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 [email protected] Any opinions expressed are those of my invisible friend, who really exists. Honest. -----Original Message----- From: HarrisK@anetMHS (Kevin Harris){MHS:[email protected]} Sent: Monday, September 15, 1997 6:20 PM To: emc-pstc@anetMHS ("EMC-PSTC E-mail"){MHS:[email protected]}; 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 [email protected]
