0805 Kevlin sense connections

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Is it possible to Kelvin-connect an 0805 sense resistor?  The layout
guy didn't think so.

             .-----.
    ------+--| Rs  |--+----
         /   '-----'   \
         |             |
         O             O

There isn't enough room between the pads to make the ideal connection--

    .-.--------.-.
    | |   Rs   | |       .-.----.-.
    | |  1206  | |         | |0805| |
    '-'--------'-'       '-'----'-'

  .----.      .----.      .---.   .---.
  |    |      |    |      |   |   |   |
  |    |-.  .-|    |      |   | .-|   |
  |    | |  | |    |      '---' | '---'
  '----' |  | '----'          
         |  |

   center connected = ideal


I told the layout wiz that connecting sense wires to the pads'
inner corners was just as good, as long as each Kelvin connection
was made past the solder fillet.

    .-.--------.-.
    | |   Rs   | |
    | |  0805  | |
    '-'--------'-'
  .----.      .----.
  |  # |      | #  |  # = solder fillet
  |  # |      | #  |
  |  # |      | #  |
  '----+      +----'
       |      |
       |      |

   corner connection

He didn't believe me.

I calculated a pad's resistance to be about 500u ohms, with about
250u ohms up to and including the fillet, and 250u ohms after.

Any resistance error would certainly have to be less than the pads'
overall after-fillet 250 u ohms (each) resistance. But
past-the-solder-joint pad portions should see only a small fraction
of full current, so the voltage drop error should be much smaller.

Invoking "Never wonder when you can measure," I prototyped it.

I connected a 0-ohm 0805 to two pieces of 1oz foil, one foil in air
and the other foil as-laminated (to a piece of FR-4).  I bent the
latter foil around the FR-4's end so we could probe the pad voltages
post-fillet.


(top view)
--------------------. .-----------------------
                    | |  copper foil
                    | |  1oz (35um) x 1.8mm
--------------------'^'-----------------------
                     |
                .-.-----.-.
                | |     | |
                | | 0805| |
                '-'-----'-'



(side view)
        .--.--------.--.
       #|  |  0805  |  |#  <~~solder fillet
      ##'--'--------'--'##
  ========         ======================
  -------.\             /
    FR-4 | ||           /
  -------'//           /
        ==  \         /
           1oz Cu foil


I applied 1/2A and measured the voltage drops to 5uV.

The upshot:
 o 63/37 solder fillet from the foil to the top of the part: 600 microhms.
 o | V(inner center) - V(inner corner) | <= 5uV.  No measurable difference.
 o Thermocouple voltages from unequal heatsinking were larger than any
   potential differences across the post-fillet surface of the pads.

Conclusion:
For our 50 milliohm sense resistor, wires to an 0805's inner corners
provides an accurate Kelvin connection with total error less than
50 u ohms-equivalent for the two pads, i.e., <= 0.1%.


Cheers,
James Arthur

Re: 0805 Kevlin sense connections
:
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e.  
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I don't have a way to view your ascii art.  It's one of the casualties of t
he Google groups "improved" user interface.  They rip out multiple spaces.  
  

Since I can't see what you are talking about, I didn't read the details of  
your attempt, but that is entirely practical.  But the mention of "corners"
 concerns me.  If the traces feeding current to the 0805 resistor are from  
the far ends of the pads, you can be confident there will be a relatively c
onsistent connection to the resistor ends at the near edges of the pads and
 so a very good kelvin connection.  But if you are talking about picking of
f the voltage at the corner of the pads that may be leaving a bit of accura
cy on the table.  I would have my sense traces run between the pads and att
ach at the center of the inner edges to minimize any impact of the solder j
oint being unsymmetrical.  

One web site says the pads for an 0805 resistor have 47 mils between the in
ner edges.  That's wide enough to run 9 mil track/space.   Wikipedia shows  
what I'm talking about.  

https://en.wikipedia.org/wiki/Four-terminal_sensing#Operating_principle

With the current flowing to the part from the long edges, the near edges ha
ve a nearly kelvin connection to the component.  

--  

Rick C.

- Get 1,000 miles of free Supercharging
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Re: 0805 Kevlin sense connections
On Monday, January 18, 2021 at 8:24:14 PM UTC-5, Rick C wrote:
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the Google groups "improved" user interface. They rip out multiple spaces.
  
