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Design007-June2021

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80 DESIGN007 MAGAZINE I JUNE 2021 etch the circuit layers using standard sub- tractive PCB processing. As far as chemistry, the printed circuit board industry commonly employs cupric chloride etchants for the pri- mary circuit image and resistive layer width definition. A second etch step is required to selectively remove copper to define the length of the resistor elements. is process requires a different chemistry to ensure only copper removal without altering or degrading the remaining resistive element geometry. Power Dissipation As far as handling power, the manufacturers state that power density is defined as the total power dissipated divided by the effective sur- face area. e power density of the resistor ele- ment increases as the area decreases. All other conditions illustrate that for the same power input, the temperature rise will depend on the area of the resistor. In other words, the resis- tors with a larger surface area can dissipate more power than a narrow geometry, provided that all conditions remain the same. If space is available, design the resistor element to be as wide as possible. Additionally, when the resis- tors are buried within the layers of the circuit board, the physical and thermal characteristics of the substrate material (the total thickness of the substrate and the collective number of copper layers) will directly affect heat dissipa- tion from the fully assembled circuit board. Omega Technologies notes that electrical current should not exceed the rated current- carrying capacity of the resistor because exces- sive current could cause permanent damage to the formed resistor element. [2] In-Process Testing Prior to further circuit layer lamination, the resistor elements' value and tolerance must be validated. e "flying probe" test is widely used to validate the embedded resistor elements' target value before lamination of additional circuit layers. Most flying probe testers can also perform signal integrity testing as well as iden- tify the location of opens and shorts. To enable test probe access, dedicated land features must be provided for each formed resistor element. Lands provided for test probe access must not be arranged in a way that would require probes to cross over or contact other probes. When laser trimming is required to fine-tune a resis- tor element, the probe contact must not inter- fere with laser access while making a trim. For best test and trim accuracies, the test lands should be near to the resistor-to-circuit termi- nation point to ensure precise resistor value measurement. When developing land geometry for test- probe access, probe tip shape and dimensions will determine the minimum land size. Test system manufacturers state that probes have a placement tolerance of 50 μm (0.002") in each of the X, Y, and Z axes. e land pattern diameter must be greater than the probe tip to accommodate probe positional tolerances and allow for uniform probe pressure and pad scrub. First-time users should refer to their cir- cuit board supplier to determine the suitability of their test equipment and availability of the required soware for testing the embedded resistor elements. Design Tools Most soware developers are currently fur- nishing the necessary tools to implement the embedded resistor elements within the multi- layer circuit board. Material suppliers suggest that the CAD designer consider using Mentor, Allegro, Intergraph, and PAD Power PCB in conjunction with an Excel program to aid in developing the more complex resistor element geometries. While several resistor elements will likely remain on the outer surface of the finished circuit board, the embedded resistors will require a unique reference designator to avoid procurement errors. For example, a sur- face-mounted resistor will be defined as R110 while the embedded or buried resistor will be labeled BR110 on the schematic diagram and material list.

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