Re: EN 60950 and component heating
Hello from San Diego: Kevin Harris asks some questions about temperature measurements. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . For safety purposes, we are concerned with the performance of insulation. Insulation performance is a function of its temperature. (We may also -- for safety purposes -- be concerned with the physical support of the conductors by a plastic material, which may sag as a function of temperature.) Most electrical insulators are poor thermal conductors. To get an accurate measurement of insulation temperature, it is better to measure the temperature of the conductor in contact with the insulator. This MAY give a slightly pessimistic value, but that is better than a lesser value. So, measuring the diode pad gives a reasonable measurement of the insulation temperature. 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? Because the insulator is a poor thermal conductor, no matter how close you get your thermocouple (or probe) to the PCB pad, there will be a thermal drop between the pad and the thermocouple, and the measured temperature will be less than the temperature at the pad. 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. There are several ways of reducing pad temperature: 1. Use long leads which increases the thermal resistance from the diode junction to the pad (previous mentioned). 2. Mount the diode away from the PCB to reduce radiated heat (previous mentioned). 3. Use maximum pad size (i.e., don't worry about keeping it circular) on BOTH sides of the board to increase radiation from the pad to the surroundings. This (heat-sinking) is probably the single most effective way of reducing temperature at the PCB. (Of course, this makes it harder to get a good solder joint because it takes more heat to reach solder temperature!) 4. Change to a Schotky diode (to reduce the forward resistance, which reduces the I*I*R power dissipation). (Changing to a higher power diode will not change the power dissipation and therefore will not change the temperature. Changing to a physically larger diode will increase the radiation, thus reducing temperature.) 5. Change to a higher temperature rated PCB (previously mentioned). One more point. Often, running a PCB at a temperature slightly above its rating will result in permanant discoloration. This shows that a chemical change has occured in the PCB material. The discolored PCB now no longer has the same electrical or physical characteristics as a normal colored board. You should not expect a discolored board to have the same dielectric properties as a normal colored PCB. Best regards, Rich - Richard Nute Quality Department Hewlett-Packard Company Product Regulations Group San Diego Division (SDD) Tel : 619 655 3329 16399 West Bernardo Drive FAX : 619 655 4979 San Diego, California 92127 e-mail: ri...@sdd.hp.com -
Re: EN 60950 and component heating
Hi Glenn, Appreciate your response. but, I believe you raised two separate and equally valid points. 1. Insulation breakdown. 2. Some other safety hazard caused by heat. Understand your primary concern well. As far as the secondary point, the two of them fought it out and one had it one way, and the other had it another. I believe one of them (CSA) had some trouble with a product whereby a transformer had come loose. Not sure of the specifics of that situation, but I believe that was his resolution for it. Doug -- From: Lesmeister, Glenn glenn.lesmeis...@compaq.com To: 'dmck...@paragon-networks.com'; 'IEEE Product Safety Technical Committee -' emc-p...@majordomo.ieee.org Subject: RE: EN 60950 and component heating Date: Wednesday, September 17, 1997 4:27 PM Doug, To answer your question of which is better: The primary concern should have been the temperature of the windings to see if the insulation (between primary and secondary?) would have exceeded the allowable limits and thus possibly break down. A (much lesser) secondary concern would have been the risk of a fire starting either in the transformer or in other components due to the heating of the transformer. Glenn Lesmeister -Original Message- From: dmck...@paragon-networks.com [SMTP:dmck...@paragon-networks.com] Sent: Tuesday, September 16, 1997 5:35 PM To: IEEE Product Safety Technical Committee - Subject: Re: EN 60950 and component heating Hi Kevin, I had a rather bad experience between UL and CSA in the older days when there wasn't so much discussion and agreement between them. I had set up an MOU between them with UL as the test location. Went like this ... Switching power supply. Has a transformer. Must do abnormals on it. UL does the abnormals. Temp probe on the *windings* of the transformer. CSA said, No way. Since the real concern is the PCB flaming from over temps from the transformer, we want the probes on the *bobbin*. UL said, No way. Since the real concern is what generates the heat, we want the the probes on the *windings*. To this day, I can't say absolutely which way is the better. Guess I didn't help you much either. Regards, Doug From: Kevin Harris harr...@dscltd.com To: EMC-PSTC (E-mail) emc-p...@ieee.org Subject: EN 60950 and component heating Date: Monday, September 15, 1997 6:14 PM Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com
