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I have soldered a simple adapter to connect my AVRISP mkII to my bradboard.

The AVRISP mkII couldn't communicate with the ATmega on my bradboard. After a while I figured out, that the voltage at the reset pin of the ATmega is only about 3.2 volts. I disconnected the AVRISP and still the voltage was at 3.2V. After plugging out my adapter the voltage was back at 5V.

After inspecting the solder pads and some cleaning (I even used a knife to be sure nothing shorted together) I reconnected the adapter and measured again. The voltage is at about 3.7V.

What is causing the voltage drop? Can the remains of flux conduct?

The picture shows my adapter (back and front) the red line is connected to the reset pin, the blue line to the GND pin.

enter image description here

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Do you measure continuity between those pins? – Kevin Vermeer Jul 22 '11 at 20:29
When I measure continuity my multimeter is not beeping. If there would be a direct connection (less then 10 Ohms, according to the manual of my multimeter) the voltage should drop to 0V instead of 3.7V, right? All voltages are measured between GND and the reset between of the ATmega. – PetPaulsen Jul 22 '11 at 20:45
Less than 10 ohms is considered continuity, but your voltage regulator would likely complain about an 11 ohm impedance, wouldn't it? Somewhere on your breadboard, in your AVRISP, in your cabling, or (most likely) on this PCB, there is a path for current between those pins. With power off, plug everything together and measure the resistance. – Kevin Vermeer Jul 22 '11 at 21:48
It's also possible that whatever short you have isn't behaving like a resistor. For instance, if your microcontroller is powered by 3.3V, applying 5V to a pin would cause internal diodes to conduct, possibly to 3.7V. – Kevin Vermeer Jul 22 '11 at 21:49
Its my little PCB for sure. When I remove it, the voltage is at 5V, because of the 10K pullup resistor. The microcontroller is powered by 5V. – PetPaulsen Jul 22 '11 at 22:05
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1 Answer

up vote 1 down vote accepted

I solved my problem.

The solder wire was water solvent and has to be cleaned (according to the manufacturer).

After cleaning with water instead of alcohol the flux was removed. My board is working now.

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Good lesson. Change your solder type NOW ! :-). – Russell McMahon Jul 23 '11 at 1:07
@Russell McMahon - What? Water-soluble flux is more pleasant to clean than alcohol-soluble. As long as you use the correct solvent to wash the board, there shouldn't be a problem. – Connor Wolf Jul 23 '11 at 4:16
4  
I meant that any flux that affects insulation properties enough to need cleaning off for successful operation is better not used. In a production environment you may have a flux that has enough good properties that you are happy to need to follow rigorously follow formal cleaning procedures. In an amateur and prototyping situation it should almost never be necessary to clean flux off after soldering. – Russell McMahon Jul 23 '11 at 5:09
Only "no-clean" flux comes close to being electrically inert, and it still has limitations such as being able to "throw off" the value of a megohm-range resistor it crosses. Rule #1 is always clean your work. – Mike DeSimone Jul 23 '11 at 5:18
I use no-clean, and I still clean the crap out of the boards. – John U Feb 7 at 9:54

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