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44 The PCB Magazine • July 2014 EMBEDDED RESISTORS IN LOW OHMIC APPLICATIONS continues Terminating resistors are used to reduce sig- nal reflection and maintain proper signal in- tegrity. OhmegaPly embedded resistors can be placed very close to the end of the driveline, improving impedance matching and reducing line delay. For fine-pitch BGA packages where the channel widths between the pads surround- ing the interconnecting vias are insufficient for standard resistor footprint designs, resistors can be formed as part of the internal logic trace. A 10-ohm per square sheet resistivity product spe- cifically designed for low value series termina- tion has been used for more than 10 years in this manner. Built-in-trace resistors simplify designs by eliminating the resistor footprints under the SMT components. The resistor width equals the trace width, but the resistor length is not lim- ited (because the resistor can always follow the trace). Typical trace widths, and hence resistor widths, are greater than 125 µm. With the introduction of micro-ball grid ar- rays (µBGAs) with pad pitches of less than 500 microns, the channel width is reduced to the point where there is insufficient space for stan- dard built-in-trace resistor elements. Resistors with line widths less than 100 microns are re- quired [1] . The necessity for smaller line width resis- tors with good tolerances and power lead to the development of an optimized OhmegaPly alloy with improved chemical and physical stability. This improved material has the ability to cre- ate built-in-trace resistors less than 50 µm wide. Embedded resistors within 0.3 mm pitch µBGA or PBGA packages are therefore achievable where line and space widths are about 40 µm [2] . Embedded Resistors in Dual In-Line Memory Modules Higher memory speeds in DIMM modules requires termination of the bus to manage ring- ing and reflections. Typically these termination values will be 15–55 ohms. For example, ap- proximately 100 resistors can be embedded on a DDR3 RDIMM module and roughly 90% of those resistors are below 40 ohms [3] . The use of embedded resistors allows designers to place re- sistors where traditional SMT components can- not be placed. This improves signal integrity by placing the series termination or buffer resis- tors closer to the source and removes the restric- tion of placing along the board edges. A prime example of performance improvement is seen when making a qualitative comparison of noise levels on signals when using an embedded resis- tor and an equivalent SMT component [3] . Embedding low-ohm resistors frees valuable board area which can result in larger memory modules by using larger DRAM chip footprints or production of a simpler board by removing the resistors from the assembly process. Earlier this year, JEDEC released a standard for design of DDR3 204-pin unbuffered SO- DIMMS using embedded resistors [5] . Multi-Chip Modules Multi-chip modules commonly referred to as MCMs or SiPs are devices that combine multiple integrated circuits, semiconductor dies or other discrete components on a unifying substrate. Layout routing in such small footprints need to minimize vias and interconnections between layers. Some of the challenges of designing an MCM have been solved by using embedded re - sistors. Embedded termination resistors built-in- traces have the benefits of improving signal in- tegrity and eliminating the extra space for SMT resistors. Again, typical resistor values needed for impedance matching are in the range of 15– 51 ohms [4] . High-speed digital and RF SiPs such Figure 1: example of orBit layout.

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