Re: EN 60950 and component heating
Mark: I don't really think that you'd want to have a shiny, conductive piece of aluminium foil bouncing around inside your power supply, shorting things out and compromising your spacings. These self-adhesives don't last too long under ageing ... Cheers, Egon :-)
EN 60950 and component heating
Hello Kevin, The heating test performed on your PCB is to verify the compliance of the PCB usage. In some cases, the temperature on the body of non-safety critical components must verify to determine the appropriate spacing of the surrounding components, such as wires (rated 105) or capacitors (rated 85). Following are some of other ways to work around you would want to try: 1. Change the diode in question by increasing the current rating of diode. 2. If the vertical clearance is permitted; mount the diode vertically with the use of a plastic spacer (94V2 or better) between diode body and PCB. This spacer is to maintain the position of the component after 3 Newton push test. 3. If temperature is marginal, change the PCB rating to 130 degree C operating temperature. Consider that the real estate on the PCB is better used when diode is in vertical position. If the above still does not help, just take two Tynenol and call the doctor in the morning. Good luck. Tac Pham ph...@compuserve.com
Re: EN 60950 and component heating
Hi Kevin, I had a rather bad experience between UL and CSA in the older days when there wasn't so much discussion and agreement between them. I had set up an MOU between them with UL as the test location. Went like this ... Switching power supply. Has a transformer. Must do abnormals on it. UL does the abnormals. Temp probe on the *windings* of the transformer. CSA said, No way. Since the real concern is the PCB flaming from over temps from the transformer, we want the probes on the *bobbin*. UL said, No way. Since the real concern is what generates the heat, we want the the probes on the *windings*. To this day, I can't say absolutely which way is the better. Guess I didn't help you much either. Regards, Doug From: Kevin Harris harr...@dscltd.com To: EMC-PSTC (E-mail) emc-p...@ieee.org Subject: EN 60950 and component heating Date: Monday, September 15, 1997 6:14 PM Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com
RE: EN 60950 and component heating
Doug, To answer your question of which is better: The primary concern should have been the temperature of the windings to see if the insulation (between primary and secondary?) would have exceeded the allowable limits and thus possibly break down. A (much lesser) secondary concern would have been the risk of a fire starting either in the transformer or in other components due to the heating of the transformer. Glenn Lesmeister -Original Message- From: dmck...@paragon-networks.com [SMTP:dmck...@paragon-networks.com] Sent: Tuesday, September 16, 1997 5:35 PM To: IEEE Product Safety Technical Committee - Subject:Re: EN 60950 and component heating Hi Kevin, I had a rather bad experience between UL and CSA in the older days when there wasn't so much discussion and agreement between them. I had set up an MOU between them with UL as the test location. Went like this ... Switching power supply. Has a transformer. Must do abnormals on it. UL does the abnormals. Temp probe on the *windings* of the transformer. CSA said, No way. Since the real concern is the PCB flaming from over temps from the transformer, we want the probes on the *bobbin*. UL said, No way. Since the real concern is what generates the heat, we want the the probes on the *windings*. To this day, I can't say absolutely which way is the better. Guess I didn't help you much either. Regards, Doug From: Kevin Harris harr...@dscltd.com To: EMC-PSTC (E-mail) emc-p...@ieee.org Subject: EN 60950 and component heating Date: Monday, September 15, 1997 6:14 PM Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB
EN 60950 and component heating
Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com
RE: EN 60950 and component heating
