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PCB Design Strategies in Handheld Devices

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Overview: This presentation reviews factors influencing thermal design in consumer handheld devices from touch temperature constraints, dissipating heat effectively from high power density components, tight tolerance geometry, through to varying power modes from user operation. Thermal management studies ideally allow accurate, rapid, physical modeling to make decisions on materials, components and layout beside power control feedback to achieve performance & target lifetime with reduced testing requirements. Simulation methods are discussed and closer integration with both electronic and industrial design functions as part of overall product development.

John WilsonJohn Wilson
John Wilson joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) after receiving his BS and MS in Mechanical Engineering from the University of Colorado at Denver. Since joining in 1999, John has worked on or managed more than 70 thermal and airflow design projects. His modeling and design knowledge range from component level to Data Centers, heat sink optimization and compact model development. John has extensive experience in IC package level test and analysis correlation through his work at Mentor Graphics’ Fremont based Thermal Test Facility. Currently John serves as an Electronics Product Specialist.

The following are questions presented to the speaker by the attendees during the webinar, along with answers to each.

Is there any optimum thermal conductivity needed for the heat spreader to have an effect?
Answer: Generally speaking higher conductivity is better. There is a conductivity above which there will be diminished thermal performance gains. The effect of the heat spreader will be influenced by the adjacent thermal conductivities.

There are different “limit” temperatures before injury depending on the material. There is a confort temperature of 41°C. What would be the heat flux corresponding to the confort limit?
Answer: The heat flux will depend on whether the surface in that area is being touched. The grip pressure and other interfacial parameters could also influence the heat flux.

Can you think of any benefits of heat spreaders in this application with different thermal conductivities in x & y in-plane directions?
Answer: I think the result would be generally higher temperatures more than a controlled heat flow direction in this application.

For outer case temperatures presented, was the case modeled as mulitple materials, or only polycarbonate on all surfaces?
Answer: The outer case body was modeled as a polycarbonate.

What options are availble for isolation in handheld?
Answer: The two lowest conductivity materials inside the phone were FR4 and air. Air is a good insulator, 15x lower than FR4. I am not sure what benefit could be gained by using an insulator with a lower conductivity than air.

Does your analysis assume perfect heat transfer accros interfaces?
Answer: All perfect contacts were assumed. It would be interesting to study various thermal resistances across critical joints to determine the thermal significance.

Did you know if the heat radiation effects the temperatures of the mobile phone?
Answer: About 50% of the heat lost from the phone was from thermal radiation.

What is the thickness of heat spreader sheet in your test and hwo did you bond it on case surface?
Answer: 50 micron

How much phase change material was assumed? (mass)
Answer: It was approximately 25x25x5 mm3 volume.

How is the PCM encapsulated, or is this just a computer simulation?
Answer: The PCM was assumed to be encapsulated in Aluminum and was only considered through simulation.

Can you tell us if paste TIM is good or the phase change?
Answer: Both have advantages.

I was always told that radiation was only a fraction of the overall heat loss for electronics cooling, Your presentation showed about 50% of the heat was dissipated for hand held phones and tablets through radiation. Is it therefore important to have the right emmissivity on the outer surface?
Answer: It would be important to know the emissivity for this reason.

Is pate temperature interferance material good or the phase change one
Answer: Similar to above, use that answer. [I won’t recommend one over the other]

Why cant we use micro heat pipes to transfer heat from heated case and then use small thermoelectric generators to generate power from it?
Answer: It is an interesting idea but ultimately the heat needs to be rejected to outside of the case. It might be beneficial but ultimately will require more heat to be rejected from the outer surface.

Do you know any material which works as thermal diode (heat transfer in one direction and zero heat transfer in reverse direction)?
Answer: I’m not aware of any materials like this.