then they can
easily survive what your customer may expose them to. The military required
a 50% de-rating on all components and board materials to add reliability to
their equipment which MUST work... :)
If you are unfamiliar with the term "de-rating" an example would be if a
circuit required a 35 Watt part to operate normally, you would specify one
that would have a 70 Watt rating to 'de-rate' it by 50%, basically doubling
the requirement or specification on the part.
More info on this practice is available in the Rome Air report
RADC-TR-82-177 published June 1982. It has been sighted as a guideline for
de-rating for many years in numerous designs across the country.
I'm not sure where you can get a copy, I have kept a paper copy of it for my
reference that I have had since the 1980's... published by the National
Technical Information Service, US Dept. of Commerce in Washington.
It's about 255 pages...
Other references I have seen that are somewhat related and lend background
and reference to the reliability requirements are MIL-HDBK-338, MIL-STD-975,
RADC-TR-84-254, and RL-TR-92-11.
Building Reliability into your products only strengthens your company's
competitive edge...
Given the choice between a low reliability product for less money and a
higher reliability product I tend to pay a little extra so I don't have to
deal with the failures and replacement costs... wouldn't you? It's just good
economic and business sense. And as a supplier of robust products you get
the extra bonus of a reputation for being a company that builds reliable
equipment... that's kind of fun to brag about too... :)
Best regards,
Bill Brooks
-----Original Message-----
From: Ted Tontis [mailto:[log in to unmask]]
Sent: Monday, June 25, 2007 7:36 AM
To: [log in to unmask]
Subject: Re: [TN] UL specification on temp rating for bare board material
David,
Thanks for the response and the image of your fabrication notes. All of
our designs are Pb so I do not have to worry about the elevated temps of a
Pb free process.
My problem is that the boards I normally deal with are large in size
with a great deal of copper pours all over the board. I know from a process
and field stand point the boards would be more reliable using a higher Tg.
The response I get is that we have large vias to help offset problems with
barrel wall strength.
Regards,
Ted
Regards,
Ted
----- Original Message -----
From: "David Greig" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, June 25, 2007 9:10 AM
Subject: Re: [TN] UL specification on temp rating for bare board material
> Ted,
>
> Here is a snap of part of the notes I use. If your assembling lead free
then most < 170C materials are unlikely to be suitable.
>
>
> Best Regards
>
> David Greig
> ______________________________
> GigaDyne Ltd
> 5 Albany Business Centre
> Gardeners Street
> Dunfermline KY12 0RN
> United Kingdom
> t: +44 (0)1383 624 975
> www.gigadyne.co.uk
> ______________________________
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Ted Tontis
> Sent: 25 June 2007 14:50
> To: [log in to unmask]
> Subject: [TN] UL specification on temp rating for bare board material
>
> I have just sent out two designs to be quoted both are high power designs
for industrial applications. now I am new here and
> thought it would be a good idea to add more detail to the fabrication
notes. The material I called out for was 4101/23 or 24
> with a minimum Tg of 170, the old notes didn't call out for anything. So I
get the quotes back and my employer said to go back
> to the 140 Tg, the new material isn't needed. I tried to explain to him
that a 140 Tg used in the environment you expect to have
> this product operate in is risky. He explained to me that UL specifies an
operating temperature of the material and that we will
> never reach that level. I tried to explain to him that's when the material
fails in a ball a flames. UL is not concerned if the
> product will function correctly or not, their concern is will someone get
hurt or die. Is my understanding correct on the UL
> requirements? UL is just a flame and voltage limit not a limit on normal
operating temperature ranges?
> He pays the bills so he can do what he wants however, I am getting
tired of trying to explain to him not to confuse we have
> never had a problem so far with you have just been lucky thus far.
>
> Thank you,
>
> Ted Tontis CID
> Manufacturing Engineer
> Custom Power Technology
> N93 W14605 Whittaker Way
> Menomonee Falls, WI 53051
> [log in to unmask]
> www.custompowertech.com
> Phone: 262-253-0880
> Fax: 262-253-0948
>
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