When to embed: advice on factors to consider and questions to ask before layout.
What is the best way to decide? The available cost and performance modeling tools (1,2,3) are very sophisticated and effective but they are also relatively expensive and complex. Once the manufacturing process costs, material properties and costs, and performance characteristics are input for the chosen material set, the modeling process is quite fast. These tools are worthy of your investigation and the authors/developers are good guys with whom to consult.
In the absence of sophisticated tools, however, a simple logical decision process can be used (Figure 1). It is an intuitive process. The more questions you can ask and the more answers you can find, the greater the accuracy in your result. This discussion is not intended to ask all the questions or even to provide any answers. Rather I want to provoke your thoughts and get you on the road to a proficient thought process.
[FIGURE 1 OMITTED]
Let's walk through FIGURE 1. Start with the bill of materials (BOM). A separate review of the resistor and capacitor values and the tolerances and performance requirements should be made. Are there a large number of parts of similar values and tolerances? How many layers will be required? Will planar or discrete capacitors be required? Will a single resistor material set cover the majority of the resistor values? Which parts does it make sense to embed and which should be left alone?
If there are a large number of parts that can be embedded, significant board size reduction can be realized. Conversely, the board size may stay the same and functionality added to the freed surface real estate. In either case, reductions can be seen in the assembled board cost or the cost per function on the board. Note: these are system-level costs, not bare-board costs.
If the system costs cannot be reduced but signal speed is a show-stopper, then embedding may be required. Or if board size is a show-stopper and size reduction cannot be achieved with full implementation of HDI technology, then embedding may be required.
At this point, assuming embedding resistors and/or capacitors appear to be the proper choice, we review the R and C paths. If planar capacitance materials will meet the requirements, select the appropriate material set and analyze it in the PCB stack-up. If discrete capacitors are required, select the material set and create preliminary part libraries based on material properties and design guidelines, and analyze them in the PCB stack-up.
Resistors are discrete components even though some materials are referred to as planar or sheet resistors. Select the resistors to embed and the appropriate material set. Create preliminary part libraries based on material properties and design guidelines, and analyze them in the PCB stack-up. Some resistor materials cover a broad range of values; others are more limited. Varying the aspect ratio of the resistor element can also provide a range of values with a single material.
Assess the preliminary design for component density, added layers, stack-up, board thickness and board size. Select a PCB manufacturer experienced with the chosen material set(s) and work with that vendor during the preliminary design phase, incorporating DfM techniques to ensure a manufacturable design. Perform a bare-board cost analysis with and without embedding. Determine the component and assembly cost of the passives to be embedded. Determine the relative system costs with and without embedding. Confirm the expected performance benefits. Compile the assessment and coordinate this with your customer.
If the decision is to continue embedding, proceed cautiously, ask more questions, gather more intelligence, analyze thoroughly and focus on success.
(1.) Dr. Peter Sandborn, University of Maryland CALCE, Electronics Products and Systems Center (www.enme.umd.edu/ESCML)
(2.) Chet Palesko, Savansys Answer Systems (answer-systems.com).
(3.) Dr. Q. J. Zhang, Carleton University (www.doe.carleton.ca\~qjz).
RICHARD SNOGREN is a member of the technical staff at Coretec Inc. (coretec-inc.com). He can be reached at firstname.lastname@example.org.
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|Title Annotation:||Getting Embedded|
|Publication:||Printed Circuit Design & Manufacture|
|Date:||Feb 1, 2004|
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