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: [email protected] ________________________ _______________________________________________________________________________ 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 [email protected] ------------------ 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 <[email protected]> To: "EMC-PSTC (E-mail)" <[email protected]> Subject: EN 60950 and component heating List-Post: [email protected] 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: [email protected] Precedence: bulk Reply-To: Kevin Harris <[email protected]> X-Resent-To: Multiple Recipients <[email protected]> X-Listname: emc-pstc X-List-Description: Product Safety Tech. Committee, EMC Society X-Info: Help requests to [email protected] X-Info: [Un]Subscribe requests to [email protected] X-Moderator-Address: [email protected]
