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Thermal issues and properties of embedded resistors: a study of power dissipation characteristics and their effect on laminates is launched.

IT ALL STARTED in 1998. I was building circuit boards and Mike Jouppi was analyzing "black boxes" for aerospace use. Mike ran into a situation where he was considering the power dissipation effects of the copper traces on the overall thermal behavior of the electronics. His inherent curiosity and passion for discovery caused him to study the rules by which copper traces were sized. Long story short, he discovered the rules were based on data going back to the '50s. He wanted to verify the data and determine if the evolutionary changes in PCB materials and construction changed the rules. At that time, he was with Martin Marietta and I was with SAS Circuits, a PCB supplier to his company. Both of us were in Colorado and lived and worked only a few miles apart. He contacted me to brainstorm test vehicle design and fabrication. We have been working closely ever since. Since my entrance into the world of embedded passives (and resistors in particular), when Mike and I get together our conversation invariably turns to embedded resistors and their power-handling capability and effects on overall PCB temperature.

Last month during the EPUG (Embedded Passives Users Group) telecon, Mike made an excellent presentation on power issues of embedded resistors. During a subsequent day of skiing at Keystone, the conversation, as always, turned to embedded resistors and current. It's time to take action, we decided. While riding the lift and during occasional rest stops (mostly for me) to enjoy the great Colorado mountain vistas, we formulated a plan.

We decided to keep it simple. Also, to design the work in phases that would maintain focus on the desired direction and result, but provide for learning and mid-course corrections. This is not unlike my going around mogul fields: from experience, I have learned that moguls and my legs are not compatible and, for me, they usually cause painful "face plants." By going around them, 1 maintain my objective of getting to the bottom of the run pain-free and with minimal energy expended.

Mike and I will coauthor the resistor discussions in this column for some time. (As a side note, Mike has been running tests, performing analysis, studying conductor heating and is leading the development of a new IPC standard on current carrying capacity in printed circuits.)

We decided on a three-phase approach. Phase 1 will include fact-finding, test vehicle development and temperature measurement techniques. Phase 2, built on the learning from Phase 1, will test the fundamental resistor and power dissipation characteristics in PCBs and correlate these characteristics to PCB materials, stackups, and manufacturing processes. Phase 3 will incorporate the findings from Phases 1 and 2 into the IPC standards program.

We will maintain an active role in the EPUG as Well as IPC's standards development program.

Resistors are Poor Conductors

Yes, this is true. It is easier to visualize as we place them inside a PCB, sometimes, as with thin films, in line with the copper conductor. However, the resistor is still a resistor and a discrete component as well. Resistor components are designed, manufactured and qualified to rigid performance standards. For instance, MIL-PRF-55342G establishes 12 test conditions with 48 specific characteristics. These test conditions are:

* Maximum ambient temperature at full wattage.

* Maximum ambient temperature at zero wattage.

* Thermal shock.

* Power conditioning.

* Low temperature operation.

* Short time overload.

* High temperature exposure.

* Resistance to bonding exposure.

* Moisture resistance.

* Resistance temperature coefficient.

* Resistance tolerance.

* Life.

All of these involve power and temperature. It only makes sense that the resistors we design using the commercially available resistor materials to embed inside the PCB meet the same performance standards.

In the embedded world today we have two resistor material technologies commercially available, and a third emerging material close to commercialization. These are thin films, polymer thick films and ceramic thick films. Each material technology has its particular performance characteristics and manufacturing processes for integration into the PCB composite and cost. Although the physics is the same for the resistance of all of the materials, the thermal behavior of each material set may be different and will be affected by the design and manufacturing processes.

Next month, Mike Jouppi will begin our discussion of power dissipation and temperature limits.

RICHARD SNOGREN is a member of the technical staff at Coretec Inc. (coretec-inc.com). He can be reached at rsnogren@coretec-denver.com.
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Title Annotation:Getting Embedded
Author:Snogren, Richard
Publication:Printed Circuit Design & Manufacture
Date:May 1, 2004
Words:726
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