[SI-LIST] Re: How to solve short circuit issues in high dense pcbs
- From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 31 Oct 2011 06:10:27 -0500
> Apply power using a high current capable supply set to 0.15V or less
at
> the board connection and slowly increase the current while observing
any:
Great advice. In addition to keeping the ESD protection from turning
on, it's best to keep the ICs from turning on as well. That is, keep
the currents to a minimum such that the shorted part draws the most
current. This approach will heighten the distinction between the
shorted part and all the other parts.
> 1. FLIR image for hot spot.
> 2. Voltage drops by region.
> 3. Manual temperature monitor by region.
We have had limited luck with these approaches. When power planes are
thick, localized heating quickly dissipates across the board, and the
temperature at the shorted part will be very-slightly elevated relative
to the rest of the board. Similarly, the voltage drop across the board
will likely be minimal when the power planes are thick (due to their low
DC resistance), so voltage differences will be difficult to measure. If
you attempt either of these approaches, use high-resolution equipment
that can measure sub-degree and sub-millivolt differences.
Using short bursts of current followed by a quick snap-shot IR camera
can sometimes capture the localized heating before it spreads throughout
the board. However, the approach takes some experimentation of current
profiles and camera timing. Again, our luck has been limited.
> The most likely problem is solder shorts with the BGAs, or cracked
> capacitors due to poor solder processing, or counterfeit parts.
> However, your board could also have warped during soldering, and/or
> process chemicals wicked causing a short between Vdd and Vss planes.
Agreed. For these types of shorts, fluoroscopy (real-time X-ray)
inspection works well for thin boards (i.e., boards with thin copper
planes). Board-assembly shops often have fluoroscopy capability, and
they can inspect every BGA and capacitor for pad-to-pad shorts.
One additional thing to look for. If this is a new board design, the
short might be due to something as simple as a part being placed wrong.
When working with a new board design, we have seen a few board assembly
shops place ICs on the board incorrectly, such as FPGAs rotated by 180
degrees, QFPs 90 degrees, LEDs rotated 180 degrees, etc. A manual
inspection of the part placement relative to the original CAD drawing
should catch these easily-fixable issues.
Good luck,
Pat Zabinski
Mayo Clinic
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