[SOLVED] 1/10 th watt SMD resitor going bad after soldering is done

[shivakumar.r]

Dear readers,
I am doing project which is used to controll heavy HP motors (nearly 10 HP).
I use 1/10th watt SMD resistors for my project.Recently i am facing issue of SMD resistor going bad.
After all the PCB soldering is done and if i measure the resistance value it is showing large values.

Example: I am using 100 ohm SMD resister at the output of current transformer for measureing the current.
after the PCB soldering is done if i measure this value it is showing 300 ohms.please help

 

[niri]

don't measure after mounting, maybe its measuring over all resistor

 

[keith1200rs]

If a resistor reads low in-circuit then it could be because of other components on the board. If it reads high then it sounds like you have damaged it during soldering. Too hot for too long can make the resistor connections detach.

Keith

 

[niri]

If a resistor reads low in-circuit then it could be because of other components on the board. If it reads high then it sounds like you have damaged it during soldering. Too hot for too long can make the resistor connections detach.

Keith

Change the package of resistor i mean change the wattage of resistor and check, calculate the value and also power on the resistor and use

 

[FvM]

Damaging SMD resistors during soldering isn't impossible but hard to achieve with suitable tools and correct processing.

Did you check the resistor marking?

I won't guess about correct resistor rating and chances to check a specific component in place without seeing a schematic.

 

[pranam77]

SMD resistors tend to be ok even when exposed to high temeratures with manual and wave soldering techniques, and would not change values in normal circumstances. Once you have mounted the resistors, resistances of other componenets will affect the reading a particular resistor. So just power it and check its working provided you are sure about the assembly and deisgn flaws in.
Cheers

 

[tpetar]

Maybe You get bad Chinese components, or try to clean contacts before measuring, clean end of instrument probe before measurement. Check your instrument.

 

[gongdori]

I agree to Niri. You can get very different value by measuring it on the board.

 

[tpetar]

What temp You set on soldering station ? Did You use flux ?

 

[doraemon]

Hello!

don't measure after mounting, maybe its measuring over all resistor

Yes, I would guess that this is the problem. See attached picture.
Problem: measuring a resistor without unmounting it.
If you use your tester, then you will inject come current in resistor R1. But
as there may be other resistor or components, some current will also go in the
other parts and you will never be sure of the resistance you are currently measuring.

The solution is illustrated with one particular example. You want to measure R1, but
there are other resistors R2 and R3. The solution: you add an op-amp that copies the
voltage at point A to point M. In this case, if VA = VM, the current i1 will be 0
(Ohm's law). Therefore if you measure VA and know the current you are injecting,
you can calculate R1 accurately.



Important note: at first, you may think that copying voltage of point A to point M
could be easily done by using a plain wire. But this will not give you the right measurement
because in this case, I1 will not be 0, but IA/R3. Why is it different? Because the amplifier
brings the right amount of current that balances exactly the system so that nothing
is going through R2. Using a wire, you have indeed VA = VM, but the current is not 0
and therefore the resistor measurement is not right.

Dora.