Resistor change value

Hi,

Has anybody encountered a case where after soldering SMD resistors their value somehow changes. I soldered them on different boards with same design and some came out right, most came out wrong. What's going on?

Thanks!

Temperature coefficient of resistance, if they are still hot after soldering?

What resistor values are you working with? Is it a few ohms, or 100 ohms+? For low ohms, poor soldering joints could be significant (cold joint, solder contaminated with junk from dirty soldering iron, etc.).

Some cheap soldering irons get way too hot for soldering. My temperature controlled Weller soldering iron never gets too hot.

Hello,

It could be that the resistors did became to hot during the soldering process.
I have here a link with a PDF with some tips from AVR Freaks.
http://www.avrfreaks.net/modules/FreaksArticles/files/15/Low Cost SMD Soldering Guide.pdf

Bertus

colinb said:

Temperature coefficient of resistance, if they are still hot after soldering?

What resistor values are you working with? Is it a few ohms, or 100 ohms+? For low ohms, poor soldering joints could be significant (cold joint, solder contaminated with junk from dirty soldering iron, etc.).

Click to expand...

10k, 5.67k and 590ohms. The 100 ohms actually remain at 100ohms.
I thought the same thing about the soldering but I only kept the iron there not even second. I was using flux so the solder sticked almost instantly.

Did you measure the resistor before and after soldering?

Is the change more than the resistor tolerance?

Are you sure there is no other conductance as part of the circuit to which the resistor has been soldered? If you've installed other components, chances are there is another conductive path that is altering the resistance measurement. Try desoldering the resistor and measure it out of the circuit to confirm whether the resistor has actually changed, or if the circuit is interfering with the measurement.

To accurately measure resistance, you need to desolder them from the board, you cannot measure the value of a resistance when it is actually part of a circuit.

crutschow said:

Did you measure the resistor before and after soldering?

Is the change more than the resistor tolerance?

Click to expand...

Yes, it's about half for the 10k. It's weird because on another board of same design it reads 10k.

Are you measuring resistance in-circuit?
You can only get an accurate reading if you are certain that there are no other components in-circuit that can affect the reading.

Heat can definitely permanently change the resistance of resistors. Cutting the resistance in half is pretty extreme, but I wouldn't figure it impossible. Are you sure they went in with the correct resistance? Maybe you've got a few mislabeled or misplaced components.

Do you know the type or resistors you're using? Size, rating, where you got them, etc;? All of that might help us in understanding the issue as well.