Kevin, Good question. By measuring the pad there are several items to consider. 1st, is the thermocouple in electrical contact with the pad? If so, you may have an incorrect reading caused by currents from the pad through the probe. 2nd, In measuring the pad, you are measuring the junction temperature of the soldered connection. This may or may not be a valid measurement. It is if you want to find out if the solder is going to melt or get stress cracks from repeated heating and cooling. It does not necessarily represent what the pcb material itself is seeing for temperature. 3rd, my practice is to measure temperature of the pcb near the device, either next to or underneath it, depending. 4th, to decrease the temperature of the pad, try adding more copper aorund the pad. A larger surface area, especially if on both sides of the board, will spread the heat out more. You may need to provide multiple current paths to the pad to keep one of them from heating up more than it should. You can also add ripples or bumps to the copper to increase surface area even more. 5th, if you are more concerned with the pcb and not the pad, then you might try moving the component off the board using longer leads, standoffs, etc. I have placed up to 25 watt resistors on boards by using longer leads and leaving an air gap of 1/4 to 1/2. Last, there are always heatsinks and fans to use to keep component temperatures, and thus pads and boards, cool enough to eliminate problems. And since it is a non-user access area, that makes it even easier. Regards, Scott Douglas Principal Compliance Engineer ECRM Incorporated Telephone: 1-508-851-0207 Facsimilie: 1-508-851-7016 e-mail: sdoug...@ecrm.com ___ From: Kevin Harris on Tue, Sep 16, 1997 12:35 AM Subject: EN 60950 and component heating To: EMC-PSTC (E-mail) Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com -- RFC822 Header Follows -- Received: by macgtwy.ecrm.com with SMTP;16 Sep 1997 00:34:53 -0400 Received: by highlight.ecrm.com (AA10846); Mon, 15 Sep 97 23:30:10 EDT Received: from ruebert.ieee.org by maildrop.ecrm.com (XAA13124); Mon, 15 Sep 1997 23:31:42 -0400 (EDT) Received: (from daemon@localhost) by ruebert.ieee.org (8.7.5/8.7.3) id SAA03229 for emc-pstc-list; Mon, 15 Sep 1997 18:11:39 -0400 (EDT) Message-Id: 21ED5A1AFBBFD011B07000805F49DF4309252E@NT_MAIL From: Kevin Harris harr...@dscltd.com To: EMC-PSTC (E-mail) emc-p...@ieee.org Subject: EN 60950 and component heating List-Post: emc-pstc@listserv.ieee.org Date: Mon, 15 Sep 1997 18:14:01 -0400 X-Priority: 3 Mime-Version: 1.0 X-Mailer: Internet Mail Service (5.0.1457.3) Content-Type: text/plain Sender: owner-emc-p...@majordomo.ieee.org Precedence: bulk Reply-To: Kevin Harris harr...@dscltd.com X-Resent-To: Multiple Recipients emc-p...@majordomo.ieee.org X-Listname: emc-pstc X-List-Description: Product Safety Tech. Committee, EMC Society X-Info: Help requests to emc-pstc-requ...@majordomo.ieee.org X-Info: [Un]Subscribe requests to majord...@majordomo.ieee.org X-Moderator-Address: emc-pstc-appro...@majordomo.ieee.org
Re: EN 60950 and component heating
To add to Art Michael's ways used to work around the problem of PCBs getting too hot, if the heat is being radiated (rather than conducted through the leads) from the component to the PCB, put a shiny reflective surface on the PCB (self adhesive aluminium foil, or just an area of copper on the top of the board under the component). Every little helps. Regards, Mark (whose PCBs went black and crunchy when they got a little too hot...) -- Mark Hone Wellman CJB Limited Email: m...@cjbdev.demon.co.uk Airport Service RoadTel: +44 (0)1705 664911 Portsmouth, Hampshire Fax: +44 (0)1705 697864 PO3 5PG, ENGLAND
RE: EN 60950 and component heating
We run into this quite often, and yes, I do consider the PCB measurement
to be a valid measurement of the PCB temperature whether you are on a
pad, a trace, or laminate. My experience with agencies is that they
agree, and will allow (for example) a power resistor to be as hot as it's
manufacturer says it can be, as long as the PCB underneath is not
exceeding its limit.
You may already be aware of this, but be careful of making measurements
right on the pad (or any bare live part) for 2 reasons. The first is
that some (inferior) temperature measurement equipment gives bad readings
when noise is introduced onto the thermocouple by placing it on a noisy
bare live part. It is easy to check for this by turning the EUT on and
off and seeing if the temperature reading instantly changes by a large
amount. The second thing to watch out for are the common-mode and
channel-to-channel voltage ratings of the temperature meter
We use one of 2 methods to get around the problem you describe. The
first, if overhead clearance will allow it, is to raise the part off the
board (usually needs 1/8 or more) using a spacer or by lead-forming a
kink into the leads. The second is to put a barrier of some sort between
the part and the PCB. We've seen as much as a 10C improvement on PCB
temperatures when mounting hot parts flush on a piece of 0.010 Nomex.
Hope this helps.
Regards,
Jim Eichner
Statpower Technologies Corp.
Burnaby, B.C., Canada
jeich...@statpower.com
Any opinions expressed are those of my invisible friend, who really
exists. Honest.
