Posts Tagged ‘IR Drop/Thermal co-simulation’

6 June, 2012

What?  Isn’t that backwards?  Technically, yes.  The board is merely a pathway through which ICs talk to each other, and receive power to do so.  However, if the power distribution network (PDN) of the board is inadequately designed, it can actually be heating up the ICs.

ICs are supposed to be the main source of heat on a PCB.  Heat is conducted from the ICs to the board through their pins.  Sometimes a metal slug or thermal glue is placed at the base of the component to enhance this effect.  This method of component cooling is sufficient for most components.  (The really power-hungry components will also require a heat sink to help cool them).  So this means that the hottest locations on the board are right beneath the components.  What is beneath the component?  In the case of a BGA, there is usually a dense pinfield.  This pinfield creates a web of copper which will choke the current feeding the IC.  The current has to find its way through narrow pathways created by the “swiss cheese effect” of all the pinfield anitpads in the power and ground planes.  These areas of high current density will start emitting power in the form of heat, which means a drop in voltage for the IC as well as additional heat in that area of the board.

This article discusses the problem in more detail.

Don’t heat up your ICs even more by designing an inadequate PDN.
How do you avoid that?  Simulate it!

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5 June, 2012

For over 50 years, designers have been calculating current-carrying capacity on PCBs using charts created by the Navy in 1956 (more specifically NBS (National Bureau of Standards) Report #4283 “Characterization of Metal-insulator Laminates”, D.S. Hoynes, May 1, 1956. Commissioned by Navy Bureau of Ships).  These charts are still in use today; many designers have probably seen them in the IPC-221 spec.  The problem with using these charts is that they are based upon fixed-width conductors in a specific set of environmental conditions.  The newly released standard pertaining to current-carrying capacity, IPC-2152, has expanded the number of charts to include different scenarios.  However, even the IPC-2152 spec advocates simulation as the most accurate means of predicting current-carrying capacity.

The issue has to do with a number of factors, including the co-dependence of current and temperature.  As the current through a conductor increases, so does the temperature in that conductor.  Since temperature affects conductivity, it will also affect the current through a given conductor size.  This co-dependence spawns the need for co-simulation between power integrity and thermal analysis.  Such analysis can also take into account the complicated, non-uniform shapes that are used to carry current in most modern PCBs, including the non-uniformity of current distribution as well as temperature distribution.

To find out more, take a look at my recent article on the subject in Printed Circuit Design and Manufacture Magazine.

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4 June, 2012

Simulation is a way of predicting reality.  The more information we put into the simulation, the better our prediction of what is really going to happen.  Certain aspects of electrical simulation, like signal integrity, can be simulated relatively “independently” of other influencing forces.  Sure, there are some temperature dependencies on silicon behavior, and those are typically represented by IBIS models created at different temperatures for fast, typical, and slow silicon behavior.  And also, the issues of vias tend to complicate things, as they blur the world between traditional signal integrity and power integrity as well as 3D electromagnetic simulation.  However, no two disciplines are more closely related than thermal analysis and power integrity, more specifically DC Drop.

The voltage drop across a plane is determined by the conductivity of the copper.  Copper conductivity changes 4% for every 10degC of temperature change.  That equates to a 32% change for an 80degC temperature rise, which is pretty significant.  So, in order to find an accurate measure of the voltage drop, temperature needs to be included.  The other interesting aspect of this is that drops in voltage mean power is being dissipated by the plane, power that is being dissipated as heat.  So, the results of each of these analyses will affect one another.  Hence, the need for PI/Thermal Co-simulation.

You can read more about this in my recent article in Printed Circuit Design and Manufacturing Magazine.

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