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JANUARY 2024 I DESIGN007 MAGAZINE 45 be common. For smaller packages and higher speeds, smaller pitches are available. When we route differential pairs through a package, it is a good idea to place the balls connecting the two legs of the differential pair on adjacent grid points and surround them with enough return (ground) vias. Return vias help to keep the impedance bet- ter defined through the transition between the board and package and provide isolation to and from nearby signal and power connections. During the solder reflow process, depending on many factors, the size and shape of the sol- der balls will end up being slightly different. ese solder balls, no matter how tiny they look, still tend to be bigger than the connect- ing via barrels and traces. erefore, seemingly small absolute variations in geometry num- bers may result in very noticeable impedance, insertion loss, and crosstalk performance dif- ferences. Reproduced 2 , Figure 3 shows the simulated performance as a function of solder ball dimensions. Our final illustrations are related to power distribution. As opposed to the early days of electronic circuits, when carrying the power and ground connections around a board was done with wires or fat traces, to feed power- hungry chips consuming tens and hundreds of amperes today, we use power planes in our boards. When we define our stackup, the num- ber and thickness (weight) of our power and ground layers, it is very useful to know a little bit about the processes of how printed circuit boards are fabricated. To determine the current-carrying capabil- ity of a power-plane shape, we need to know the thickness and conductivity of the copper. Instead of thickness and conductivity, in the trade of copper foils, people use weight (you may hear about one-ounce or two-ounce cop- per) and method of production (electrodepos- ited or rolled/annealed copper). e recent "Mind Your Units" article 3 is a reminder that units matter, and the units used for PCB fabri- cation and trade may be counter-intuitive. How the PCB copper is patterned during fabrication will also influence the current-car- rying capability. During wet etching, the side- walls of traces and plane shapes will become tilted. On a single large plane shape, this may matter very little, but when we have large Figure 3: Effect of solder ball height on insertion loss and TDR profile (reproduced 2 ).