My last column, "Your Layout is Skewed" (EDN, April 18, 2002), concerns various styles of corners and bends you normally use on differential edge-coupled pairs. It points out that all corners, whether chamfered or not, add extra length to the outside trace as it rounds the bend. The length added by a 90? bend ranges from 1.5 to 2 times the intrapair trace pitch, depending on how the corner is chamfered. The extra time added to the outside trace is a form of intrapair skew.
This column considers two strategies for minimizing the intrapair skew that a differential net accumulates. The six BGA chips within the dotted line in Figure 1 illustrate the first strategy.
Pair A exits the bottom chip heading north. It enters the receiver (top chip) also heading north. Along the way, this pair takes one right turn and left turn. The skew accumulated in the two successive turns cancels to zero.
In a general routing problem, the required number and types of turns depend on the relative orientations of the driver and receiver. Because pair A starts and ends going in the same direction, this pair always makes equal numbers of right- and left-hand turns, no matter what happens in the middle (unless it makes a spiral). The skew that accumulates on any pair with a chip floor plan like A is zero.
Pair B doesn't fare as well. It exits the bottom chip heading east. It takes one left turn to head north (the orientation of the receiver), after which the number of left and right turns are balanced. The total skew that pair B accumulates equals the amount that one left-hand turn generates.
Pair C is the worst of all. It exits to the east and enters to the west. It therefore requires two extra left turns to achieve the correct orientation. If you are going fast enough so that every turn matters, you should carefully plan your chip orientations so that the accumulated skew naturally balances to zero.
The second strategy concerns the precise manner in which your pair enters or exits a BGA (or any field of connector pins, package pins, or vias). This strategy works best when the ball pitch exceeds the intrapair trace pitch. It works by offsetting the center line of the pair as it enters (or exits) the BGA (D in Figure 1). By offsetting the top pair toward the pink solder ball, extra time delay accrues to the blue trace. Offsetting the bottom pair toward the blue solder ball adds time to the pink trace. This strategy buys some time, which you can use to pay for other floor-planning inadequacies. It isn't perfect, but it delivers what you need-balanced skew.
When you have to adjust the skew, I favor doing it near either the driver or the receiver, whichever has the poorest termination. That way, the skew adjustment can't possibly affect the quality of the good termination at the other end of the line. If both ends have high-quality terminations, then you place the adjustment at either end. In an imperfect world, that's as well as you can do.
To those who yearn for a perfect layout with zero bends, I say, with all due credit to The Rolling Stones, "You can't always get what you want, but if you buy some time, you just might find, you get what you need."