| Hi | I have been conducting my first reflow profiles. Most of the texts that I have read suggest soldering the thermocouples to a populated pcb. I am finding this very difficult. I have been using a high melting point (Sn5 / Pb95) solder and the difficulty is in getting the thermocouple junction wetted. I have even tried using an active (acid) flux without success. The only way that I managed to make a joint was to end up with a large ball of solder at the point of contact between pad / leg / thermocouple. I have set my profile under these conditions but it does not seem correct. Any comments, suggestions on connecting thermocouples please | | Regards | | Terry | Hi Terry,
Yes, there is a trick to geting these thermocouple beads into a solder joint. It's taken me a few years to figure it out, but I've finally gotten it down.
First, the smaller beads are easier than the larger ones, but I have a KIC and usually use the larger ones without much difficulty.
That high temp solder's a bear to work with, ain't it? The second you pull the iron off it freezes, and don't even try using conventional wick with it. So you only get one chance per joint to get it right. If you muff it up, just go to another joint.
Here's my Not-So-Secret-Anymore Method:
1) Pick a joint at the very end of the device. If you're right-handed, this is the last two leads on the right side of the plcc or soic. If you don't take these, you might just make one big high temp bridge up the entire side of the device. Use a large tip for your iron to make your life easier.
2) Get your thermocouple ready. No kinks or bends near the junction bead. Tin the iron with the high temp.
3) Hold the iron on the lead, and with your left hand, run the high temp solder in 'till you get a nice little blob. Pull the iron back SLIGHTLY (I mean ever so slightly), keeping a nice web of wet solder between the leads and the iron.
4) Now insert the thermocouple bead into the wet blob with your left hand.
5) CRITICAL STEP> Drag the iron and molten solder over the thermocouple to the left, encapsulating the bead.
Now, obviously if you are left-handed, reverse everything.
This is a foolproof way to attach thermocouples. I used to have a lot more difficulty with them until I figured it out. It would take me half an hour to put down 6 thermocouples; now it takes about 2 minutes. I teach engineer trainees with very little hand soldering experience and they pick it up very quickly.
I wish I could show you instead of describibg it, but after a few tries, you should be a pro.
reply »