# What is the best way to solder a large flat SMD shunt resistor so as to minimize joint resistance?

I have a BLDC motor controller that uses a high side 0.001 ohm current sense resistor (CSS2H-5930K-1L00F) and a MAX40201FAUA (50 V/V) to measure the current during motor operation via a Teensy 3.6 ADC. I am trying to calibrate the current sense hardware to the Teensy 3.6 microcontroller, but I am having issues with the MAX40201 outputting values way too high for the known current passing through the resistor. I measured voltages on each side of the resistor with my oscilloscope and it showed a voltage drop of 30-80 mV for a 2.5 A known load across the 0.001 ohm resistor!

So I am thinking that the solder joint is bad. During soldering, I used a stencil and a rework gun to solder the SMD components. For the high amperage traces, I later added higher temp silver bearing solder with a hand soldering iron to reinforce them. I thought this would help the traces hold up to the high intermittent current.

Since my process apparently led to either incomplete fusion of the large flat current sense resistor or capturing flux between the resistor and the PCB, what is the best method for hand soldering these types of components so as to minimize joint resistance?

Image of the PCB section housing the 0.001 ohm resistor:

• Comment not an aswer as not quite the question and probably too late now anyway: 1mΩ is very small and this was always going to be tricky. You can get four-terminal resistors for current measurement, which make this much easier. – Jack B May 27 '20 at 12:45
• It's not clear where the current flows in the three branches off pin 2 of the resistor. – Spehro Pefhany May 27 '20 at 12:57
• "So I am thinking that the solder joint is bad." It would be helpful if you could take a picture of the joint then, zoomed in as much as possible. But looking at the board from the side would likely tell if the joint is bad, you'd likely have a gap between the component and the board. – Lundin May 27 '20 at 13:16
• @JDD If it is a high side resistor then you probably used 2 scope probes to measure the voltage on either side of the resistor and took the difference to measure the current.This does not work with voltages as small as you're describing. The difference of 2 probes is not accurate enough to measure a difference this small. There may be nothing wrong with the solder. – scorpdaddy May 27 '20 at 13:28
• @scorpdaddy Especially if he was scoping the main terminals and not the kelvin terminals. Oscilloscopes aren't known for their accuracy to begin with. – DKNguyen May 27 '20 at 13:30

1mm of a 9mm wide 2oz trace is about 27$$\\mu\Omega\$$ at room temperature so about 3%- so I think < 5% should be achievable, with care.