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f your attempt, but that is entirely practical. But the mention of "corners
" concerns me. If the traces feeding current to the 0805 resistor are from  
the far ends of the pads, you can be confident there will be a relatively c
onsistent connection to the resistor ends at the near edges of the pads and
 so a very good kelvin connection. But if you are talking about picking off
 the voltage at the corner of the pads that may be leaving a bit of accurac
y on the table. I would have my sense traces run between the pads and attac
h at the center of the inner edges to minimize any impact of the solder joi
nt being unsymmetrical.  
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inner edges. That's wide enough to run 9 mil track/space. Wikipedia shows w
hat I'm talking about.  
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have a nearly kelvin connection to the component.  

I nosed around a bit more and found another approach that helps to exclude  
the effect of the solder joint, but I'm not sure it would be easy to use on
 an 0805 part.  

https://www.electropages.com/blog/2016/01/10-tips-for-designing-with-curren
t-sense-resistors

They split the pad into three "zones" with the current flowing on the two w
ide ones along the edges.  Then a trace wide pad is used to contact the res
istor caps in the center with a separate connection.  Unless the solder joi
nt gets very ugly, this should work well.  

--  

Rick C.

+ Get 1,000 miles of free Supercharging
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Re: 0805 Kevlin sense connections
snipped-for-privacy@yahoo.com wrote:
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How about

 >
 >      .-.--------.-.
 >      | |   Rs   | |
 >      | |  0805  | |
 >      '-'--------'-'


 >              |
 >    .----.    | .----.
 >    |  # |    | | #  |  # = solder fillet
 >    |  # |-\  \-| #  |
 >    |  # | |    | #  |
 >    '----+ |    +----'
 >           |
 >           |

?

Cheers

Phil Hobbs



Re: 0805 Kevlin sense connections
On Monday, January 18, 2021 at 8:52:33 PM UTC-5, Phil Hobbs wrote:
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I considered that. I also considered one Kelvin trace to one pad-center
and the other to the other pad's corner. (One ideal, one compromise
connection.)

The layout guy rejected two inner connections as a solder-shorting hazard, but
it doesn't look bad to me.

The inner gap is 0.023", so you could slip a 0.008" trace in there with 0.008"
air gaps. I just now managed to squeeze in a 0.006" trace down the center
to each pad with your 45 deg diagonal jog just before reaching the part's
center line.

But the real revelation to me was, it doesn't matter.  Being able to probe
the potential gradients on the copper past the solder fillet was revealing. To
within my 50 microohm measurement resolution, corners were just as good
as connecting to the dead center of the pads.

Cheers,
James Arthur

Re: 0805 Kevlin sense connections
On Monday, January 18, 2021 at 9:51:05 PM UTC-5, snipped-for-privacy@yahoo.com wrote:
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Oops. 5uV / 0.5A = 10 micro ohm equivalent resolution.  Doh!

--James

Re: 0805 Kevlin sense connections
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, but  
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Did you take into account soldering variations?  Not every solder joint tha
t is adequate for electrical purposes will be completely uniform potentiall
y giving a higher or lower voltage to a corner connection than a center con
nection.  This conversation made me take a look at the power supply board o
n the project I'm working on and they guy used every variation of Kelvin co
nnection you can think of.  One resistor uses a good Kelvin connection to o
ne pad and the "ground sense" connection goes directly to the ground via an
d not the pad at all!  

I expect this is because he is thinking the importance of the Kelvin connec
tion is related to the current flowing in the connection, but in reality it
 is related to the resistance of the current sense element.  The ratio of t
he current resistor to the parasitic resistance in the connections is what  
determines the need for a Kelvin connection.  In this case, it is the tinie
st sense resistor on the board at 15 milliohms.  

In the process of reviewing the design for the current sense resistors I fo
und he has the power FETs wired in backwards.  

--  

Rick C.

-- Get 1,000 miles of free Supercharging
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Re: 0805 Kevlin sense connections
On Mon, 18 Jan 2021 16:51:42 -0800 (PST), " snipped-for-privacy@yahoo.com"

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That one above looks about as good as you're going to get. It should
be fine with 0805's and 6 mil traces. Our 0805s have 30 mils between
the pads, which just works at 6/6. Tell your layout person to man up.

This worked very well:

https://www.dropbox.com/s/baud78hpa9o86ql/P902B_Shunts_1.jpg?raw=1

https://www.dropbox.com/s/zugeh5orberbt0k/P902B_Shunts_2.jpg?raw=1

The resistors are 39 mohm 2512's to get about 10 mohms net. We
calibrate around that. More resistors helps reduce the solder
resistance inherent in a 2-wire resistor, and I expect 15 amps or so.

To do much better, you'd need a proper 4-lead resistor.




--  

John Larkin      Highland Technology, Inc

The best designs are necessarily accidental.


