[SI-LIST] Re: PCB Leakage Currents

I agree with Tom, and would go further - this can be caused by general 
contamination during the fabrication process, unless they fab the board 
in a clean room. It's a common hazard when routing thermal sensor 
circuits where the sensor diode currents are on the order of 100-250 nA, 
although 200 nA of leakage seems on the high side. .

You may have some power lines for the active circuitry close to the 
sense lines.The contamination could be on the surface (underneath the 
soldermask) or on the internal layers. The solution is to keep the sense 
lines far from other circuitry, and isolate them with guard traces at a 
voltage close to that of the signal being measured. The separation will 
keep the surface resistance between the aggressors and victim high, and 
the guard traces will drain off any remaining leakage current.

You will need to work a little closer with your layout house.

Alan Hilton-Nickel

Tom Dagostino wrote:

>This sounds like contamination on the board. Finger prints, poorly cleaned
>boards after soldering or contamination in the board's manufacturing process
>could all lead to leakage currents.
>
>Try washing and rinsing the board thoroughly and retest. Also check some
>un-stuffed boards to see if there is any leakage on those.
>
>Tom Dagostino
>Teraspeed Labs
>13610 SW Harness Lane
>Beaverton, OR 97008
>503-430-1065
>http://www.teraspeed.com
>tom@xxxxxxxxxxxxx
>
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>401-284-1827
>
>Teraspeed is the registered service mark of
>Teraspeed Consulting Group LLC
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
>Christopher.Jakubiec@xxxxxxxxxxxx
>Sent: Wednesday, April 20, 2005 8:56 AM
>To: si-list@xxxxxxxxxxxxx
>Cc: Christopher.Jakubiec@xxxxxxxxxxxx
>Subject: [SI-LIST] PCB Leakage Currents
>
>
>All,
>
>I am currently working with a PCB DUT board that interfaces to an ATE
>(tester).  I am using this board along with a single socket to measure
>power MOSFET DC characteristics.  The PCB is a simple 4 layer board that
>routes the force/sense resources of the tester to the MOSFET socket.  I
>currently do not know the exact layer stackup of the board since I
>merely created the schematic, and the layout was done elsewhere.  I do
>know that there are a set of force and sense traces on the board for 5
>individual tester resources.  1 resource connects to the gate, 3
>resources to the drain, & 1 resource to the source(source is also tied
>to ground).  There are also some digital traces on the board that bring
>digital power for the EEPROM and the relays, and these have tantalum and
>electrolytic capacitors included.  Everything on the board, analog and
>digital are using 1 single point ground.
>
>My problem is when I am trying to measure off-state leakage from drain
>to source of the MOSFET (Vgs =3D 0V, Vds =3D 55V), I measure >200nA of
>current before I even put the device in the socket.  When I repeat the
>measurement with a blank PCB board I measure 20nA, so I don't believe
>that it is coming from the tester or cables.  The tester resource that I
>am using for the measurement has accuracy & resolution <100pA.  Any
>ideas of where this 200nA leakage current may be coming from?
>
>Thanks,
>
>Chris
>Infineon Technologies
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