I totally agree: we measure temperature on the PCB top side to know if we 
     have enough heat at the end of the preheating zone to perfectly activate 
     the flux, so we can get good top side solder wicking. 
     I don't see the significance of attaching a thermocouple on a component 
     lead: you will know the temperature on the lead, but it doesn't really 
     matter as when the lead enters the wave, it heats up very fast. You still 
     don't know the temperature on the walls of the barrel (which is much more 
     critical than the lead itself, especially in the case of ground pins). If 
     you are planning to place a thermocouple on a wall, where do you put it? On 
     the top side, but still in the barrel? Between the last ground plane and 
     the top of the PCB? All of course without disturbing the heat distribution, 
     so you measure a real case. Pretty hard!
     
     If you could give me good reasons for putting thermocouples on leads, I 
     would appreciate it!
     Thanks
     Emmanuelle Crombez

______________________________ Reply Separator _________________________________
Subject: Re: ASSY: MOLE temperature probes
Author:  [log in to unmask] at uucpmail
Date:    5/30/97 11:32 AM


Hi Jan, 
Where you put the thermocouple depends as to what it is you want to measure. 
In wave soldering you will get a definite temperature gradient from the 
bottom side to the top side of the PCB. The gradient is less for thicker 
copper plating in the hole because of the better heat transfer. The gradients 
in the epoxy of the PCB will be larger because of the significantly lower 
thermal conductivity.
So, if you are interested in solder reaching the top surface and to wet 
properly, you place the thermocouple on the top land. If you are interested 
in the overheating of the PCB you would want to place the thermocouple into 
the epoxy at the bottom side of the PCB.
     
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting 
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]
     
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