  

Re: 0805 Kevlin sense connections
On Monday, January 18, 2021 at 10:42:31 PM UTC-5, snipped-for-privacy@highlandsniptechnology.com wrote:
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I've seen people recommend trifurcating the inner aspect of the pad
and connecting the Kelvin sense to the center trifurcation. I'm skeptical
that does much more than assure a connection to the center of the
resistor's termination, which you got anyhow by just running the
Kelvin sense to ... the center of the pad's inner aspect.

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The lower connection (to pins 1 of R17-20) connects to that pad-bus just north of the solder
fillet, but at a position where I still measured noticeable voltage gradient in my pads running
at 1/2A. Current flow at that position is still non-zero.

Connecting at the top corner past all the current flows, just as the pins 2 Kelvin goes to the corner
above it, would avoid any trace-drops in that area.

You might arguably have wanted to tap off early like you did, to compensate for the drop across the
component's end termination from solder fillet to the top of the part, though.  Doesn't make all
that terribly much difference for a 39 m-ohm shunt.

(I really like the sense resistors with resistive film top and bottom.  Makes 'em tougher.)

The gap between your 2512s' pads is pretty small.  You could widen that up and sneak in connections
to the center of each bussed shunt connection, and add some tiny heat sink fins to make up for the lost
copper pour. :-)

Cheers,
James

Re: 0805 Kevlin sense connections
On Mon, 25 Jan 2021 06:20:47 -0800 (PST), " snipped-for-privacy@yahoo.com"

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Solid metal is even tougher!

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Unless you buy manganin or zerenin resistors, or some Vishay or Susumu
sort of thing, the details of the Kelvin connection are lost in the
resistor errors.

PCB traces are about 500 uohms per square, so a Kelvin connection
often makes sense. But don't get crazy about the details. Via in pad
is I guess ideal.

When we were in the NMR business, we made our own 6-wire shunts and
epoxied them into temperature-controlled aluminum blocks. That's a
whole nother story.

https://www.dropbox.com/s/nlwbub0yx96krgl/Manganin_Bits.JPG?raw=1







--  

John Larkin      Highland Technology, Inc

The best designs are necessarily accidental.


  

Re: 0805 Kevlin sense connections
On Monday, January 25, 2021 at 9:20:53 AM UTC-5, snipped-for-privacy@yahoo.com wrote:
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That would probably be correct if the solder wicking was always uniform.  But when it isn't, the trifurcated pad will most likely give a better connection than any other.  

--  

Rick C.

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Re: 0805 Kevlin sense connections
Rick C wrote:
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The end caps are made of plated nickel, iirc, which has a huge range of  
conductivities depending sensitively on conditions and composition--I've  
seen numbers quoted ranging from about 7 uohm-cm to 110 uohm-cm.  
(Copper comes in at about 1.7 uohm-cm, and solder somewhere between  
about 8 and 40 depending on composition.)

So it isn't instantly obvious that the end caps are going to be much  
more accurately equipotential than the pad if there are wetting or  
solder-volume issues.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: 0805 Kevlin sense connections
On Friday, January 29, 2021 at 5:15:09 AM UTC-5, Phil Hobbs wrote:
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rote:  
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echnology.com wrote:  
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n--  
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cal  
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. But when it isn't, the trifurcated pad will most likely give a better con
nection than any other.  
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Of course it is a small order effect.  I don't think it is a matter of resi
stivity.  It would be simple geometry.  But maybe this not a real thing.  I
 didn't sit down and plot our potential lines.  I wouldn't go to such measu
res myself.  I can't imagine connecting at the inner edge of the pad isn't  
good enough.  But then it all depends on the value of the sense resistor.  
Someone was trying to tell me it had to do with the current, but it doesn't
 really.  How much impact the connection makes on the measurement (the need
 for the Kelvin connection) depends only on the resistance of the joint com
pared to the value of the sense resistor.  

--  

Rick C.

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Re: 0805 Kevlin sense connections
On Fri, 29 Jan 2021 05:15:00 -0500, Phil Hobbs

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Imagine some software that monitors the PCB base temperature and the
current-squared history of the resistor. It runs a thermal model to
compensate TCR and thermoelectrics, with suitable cal factors.


Re: 0805 Kevlin sense connections
On Saturday, January 30, 2021 at 7:21:08 AM UTC+11, John Larkin wrote:
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<snip>

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The fantasy does have to include well-defined and  stable Peltier/Seebeck coefficients.

Once you put your blob of solder on the board and the component it ought to have fairly stable properties, but you'll probably have to measure them separately for each board and sorting one from the other might be tricky.

--  
Bill Sloman, Sydney

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