-Original Message-
From: HarrisK@anetMHS (Kevin Harris){MHS:harr...@dscltd.com}
Sent: Monday, September 15, 1997 6:20 PM
To: emc-pstc@anetMHS (EMC-PSTC E-mail){MHS:emc-p...@ieee.org};
JEichner; bceresne
Subject: EN 60950 and component heating
Hello All,
In testing some product for excessive temperatures I have come up
against the following problem. Consider a diode (part of a bridge
rectifier circuit) and the PCB underneath the component. If one measures
the temperature of the diode it does not come close to the specification
for the part. However if we place a thermocouple on the pad where the
diode is attached to the PCB and we consider that as a temperature
measurement for the PCB material itself ,then the temperature obtained
is above the board manufacturers spec of 110 C (when we take into
account our maximum permissible ambient temperature of 49 C). By the by
all this is NOT operator accessible if that makes any difference.
Questions.
1. Is this a valid temperature measurement for the PCB? I'm of two minds
on this. It could said that I'm really measuring the diodes temperature
and not the PCB. On the other hand the diode pad does touch the PCB .
2.Would it be more reasonable to measure the temperature in the same
neighborhood as the pad but make sure that the probe does not touch the
PCB pad? Would a notified body be of the same opinion?
3.If you feel that the first method is a valid measurement technique
then do you know of any ways to work around the problem? There are many
power devices that can easily and safely exceed a PCB material spec of
only 110 C.
Thanks for your opinions!
Best Regards,
Kevin Harris
email harr...@dscltd.com
RE: EN 60950 and component heating
Good point. Local heating can damage a board locally. Although people often do not test the way you indicate, I've seen several failures due to long-term heat aging of the board under a hot component. Some things I've done: 1. Space the component body off the board and/or use longer leads to reduce heat flow to the board. Preformed leads or spacers help. 2. Use oversize pads and burried layers as a heat sink (heat spreader) to minimize local hot spots. 3. Use higher rated components which are physically larger (to get rid of heat better) or more efficient (to produce less heat). -- From: Kevin Harris To: EMC-PSTC (E-mail) Subject: EN 60950 and component heating List-Post: emc-pstc@listserv.ieee.org Date: Monday, September 15, 1997 3:14PM Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com
Re: EN 60950 and component heating
Hello Kevin, This is not an uncommon problem as you have probably guessed. Rather than get embroiled in discussing the details of what one should measure, I'd rather offer a couple of ways I've seen used to work around the problem. A) Assuming you are using leaded diodes; Raise the diode off of the board and pass the leads thru tubular standoffs (rivets) which are staked to the board prior to wave soldering or pass the leads thru glass or ceramic beads prior to insertion in the board. Another standoff scheme is to bend a V into the leads prior to insertion into the board. I've seen diodes raised as much as 2 cm off the board. B) Use a higher temperature-rated circuit board. C) Use a combination of A and B, noted above. Let us know how this works out. Regards, Art Michael, Editor * * * * * * * * * * * * * * * * * * * * * * * International Product Safety News * *Check out our current offer on the * * Safety Link at http://www.safetylink.com * * * * * * * * * * * * * * * * * * * * * * * --- On Mon, 15 Sep 1997, Kevin Harris wrote: Hello All, In testing some product for excessive temperatures I have come up against the following problem. Consider a diode (part of a bridge rectifier circuit) and the PCB underneath the component. If one measures the temperature of the diode it does not come close to the specification for the part. However if we place a thermocouple on the pad where the diode is attached to the PCB and we consider that as a temperature measurement for the PCB material itself ,then the temperature obtained is above the board manufacturers spec of 110 C (when we take into account our maximum permissible ambient temperature of 49 C). By the by all this is NOT operator accessible if that makes any difference. Questions. 1. Is this a valid temperature measurement for the PCB? I'm of two minds on this. It could said that I'm really measuring the diodes temperature and not the PCB. On the other hand the diode pad does touch the PCB . 2.Would it be more reasonable to measure the temperature in the same neighborhood as the pad but make sure that the probe does not touch the PCB pad? Would a notified body be of the same opinion? 3.If you feel that the first method is a valid measurement technique then do you know of any ways to work around the problem? There are many power devices that can easily and safely exceed a PCB material spec of only 110 C. Thanks for your opinions! Best Regards, Kevin Harris email harr...@dscltd.com
