[SI-LIST] Re: How to solve short circuit issues in high dense pcbs
- From: "N. Paul Taddonio" <paul.taddonio@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 1 Nov 2011 08:25:01 -0400
You may also be able to apply freezer spray to various locations on the
board while observing the resistance from power to ground.
----- Original Message -----
From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
To: "rskiruban" <rskiruban@xxxxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Monday, October 31, 2011 5:14 PM
Subject: [SI-LIST] Re: How to solve short circuit issues in high dense pcbs
> Hi,
>
> You might have known this already, but I think it's worth mentioning:
> If you measure power planes with a multimeter, you would find that most
> low
> voltage rails appear to be a short circuit, like 1...10 Ohms. For example
> a
> 1W+ processor's core and I/O rails normally look like this. That is not a
> short circuit, but it's normal.
>
> Sometimes there is really a short circuit. Then what I do is either use a
> thermal camera, or use a logic analyzer on the power sequencing logic to
> find out which power rail has the short. I normally use an FPGA for
> sequencing and use the Actel-Identify on-chip debugger and some tricks in
> the sequencer's VHDL code. The sequencer turns every rail on, and if a
> powerOK signal doesn't get asserted on one rail (due to a short circuit
> load), then the sequencer shuts down within milliseconds and latches all
> signals (Power_OK and Power_Enable), then it displays them on my screen.
> This way its easy to see which rail has really a short, without leaving it
> on for seconds or minutes (risking a damage) to inspect by a human (me in
> the lab). So its safe enough. If you dont have an FPGA on your board for
> power sequencing, then you can still use an oscilloscope with lots of
> channels.
> You might end up lifting up inductors or MOSFETS for seeking the rail with
> a
> short. Lots of power rails feed subordinate power rails, wherre each rail
> is
> "generated" by a DC/DC converter or a powerswitch (Mosfet, especially for
> ACPI motherboards). Any of the devices on any of those rails can be the
> sinner, but you can isolate the rails by desoldering components (that
> would
> connect them, like the inductors, mosfets).
>
> regards,
> Istvan Nagy
> Bluechip Technology
> UK
>
>
> -----Original Message-----
> From: rskiruban
> Sent: Monday, October 31, 2011 8:01 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] How to solve short circuit issues in high dense pcbs
>
> Hi All,
> One of my board having a short circuit (Zero Ohms) across a power supply
> (That supply connects to thousands of decoupling capacitors and 44 BGAs)
> and Ground. Is there any method to identify the route cause for the short
> without removing any components?
>
> Note:
> ####
> 1. The board is not yet powered on.
> 2. It was verified that the PCB doesn't contains any short across supplies
> before board assembly.
>
>
> Thanks and regards
> Kiruba Sankar
> Project leader
> Hardware Design & Development
> Email: rskiruban@xxxxxxxxxxxxxxxxxx
> web: www.datapatternsindia.com
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