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
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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
>
>
>
